Research history of climate change

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The research history of climate change describes the discovery and investigation of climate change events in the context of geological and historical time periods, including global warming that has occurred since the 20th century . Systematic research into natural climatic changes began in the first half of the 19th century with the gradual reconstruction of the Ice Age cycles and other climatic environmental changes in the context of paleoclimatology and Quaternary research . As early as the end of the 19th century, human influences on the earth's climate system via greenhouse gases were suspected, but corresponding calculations were strongly doubted until the 1960s. Detailed descriptions of the research history of climate change , in particular of the anthropogenic climate change ascertainable in the course of the 20th century , can be found, for example, in Chapter 1 of the IPCC's Fourth Assessment Report and in more detail by the US physicist and science historian Spencer R. Weart . A German-language version based on the work of Spencer Weart can be found on the Skeptical Science homepage .

While the greenhouse effect was discovered as early as 1824, the climate-warming effect of the steadily increasing concentration of carbon dioxide in the earth's atmosphere could only be quantified towards the end of the 1950s due to improved measurement methods and a broader database. Although some scientists found that man-made air pollution could also cool the climate, climate research increasingly favored the assumption of warming from the mid-1970s. In the 1990s, through advanced computer models and a deeper understanding of the cold ages, the following consensus emerged: Greenhouse gases play a major role in climate change, and emissions caused by humans are primarily responsible for ongoing global warming .

Two hundred years of research into climate change

Discovering the basics

The first ice age theories

As one of the earliest pioneers of the concept of prehistoric times, the English polymath Robert Hooke suspected as early as the late 17th century based on fossils from the Jurassic (such as ammonites and sea ​​turtles ) that the southern English climate must have been considerably warmer in early geological history. Building on this, he suggested determining the climates of primeval habitats with the help of fossils . Against the then widespread belief in the biblical creation myth , the assumption of a primeval epoch, which encompassed considerably longer periods than the historically documented human history, was not able to prevail until a century later. In the course of the Enlightenment and with the development of geology into modern science from 1750, the idea of ​​prehistoric times gradually gained ground. Nevertheless, many scientists were still influenced in their thinking by religious ideas, as the basalt dispute showed. The engineer and geographer Pierre Martel made a first contribution to establishing the Ice Age theory in 1742 . In his view, the Chamonix glaciers were once much more extensive, suggesting a colder climate in the past. His Swiss compatriot Gottlieb Sigmund Gruner held a similar opinion , who in 1778, in his book Journeys through the Strangest Regions of Helvetia, associated the conglomerate of old terminal moraines with earlier glacier stands. P. 69 With these findings, Martel and Gruner were decades ahead of their time. The possibility of extensive glaciation as a result of a climate shaped by the Ice Age was too revolutionary an idea at the end of the 18th century to be accepted by science.

Erratic block deposited by the Fennoscan Ice Sheet on the island of Rügen .

Between 1780 and 1830 a fundamental debate, partly religiously motivated, took place between Neptunists and Plutonists (basalt dispute ). A central theme of the Neptunists was the Flood , which was often regarded as a real geological event or as a synonym for several global flood disasters until the first half of the 19th century. The controversy between Neptunists and Plutonists also became apparent in the dispute about the origin and "migration" of the erratic blocks ( boulders ) deposited by ice age glaciers in the Alpine region, in the north German lowlands and in Scandinavia and which are characteristic of glacial landscapes. The riddle of the widely scattered boulders was discussed more intensively from 1760, whereby in addition to the favored drift theory, above all water, mud and rubble floods as well as volcanic eruptions were used as explanations for the transport of the erratic blocks. P. 108 ff At this time there was still a long way to go from a deeper understanding of glacier dynamics and glacial morphology , and it was only the works and studies of Louis Agassiz , Johann von Charpentier , Karl Friedrich Schimper and Ignaz Venetz that began to paint an increasingly differentiated picture of the Ice Age climate from 1830 onwards and the associated processes.

However, in this early phase of research, the postulated climate changes could neither be limited in time nor even rudimentarily dated, and there was just as little clarity about the possible causes.

The sun as a cause of climate change?

In 1801 the astronomer Wilhelm Herschel discovered that between 1650 and 1800, a period later known as the Little Ice Age , a small number of sunspots seemed to be associated with poor wheat harvests and, he concluded, unusually low temperatures. The connection he postulated between the cyclical changes in solar activity and natural climatic fluctuations was, however, already controversial at the time and was subsequently discussed again and again in science until the end of the 20th century.

Research focuses on the Quaternary Ice Age

Milestones in research into the science of climate change between 1820 and 1930

By the middle of the 19th century, the now more numerous proponents of the Ice Age theory had gathered so much evidence and “ climate witnesses ” for the existence of an earlier Ice Age that it was gradually becoming more difficult to ignore the arguments put forward. In the course of the geological exploration of North America it also became clear that the cold phase observed in Europe was not a regional phenomenon, but apparently had affected the entire northern hemisphere . The ice age model was further confirmed by the discovery of very old glaciation traces in Africa, Australia and India, which, according to current knowledge, are assigned to the permocarbon glaciation about 300 million years ago.

As one of the most tireless exponents, the Swiss naturalist Louis Agassiz (1807–1873) campaigned for the scientific acceptance of the Ice Age concept. On numerous trips, combined with lectures in front of an academic audience, as well as through the publication of several books, he made a decisive contribution to the popularization of his ideas. However, around 1850 a scientific consensus on this topic was not yet in sight. This was delayed mainly for the following reasons: p. 532 ff

  • A “world winter”, as postulated by researchers such as Karl Friedrich Schimper , meant for the majority of contemporary geoscientists a relapse into the catastrophism founded by Georges de Cuvier and the associated cataclysmic theory . This view was now considered outdated and refuted and had been replaced by the "modern" actualistic concept of the English geologist Charles Lyell .
  • At the same time as the geological findings of a glacial period, clear indications of earlier warm periods were found in the corresponding stratigraphic layers . This apparent incompatibility became obsolete with advances in chronostratigraphy and geochronology , most notably the discovery that the Quaternary Ice Age had been interrupted several times by interglacials such as the Eem Warm Age .
  • The ideas about the possible extent and the flow behavior of glaciers were based on the example of the Alpine glaciers for decades. Drawing global conclusions from this local perspective, the geoscientists of the time almost unanimously rejected an increase in the size of ice fields that covered half continents. This doctrine changed dramatically when the exploration and measurement of the Greenland ice sheet began in the second half of the 19th century.

With a few exceptions, the Ice Age theory was generally accepted by 1880 at the latest and developed into an important pillar of the geosciences in the form of Quaternary research . However, for a long time there was no well-founded theoretical model that could physically correctly describe the causes of the various warm and cold periods in the history of the earth. Regardless of this, the foundations of today's climatology emerged in part parallel to the Ice Age theory and in their beginnings go back well into the 19th century.

Temperature, heat and thermal radiation

The widespread use of thermometers , including in greenhouses , began in the first half of the 18th century (temperature scales according to Fahrenheit , Réaumur and Celsius in 1724, 1730 and 1742, respectively). In 1767, Horace-Bénédict de Saussure measured the intensity of solar radiation in valleys and at height as the temperature in glass cases placed one on top of the other. In an improved version, a first “ solar cooking box ”, it reached temperatures of over 100 ° C.

The observation of the dynamics of temperature changes led Joseph Black , the discoverer of carbon dioxide , in the second half of the 18th century to distinguish the amount of heat from temperature . He founded the concepts of latent heat and heat capacity , but had a wrong idea of the drive for the heat flow that compensates for temperature differences , see caloric theory . In 1791 Pierre Prévost concluded from experiments by Saussure and Marc-Auguste Pictet , who had mapped hot and cold bodies with metal concave mirrors on thermometers , that a thermal equilibrium between bodies can be established through radiation alone , see Prévost's theorem .

Jean Baptiste Fourier

Jean Baptiste Joseph Fourier, Portrait of Julien Léopold Boilly (1796)

Jean Baptiste Joseph Fourier (1768-1830) explained the atmospheric greenhouse effect in 1824. He noticed that the earth was much warmer than a rough estimate without an atmosphere should be. He found that the atmosphere is very “ transparent ” to visible light, but not to the infrared radiation emitted by the heated ground . Clouds would make nights milder by absorbing this radiation. He compared the effect with that of the Saussure cooking box.

Fourier correctly recognized that most of the resulting warming of the cooking box is not due to the greenhouse effect, but to the suppressed convection . The warming of the box was mainly based on the fact that the solar radiation acted as a heat source and that the circulation between the outside and inside air was prevented. The term derived greenhouse effect (English greenhouse effect ) is used in climatology to this day, although the atmospheric greenhouse effect , especially on the climate impact of various greenhouse gases based. Fourier also correctly stated that both natural changes and the influences of human civilization can have an impact on the climate. However, he only expected such changes through changes in reflectivity, i.e. the albedo of the earth. Although Fourier was undoubtedly one of the best mathematicians and scientists of his time, he was unable to describe the warming effect of the greenhouse effect mathematically.

John Tyndall

John Tyndall

"Just as a dam causes a local river to swell, our atmosphere, which acts as a barrier for radiation from the earth, creates a rise in temperatures on the earth's surface."

In 1862 , John Tyndall (1820–1893) aptly described the natural greenhouse effect . In the course of extensive measurements carried out with the precision possible at the time, he identified the gases responsible for this. He found that water vapor is responsible for most of the greenhouse effect. He also correctly described the contribution of the other gases such as carbon dioxide (CO 2 ) or ozone (O 3 ) as significantly weaker, but not negligible.

As early as 1856, the American Eunice Newton Foote had observed in experiments a stronger warming in air-filled glass cylinders by solar radiation, if the air contained was moist or replaced by CO 2 . She noted that higher concentrations of CO 2 in the atmosphere would be associated with higher temperatures on earth. However, other properties of the gases dominated the measured temperature differences in their experimental setup.

Tyndall's measurements were based, among other things, on preliminary work by Macedonio Melloni , who had done pioneering work in relation to the measurement technology required for this. In Tyndall's apparatus, a tube about one meter long was used, the ends of which he covered with windows made of rock salt , as these, unlike glass panes, are transparent to infrared radiation. At one end he placed boiling water, the temperature of which is very easy to keep stable at the boiling point, and at the other end a thermocouple connected to a sensitive ammeter. The deflection of the ammeter was a measure of the amount of infrared radiation that could get through the tube to the thermocouple. Investigations of the absorption spectrum of the gases in the earth's atmosphere were not the subject of his measurements; he focused on a quantification of the absorption capacity for infrared radiation.

Convinced of the correctness of the then controversial ice age theory, he traveled several times to Switzerland from the mid-1850s (in 1856 together with the biologist Thomas Henry Huxley ), where he studied the plasticity of ice and the flow behavior of glaciers on site. This resulted in a large number of articles on this topic in the following years, which appeared in English, German and French-language magazines. Based on geological and geophysical issues, Tyndall devoted himself to meteorology and the impact of greenhouse gases on the climate. P. 495 ff He argued that a slight decrease in the concentration of carbon dioxide in the earth's atmosphere would cause a slight global temperature decrease. However, this affects the concentration of the far more effective greenhouse gas water vapor, which ultimately results in a strong cooling.

In order to understand the climate mechanisms of earlier warm and cold periods in detail, however, further physical knowledge was required, which was essentially only gained in the course of the 20th century. Scientists who pointed out towards the end of the 19th and beginning of the 20th century that humans were able to change the earth's climate through their work received little attention for a long time. According to general estimates, no warming was to be expected in the next few centuries, nor would an anthropogenic influence on the earth's climate system have been measurable. In addition, due to the lack of systematic measurements, there was no significant evidence of a change in greenhouse gas concentrations in the earth's atmosphere until the middle of the 20th century.

James Croll

Diagram of the Milanković cycles with an overview of the
precession (precession) , skewness of the ecliptic (obliquity) ,
eccentricity (excentricity) , fluctuations in solar radiation on the earth (solar forcing) and the cold and warm periods (stages of glaciation) .

Already in the early 19th century there was speculation about various astronomical causes of the ice ages. In 1824 , the Danish geologist Jens Esmark published the hypothesis that the orbit of the earth around the sun was strongly eccentric in prehistoric times and resembled that of a periodically recurring comet . In the 1830s, the French mathematician Siméon Denis Poisson suspected, on the basis of the then prevailing ether theory, that the universe was divided into warmer and colder regions through which the solar system moved over long periods of time. P. 475 ff The first well-founded and well-founded ice age theory was formulated by the Scottish naturalist James Croll (1821–1890). Drawing on the calculations of the mathematician Joseph-Alphonse Adhémar and the astronomer Urbain Le Verrier , in a sensational paper in Philosophical Magazine in 1864 he advocated the idea that changes in the earth's orbit in connection with the strong ice-albedo feedback are responsible for the formation of the Ice ages could be responsible. He was the first to point out the power of this feedback link in the global climate system. From around 1870 the possibility of cosmic or solar influences on the earth's climate was scientifically discussed on a broader basis.

Croll's theory was supported by concrete calculations in the 1920s and 1930s by Milutin Milanković and Wladimir Köppen . Until the 1960s, however, only a few climatologists believed that the cause of the Ice Ages was to be found in the Milanković cycles : the change in the intensity of solar radiation was very small compared to the observed temperature fluctuations. It was too small even if the water vapor and ice albedo feedback were included in the considerations. In addition, geological findings about past ice ages were found that apparently contradicted the theory. In the first half of the 20th century, the climate data on past ice ages and their cyclical processes were also too imprecise to prove or refute the theses of Croll and Milanković.

Physical laws

In addition to Tyndall's work, the Kirchhoff radiation law formulated by Gustav Robert Kirchhoff in 1859 and the Stefan-Boltzmann law developed by Josef Stefan and Ludwig Boltzmann in 1879 formed essential foundations. The latter made it possible to calculate the power that is emitted by a heater at a certain temperature. Wilhelm Wien supplemented the Stefan-Boltzmann law in 1893. With the help of Wien's displacement law , it was now possible to calculate the wavelength of the highest photon flow rate emitted by a radiator at a certain temperature. In 1900, Max Planck finally combined these laws in Planck's radiation law , which to this day represents the most important physical basis for understanding the Earth 's radiation budget.

Svante Arrhenius

Svante Arrhenius , one of the pioneers in the history of the science of global warming

The Swedish physicist and chemist Svante Arrhenius (1859–1927) was fascinated by Tyndall's idea that changing concentrations of carbon dioxide could be an essential factor in explaining the large temperature jumps between the warm and ice ages. Building on the preliminary work of Samuel Pierpont Langley , he was the first to do extensive calculations. Ultimately, he calculated a greatly simplified climate model , which he calculated over several months without any machine help. In 1896 he published his results together with the hypothesis that halving the carbon dioxide concentration would be enough to start an ice age. It was outstanding that he took into account the ice albedo feedback described by James Croll in his calculations.

He received prominent support for his theory from Nils Ekholm and Thomas Chrowder Chamberlin, among others . In a publication published in 1899, Cyrus F. Tolman estimated that the world's oceans contain about 18 times more carbon dioxide in the form of carbonic acid than in the atmosphere; however, the solubility of carbon dioxide is temperature dependent. It is therefore quite possible that these are the reservoirs in which the atmospheric CO 2 was dissolved during the ice ages. It could be released with increasing global warming and thus have an intensifying effect on the respective trend in global average temperatures.

Arrhenius initially only mentioned as a secondary aspect that anthropogenic CO 2 accumulation in the atmosphere could further increase the current earth temperature. It was only in a publication published in 1906 that he discussed this in detail. For the climate sensitivity he determined 5 to 6 ° C. Based on the worldwide emission rates of 1896, he expected the atmospheric carbon dioxide content required for such a temperature increase to be twice as high in about 3000 years, and only in a few centuries did he expect that a temperature increase would be measurable at all. He hoped for “more even and better climatic conditions” and “many times more harvests”. But he also understood that permanent use of fossil fuels would cause problems in the long term due to the associated global warming.

Arrhenius 'contemporary Walther Nernst took up Arrhenius' thoughts and suggested producing additional carbon dioxide to warm the earth's atmosphere. He wanted to burn coal for it that could not be extracted economically.

Early 20th Century: Criticism and Rejection

In the first half of the 20th century, Arrhenius' theory was initially predominantly opposed to it. His assumptions were based on too many unconfirmed and simplistic assumptions that the skepticism was justified. In his calculations, Arrhenius had only considered ice-albedo feedback and water vapor feedback in the absence of specific data using estimated values. He did not even consider heat transport through convection and ocean currents, and he lacked knowledge of the ice age atmospheric greenhouse gas concentrations to substantiate his Ice Age theory. In his considerations, he also did not include possible changes in cloud formation caused by warming in his calculations. Clouds can, however, significantly change the Earth's radiation balance , and some scientists of his time assumed that warming through increased cloud formation would compensate for this completely.

In 1900 a publication by the well-known physicist Knut Ångström appeared . In this he explained that halving the atmospheric carbon dioxide content would only change the infrared absorption by 0.4%, which could not have any significant influence on the climate. As it turned out later, Ångström's laboratory assistant carried out the measurement incorrectly, the spectrometers available at the time were too imprecise for the task, and he also misinterpreted the measurement results. Ångström wrongly assumed that the absorption spectra of water vapor and carbon dioxide largely overlap and that the absorption effect of the trace gas is therefore negligible. However, this was the result of the inadequate measuring devices for this measurement at the time. With a correct measurement, Ångström's assistant would have found a 1% change in absorption resulting from halving the carbon dioxide concentration. Another mistake arose from the fact that Ångström's assistant was taking his measurements at sea level. Even if there was no measurable difference in absorption there, the effect of a change in the concentration of the greenhouse gas carbon dioxide would not change anything: The greenhouse effect is decisive for the strength of the overall greenhouse effect in higher atmospheric layers, where the air is very dry due to the prevailing cold. Therefore, the overlap of the absorption band of carbon dioxide with that of water vapor has little effect overall. Since the air at high altitudes is not only very dry, but also considerably less dense than on the ground, an increase in the carbon dioxide concentration there definitely increases the greenhouse effect in the form of increased absorption. On average, the earth radiates its heat into space at an altitude of 5500 m. An increase in mean global greenhouse gas concentrations means that the area in which the earth radiates is shifted to greater heights. However, since it is colder there, heat is radiated less effectively there; the additional heat build-up causes all layers of the atmosphere below to become warmer until the radiating layer again loses as much energy in the direction of space as is radiated from the sun. Arrhenius recognized the flaws in Ångström's argument and vehemently disagreed.

1930s: Guy Stewart Callendar

Chronological sequence of climate science, 1930–1960

In the 1930s it was noted in the US that temperatures in their region had increased over the previous decades; The majority of scientists assumed a natural climate cycle, and an increased greenhouse effect was just one of many possible causes.

After evaluating the temperature data of the last 50 years from 200 meteorological stations, Guy Stewart Callendar (1898–1964) determined a statistically significant global annual warming rate of 0.005 ° C. He believed that this warming was too pronounced and extensive to be based on natural climate fluctuation. In a paper published in 1938, he estimated the amount of carbon dioxide emitted over the past 50 years at 150,000 million tons. He assumed that about three quarters of them were still in the atmosphere. He estimated the warming resulting from this amount of greenhouse gas to be 0.003 ° C / year (as of 2011: 0.02 ° C / year) and assumed a CO 2 concentration of 274 ppm in 1900; By extrapolating the then estimated annual emission rate of 4,500 million tons of carbon dioxide, he expected an atmospheric carbon dioxide concentration of 396 ppm for the year 2100 (this value was reached in 2013). He estimated the warming resulting from the burning of fossil fuels to be 0.16 ° C for the 20th century, 0.39 ° C for the 21st century and 0.57 ° C for the 22nd century. Callendar also viewed global warming as something positive, as anthropogenic warming seemed to banish the risk of a soon recurring ice age for the foreseeable future.

1940s: Hermann Flohn

Hermann Flohn was the first German climate researcher who represented the global climate impact of anthropogenically increased CO 2 concentrations or anthropogenic climate change since his inaugural lecture in 1941 at the University of Würzburg and published numerous publications on this until his death in 1997. Flohn is regarded internationally as one of the pioneers of international and national climate research and has repeatedly referred to the CO 2 problem since the post-war period. Although this position was not without controversy among climatologists at the time, he received support from experts, including the leading climatologist Michail Ivanovich Budyko .

Middle of the 20th century: First rejection, then acceptance

Even if individual scientists emphasized the climate relevance of increasing carbon dioxide concentrations at a very early stage, Callendar's work was largely criticized. At the time there was no solid evidence that atmospheric CO 2 levels actually increased. The available data on atmospheric CO 2 content were very imprecise. Measurements resulted in values ​​that deviated so strongly from one another, depending on the measuring location and measuring time, that neither an average concentration was known nor a possible increase was detectable. In the world's oceans there is 50 times more carbon dioxide dissolved in the form of carbonic acid than the entire atmosphere contains. Since carbon dioxide dissolves well in water, the overwhelming number of scientists assumed that all additional amounts of the trace gas carbon dioxide brought in by humans would disappear in the sea. Especially since it was known that the amount of CO 2 emitted by burning fossil fuels is only a tiny fraction of the amount that is converted within the framework of metabolic processes such as photosynthesis and respiration .

The work of Tyndall, Arrhenius and Callendar was hardly discussed any more. They also contained too many theses that could not be checked for the foreseeable future. The undeniable findings of the ice ages were still waiting for a solution, but the ice ages were explained by geological causes that had influenced the climate locally through changed wind and ocean currents. Hardly anyone thought global climate change possible at that time.

In 1951 the American Meteorological Society wrote in the Compendium of Meteorology : “The idea that increasing the amount of carbon dioxide in the atmosphere could change the climate was never widespread and was eventually discarded when it was found that all infrared radiation absorbed by carbon dioxide was already from Water vapor is absorbed. ”That this is wrong and Arrhenius was right with his objection, however, had already been published almost 20 years earlier - among others by EO Hulburt and Guy Callendar.

Late 1950s: Theory is revived

The 1950s brought an enormous increase in knowledge in all areas of science. As a result of the Cold War , the American government increased research spending in many areas of science and technology, including geology, oceanography, and meteorology. The military was interested in how the radiation from the atomic bombs is absorbed and how the fallout is distributed in the atmosphere and the oceans. They also wanted to know when someone, somewhere, was going to carry out an unearthly atomic bomb test. There was hardly an area that could have been unimportant to the military.

One of the most important achievements in paleoclimatology was the combination of radiometric dating with chromatography and mass spectrometry . This made it possible to determine the absolute age and thus the time of origin of many fossils.

Key technology radiocarbon dating

Willard Libby had already developed counting methods for very low levels of radioactivity in the 1930s. Building on this, he introduced radiocarbon dating in 1949 . With this revolutionary method, one could determine the age of carbonaceous fossils, which were not older than 50,000 years, with previously unknown accuracy. Above-ground tests of nuclear weapons led to a strong increase in the concentration of radioactive 14 C, the nuclear weapon effect . With the help of Libby's new technology, it was now possible to detect the 14 C generated by atomic bomb tests .

The director of the US Scripps Institution of Oceanography , Roger Revelle , had studied the chemistry of the oceans intensively at the beginning of his career in the 1930s. He was considered an expert in this field and had gathered enormous knowledge about the gas exchange between the atmosphere and the oceans. However, he did not have adequate methods to study the gas exchange of carbon dioxide, so he turned to other things.

In order to be able to absorb additional carbon dioxide produced by burning fossil fuels, the oceans had to mix. As part of a research project, Revelle accidentally found out that radioactive carbon, which was formed in an underwater bomb test, moved in a layer that was only one meter thick, but stretched over hundreds of square kilometers. To his great astonishment, this proved that there was hardly any vertical mixing of the water layer. If this was true of 14 C from nuclear tests, then it had to apply to every other substance that was introduced into the oceans - including carbon dioxide.

One day Revelle became aware of the work of Hans E. Suess , who was concerned with optimization methods for the new radiocarbon dating. This went well with his research projects on ocean mixing and gas exchange; Fortunately, he had budgets to recruit Suess to work with him to tackle the unresolved issues of ocean carbon dioxide exchange.

Conflicting results

After evaluating the 14 C studies, Revelle and Suess published in 1957 that the average retention time of carbon dioxide in the atmosphere was around 10 years. This was in good agreement with the results of the research group led by James R. Arnold , who had previously worked at Willard Frank Libby and was currently working at Princeton University . In 1958, Arnold moved to Revelle at the then newly established campus of the University of California in San Diego .

The researchers estimated the duration of a complete overturning of the oceans to be a few hundred years. The results suggested that carbon dioxide produced by burning fossil fuels dissolves very quickly in the oceans, making it unlikely to accumulate in the atmosphere. However, this would render speculations about a possible, man-made global warming, which was caused by an increase in the concentration of the trace gas, insubstantial.

However, these results contradicted Guy Callendar's analyzes. He never tire of pointing out that the (rather imprecise) measurement series of the trace gas carbon dioxide available to him clearly indicated that this was accumulating in the atmosphere. There was, however, a much more important indication: In his studies of radiocarbon dating, Suess had discovered that younger wood samples had a shifted 12 C / 14 C ratio: the younger they were, the less 14 C they contained. And less than could be justified by radioactive decay. This effect could be explained if the carbon dioxide in the atmosphere had been mixed with carbon dioxide that came from the burning of fossil fuels, in which 14 C had almost completely decayed due to its old age. This effect later became known as the Suess effect . The arguments could not be dismissed out of hand. Revelle and Suess looked for mistakes in their work. First they tried to explain the contradiction by not including the uptake of carbon dioxide by plants in their considerations. Ultimately, however, Bert Bolin and Erik Eriksson found the main problem: The researchers had investigated the exchange of substances in the presence of an equilibrium concentration between the atmosphere and the oceans. Burning fossil fuels, however, leads to a steady flow of CO 2 , and there is no equilibrium. If the very slow rate of circulation of the oceans is also taken into account, this leads to a completely different result: According to this, although atmospheric carbon dioxide would be dissolved quickly, it would also be re-emitted into the atmosphere just as quickly, so that only about 25% would be absorbed by the oceans. The relationship between atmospheric emission and uptake by the oceans was named after Roger Revelle and called the Revelle factor .

Everything now indicated that Callendar was right, that carbon dioxide was indeed building up in the atmosphere.

Keeling's measurements

In order to clarify the question of whether the increase in the concentration of the greenhouse gas carbon dioxide in the atmosphere predicted by Revelle and Suess was actually measurable, the Scripps Institute applied with the project of an atmospheric carbon dioxide measurement for the international geophysical year 1957/58 . The young chemist Charles David Keeling was entrusted with the project; Just one year later he came up with the “Keeling curve” named after him, which was the first unequivocal evidence that the concentration of this greenhouse gas is increasing. In contrast to his predecessors, who failed in this task, Keeling carried out his measurements far away from sources and sinks of the trace gas and for the first time used a non-dispersive infrared sensor with a measurement setup that provided results of the highest precision. His measured values ​​received additional accuracy from the fact that he did not record them selectively, but continuously at several stations that were far apart.

The greatest experiment in human history?

Absorption spectra of the gases in the earth's atmosphere

In 1956 Gilbert Plass used computers for the first time to calculate the expected warming, and for the first time precise absorption spectra of the CO 2 were included in these model projections . Physicists from Johns Hopkins University had carried out corresponding measurements , and Plass was able to access this data as part of a cooperation. He was the first to prove that the absorption bands of water vapor and carbon dioxide do not overlap. He also found that global warming caused by an increase in the concentration of carbon dioxide would not be prevented even if the absorption bands were completely superimposed. He calculated global warming of 3.6 ° C for an assumed doubling of the atmospheric carbon dioxide concentration. For the year 2000 he assumed a 30% higher carbon dioxide content in the atmosphere and expected a global warming of about one degree as a result.

Based on these data, measurable anthropogenic global warming was no longer to be expected in centuries, but in decades. The greenhouse effect was quantified more precisely by Plass's calculations, and the increase in the concentration of the greenhouse gas carbon dioxide was now clearly documented. Roger Revelle commented on this with the frequently quoted words: "Mankind has started a large-scale geophysical experiment that has never existed in this form in the past, nor will it be repeated in the future."

1960s

Since the 1940s and throughout the 1960s, mean temperatures decreased globally. Doubts about the theory of man-made warming were confirmed, because the carbon dioxide concentrations rose during this time. There was talk of global cooling through aerosols . Some researchers blamed increasing environmental pollution of the air for the cooling . At the time of the war-related arms industrialization up to 1945, the winters with extreme cold increased in Europe. In the period up to 1960, residential houses were also heated entirely with coal, and heating oil was not yet available in sufficient quantities. This smog problem caused by coal burn-up was to be repeated 50 years later with the rapid industrialization of China.

First climate models

The history of climate science from 1960 to 2010

The availability of the first computers led to the first numerical weather forecast in the 1950s , and of course one wanted to use computers to calculate climatological processes. However, this initially brought more confusion than clarification and fueled doubts about the correctness of the thesis of global warming.

With the help of the exact absorption data of water vapor and carbon dioxide that Gilbert Plass had published a few years earlier, Fritz Möller calculated a one-dimensional climate model in which he not only included the water vapor released by additional heating, the so-called water vapor feedback , but also the heat exchange between the soil and the atmosphere. To his astonishment, his calculations showed massive warming and, under certain conditions, even unending warming, which continued to intensify until all the oceans evaporated. But assuming that the warming resulted in an increase in cloud cover of one percent, this would have completely offset the warming effect of an increase in carbon dioxide concentration itself by 10%. And nobody knew the reaction of cloud formation to a change in temperature. The correct description of the influence of the clouds was a big problem and should remain so in the following decades.

The reason for the warming spiral identified by Möller was quickly found: In his one-dimensional climate model, he had taken into account the heat transport between the ground and the air, but not the heat transport through convection . Syukuro Manabe realized this as early as the mid-1960s and developed it further together with Richard Wetherald . The “Manabe-Wetherald one-dimensional radiative-convective model” created in 1967 is considered to be the first reasonably realistic atmosphere model that took into account both the earth's radiation budget and the convection taking place. This model showed a warming of 2.3 ° C that would result from a doubling of the concentration of carbon dioxide in the atmosphere.

First earth observation satellite

Representation of the Nimbus-3 satellite

In the mid-1960s, another key technology became usable for climatology : Earth observation satellites . The second generation of TIROS satellites was operationally used for climate research from 1966 onwards and had radiometers and spectrometers . From now on one could measure the heat balance of the earth, its ice cover or the spectrum and intensity of the solar radiation from space. For the first time, solar-related measurements were completely free of falsifying atmospheric influences and led to a precise definition of the solar constant , which previously could only be approximately determined.

With the help of the Nimbus III satellite , Manabe was able to verify his climate model with measurement data from space in 1969. It showed a good match.

The number and quality of the built-in satellite instruments was to increase sharply in the coming decades, with considerable progress also being made in miniaturization .

First warnings

Mikhail Ivanovich Budyko did further pioneering work . He calculated the radiation balances for incoming and outgoing radiation in arctic regions and provided quantitative information for the ice-albedo feedback, which was previously only described qualitatively . He emphatically warned of the resulting climate changes, which, however, were not to be expected until the next century. An advisory body to the US government also expressed concern in 1965 that global warming was a serious threat ("... a matter of 'real concern"). The experts therefore recommended that the opportunities and risks of geoengineering be examined. This was intended to increase the albedo of the earth's surface in order to compensate for the warming effect of the increasing carbon dioxide concentration in the atmosphere.

Around 1965 an assessment was as follows:

“People's ovens and combustion machines emit around 12 billion tons of carbon dioxide into the earth's atmosphere every year. In the next fifty years the amount will quadruple. Such a growth rate could raise the mean temperature on earth by about 1 ° C and thereby, in the long run, melt the Greenland Ice Sheet and the vast Arctic ice fields, raise the sea level by fifty meters and shut down all the ports and coasts in the world Put water. "

1970s

The contradiction of increasing carbon dioxide concentrations in spite of falling temperatures worldwide prompted John D. Hamaker to develop a theory according to which an increased greenhouse effect via changed cloud formation, changed precipitation patterns and processes in the biosphere would initially cause warming, but then increased icing on the Poland and through the ice-albedo feedback would trigger the beginning of an ice age. However, on the basis of research results from later years - especially the data from the Vostok ice core - his theory was refuted.

Global average temperatures continued to decline until the mid-1970s, leading to heated controversy in climatology. Even then it was assumed that the massive aerosol inputs into the atmosphere could be the cause of the observed cooling. The US president was warned by George Kukla and Reid Bryson , among others, of an ice age that would result. In a work published by Stephen Schneider among others , it was speculated that the cooling effect of the aerosols could mask the warming effect of the greenhouse gases. The problem was that at that time there was a lack of knowledge about the exact extent of cooling or warming effects and therefore no one knew which effect was stronger.

On the other hand, however, a significantly larger group of researchers warned of significant global warming to come. Given the current carbon dioxide emissions, the warming could possibly lead to an ice-free polar sea as early as 2050. The German Physical Society also warned of man-made climate change in a press release for the 36th Physics Conference in 1971. "The observable effects are still very small", but if industrialization and population growth continue unchecked, "in two or three generations at the latest the point will be reached at which irreversible consequences of global proportions will inevitably occur". If the increase in fossil fuel consumption continues, then in the year 2000 an atmospheric CO 2 concentration "between 370 and 380 ppm" would be reached.

In 1975 Wallace Broecker wrote in the abstract of one of his publications:

“If man-made dust is not the main cause of climate change, there are compelling arguments that the current cooling trend will end in about a decade and be replaced by carbon dioxide-induced warming. Similar to similar events in the past, the natural cooling of the climate, which had masked the effect of carbon dioxide since 1940, will end. Once that happens, the exponential increase in atmospheric carbon dioxide concentration will become a significant factor and, by the beginning of the next century, will bring the planet's temperatures outside of the ranges seen for the past 1000 years. "

Broecker was to be proved right with his prognosis - even without the reinforcement by an approximately 80-year natural cycle, which he erroneously assumed at the time, rising CO 2 concentrations in particular caused such a rise in temperature. Not only was his work frequently quoted, the term global warming used in it was also taken up. Global warming or its translation global warming became a synonym for man-made climate change.

Erratic climate change and short warm periods

The picture of past ice ages could be drawn more and more clearly and showed that climate changes can happen very quickly. In complete contrast to the assumption of an unchangeable and stable climate, which had been widespread for decades, everything now indicated that even small parameter changes could result in sudden climate change . Preliminary work from 1966 had already provided evidence that rapid and violent climatic changes had occurred at the end of the last Ice Age. The findings, which came exclusively from sediment cores from the sea floor around Greenland in the 1960s, could now also be found in other places on earth and with other detection methods such as B. Ice cores are reconciled. They also consistently showed that a warm period like that of the Holocene was not the rule but an exception in the history of the Quaternary climate . Short warm periods alternated with long cold periods. Even in the 1970s, no metrological evidence could be provided for the long-predicted but never confirmed global warming. In addition, the current warm period, the Holocene, lasted for 11,700 years, while the last warm period, the Eem warm period , ended after a period of 11,000 years. For some, an imminent ice age seemed more likely than warming.

First global climate models

Drill core surveys from Greenland showed that along with the climate, the salinity of seawater had fluctuated in the past. The North Atlantic Current had apparently changed several times. This supported the assumption that ocean currents play an important role in climate change because of the very large amount of energy they can transport. Syukuro Manabe recognized the great importance of the oceans for understanding climate change and in 1969 designed the first climate model with which he modeled the behavior of the oceans. Unfortunately, even in the 1970s, computers were far from being powerful enough to calculate such a complex climate model over long periods of time. The 14 C investigations by Revelle and Suess had shown that the oceans needed almost 1000 years for a complete revolution. This was short in geological timescales, but the duration of a single ocean revolution was clearly too long as the calculation period for a complex climate model. Climate models, which, in addition to the radiation balance and convection, also take into account the behavior of the oceans, therefore had to be greatly simplified in order to remain predictable.

Together with the oceanographer Kirk Bryan , Manabe managed to design a simplified climate model that included the radiation balance and convection as well as the seasons and the behavior of the oceans. In 1979, their model could be calculated over a period of 1000 years. While it had many shortcomings, it had some characteristics of our earth's climate; For example, the desert region of the Sahara and the heavy precipitation in the Pacific region developed without the researchers having specially designed the model to show these phenomena.

paleoclimatology

Isotope stages in the temporal area of ​​the Neogene - Quaternary boundary

With the help of climate models, researchers have now attempted to correctly reproduce the climate during the ice ages as well as that of modern times. If this were successful, it would be possible to know which feedbacks are affecting the climate system to what extent, and these parameters could be used to estimate the extent of future warming. The prerequisite for this, however, was to know the climate of past ice ages. This is exactly what was attempted with the CLIMAP project in the 1970s , as advances in isotope analysis and mass spectrometry made it possible to reconstruct the past climatic conditions better and better.

In 1953 Willi Dansgaard had shown that the composition of 18 O (oxygen-18) and 2 H (hydrogen) in rainwater fluctuates depending on the prevailing temperature. This principle of the so-called oxygen isotope level was used in the 1970s by Cesare Emiliani , John Imbrie and Nicholas Shackleton to analyze the temperatures of the Cenozoic . The analysis, covering a period of one million years, revealed in an excellent way that fluctuations in solar intensity caused by changes in the earth's orbit were responsible for the strong climatic fluctuations during this time. The work of Imbrie and Shackleton removed the last doubts about the correctness of the theory of Croll and Milankovic; this became popular under the term Milanković Cycles . Slightly modified, the theory has been an integral part of paleoclimatology and Quaternary research since the 1980s and is an indispensable instrument both in the reconstruction of the Ice Ages and in researching various climate change events during the Phanerozoic .

In addition, future climatic developments can also be forecast with the help of the Milanković cycles . Based on his analyzes, Shackleton expected a new ice age within the next 20,000 years.

Danger from collapsing ice sheets

In the early 1970s, theoretical considerations about the structure of ice sheets showed that they are inherently unstable and, under certain conditions, tend to collapse. The glaciologist John Mercer then discovered in 1978 that the West Antarctic ice sheet has a special topology that can lead to such a collapse. The ice sheet of West Antarctica rests on rock surfaces that are below sea level; the sea floor there rises the further one moves away from the continental shelf towards the sea, only to then drop again. The grounding line is the point at which the ice sheet loses contact with the solid ground and begins to swim. From this point on, we no longer speak of an ice sheet, but of an ice shelf. If the contact line of the ice sheet, caused by a melt, were to overcome the highest point of this profile, an unstoppable dynamic would set in, which would lead to an accelerated and unstoppable collapse of the glacier. Mercer stressed that such a collapse would be one of the first disastrous consequences of man-made climate change. He mentioned in the same publication that such an event would be heralded by the breaking of several large Antarctic ice sheets.

Other sources of warming

In the years that followed, the researchers found a number of other factors that also contribute to global warming.

Other greenhouse gases

The atmospheric chemistry made great progress. The planned construction of a fleet of high-flying supersonic planes, as well as a large number of expected space flights, drew the researchers' attention to the effects of the associated emissions in the stratosphere. Investigations showed that the ozone layer would be damaged by nitrogen oxides and CFCs , which, in addition to having a very long life in the atmosphere, also had enormous potential as a greenhouse gas . For the first time, the effects of previously neglected greenhouse gases such as methane and nitrous oxide were also pointed out. However, these voices received little attention, after all, they were only components of the air whose concentration was very low even compared to the trace gas carbon dioxide. People preferred to speculate about the extent to which sulfuric acid could change the reflectivity (i.e. the albedo ) of the earth via a change in cloud formation and thereby have a cooling effect.

Waste heat

In the first two reports to the Club of Rome from 1972 and 1974, in addition to the anthropogenic greenhouse effect, the causes of global warming were also discussed for the first time as “thermal pollution” from waste heat. With their hypothetical continuation with maximum use of photovoltaic energy, the global growth limit determined by just tolerable warming would be reached in the coming centuries. With the exclusive use of non-renewable energies with an annual increase of 2%, an anthropogenic waste heat contribution to global warming of at least 3 degrees in the year 2300 was calculated, which, given the simplicity of the model used, agrees surprisingly well with more recent, more complex simulations.

First World Climate Conference

Syukuro Manabe (May 2018)

A central milestone for the recognition of climate change as a “serious problem” and a breakthrough for international climate research was the 1st World Climate Conference in 1979, which was carried out on the initiative of Hermann Flohn (member of the WMO expert group ). The result of the world climate conference was a fundamental declaration and the initiation of the world climate research program and the IPCC .

Events such as the drought in the Sahel zone increased the political pressure on the decision-makers, who, however, were undecided as to how concrete which threat actually was, because the climate researchers themselves were divided. So the science advisor to the US government (a geophysicist!) Decided to appoint a panel of experts that was unencumbered in the discussion. Under the direction of Jule Gregory Charney , experts were interviewed who were not yet involved in the ongoing debate. Charney's group compared two climate models, one by the Japanese climate and meteorologist Syukuro Manabe , the other by the US climate researcher and later NASA director James E. Hansen . Both models differed in details, but not in the key statement that an increase in the concentration of the trace gas carbon dioxide would undoubtedly lead to a significant increase in temperature. The experts checked u. a. Using simple, one-dimensional atmosphere models to determine whether the previous models could have neglected a significant effect - they found nothing. For the warming to be expected when the carbon dioxide content of the atmosphere doubled, Manabe's model had shown 2 degrees, Hansen's model showed a warming of 4 degrees. It was finally agreed that the most likely value was 3 degrees, knowing full well that this was ultimately only an estimate.

In the 1979 “Report of an Ad hoc Study Group on Carbondioxide and Climate” with the title “Carbon Dioxide and Climate, A Scientific Assessment” by the National Research Council , it was read next to it that significant warming due to the thermal inertia of the oceans is only in a few decades can be expected. The report was later called the Charney Report for short and its content was in good agreement with a report by the JASON group of experts that appeared in the same year.

Start of the earth system analysis

As early as the 1960s, scientists had recognized that the processes in the climate system are the result of a large number of interacting processes. It can only be understood if the mutual influence of all components and processes involved can be understood and mapped in a suitable form. In 1972 the International Institute for Applied Systems Analysis was founded in Vienna to promote research in this area . As a result, the US NASA founded the Earth System Sciences Committee in 1983 and the Potsdam Institute for Climate Impact Research in Germany in 1992 . Since then, the earth system science carried out in these institutes has been researching the development and effects of global environmental changes.

1980s

The number of scientific publications on climate change in the 1980s was approximately twice as high as in the 1970s. Many details on the history of the climate came to light: For example, the sudden changes in climate that had already been discovered in the 1970s were described in more detail as Heinrich events and Dansgaard-Oeschger events . So good was our understanding of the magnitude of the greenhouse effect and changes in greenhouse gas concentration that T. Wigley and Philip D. Jones wrote in an article published in Nature in 1981 : “Although the view that an increase in carbon dioxide Concentration leads to a warming of the climate, is widespread, this warming is not yet detectable due to the noise in the climate system. ” In their work they explain that this warming will only be pronounced enough for it to become clear towards the end of the century stands out from the background noise.

In Germany there were serious traffic restrictions due to smog , and the problem of increasing air pollution was recognized. The Helsinki Protocol , which came into force in 1987, reduced pollution around the world, reversing the cooling trend observed since the 1940s. But not only has it been warmer since 1974, 1988 went down in history as the warmest year since systematic weather records began.

Control mechanism of the earth

In the early 1980s, researchers found that the carbon cycle had some sort of regulating mechanism that kept the earth in a temperature range that was favorable for the development of life for most of the time. It has long been known that the greenhouse gas carbon dioxide enters the atmosphere through volcanic activity and is removed again through the weathering of rock. The weathering process of rock has two essential properties. On the one hand, its intensity depends on the average temperature of the earth, and at higher temperatures more carbon is bound, and on the other hand, it runs very slowly. So it was to be assumed that there must have been phases in the history of the earth in which this process had failed because it reacted too slowly. Geologists finally found suitable rock layers in Namibia: the older layers were covered by a long-lasting glaciation which, as was known, affected large parts of the globe. Since ice surfaces reflect incoming sunlight very efficiently, a very high greenhouse gas concentration was necessary to free the earth from this icing again. In fact, the younger layers above it testified to a very high rate of weathering afterwards, as was to be expected in a warm climate. During the long Ice Age weathering was very low, while volcanoes screwed the carbon dioxide of the atmosphere continuously getting higher, began to melt through the ice and thus the cooling "mirror" of the white ice was becoming increasingly smaller, what about the ice-albedo feedback to The warming process accelerated until all the ice had melted and the earth was in a hot climate. This extremely warm climate lasted for tens of thousands of years, exactly as one would have expected based on the dynamics of rock weathering. The reason was that the ice masses melted considerably faster than the carbon dioxide could disappear from the atmosphere. With the discovery of the regulating mechanism, an essential element for clarifying the so-called paradox of the weak young sun was found. Scientists had long sought an explanation for the fact that liquid water existed throughout the history of the earth, although the solar radiation in the Archean was about 25% weaker than today and had only increased steadily over the course of a few billion years.

Climate models

Researchers now also included fluctuations in solar activity in their calculations and began to consider details of land masses. For example, they parameterized the speed with which rain flows off on different soils and the lower reflectivity ( albedo ) of forests compared to deserts. Despite great efforts, climate models were deficient in many respects even in the 1980s. Several years of simulation runs usually ended in unrealistic conditions; In the absence of alternatives, modelers often selected parameters without an empirical basis, just to rule out such impossible states.

One of the unsolved problems in the 1980s was the low temperature difference between polar and equatorial regions, which was not found in the climate models and which apparently existed during the ice ages. The CLIMAP data did not match the models, regardless of how the researchers tried to parameterize them. A comparison of 14 climate models also showed that clouds were in no way adequately depicted in the models. Unfortunately, the available measurement data from satellites were not accurate enough to correct this deficiency on the basis of observations.

The danger of the nuclear winter

When the cold war seemed to escalate in the 1980s , atmospheric physicists began to investigate the possible consequences of a global nuclear war. Several independent studies immediately pointed out the danger of a nuclear winter , which would result from the massive aerosol input that the explosion of a large number of atomic bombs would cause. Since such an event would threaten the continued existence of mankind, the discussion about it gained public attention. The topic was also processed in the media and was, among other things, the subject of the successful television film The Day After , which was subsequently shown in many cinemas around the world.

Arrhenius confirmed

Even Alfred Wegener took the Greenland ice in the 1930s ice cores to gain valuable information from it about the past climate. Advances in physical and chemical analysis made it possible for researchers in the following years to extract more and more information from the samples. After years of unsuccessful efforts, at the beginning of 1980 it was finally ready to reliably reconstruct the carbon dioxide concentration of days gone by from tiny air bubbles stored in the ice. What they found was a sensation: at the height of the last ice age 20,000 years ago, the carbon dioxide concentration was only half that of the warm period of the 20th century. This was the first proof of what John Tyndall, Svante Arrhenius and Thomas Chamberlin had suspected 80 years earlier, but could not prove during their lifetime: a drastic drop in atmospheric carbon dioxide concentration was essential for the formation of the Ice Ages. A drilling in the Antarctic, in which a drill core made it possible to reconstruct the last 150,000 years, brought further certainty. It showed the course of the carbon dioxide concentration in the course of an entire ice age cycle: warm - cold - warm. The carbon dioxide concentration in the atmosphere was amazingly synchronized with the temperature curve, it was low during the Ice Age and high during the warm phase.

methane

Ice cores not only showed an up and down in the CO 2 concentration, but almost exactly parallel to this also an up and down in the methane concentration. It was high when it was warm and low when it was cold. Isotope studies showed that living things were the source of this methane. In the search for possible candidates, many possible sources were found: rice fields, bacteria in the stomachs of ruminants, in the soil of bogs and swamps. Living beings obviously had a significant influence on the development of the global climate.

Although the concentration of this greenhouse gas was significantly lower than that of CO 2 and only had an average residence time of 12 years in the atmosphere, the effect of methane as a greenhouse gas over a period of 20 years is 72 times greater than that of CO 2 . Atmospheric methane concentration increased 1% per year in the 1980s. It has been increasing since the late 16th century.

Even more greenhouse gases

The oceanographer Veerabhadran Ramanathan belonged to the group of those who warned of neglected greenhouse gases in very low concentrations in the mid-1970s. In 1981 Ramanathan wrote that the very strong greenhouse effect of CFCs alone could warm the earth by a whole degree by the year 2000, if the emissions of this gas continued as before; In 1985 he published in a sensational work that no fewer than 30 trace gases act as greenhouse gases and that humans have already significantly increased the concentration of a number of these gases and continue to do so. Taken together, the gases would have almost the same global warming potential as carbon dioxide, which until now was the only focus of the considerations.

It so happened that in the year of its publication, the ozone hole over Antarctica was discovered. So atmospheric chemists were right in their warnings about the threat to the ozone layer. And politicians from outside the field could now also see how great the influence of trace gases in the smallest concentration can be on the atmosphere. If global warming from carbon dioxide alone was a threat, it was now clear that the core problem was much larger. International action was required. Two years later, in 1987, the Montreal Protocol decided to ban the manufacture of CFCs, and in 1988 the Intergovernmental Panel on Climate Change , or IPCC for short, was established.

The one-degree goal

After the warming that had been expected for many years began to show in the records of global temperature data in the 1980s, the scientists asked themselves which effects of man-made climate change were still acceptable and where the limit to dangerous climate change was to be seen. The German Physical Society pleaded in December 1985 and together with the German Meteorological Society in 1987 for compliance with a one-degree target. If global warming is exceeded by one degree compared to the average value as it existed before human intervention in the world climate, serious negative consequences can be expected.

The IPCC is founded

In November 1988, the United Nations Environment Program (UNEP) and the World Meteorological Organization (WMO) the Intergovernmental Panel on Climate Change ( Intergovernmental Panel on Climate Change , IPCC) was established. The IPCC was founded under the leadership of the conservative Reagan administration with the task of summarizing reports and recommendations from all leading scientists in the field of climatology, whereby the consensus of the governments involved was imperative for each report.

1990s

The number of scientific publications on climate change doubled again in the 1990s. In 1990 there were only 40 conferences at which papers on global warming were presented, in 1997 there were already more than 100. The increase in knowledge was correspondingly large.

The World Radiation Monitoring Center was founded at the Swiss Federal Institute of Technology in Zurich (ETH Zurich) in 1992 and subsequently expanded. This resulted in a worldwide network of more than 50 ground stations, the measurement results of which can be called up almost in real time and which enable the evaluation of all relevant radiation components , including global , reflex and direct radiation , as well as terrestrial components such as atmospheric counter-radiation . This made it possible to precisely examine, document and archive changes in the greenhouse effect or radiative forcing within the framework of the Global Climate Observing System (GCOS).

The Vostok ice core

Evaluation of the Vostok ice core: The temperature curve, the carbon dioxide concentration, the methane concentration and the strength of the solar radiation, the so-called insolation over the last 400,000 years, are shown

At the end of the 1990s, a Russian-French research team from the East Antarctic Vostok Station succeeded in recovering an ice core in the new record length of over 3000 meters. This showed four complete ice age cycles with a duration of 100,000 years each over the course of the last 420,000 years. By means of improved analytical methods, it was possible to understand the surprisingly good agreement with the Milanković cycles in Greenland and the parallel rise and fall in carbon dioxide and methane concentrations. A closer analysis confirmed an assumption made years earlier: the increase in the carbon dioxide concentration always took place after the temperature rise. While earlier results suggested a time lag of 600 to 800 years, more recent work suggests that there was little or no time lag of a few years or decades between the warming and the rise in CO 2 concentration.

The warming was not synchronous; there was a significant time difference between the northern and southern hemispheres, with the warming of the southern hemisphere beginning before the warming of the northern hemisphere.

The evaluation of the ice core again demonstrated the importance of greenhouse gases as well as the other feedback mechanisms: The slight change in the earth's radiation balance triggered by the Milanković cycles was reinforced by a change in the concentration of atmospheric greenhouse gases. Together with the ice albedo feedback , the water vapor feedback and other, weaker feedback elements, the effect was so great that it led to the coming and going of ice ages. It remained unclear whether the carbon dioxide released came from the world's oceans, permafrost, methane hydrates or other sources. What was certain was that the increase in the concentration of these gases was a consequence of this slight warming and had intensified it further.

While the greenhouse gas concentration often increased during the Quaternary Ice Age as a reaction to the warming tendency of the Milanković cycles, human (anthropogenic) emissions are currently responsible for the greenhouse gas concentration preceding the actual temperature increase. The effect is, of course, the same in both cases: increasing warming coupled with a further release of greenhouse gases, as could be the case, for example, in thawing permafrost regions. The same applies to the methane hydrate present in many oceanic areas, which is stored in solid form on shelf bases and in the deep sea and binds extensive quantities of methane in the order of magnitude of around 10 trillion tons. In the coming decades, an increased release of methane from methane hydrates or the permafrost could be a clear warning signal for a self-reinforcing warming spiral.

The 1st and 2nd Assessment Report of the IPCC

The first assessment report of the IPCC , published in 1990, stated that it is certain that there is a natural greenhouse effect and that humans are increasing the concentration of some greenhouse gases, which will lead to a global increase in temperature. So far, however, there is little empirical evidence for man-made climate change (“little observational evidence”).

In the second assessment report published six years later, chaired by Benjamin D. Santer, it was stated for the first time: The weighing of the data suggests that humans have a noticeable influence on the global climate of the 20th century (“the balance of evidence” suggested there had been a 'discernible' human influence on the climate of the 20th century ”).

Aerosols

The effects of aerosols on the climate have been discussed since the 1950s : the greenhouse effect due to the interaction with infrared radiation and the scattering and absorption of sunlight as direct effects as well as the indirect effect as condensation nuclei for water vapor, which could actually have a cooling effect on dark aerosols - the sign of the overall effect was still uncertain even after the 1990s.

The situation was different with the bright sulphate aerosols. James E. Hansen used data from volcanic eruptions from Mount Agung in 1963 and El Chichón in 1982 to quantify the cooling effect of volcanic eruptions. It was therefore already clear since the 1960s that sulphate aerosols have a cooling effect on the climate, which could also be well understood with the help of ice cores for eruptions far back in the past.

The 1991 Pinatubo eruption turned out to be a godsend for climatologists. Now they were able to check whether their assumptions about the effects of sulfates were correct, because the volcano emitted almost 20 million tons of sulfur dioxide, a sulfate cloud the size of the US state of Iowa . Hansen's group predicted a half-degree cooling, which would primarily present itself over higher northern latitudes and which would last for a few years. This is exactly what was observed.

Climate models

In the 1990s, climate models were parameterized with knowledge of the cooling sulphate emissions from volcanic eruptions. This resolved a contradiction: Should the climate sensitivity, i.e. H. the expected warming when the concentration of carbon dioxide doubles, actually in the range of three degrees, this should have shown itself in the course of the global mean temperature in the 1960s and 1970s, but this could not be observed. After the cooling effect of sulfur dioxide had become part of the models, the temperature profile of the 20th century was also easy to display.

The problem of the excessive temperature difference between polar and equatorial latitudes could also be solved in this decade: Investigations on ice cores showed that reconstructions of the CLIMAP study, which indicated almost unchanged temperatures in equatorial latitudes during the ice ages, were probably incorrect.

While in the previous decades some parameters in climate models had to be selected without a physical basis in order to prevent the model from assuming unrealistic conditions, in the 1990s climate models had apparently reached a quality that was so good that they can no longer be brought about were able to reproduce incorrect measurement data by choosing suitable parameters.

The two-degree goal

The two-degree target was then formulated in the late 1990s as the internationally applicable limit for a just acceptable climate change . It may have been proposed for the first time by the German Advisory Council on Global Change (WGBU) . The WBGU approved the limit in a 1995 report. The two-degree target was adopted by politicians and made the focus of European climate protection policy. It is based on the assumption that when exceeding a global warming of two degrees tipping points ( tipping points would be achieved), the irreversible and hardly predictable in its consequences negative consequences might spawn themselves.

2000s

Spatial distribution of global warming: The graph shows the temperature anomalies in the period 2000–2009 (above), the warmest decade recorded so far, and in comparison the years 1970–1979. Anomalies are shown, ie deviations from the long-term mean for the period 1951–1980, not absolute temperatures.

The first decade of the 21st century was the warmest since systematic temperature records began. Looking at the individual years, 2005 and 2010 were the warmest years since measurements began.

In climatology it has been known for a long time that over 90% of the heat brought into the climate system by humans via the increase in greenhouse gas concentration does not end up in the atmosphere, but in the oceans. Unfortunately, ocean temperature data were only available very sparsely, and there was only a limited amount of data from the deep sea in particular, so that no reliable statement could be made about a possible warming of the oceans.

The Argo project started in 2000 precisely addressed this problem. With the help of a fleet of automated diving buoys or drifting buoys, it was now possible to record the temperature development and the heat content of the oceans in a previously impossible quality and quantity. In November 2002, the one millionth data profile was transferred as part of the Argo project. This means that the number of oceanographic measurements carried out by research vessels in the entire 20th century was already doubled at an early stage in the project. There are currently around 3,960 Argo buoys in use in all of the world's oceans (as of June 2020), whereby so-called "Deep Argo" floats are increasingly being used, which record the temperature, heat content and flow patterns of the deep-sea areas as well.

The 3rd assessment report of the IPCC

In the third assessment report published by the IPCC in 2001 , the impact of humans on the climate could not only be demonstrated with greater certainty, but thanks to the improved data situation it was now already in a position to quantify the extent of human impact on climate change. While the second assessment report reported a noticeable impact of humans on the climate, it was now written of clear indications (English strong evidence) that humans are changing the earth's climate. To visualize how great the influence of humans is, the report contained a temperature reconstruction by Michael E. Mann , which became widely known as the hockey stick diagram .

Tilting elements

It has been known since the 1970s that the earth's climate often reacts chaotically: Small changes can have major effects; this has often happened in the past with abrupt climate changes . In the 2000s, Hans Joachim Schellnhuber pointed out that there are also a number of elements in the climate system and in ecosystems that tend to undergo changes that are difficult or irreversible; This means that they remain in their new state even when the effect that triggered the change has disappeared again. This behavior is known as hysteresis in systems theory . Since it was first mentioned in the scientific literature, a number of tipping elements have been found in the Earth system , including the Greenland ice sheet and the Amazon rainforest .

Confirmation of decades-old predictions by climate researchers

By comparing satellite data recorded in 1970 with measurements from 1997, it was possible to prove for the first time in a publication in 2001 that the emission spectrum of the earth had changed. In the spectra, the increased greenhouse effect due to the greenhouse gas concentration, which has increased significantly since 1970, was clearly visible. In addition, another study could also show that the radiative forcing had increased so much through the increase in the concentration of the atmospheric concentration of the greenhouse gas carbon dioxide that this could also be proven by measurement in an 8-year series of data. The result of this study was confirmed in 2015 in the context of another work in which data were evaluated over a period of 10 years.

In 2003 another prediction came about: The British meteorologist Ernest Gold had published in 1908 that it was to be expected that the tropopause would increase with increasing CO 2 concentration due to the increased greenhouse effect. This could now also be measured.

For decades, climate researchers had assumed that a warmer world would lead to the release of carbon dioxide and methane from permafrost. As was found out in the 1990s by analyzing drill core data, this had actually happened regularly in the history of the earth. The fear arose as early as the 2000s: In the summer months, a large increase in the concentration of these gases could be observed in the large permafrost regions of Siberia and Alaska. In addition to the emissions of greenhouse gases that come from humans, there are now also emissions from fossil carbon sources that outgass the earth as a result of man-made warming.

In 2002 the collapse of the Antarctic ice sheet Larsen B attracted international attention; in 2008 the Wilkins Ice Sheet broke open ; these were the indicators that John Mercer had seen in 1978 as a sign of an impending collapse of the West Antarctic ice sheet.

The Anthropocene

In 2008 the Stratigraphic Commission of the Geological Society of London found that there are now enough arguments to suggest that humans have initiated a new stratigraphic segment. Species extinction , overfishing , acidification of the oceans , global warming and other processes triggered by humans have already influenced the earth so strongly that a clear and sustainable biostratigraphic signal has been and is being generated. The term Anthropocene (from ancient Greek ἄνθρωπος ánthrōpos ' man ') was chosen because man has become the primary factor that shapes the earth. The decision on the implementation of the Anthropocene in the stratigraphic system lies with the International Commission on Stratigraphy (ICS), in whose Working Group on the 'Anthropocene' the various aspects of the proposal are currently being discussed in detail.

Further confirmations

In the fourth assessment report of the IPCC from 2007, the main cause of global warming is given as "very likely" with a stated probability of over 90% as the emissions of greenhouse gases caused by humans. Also in 2007 the IPCC received the Nobel Peace Prize together with the former US Vice President Al Gore . In 2009, the “Copenhagen Diagnosis” was updated after AR4 was published in 2007. The authors wrote that the extent of some of the developments indicated in the last IPCC report were underestimated. In the year AR4 was published (2007), the Arctic sea ice cover was 40% lower than the computer models had predicted. Sea level rise over the past 15 years has been 80% higher than the IPCC forecast. Accordingly, the predictions regarding future sea level rise up to the year 2100 were revised upwards: in the meantime a rise was expected twice as high.

Also in 2009, the journal Nature Climate Change launched a new platform on which scientists can publish their findings on the processes and consequences of climate change.

2010s

2011 was not only the year with the highest ever measured carbon dioxide content in the atmosphere, it was also the year of the world's largest ever measured carbon dioxide emissions , an increase of 3% compared to the previous year. Due to the investments that have taken place in energy sources emitting carbon dioxide, an 80% increase in the emission rate from 2010 to 2020 seemed almost certain.

The rapid progress in radiometric dating and the development of biogeochemical detection methods in the context of palaeoclimatology led to a considerable increase in measurement accuracy and thus to a partial re-evaluation of geological, geophysical and biological events. With the help of modern dating methods it became possible to narrow down the climatic fluctuations of the geological past more precisely, to reconstruct them in increasing detail and to compare their course or extent with the current warming. These and similar investigations made a decisive contribution to the fact that the basic knowledge documented in the scientific literature is constantly expanding. According to an overview study from 2016, over 220,000 peer-reviewed papers on the topic of climatology were published between 1980 and 2014.

By analyzing satellite measurements, which now cover a period of more than 30 years, it was possible in 2013 to clearly demonstrate the human impact on ongoing climate change in another way. The measurement data showed a cooling of the stratosphere with simultaneous warming of the troposphere. This effect only occurs if the warming is caused by an increase in greenhouse gas concentrations, since increased solar activity would also have heated the stratosphere.

The 5th IPCC Assessment Report

In the fifth assessment report of the IPCC (September 2013), the statements of the previous climate reports were confirmed and uncertainties regarding the impact of humans on the climate were reduced. The experts now write that it is extremely likely that humans are the main reason for the observed global warming since 1950.

Melting West Antarctica exceeds tipping point

In 2014, it was found in several independent publications that the melting of the West Antarctic Ice Sheet has most likely already passed its tipping point ; H. the ice sheet is meanwhile so unstable that further melting can no longer be stopped (cf. tipping elements in the earth system ). An ice surface the size of France will most likely disintegrate in the next 100 to 300 years, causing the sea level to rise by one meter on a global average. These findings confirmed John Mercer's 1978 predictions.

In the area of ​​the Antarctic Peninsula , a piece of around 5,800 km² broke off the Larsen C Ice Shelf on July 12, 2017, reducing its area by around 12%. The mass of the iceberg is around one trillion tons; it is one of the largest icebergs ever observed. The demolition threatens the destabilization and dissolution of Larsen C.

In contrast, there is no clear trend in East Antarctica. In June 2018, an international team of experts, consisting of around 80 Earth system and geoscientists, published the most extensive study to date on this topic with the result that the East Antarctic ice sheet is currently stable in its core areas and, unlike other Antarctic regions, does not show any significant loss of mass .

One degree limit is exceeded

The years 2014, 2015 and 2016 were the warmest years globally since regular climate records began. It was the first time that global temperature records had been set for three consecutive years. The 1-degree target set by the German Physical Society and the German Meteorological Society at the end of the 1980s as the limit for dangerous climate change was thus achieved. At the same time, the atmospheric concentration of the greenhouse gas carbon dioxide exceeded the 400 ppm mark. As a result of this warming, extreme heat anomalies that occurred in the period 1950–1981 only with a probability of 0.13% - so-called 3 sigma events - were now found on 10% of the area each summer instead of far less than one percent of the earth's surface. In 2017, a study found that the climate models on which the IPCC reports are based are very likely to underestimate the expected warming by the end of the century. In the case of unchecked emissions, the expected increase in temperature is about 0.5 Kelvin higher than previously assumed. In this context, several current studies come to the conclusion that, in contrast to pre-industrial climate fluctuations, the current warming occurs simultaneously on all continents, has not been exceeded in its rapid development by any climatic change of the last 2,000 years and is probably also without a comparable example in recent geological history should. In addition, all paleoclimatological data series indicate that the warming that has taken place in the 21st century so far surpasses the temperature values ​​of the Holocene optimum climate (about 8000 to 6000 years ago).

Continuation of global warming

Reconstruction of the global temperature development over the last 2000 years, including anthropogenic warming (according to PAGES 2k Consortium, 2019).

There is broad consensus among scientists that the currently observed climate change will proceed more rapidly than any known warming phase of the last 50 million years. Even during the Paleocene / Eocene temperature maximum - an extremely pronounced warm climate within a geologically very short period of time - the atmospheric carbon input and the associated temperature increase had considerably lower annual average rates of increase than at present. In contrast to earlier assumptions, the additional CO 2 input will only decrease gradually, even if emissions are largely stopped, and will still be detectable to a significant extent in several thousand years. Building on this, some studies postulate a longer warm period in the range of 50,000 to 100,000 years , taking into account the Earth system's climate sensitivity . Various tipping elements in the Earth system were identified as additional hazard potentials, which would initiate short-term and irreversible processes if the temperature continued to rise. Such developments would seriously change the image of the earth, especially due to the associated shift in climate and vegetation zones and the extensive melting of the West Antarctic and Greenland ice sheets with a corresponding rise in sea levels.

On the other hand, the natural control mechanisms of carbon dioxide binding such as sedimentation or geochemical weathering processes (CaCO 3 weathering) are too slow to bring about a sustainable reduction in CO 2 within a reasonable period of time . A complete exchange of atmospheric carbon dioxide on the basis of the carbonate-silicate cycle takes about 500,000 years. Although the oceans are known to be an effective carbon sink , only a relatively small part of the CO 2 is stored in deep-sea sediments in the medium term . In addition, considerable amounts of CO 2 (together with methane) could be outgassed again when seawater temperatures rise. The relatively slow reaction of the inorganic carbon cycle to a rapid increase in greenhouse gases was already known to Svante Arrhenius . Although the emission rates of that time were of comparatively little relevance towards the end of the 19th century, Arrhenius explicitly mentioned in his work On the Influence of Carbonic Acid in the Air on Ground Temperature (1896) the long retention time of carbon in the atmosphere and in the oceans. The role of weathering processes as an important influencing factor in the climate system has long been a niche topic in the specialist literature and was only dealt with on a broader basis from the 1980s.

A major aspect of current global warming is its impact on the next glacial event within the Cenozoic Ice Age . The cooling trend averaging ≈0.1 ° C per millennium, which began after the climatic optimum of the Holocene , is regarded as a harbinger and first sign of an approaching ice age climate. The recently published studies, which are based on a precise analysis of past ice age phases, including the Milanković cycles , come to the conclusion that a cold period is caused by slight fluctuations in the Earth's climate system and, above all, by the gradual changes in the Earth's orbit parameters . According to this, under normal conditions (excluding anthropogenic emissions) the next ice age would only begin in a few tens of thousands of years. This period, which is unusually long for an interglacial like the Holocene , will with a high probability extend to a total of 100,000 years and thus almost double if the initial atmospheric CO 2 value is over 500 ppm. This means the failure of a complete Ice Age cycle due to human interference in the climate system.

Social science research on climate change

After it was recognized that the climate problem was related to human behavior and decisions within social systems , social science aspects of climate change have also been examined since the 1970s . Stephen H. Schneider was one of the first climatologists to speak out in favor of interdisciplinary research (i.e. the inclusion of the social sciences in researching climate change) and to organize workshops on the topic . In 1983 he pointed out that the basis of the CO 2 problem (rising emissions) is a social science topic. The extent of future CO 2 emissions depends largely on human behavior. a. in terms of population (reproductive behavior), per capita consumption of fossil fuels or deforestation and reforestation. In addition to the social-scientific analyzes of the causes of global warming, social reactions to anthropogenic climate change were discussed at an early stage, such as risk perception , decision-making and adaptation to climatic changes. Nicholas Stern , among others, summarized the economic aspects of climate change in 2006 ( Stern Report ). In addition, in view of the lack of climate protection activities, despite the increasing certainty of scientific findings on climate change, research was intensified on the causes of the phenomenon of "inactivity". In addition to individual factors - in the context of the global warming controversy - u. a. by Naomi Oreskes examines in more detail how economically motivated interest groups in particular spread doubts about scientific findings (see Denial of man-made global warming ) and how this in turn affects political decisions.

literature

Web links

Individual evidence

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