Climate sensitivity

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Climate sensitivity and atmospheric carbon dioxide. Hansen et al. 2013
(a) CO 2 content for a certain temperature, if rapid feedback effects and climate sensitivity account for 0.75 ° C per W m −2 and non-CO 2 greenhouse gases account for 25% of radiative forcing .
(b) As in (a), but with a resolution of 0.5 Myr and with three different possibilities; the CO 2 content would have to exceed 5000 ppm in peaks if the sensitivity were 0.5 ° C. The horizontal line marks the CO 2 concentration of the early to middle Holocene at 260 ppm

The climate sensitivity is the ratio size of the increase in temperature of the earth's surface divided by an additional irradiance expressed effect of increased greenhouse gas concentrations . In general, the IPCC differentiates between equilibrium climate sensitivity (ECS) and transient climate response (TCR). It can be specified in Kelvin per watt per square meter (K ​​/ (W / m²) = K · m² / W). More common, however, is the indication of global warming when the concentration of carbon dioxide in the earth's atmosphere doubles . This means that the average temperature of the earth rises by this amount if the CO 2 concentration increases from the pre-industrial 280  ppm to then 560 ppm.

The exact knowledge of the climate sensitivity is of elementary importance for the future development of the climate, since with its help the warming resulting from a certain greenhouse gas concentration can be calculated. The value of the climate sensitivity depends on the initial climatic condition and can theoretically be determined precisely using climate proxies .

In addition to carbon dioxide , other gases also contribute to the greenhouse effect , so that their own climate sensitivities can also be determined for each of them. For the sake of simplicity, their contribution is usually calculated using the so-called CO 2 equivalents .

background

If only the radiation effect of CO 2 that can be measured in the laboratory is considered, doubling the concentration results in a climate sensitivity of 1.2 ° C. However, the sum of all feedbacks in the earth's climate system also contributes to climate sensitivity, such as B. the reaction of the sea level (feedback in the cryosphere ) depending on the planetary energy balance. A distinction is made between fast and slow feedback. Water vapor , ice albedo and aerosol feedback, as well as clouds, are considered to be fast feedback effects. The ice sheets , changes in vegetation and the concentration of the greenhouse gas CO 2 are considered to be slow feedback effects. The reaction of ice sheets and CO 2 in the air increase the climate sensitivity by an amount that depends on the period under consideration. Today's climate models overestimate the hysteresis of the ice sheets. This means that the reaction of the ice sheets to climate change does not depend as much on whether the earth is in a warming or cooling state as is commonly assumed.

The climate sensitivity is a dynamic factor that depends on the respective climate condition. Models and the history of the earth show that climate sensitivity also increases with the increase in radiative forcing, i.e. with increasing global temperature. For example, for the strong warming phase of the Paleocene / Eocene temperature maximum 55.5 million years ago, it is assumed that the Earth system climate sensitivity during this time (including all short and long-term feedback factors ) in the range of 3.7 to 6.5 ° C. Similarly high values ​​(up to 6 ° C) are estimated for most of the rest of the Cenozoic era .

ECS and TCR

Due to the thermal inertia of the world's oceans, the global climate system generally only reacts slowly to changes in radiative forcing. A distinction is therefore made between the Equilibrium Climate Sensitivity (ECS) and the Transient Climate Response (TCR). The ECS describes the rise in temperature that can be observed after the climate system has reached the new state of equilibrium after a change in the radiative forcing, which takes millennia.

The Transient Climate Response is more suitable for quantifying the influence of humans on the climate. This is defined as the temperature increase that is observed when the CO 2 concentration doubles in a scenario in which it increases by 1% per year. The most likely value for the TCR is around 2 degrees after 70 years.

Range of research results (ECS)

Since the discovery of the warming effect of carbon dioxide, many different values ​​for climate sensitivity have been published.

In 1896, Svante Arrhenius assumed a climate sensitivity of 5.5 ° C. Guy Stewart Callendar reached 2 ° C in 1938. The spectrum of published values ​​ranges from 0.1 ° C (Sellers, 1973) to 9.6 ° C ( Fritz Möller , 1963). The National Academy of Sciences was the world's first major scientific organization to warn of global warming and in 1979, in the Charney Report, gave the climate sensitivity as 3 ° C (± 1.5 ° C), which is still the standard today. A study from 2006, using combined assessments based on Bayes' theorem, came up with a 95% probability of a range of values ​​for climate sensitivity between 1.5 ° C and 4.5 ° C.

In its fourth assessment report published in 2007, the Intergovernmental Panel on Climate Change (IPCC) stated values ​​between 2 and 4.5 ° C as “likely”. The best mean estimate is 3 ° C, and a sensitivity below 1.5 ° C is "very unlikely". In the fifth assessment report , which appeared in 2013, this probable range was changed to a range between 1.5 and 4.5 ° C. This information is identical to that of the third assessment report from 2001.

Determination of the climate sensitivity

Depending on the determination method, there is a different value for the climate sensitivity. In 2005 it was measured that the earth absorbs 0.85 W / m² more energy than it radiates into space. In a series of measurements lasting over 8 years, an increase in long-wave atmospheric counter-radiation due to the anthropogenic greenhouse effect could be proven by measurements. By far the largest part of the measured additional radiative forcing was, as expected, due to the positive feedback from clouds and water vapor. However, such measurements are not suitable for calculating the climate sensitivity, since many of the feedback effects in the climate system are not taken into account. Various methods are used to try to reduce uncertainties when determining climate sensitivity:

Paleoclimatological methods

As the temperature rises, the maximum amount of water vapor contained in the atmosphere also rises. This water vapor feedback is one of the strongest positive feedback in the global climate system.

Several paleoclimatological studies attempted to determine the climate sensitivity of the last several million years. A paleoclimatological study published in the journal Nature in 2007 examines climate sensitivity over the past 420 million years. The global average temperature and the concentration of greenhouse gases were subject to strong fluctuations during this very long period and the solar radiation output increased by around 4% during this time , which is a good prerequisite for an assessment of climate sensitivity based on this with a small margin of error. Unfortunately, the climate archives of the ice cores barely extend further than a million years into the past, and the arrangement of the land masses has been subject to profound changes during this time, so that there is great uncertainty about many climate-determining parameters. These investigations thus result in a comparatively large uncertainty, which gives 1.5 ° C as the lower limit value, 6.2 ° C as the upper limit value and 2.8 ° C as the best estimate.

In a paper published in 2012, the results of several studies were summarized that had the last 65 million years in focus. With a 95% probability, these provided a value for the climate sensitivity that lies in the range between 2.2 ° C and 4.8 ° C.

After a paleoclimatological analysis of the last 784,000 years with eight complete cycles of cold and warm phases within the Quaternary Ice Age , the authors of a study published in 2016 come to the conclusion that climate sensitivity is highly temperature-dependent. According to this, the climate sensitivity during a cold period such as the Würm or Vistula glacial is around 2 ° C and increases by about twice (from 1.78 K to 4.88 K) under warm period conditions such as the Holocene .

Regression analysis

With a good knowledge of all climate-determining factors one can try to isolate the climate sensitivity with the help of a regression analysis . For this purpose, the ice age cycles of the past thousands of years are examined. During this time, the CO 2 concentration and the temperatures fluctuated greatly, while other climatologically effective parameters did not differ greatly from the current situation. Ice cores, which have been obtained from various locations on earth since the 1990s, provide information about the prevailing concentrations of greenhouse gases, aerosols and amounts of precipitation as well as the temperature profiles over the last approx. 1 million years.

Climate models

The current and future climate can only be correctly simulated if the climate sensitivity has also been correctly determined. Therefore, climate models are tested to see whether they can correctly simulate the current climate as well as the climate during the ice ages. In the context of such simulations, over 1000 models are calculated, with input parameters being varied within their assumed error range. Models that do not correctly reflect the temperature profile in the period under consideration (> 90%) are rejected. Using this method, 3.4 ° C was found to be the most likely value for climate sensitivity.

Looking at the temperature changes during the past ice ages, one could use ice cores to show a temperature change of 5 ° C with a change in radiative forcing of 7.1 W / resulting from the Milanković cycles and the feedback (albedo, vegetation, aerosols, CO 2 ) Link m². The climate sensitivity calculated from this is 5 / 7.1 = 0.7 K · W −1 · m 2 . This empirically determined climate sensitivity can be used to calculate the temperature rise resulting from a radiative forcing of 4 W / m², which corresponds to a doubling of the atmospheric CO 2 concentration compared to pre-industrial values. The result shows an increase of 3 ° C. The first results of the newly developed model generation CMIP6 show, at 2.8 to 5.8 ° C, significantly higher climatic sensitivities than earlier model comparisons. The last model comparison (CMIP5) showed values ​​between 2.1 and 4.7 ° C. However, there is still no final clarity as to whether the new results are realistic and which factors have influenced the increased climate sensitivities in the CMIP6 models. One explanation for the higher climate sensitivities of the new models is the differences in the consideration of cloud feedback.

Earth system climate sensitivity

The melting of large amounts of ice - as it is, for. B. exist in Greenland or in the Antarctic - takes many centuries and the warming runs u. a. due to the ice-albedo feedback, even with a complete emission stop over these periods. In addition, climate change also leads to changes in vegetation. Forest absorbs considerably more incoming rays than B. the comparatively light surface of the tundra.

Around half of the carbon dioxide emitted into the atmosphere today ends up in the oceans in the form of carbonic acid . Since the solubility of CO 2 in water is temperature-dependent, the warming of the world's oceans will reduce their storage capacity for this greenhouse gas; Model studies indicate that the biosphere will change from a CO 2 sink to a CO 2 source from around the end of the 21st century . The analysis of ice cores shows that global warming caused the concentration of greenhouse gases to increase with a certain time lag, which further intensified the warming. Even a precise knowledge of climate sensitivity and greenhouse gas emissions therefore only allows a rough estimate of future climatic developments. In the climate report published by the IPCC in 2007, this amplification was taken into account in scenario A2 with an additional degree of temperature rise by 2100.

The Earth System Sensitivity (ESS) also contains these reactions of the climate. If the CO 2 concentration doubles , the Earth system climate sensitivity is around 4–6 ° C if the ice caps and the albedo vegetation feedback are included, and is even higher if the greenhouse gas feedbacks are taken into account. Hansen et al. 2013 calculated a value of 3–4 ° C based on a 550 ppm CO 2 scenario with the Earth system climate sensitivity . Previdi et al. 2013 calculated on the basis of the earth system's climate sensitivity around 4–6 ° C without taking into account the greenhouse gas feedback.

Meaning for the situation today

The CO 2 concentration in 2007 of approx. 380 ppm, together with the other greenhouse gases, led to a radiative forcing of 2.6 W / m². This radiative forcing would have led to global warming of 1.32 ° C if the most probable value for the climate sensitivity of 3 ° C is calculated. However, the warming would only reach its maximum after decades or centuries, as the climate reacts very slowly due to the high heat capacity of the water masses of the world's oceans. Even if greenhouse gas concentrations had been frozen at the level of the year 2000, global warming would therefore still progress by 0.6 ° C by the end of the century. And so the global warming of 0.7 ° C that occurred up to 2007 is only half to two thirds of the value to be expected for the CO 2 concentration existing at that time .

The atmospheric carbon dioxide concentration will not decrease naturally even after a period of centuries if emissions are completely stopped. A major reduction in greenhouse gas emissions is therefore not enough to stop anthropogenic global warming. This would require the immediate and complete cessation of greenhouse gas emissions.

Since the beginning of the industrial revolution, not only has the concentration of CO 2 increased; If the increase in the concentration of the other greenhouse gases is converted into CO 2 equivalents via their global warming potential , the result for 2016 is a total radiative forcing that would correspond to a CO 2 concentration of 489 ppm.

outlook

Känozoikum Kreide-Paläogen-Grenze Paläozän/Eozän-Temperaturmaximum Eocene Thermal Maximum 2 Eem-Warmzeit Letzteiszeitliches Maximum Atlantikum Jüngere Dryaszeit Globale Erwärmung Paläogen Neogen Quartär (Geologie) Paläozän Eozän Oligozän Miozän Pliozän Pleistozän Holozän Christopher Scotese James E. Hansen James E. Hansen James E. Hansen EPICA EPICA Greenland Ice Core Project Delta-O-18 Repräsentativer Konzentrationspfad
Clickable diagram of the temperature development during the 66 million years of the Cenozoic era including a scenario based on the extended representative concentration path ECP 6.0 up to the year 2300. The Earth system climate sensitivity in this section of the earth's history was mostly in a range of 4 to 6 ° C.

Burning all fossil fuels would lead to an atmospheric CO 2 concentration of approx. 1500 ppm, warming the air over the continents by an average of 20 ° C and the poles by 30 ° C. Even if the emissions of greenhouse gases were to be reduced significantly, the warming process already initiated by humans will continue to have an effect for a long time. Studies assume that climatic conditions on earth will only exist in 23,000 to 165,000 years, as they existed before human intervention in the climate system.

With regard to necessary climate protection measures , the respective climate sensitivity does not play a major role. Even if the climate sensitivity were lower than currently assumed, this would only offer scope for a slightly less energetic climate protection path. However, nothing would change about the fundamental need to decarbonise society.

See also

Web links

Individual evidence

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