Warm climate

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The Earth's Climate History . The letters “W” indicate the warm climates.
Earth mean temperatures of the last 540 million years. The last warm climate period with its maximum in the Eocene around 55 million years ago lies in the green section .

Longer warm periods in the history of the earth , which are separated from each other by ice ages , are referred to as warm climate , also non- ice age , less often warm age .

In times of warm climates, there are usually no major icy areas on earth, especially not in the polar regions (no polar ice caps ). Only in some high mountains there can be minor icing. In order to express this climate condition explicitly, the term acryogenic warm climate (aryogenic: not ice-forming ) is used.

As far as we know today, there were around four to seven Ice Ages and just as many warm climates in the history of the earth, depending on the definition. The warm climates make up about 80 to 90 percent of the earth's history. Currently, for about 33.7 million years, there has been no warm climate, but an ice age with glaciation of the Antarctic, the Cenozoic Ice Age ; or if the glaciation of both poles is used as a criterion for the Ice Age, the Quaternary Ice Age has been in place for about 2.6 million years .

While the mean global surface temperature is around 15 ° C today, it was around 20 to 25 ° C in the warm climate phases, and around 30 ° C in the hottest time of the last warm climate.

In the course of the current global warming , the Intergovernmental Panel on Climate Change ( IPCC) predicts a temperature increase of up to 5.4 ° C by the year 2100 (resulting in temperatures in the range of previous warm climates). Serious consequences u. a. for humanity is assumed.

overview

In contrast to the Ice Ages, there are as yet no names for the individual warm climate ages in the history of the earth. The typical geological traces left by icing are missing in times of warm climates. While a distinction is made between cold and warm periods within the ice ages , non- ice ages only consist of warm periods. Thus, the earth's climate is conceptually structured as follows:

 climate 
 Warm climate (poles of the earth not glaciated) 

 Ice Age (earth poles glaciated) 

Warm period (interglacial)


   

Cold Age (Glacial)




The first warm climate

Artistic illustration of the young earth

When the earth was formed about 4.5 billion years ago , it was very hot at first, so it began in a warm climate. In the first eon of earth's history, the Hadean , the planet slowly cooled down until water vapor could condense and the first oceans could form on the earth's crust . Until about 3.9 billion years ago, the Great Bombardment prevented the formation of a stable earth crust. It is controversial when it came to the formation of the first primordial ocean. However, traces of liquid water are clearly detectable from 3.8 billion years ago. The sun was initially still weak, so that the primordial ocean should actually have frozen to ice after the earth's crust had cooled down. Apparently this was not the case. There are various hypotheses to explain the warm climate with liquid water at that time (see paradox of the weak young sun ). In the oceans of the following Archaic (the primeval Earth times), the first life in the form of prokaryotes ( archaea and bacteria ) developed on earth. Evidence of life for at least 3.5 billion years is documented.

First indications of one or more glaciation phases exist from the time ago 2.9 billion years ago ( Pongola glaciation ), a significant and longer lasting interruption of the original warm climate was found by the Paleoproterozoic glaciation (or Huronic Ice Age ) in the older Precambrian about 2.3 Billions of years ago. It may have been initiated by the first formation of oxygen in the earth's atmosphere , which began about 2.4 billion years ago (see Great Oxygen Disaster ). This ice age could have been a complete and probably the longest Snowball Earth episode in the history of the earth and lasted 300 to 400 million years.

The second warm climate

This first and at the same time longest Ice Age, which destroyed almost all life that had arisen up to that point, was followed by a second warm climate phase lasting more than a billion years in which eukaryotes (living beings with a cell nucleus ) developed. At the end of the era, there was sexual reproduction for the first time . This greatly increased the complexity of the following life forms; it developed multicellular organisms .

Otherwise, the atmospheric and oceanic conditions were relatively constant during this period, which is why some scientists call it the "boring billion". Geologically, the assumed supercontinent Columbia (which existed about 1.8 to 1.6 billion years ago) slowly broke apart and a new supercontinent formed, Rodinia , which was surrounded by the ocean Mirovia .

Multiple changes

The trilobites were among the first arthropods. They lived through several warm climates and ice ages from about 521 million years ago to about 251 million years ago.

In the last billion years there was finally a multiple alternation between ice ages and non-ice ages. A reliable breakdown of this period into ice ages and warm climates is currently not possible. The Kaigas Ice Age , which is dated around 780 to 735 million years ago, is still very uncertain and may not have been global. The Sturtic Ice Age is dated from around 760 million years ago to around 640 million years ago, and there could possibly be several Sturtic Ice Ages. The Marino Ice Age is said to have ruled about 650 to 635 million years ago.

The relatively short Gaskiers Ice Age around 582 to 580 million years ago led to the widespread extinction of the first multicellular planktonic organisms, the acritars . Subsequently, in the next warm climate, the Ediacara fauna presumably developed very "primitive" first multicellular animals (Metazoa). With the Cambrian explosion at the beginning of the Cambrian , about 543 million years ago, representatives of almost all of today's animal phyla appeared "explosively" in a geologically short period of 5 to 10 million years. Here presented arthropods are the predominant animal form.

In the Ordovician first came terrestrial plants , where recent studies postulate a vegetation spread in the Cambrian period. At the end of the Ordovician, around 444 million years ago, the climate cooled down considerably and brain-like icing occurred in the southern hemisphere. The associated global cooling of around 5  Kelvin led to mass extinction . This cooling could have been caused by the plants taking carbon out of the atmosphere. According to other sources, the Andean-Sahara Ice Age began a little earlier, in the Katium about 450 million years ago, and lasted through the entire Silurian Mountains until about 420 million years ago.

In the temperate-warm Devonian fauna and flora continued to develop until the Karoo Ice Age formed the next climatic break about 360 to about 260 million years ago. It shaped the Carboniferous and the (Lower and Middle) Permian and is therefore also called the "permocarbone" or "permocarbonic" glaciation.

The last warm climate

The dinosaurs lived in the last warm climate.

The penultimate ice age ended around 265 to 260 million years ago and the last warm climate on earth began. At the Permian-Triassic border around 252 million years ago, massive volcanic activity led to extensive emissions of carbon dioxide and hydrogen chloride , to extreme acidification of the oceans with the release of methane and hydrogen sulfide, and the resulting very strong global warming, which is the largest known mass extinction caused the history of the earth. The warm climate persisted, and in the Mesozoic Era , which began about 252 million years ago and ended about 66 million years ago, the surviving species in turn evolved into novel fauna and flora, including the dinosaurs . The temperatures in the early and middle Mesozoic ( Triassic and Jurassic ) were around 2 to 4 Kelvin and during the Cretaceous 8 Kelvin higher than today, in the Cretaceous climatic optimum around 90 million years ago even over 10 Kelvin. Despite the tropical climates prevailing at this time, there are indications of at least two significant cooling phases. The first occurred at the transition from the Middle Jurassic to the Upper Jurassic about 160 million years ago and could have caused a temporary freezing of the northern polar regions. Also in the Cretaceous (Level Aptian ) there are indications that suggest a more millions permanent cooling.

At the end of the Cretaceous Period , at the Cretaceous-Paleogene border almost exactly 66 million years ago, there was another global mass extinction when an asteroid about 10 to 15 km in size hit a tropical shallow sea in the area of ​​what is now the Gulf of Mexico . The subsequent impact winter and the subsequent extreme heat phase, in combination with a few other factors, led to the extinction of 70 percent of all species, including all non-avian dinosaurs.

As the heirs of the dinosaurs, so to speak, the mammals occupied the orphaned ecological niches in the course of the subtropical Paleocene . Among these mammals were also the primates detectable since the early Eocene . The size and biodiversity of mammals increased steadily over millions of years, and birds, which were particularly hard hit by the Cretaceous-Tertiary border crisis, made rapid evolutionary progress. At the border to the Eocene around 55 million years ago, the Paleocene / Eocene Temperature Maximum (PETM) , which lasted around 180,000 years, resulted in a sudden global temperature increase of around 4 to 8 Kelvin with considerable effects on the oceanic and continental areas of life. While the oceans became increasingly acidic, some species of mammals reacted to the changed environmental conditions with a clear tendency towards dwarfism .

During the Eocene there were a series of cooling and warming phases based on a warm climate, such as the Azolla event . Due to their rapid course, these climatic fluctuations had a considerable impact on biodiversity and led to repeated changes of fauna. The climatically sharpest incision occurred on the border between Eocene and Oligocene with the extinction event of the Grande Coupure almost 34 million years ago.

In the course of a serious global cooling, the glaciation of Antarctica and with it the Cenozoic Ice Age began. Around 40 million years ago, the Drake Passage gradually began to open between Antarctica and South America . The resulting emergence of the circumpolar current intensified the icing process. The forests that had previously covered the continent were displaced.

Current and future development

The geological present is identical to the Holocene epoch, which has ruled for 11,700 years , a warm period within the Quaternary Ice Age. The cultural development of mankind falls into this climatically relatively stable phase . In the course of the current global warming , which is very likely mainly due to human influences, there was already a global mean temperature increase near the ground of 0.85 ° C between 1880 and 2012. With almost unchecked greenhouse gas emissions, a temperature rise (geologically very rapid) beyond the 4 ° C threshold is possible until the end of the century, coupled with the advancing melting of the polar ice caps and an accelerated rise in sea level . Various consequences of global warming can already be observed. Further effects of the climatic changes that could threaten humanity are predicted.

According to the unanimous scientific opinion, the anthropogenic carbon dioxide input into the atmosphere will still be detectable in significant quantities for millennia even with a future reduction in emissions, whereby several scenarios are conceivable with regard to progressive global warming. This development depends largely on the level to which the atmospheric CO 2 concentration will ultimately rise in the next few centuries. Without technological countermeasures, for example in the form of comprehensive geoengineering , the consequences associated with permanent warming would prove to be irreversible. Some studies do not rule out the possibility that a longer warm phase in the region of 100,000 years could follow a course similar to the Paleocene-Eocene temperature maximum due to long-term feedback processes in connection with a high Earth system climate sensitivity .

On geological time scales of a hundred million years and more, the increase in luminosity of the sun will lead to a permanent and gradually more intense warm climate and shape the rest of the earth's history.

Tracks in the Arctic and Antarctic

The super continents Laurasia and Gondwana about 200  mya

Warm periods formed the basis for the coal deposits in the then forested Antarctica , part of what was then the major continent of Gondwana . Reptile fossils from this period can also be found in Antarctica , including dinosaurs. The habitat of that time has no equivalent to any existing habitat on earth: the flora and fauna were particularly adapted to the long darkness of the polar night .

In the Arctic , palm trees even grew in the hot Eocene at temperatures of up to 27 ° C and ocean surface temperatures in the North Sea of ​​20 ° C, as indicated by the remains of palm pollen on the floor of the Arctic Ocean. The winter temperatures in the north polar region averaged over 8 ° C.

Climates

In the warm climate around 100 million years ago, central North America was flooded by the Western Interior Seaway

While one typically finds oscillating temperature profiles in ice ages, the temperatures in warm climates tend to be relatively uniform. However, special heat anomalies and associated abrupt climatic changes have also been identified within warm climates . These include the Paleocene / Eocene temperature maximum about 55 million years ago, a short period of global warming, and the Eocene Thermal Maximum 2 about 53.7 million years ago.

In times of warm climates, globally, the climate is generally more humid, as more water evaporates when the temperature is higher. But it can also be the other way round in the form of greater drought. This was the case in the warm, dry climate of the Triassic . The reason for this lay in the shape of the supercontinent Pangea . While Pangea's coasts were probably exposed to very strong monsoons and their precipitation, a huge desert with an extreme continental climate is assumed for central Pangea .

The sea ​​level in the non-ice ages was drastically higher than in the ice ages, since no water was bound in ice sheets ; in addition, there is the thermal expansion of the upper seawater layers. In the last warm climate around 35 million years ago, the sea level was almost 70 m higher than it is today. As a result, large areas of land were flooded by oceans. The sea surface temperatures were significantly warmer than in the ice ages, but the deep water of the oceans was also cold in warm climates. Due to the reduced circulation of the warmer oceans, large parts of the oceans were often oxygen-free and oceanic anoxic events occurred .

In the warm climate, the temperatures were by no means the same everywhere, as they are today, but rather lower in high latitudes (near the polar) than at the equator and in the high mountains lower than at sea ​​level . The temperature difference compared to today was greater at the poles than at the equator.

See also

Individual evidence

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