2.6 million years ago, the Earth was much hotter than it is now

Produced by: Science Popularization China

Author: Zang Tonggang (Master’s student in paleoclimatology)

Producer: China Science Expo

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In recent years, the issue of global warming has received increasing attention, and scientists have conducted long-term and in-depth research on this issue. In these studies, an important part is to try to help us better predict the future by studying the history of the earth. Ao Hong and Song Yougui, researchers from the Loess and Climate Change Research Team of the Institute of Earth Environment, Chinese Academy of Sciences, used China's loess records to study a major climate change event a million years ago, making a huge contribution to the understanding of past climate change and the prediction of future climate change. This research result was published in the journal Nature Communications on April 19, 2024.

2.6 million years ago, the Earth was much hotter than it is now

The Earth's climate has always fluctuated, and there have been long periods of time in the past when it was warmer than it is today. The most recent warm period was during the transition from the Pliocene to the Pleistocene, about 2.6 million years ago.

At that time, the global average temperature of the earth was 2-4℃ higher than it is now, and the sea level was about 20 meters higher than it is now (that is, coastal areas such as Shanghai, Tianjin, and Macau would all be submerged). The climate conditions at that time were completely different from now. Even at that time, even the Arctic was not covered with ice and snow like it is now, only the Antarctic was covered with ice sheets. This situation is called "monopolar icehouse" by scientists, which corresponds to the "bipolar icehouse" on the earth today, where both the North and South Poles are covered with ice sheets.

But then the Earth entered a long cooling period.

(Photo source: veer photo gallery)

Arctic ice sheet expansion and global cooling

2.6 million years ago, the ice sheets in the Northern Hemisphere had just begun to expand. The ice sheets in Greenland, North America and Eurasia not only increased in thickness, but also many small icebergs broke away from the ice sheets and drifted into the North Pacific and North Atlantic Oceans. Eventually, as the global temperature tended to be colder, the Arctic gradually became covered with snow. As the more glaciers there were, the less water there was. When the glaciers were most extensive, the sea level was about 100 meters lower than it is now.

Once the Arctic ice sheet begins to appear, the global temperature will continue to drop further. This is because the heat on the earth comes from the absorption of solar energy. After the sunlight hits the earth, part of it is absorbed by the earth's surface. In addition to being absorbed by the earth, part of it will be reflected back into space. Different surface types such as forests, soil, water, ice and snow have completely different reflectivity to sunlight, and the reflectivity of ice and snow is much higher than other terrains. Therefore, the formation of a large amount of ice and snow in the Arctic reflects more sunlight back, and the energy that the earth can absorb will be reduced. Over time, the earth becomes colder and colder. Since then, under the influence of a large amount of ice and snow in the Arctic, the earth has experienced a very long period of continuous cooling. Eventually, the current "double icehouse" distribution pattern of the Antarctic and the Arctic was formed.

Therefore, after the Arctic ice cap began to form, the climate of the entire earth began to slowly cool down. However, due to the certain fluctuations in the strength of the sun's own energy, under the general trend of the earth becoming colder, there will also be small alternations between hot and cold, with a cycle of tens of thousands to hundreds of thousands of years. Relatively warm periods are called "interglacial periods" and relatively cold periods are called "glacial periods". The earth is now in a relatively warm "interglacial period".

(Photo source: veer photo gallery)

The impact of the Arctic ice sheet and the Asian winter monsoon on us

The appearance of the Arctic ice cap has a significant impact on the climate of the entire northern hemisphere and even the world. For example, the winter wind in the northern hemisphere is closely related to the Arctic ice cap . The cold wind howls in the north in winter, and people call it the "northwest wind". This "northwest wind" is actually the winter wind, which is formed by the cold air from Siberia moving southward. If the winter wind is too strong and moves further south, hail, heavy snow and other weather will also occur in the south in winter.

Therefore, once the Arctic ice sheet melts, it will bring very serious consequences. In addition to the well-known fact that the melting of the ice sheet will cause the sea level to rise, the disappearance of the Arctic ice sheet may also cause the Asian winter monsoon to weaken or even disappear , which will cause significant changes to the climate of the entire Asia. For example, the rise in winter temperatures may disrupt the current crop cycle, and the yield of wheat, one of the current staple foods, may drop significantly; it will also cause floods in southern China, causing casualties and economic losses to residents; in addition, it may disrupt the balance of the ecosystem and may also have a collateral impact on the climate in other regions.

The change in the intensity of the Asian winter monsoon is closely related to the lives of each of us, so it has always been the focus of scientists' research. Scientists study the winter monsoon in order to better predict the future, but to achieve the goal of prediction, they first need to study the history of the winter monsoon. This is because only when we discover the historical laws of climate change can we predict the future to a certain extent.

In the historical study of winter monsoon, the most important question is: did the winter monsoon exist before the formation of the Arctic ice cap? Is there any relationship between the strength of the winter monsoon and the changes in the Arctic ice cap?

Asian winter monsoon map

(Image source: Nature Communications)

How to study past climate change

However, studying the historical changes of winter monsoons involves two difficult problems: first, how can we know about past climate change? Second, how can we determine the age of climate change?

So how do we know the changes in the Asian winter monsoon over the past million years? First of all, we know that when the wind is strong, it can move coarser particles, and when the wind is weak, it can only move some finer particles. Therefore, the coarseness of the soil can well indicate the strength of the monsoon. The Asian winter monsoon blows from Siberia to China, and the Loess Plateau is on the path that the Asian winter monsoon must pass through. Therefore, based on the changes in the coarseness of these soil particles on the Loess Plateau, we can judge the changes in the intensity of the Asian winter monsoon in the past.

Now that we have solved the problem of how the winter wind changes, how do we determine the time corresponding to the change in the winter wind? The "paleomagnetic dating method" is used here. The Earth's magnetic field affects the magnetism of minerals, and changes in the Earth's magnetic field will be recorded by the minerals. In the past, the Earth's magnetic field has reversed many times. Through the study of the reversal of the Earth's magnetic field, scientists have compiled a geomagnetic time scale. Scientists only need to study the magnetism of soil minerals and compare it with the geomagnetic time scale to determine the age of the soil material.

Based on the above two points, we can more accurately judge the changes in the strength of the monsoon and the corresponding years.

What did the study find?

Ao Hong, Song Yougui and other researchers from the Loess and Climate Change Research Team of the Institute of Earth Environment, Chinese Academy of Sciences, found that the Asian monsoon existed long before the formation of the Arctic ice sheet , but after the formation of the Arctic ice sheet 2.6 million years ago, the intensity of the Asian winter monsoon increased significantly . It was also found that the monsoon intensity has been changing in cycles of 40,000 and 100,000 years, and this periodicity has not been affected by the formation of the Arctic ice sheet. At the same time, the 40,000 and 100,000-year change cycles of the Asian winter monsoon are almost exactly the same as the global sea level change cycle. That is to say, in the past two million years, when the sea level was higher, the intensity of the Asian winter monsoon was at a stronger level, and when the sea level was lower, the Asian winter monsoon was at a weaker level. In addition, before the glaciers in the Northern Hemisphere began to affect the climate, the intensity of the changes in the Asian winter monsoon was relatively weak.

In addition, before and after the formation of the Arctic ice sheet, in addition to the 40,000-year and 100,000-year periodic changes, the Asian winter monsoon has always had significant millennial-scale fluctuations. Such millennial-scale fluctuations persisted in both the warmer (high CO₂) Late Pliocene and the colder (low CO₂) Early Pleistocene, and were mainly controlled by astronomical drivers and the Earth's internal climate dynamics.

In summary, this result is of great significance for understanding climate dynamics on a millennial scale. It shows that the size of the Arctic ice sheet only affects the intensity of the Asian winter monsoon , but does not affect the periodic changes of the Asian winter monsoon . From ancient times to the present, the intensity of the Asian winter monsoon has been fluctuating regularly with a large cycle of 40,000 years and 100,000 years and a small cycle of millennia. At the same time, the intensity of the Asian winter monsoon changes synchronously with the global sea level.

Comparison of indicators such as Asian winter monsoon intensity and sea level change

(Image source: Nature Communications)

What inspiration does it give us?

Global warming is now a recognized fact, but research has found that 2.6 million years ago, the global temperature was 2-4°C higher than it is now, while the CO₂ concentration was comparable to the current concentration. In this case, will the global temperature rise further by 2-4°C?

Research has found that the intensity of the Asian monsoon does have a 40,000-year, 100,000-year, and millennial-scale change cycle, and changes synchronously with the sea level. So, can we predict the changes in the Asian monsoon by monitoring sea level changes in the future? What impact will these changes have on Asia and even the global climate? How should we humans deal with these future changes?

These issues still require further research by scientists. One day, perhaps we will be able to fully understand the laws of climate change, solve the negative impacts of climate problems on humans, and be able to understand, master, and utilize climate.

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