Increased concentrations of greenhouse gases have cooled the atmosphere over Antarctica

In the era of global warming, caused by an increase in the content of greenhouse gases in the atmosphere, reverse phenomena may occur when a cooling effect or anti-greenhouse effect occurs. For example, it was observed in Antarctica. This means that the regional impact of global warming may be less clear-cut than expected—and what raises temperatures in one place may lower them in another.

Greenhouse gases, such as carbon dioxide (CO2) and methane (CH4), allow solar radiation to reach the Earth's surface, but block infrared (thermal) radiation emitted by our planet. As a result, the temperature rises and the surface heats up – a greenhouse effect occurs.

However, in some cases, these gases have a cooling effect, the opposite of a “greenhouse”, when the atmosphere becomes less transmittable to the sun's rays, which reduces the amount of energy received by the planet's surface. For example, this situation is typical in the upper atmosphere of Titan or Earth immediately after major volcanic eruptions. The magnitude of the anti-greenhouse effect depends on the presence of aerosol layers located at different heights and consisting of particles with different microphysical and optical properties.

Carbon dioxide and methane enter the atmosphere for a variety of reasons, but in recent decades, mainly due to human activity – the burning of fossil fuels. Over the past decades, greenhouse gas emissions have only increased. This, for example, has led to severe warming in the Arctic. There it happens three to four times faster than the world average. As a result, the area and thickness of sea ice in the Arctic Ocean basin are rapidly decreasing.

At the same time, in Antarctica the warming is not as pronounced. The ice cover of the Southern Ocean, represented mainly by seasonal ice formed during the cold season off the coast of Antarctica, remains relatively stable.

Scientists have long wondered why the Arctic is melting faster than Antarctica (which includes Antarctica and its surrounding islands and parts of the Atlantic, Indian and Pacific oceans). There are several answers to this, but for now these are hypotheses.

Some experts believe the reason is that meltwater from Antarctica stabilizes the water column and ice, protecting cold surface waters from warm deep waters. Others believe that the westerly winds blowing around Antarctica are increasing due to climate change and are “stretching” the ice over a larger area. There is another point of view: according to it, the matter is in the circulation of the ocean, due to which excess heat received from the atmosphere is “removed” from Antarctica and transferred north, to the equator.

A group of scientists from the University of Bremen (Germany) proposed another explanation – the “anti-greenhouse effect”. The results of the work were published in the journal Geophysical Research Letters.

About a decade ago, climatologist Justus Notholt and his colleagues first identified the anti-greenhouse effect over high-altitude parts of Antarctica, where the air is especially cold and dry. This cooling phenomenon was observed in the atmosphere (at low altitudes) during several months of the year when carbon dioxide concentrations increased there.

At the time, climate scientists suggested that the anti-greenhouse effect might partly explain why temperatures in Antarctica are not rising as quickly as in the wetter Arctic, where the effect appears to be rare.

In a new study, Knotholt's team tried to understand how water vapor in the atmosphere over Antarctica and the Arctic influences the warming and cooling associated with rising concentrations of methane and carbon dioxide in different layers of the atmosphere.

The scientists conducted two computer simulations. In the first, the amount of water vapor in the Antarctic air matched the levels observed over the Arctic. They found that carbon dioxide and methane had the same effect on temperatures in Antarctica as they did on temperatures in the Arctic, meaning they increased.

The second model assumed current atmospheric water vapor levels over these two regions. Climatologists modeled seasonal temperature changes under two scenarios: current levels of carbon dioxide and methane concentrations, and also doubled. It turned out that the increase in the concentration of these greenhouse gases in the atmosphere of Antarctica led to a cooling of almost the entire troposphere (altitude 10-18 kilometers) and to warming in the Arctic troposphere.

Thus, with doubled concentrations of CO2 and CH4, a warming of 0.42 kelvin occurred in the Arctic troposphere and a slight cooling of 0.01 kelvin in the Antarctic troposphere.

At an altitude of up to seven thousand meters from the surface, with double the CO2 concentration, the average temperature in the Arctic increased by 0.81 kelvin, and in Antarctica by 0.16 kelvin. However, with the same amount of CH4, a cooling of 0.06 kelvin was observed in Antarctica and a warming of 0.07 kelvin in the Arctic.

Water vapor, like greenhouse gases, contributes to the greenhouse effect. The amount of water vapor in the air depends on the temperature – the higher the temperature, the more moisture it can contain. In other words, Antarctica's colder, drier atmosphere responds differently to increased greenhouse gas emissions than the Arctic's wet, warm atmosphere.

“As humidity increases with temperature, and Antarctica gets warmer every year, the anti-greenhouse effect may eventually change to a greenhouse effect,” Notholt noted .

The authors are confident that the results of their study will help explain why Antarctica has been experiencing less pronounced warming effects than the Arctic for decades.

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