The fate of Antarctica on a thread, or the dual role of clouds in global warming

The fate of Antarctica on a thread, or the dual role of clouds in global warming

While heat waves increasingly remind us that global warming is already affecting our daily lives, climate models suggest that the greater global warming, the greater the changes in Antarctica. This is important, because the melting of the Antarctic ice sheet is currently one of the main causes of sea level rise.

But, like pedestrians on a tightrope, the future of Antarctica is uncertain: the balance can tilt in one direction or another, depending on whether the melting of the polar ice cap or the accumulation of snow becomes dominant.

Read more: Antarctic ‘Cork’ is ready to explode due to climate change

Our new study, which is currently being published, shows that clouds are an important source of uncertainty, in addition to those we already know. Under certain conditions, clouds can significantly increase surface melt, rapidly destabilize the Antarctic ice sheet, attacking it from the surface (and increasing melting from below, due to ocean warming. In the “best” case, they It will slow the melting of the Antarctic slightly by acting as a “canopy” and promoting the accumulation of snow.

Many unknown things

In climatology, there are many sources of uncertainty, in climate change itself, but also in the way models represent climate. Therefore, it is particularly complex to predict the melting of Antarctica associated with the rise in temperature.

These uncertainties also make it difficult to develop policy strategies aimed at setting a maximum warming target (eg the Paris Accords targets), according to the rates of warming and the associated risks inferred by observations and models.

Clouds play a double role

In addition to bringing moisture and precipitation to Antarctica (in the middle of which there is a very dry desert), clouds affect the energy available to cool or warm the surface.

In the polar regions, the white snow on Earth reflects solar energy towards space, particularly of short wavelengths, particularly of the visible. As long as the snow is white and does not melt, the energy of the sun is absorbed by the surface a little. But once it melts, this effect diminishes and the surface absorbs solar energy.

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Because they are white, clouds reflect some of the sun’s energy back into space. Quand il ya des nuages, plus d’énergie est renvoyée vers l’espace que lorsqu’il n’y en a pas : ils font alors l’effet d’un parasol et limitent l’énergie solaire qui arrive à la surface de land .

Understanding the effect of clouds on global warming: On the one hand, clouds can act as a canopy and limit the arrival of visible radiation from the Sun into the atmosphere; But they can also transmit infrared radiation that they emit to Earth.
Christoph KittelAnd the Introduction by the author

Snow on the surface of Antarctica, whose behavior is similar to a “black body”, emits infrared radiation towards space. In the absence of clouds, the infrared radiation emitted by the surface is lost to space. But when there are clouds, they can absorb some of that energy and release it in turn toward the surface. The infrared energy emitted by the clouds has the effect of heating the surface. This principle can be easily observed here in winter: it is always cooler at night when there are no clouds than when you are there.

The energy released from the clouds towards the surface increases the energy available to melt the Antarctic ice sheet. It is similar to the effect of greenhouse gases. Moreover, water in its various forms is responsible for 75% of global warming.

Depending on the conditions, clouds can thus cool the surface, through the canopy effect, and warm it via the greenhouse effect.

The future of Antarctica

The Clausius-Clapeyron law relates the moisture content of air to temperature. The relationship is very simple: the warmer the air, the more moisture it contains. This increases the amount of clouds, and eventually snowfall in Antarctica. The canopy effect will increase, but also the strength of the greenhouse effect. It is the balance between these antagonistic influences that will determine the role of clouds.

This balance depends on the properties of the clouds. For example, those with liquid water lead to a greater global warming effect, while those with ice and snow have a greater canopy effect.

Read more: Terrible clouds of climate kids

As a result of global warming, the snow will melt in Antarctica. This will lead to an additional process that affects the energy balance: when it melts, the snow becomes darker and reflects less energy directly from the sun (we say its albedo is decreasing). It absorbs more and dissolves more. It’s a positive feedback loop that gets stronger over time. Depending on the prevailing effect of the clouds, this can slow down the positive feedback a little (umbrella effect) or accentuate it heavily.

Schematic diagram of the feedback loop between melting snow and albedo
The melting of snow affects its albedo (its ability to reflect solar radiation) and vice versa. This is called a “feedback loop”. In A: When the snow melts, its whiteness decreases which increases the melt again because the snow absorbs more energy, and so on. In case B, the snow melts, and its albedo decreases, but the clouds prevent part of the solar energy from reaching the surface, limiting further melting (the canopy effect). In case C, the clouds amplify the melt and thus amplify the reactions (the greenhouse effect).
Christoph KeitelAnd the Introduction by the author

According to our study, one of the main sources of uncertainty in the projections is the knowledge of which clouds will become more frequent in the future, and therefore in which direction the balance will tilt. All forecasts point to an increase in clouds with a strong greenhouse effect (containing liquid water) having consequences for increased melt, but in different proportions, leading to significant uncertainty in forecasts about the amounts of melting ice.

How does a cloud enter into a climate model?

A climate model is a set of mathematical equations for the physical laws of the atmosphere. To these equations, parameters have been added to represent processes for which we do not (yet) have physical laws. Among these processes are clouds and their transformation into precipitation. Models diverge more on cloud parameters, and where uncertainty is greatest. Typically, most climate models have difficulty representing clouds in polar regions.

By increasing melt, clouds can allow tipping points to be reached and so destroy the ice shelves that stabilize Antarctica. These same clouds have also contributed significantly to the recent temperature record in East Antarctica and their role could be even more decisive in the future. However, they are still very poorly represented in climate models. None are more predictable than others, but there is every indication that the greater the warming, the more likely tipping points will be reached.

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