As heat waves increasingly remind us that global warming is already impacting our daily lives, climate models indicate that the greater the global warming, the greater the changes in Antarctica will be. This is important, because the melting of the Antarctic ice cap is currently one of the major causes of sea level rise.
But, like a tightrope walker, the future of Antarctica is uncertain: the balance could tip one way or the other, depending on whether the melting of the polar ice cap or the accumulation of snow will become predominant.
Our new study, currently being published, shows that clouds are an important source of uncertainties, in addition to those we already know. Under certain conditions, the clouds could greatly increase the surface melting, and cause a rapid destabilization of the Antarctic ice sheet, attacking it from the surface (and adding to the melting from below, due to the ocean warming. In the "best" case, they would slow down the melting of the Antarctic ice a little by serving as a "parasol" and promoting the accumulation of snow.
In climate science, there are many sources of uncertainty, in climate change itself, but also in the way models represent the climate. It is therefore particularly complicated to predict the melting of Antarctica associated with an increase in temperature.
These uncertainties also make it difficult to establish political strategies aimed at defining a maximum warming objective (like those of the Paris agreements for example), according to the rates of warming and the associated risks inferred by observations and models.
Clouds play a dual role
In addition to bringing humidity and precipitation to the Antarctic continent (the center of which is a very dry desert), clouds affect the energy available to cool or warm the surface.
In the polar regions, the white snow on the ground reflects solar energy towards space, in particular the short wavelengths, and in particular the visible. As long as the snow is white and not melting, the sun's energy is only slightly absorbed by the surface. But as soon as it melts, this effect decreases and the surface then absorbs solar energy.[Nearly 70 readers trust The Conversation newsletter to better understand the world's major issues. Subscribe today.]
Because they are white, clouds reflect some of the sun's energy back into space. When there are clouds, more energy is sent back to space than when there are none: they then have the effect of a parasol and limit the solar energy which reaches the surface of Earth.
The snow on the surface of Antarctica, whose behavior resembles 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 this energy and emit it in turn towards the surface. This infrared energy emitted by the clouds has the effect of heating the surface. This principle can easily be observed here in winter: it is always much colder at night when there are no clouds than when there are.
The energy emitted by 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 the greenhouse effect.
Depending on the conditions, the clouds can therefore cool the surface, via the parasol effect, and warm it, via the greenhouse effect.
The future of Antarctica
The Clausius-Clapeyron law relates the humidity content of the air to the temperature. The relationship is quite simple: the warmer the air, the more moisture it contains. This increases the amount of clouds, and ultimately the snowfall in Antarctica. The parasol effect will increase, but also the power of the greenhouse effect. It is the balance between these antagonistic effects that will determine the role of clouds.
This balance depends on the properties of the clouds. For example, those containing liquid water induce a greater greenhouse effect, while those containing ice and snow have a greater parasol effect.
As a result of global warming, the snow in Antarctica will melt. This will trigger an additional process influencing the energy balance: as it melts, the snow becomes darker and reflects less direct energy from the sun (we say that its albedo decreases). It absorbs more and melts more. It's a positive feedback loop that gets stronger over time. Depending on the predominant effect of the clouds, these can slow down the positive feedback a little (umbrella effect) or strongly accentuate it.
According to our study, one of the main sources of uncertainty in the projections is knowing which clouds will become more frequent in the future and therefore in which direction the balance will tip. All projections suggest an increase in clouds with strong greenhouse effect (containing liquid water) having the consequences of increasing melting, but in different proportions, which leads to a large uncertainty in the projections on the amounts of melted ice.
How do you enter a cloud into a climate model?
A climate model is a set of mathematical equations of the physical laws of the atmosphere. To these equations, parametrizations are added to represent processes for which we do not (yet) have physical laws. And among these processes are the formation of clouds and their transformation into precipitation. It is on the parameterizations of the clouds that the models diverge the most, and where the uncertainty is the greatest. Typically, most climate models have difficulty representing clouds in polar regions.
By increasing the melt, the clouds could allow tipping points to be reached leading to the destruction of the ice shelves that stabilize Antarctica. These same clouds have also greatly contributed to the recent record temperature in East Antarctica and their role could be even more decisive in the future. However, they are still very poorly represented by climate models. No projection is more likely than another, but there is every indication that the greater the warming, the greater the likelihood of reaching tipping points.
Christoph kittel, Post-doctoral researcher in climatology, Grenoble Alpes University (UGA)
This article is republished from The Conversation under Creative Commons license. Read theoriginal article.