While the Greenlandic ice sheet has warmed by nearly 3 ° C since 1982, its albedo has continuously declined. Thus, ice reflects solar radiation less efficiently, which further increases the speed of its decline. Recent work associates this darkening with a seemingly anecdotal property: the shape of the snowflakes deposited on the cap.
TheAlbedo of a surface denotes the ratio between the quantity of light which arrives on this surface and the reflected fraction. The higher it is (close to 1), the greater the reflected radiation. On the contrary, an albedo close to 0 indicates that the energy is mainly absorbed by the surface. For these reasons, we also speak of reflective power.
At the origin of a declining albedo
Composed of ice and snow, the Greenland ice sheet naturally has a very high albedo. In fact, a large fraction of the incident solar energy is sent back to space, perpetuating the cold conditions that hold the ice sheet in place. However, with climate change, there is a general darkening of the ice surface.
Although still high, the albedo is declining, testifying togreater absorption of solar radiation in summer. Result: the melting accelerates, which makes the surface a little darker and further reduces the albedo of the cap. It’s a real vicious circle. However, while satellite observations have shown the existence of a darkening, the question of its precise engine has so far remained unresolved.
In new study, researchers found that parts of Greenland had become less exposed to snowfall. However, since fresh snow has an extremely high reflectivity, this means that these areas darken. ” As the snow ages, even in a few hours to a few days, you get this reduction in reflectivity, which is why fresh snow is so important. », Notes Erich Osterberg, co-author of the paper.
The granulometry of the snow, a detail with major consequences
Scientists note that the reduction in precipitation is explained by an increase in high pressure blockages in the region since the mid-1990s. The disturbed rail then travels too north or too south to really affect Greenland. In addition, these high pressure bubbles are accompanied by a clear sky, therefore high sunshine and an abnormally mild air mass at altitude.
” It’s like a triple sentence “, Advises Erich Osterberg. ” All of this contributes to an increasingly rapid melting of Greenland. “. And you don’t have to change the albedo much for the impact to be significant. Indeed, a decrease of only 1% would be enough to destock 25 billion tonnes of additional ice over three years. But why does the recently fallen white gold have such a different reflective power than that dated just a few days ago?
” The fresh snow looks like what you would draw in a kindergarten class or cut out of a piece of paper. She has all these really sharp points because it’s extremely cold in the atmosphere when the snow falls », Explains Gabriel Lewis, main author. ” Once it falls and rests on the surface of the ice cap in the sun, it changes shape and the snow grains get larger over time. “.
According to the data in situ, the decrease in Greenlandic albedo would therefore be mainly due to a change in the texture of the snow. We speak of an increase in grain size, the flakes becoming larger and rounded. Finally, the observations also show that a possible increase in dust in the ice cannot be blamed. ” In our area of research, impurities do not seem to be sufficient to explain the change in albedo that other research teams have reported. », Reports the co-author.