The amount of solar energy absorbed by the surface of the Antarctic continent is determined by the snow's albedo, in other words by its 'whiteness'. The albedo, in turn, depends on the size of the snow grains. In a process that is familiar to physicists, once the fine particles of snow are deposited on the surface they tend to become larger, at a rate that increases with temperature. The coarsening of the grains leads to a fall in albedo, which causes temperatures to rise. Climatologists have long been aware of the importance of this positive feedback mechanism.
However, according to recent experiments by researchers at the Laboratoire de Glaciologie et Géophysique de l'Environnement (CNRS / Université Joseph Fourier Grenoble -1) and the Takuvik International Joint Unit (CNRS / Université de Laval), this effect is partially offset by a hitherto underestimated negative feedback mechanism. Using data from satellites observing the Antarctic surface at microwave wavelengths, the scientists showed that, in summers marked by heavy snowfall, albedo did not change significantly: the surface was covered with fine snow grains that were constantly renewed. Snowfall is expected to increase in Antarctica in the future and it is well known that, when temperatures are very low, the air is dry and snowfall is scarce. In Antarctica therefore, warming will also result in increased precipitation.
According to the researchers, in a climate scenario where the temperature of Antarctica rises by 3°C, heavier snowfall would increase the albedo by 0.4%. This would offset the 0.3% fall in albedo caused by the rising temperatures (positive feedback loop). Thus, despite significant warming in Antarctica, albedo will vary only very slightly over a large part of the continent. Since the positive temperature-albedo feedback loop will not be established, warming in Antarctica will be lower than expected. Warming predictions should be revised downwards by 0.5°C for central Antarctica. This research highlights the need to take better account of snow in models used to predict future climate change.
The work was supported in particular by the French Polar Institute (IPEV), the French National Research Agency (ANR) and CNRS, via the LEFE and PNTS programs.
© Jean-Charles Gallet, LGGE
A typical view of the snow-covered Antarctic plateau
© LGGE (CNRS / UJF)
Changes in the size of snow grains since 1999 at the French-Italian Concordia Station in Antarctica (75°S, 123°E), as shown by microwave satellite data. The years 2002 and 2008 show a remarkably small increase in grain size. Meteorological data also indicates exceptionally high precipitation for both years. The 'grain index' introduced in this study is a combination of satellite data approximately representing the size of grains in the top few centimeters of snow cover.
(1) LGGE is a laboratory belonging to the Observatoire des Sciences de l'Univers de Grenoble (OSUG).
Inhibition of the positive snow-albedo feedback by precipitation in interior Antarctica. G. Picard, F. Domine, G. Krinner, L. Arnaud and E. Lefebvre. Nature climate change. 1 July 2012.