|The absence of clouds is causing the Greenland ice sheet to melt||Tweeter|
A study conducted by researchers at the University of Bristol (UK) and the University of Liege has just revealed that an increase in temperature is not the only element responsible for the recent melt increase over the Greenland ice sheet. Since the middle of the 1990s, the marked increase in cloud over the second largest island in the world during the summer period, has accounted for about two-thirds of this acceleration of melt. This study, which has just been published in the journal Science Advances (1), has caused a stir among the scientific community because it casts doubt on another recent study (Van Tricht et al. 2016) which found that the clouds (and the greenhouse effect they cause) were responsible for the melt over the ice sheet.
The role of clouds could be schematically compared to a thermostat in a house. The more abundant the clouds are, the more they reflect the light of the sun, and therefore, filter its heat. Conversely, the less abundant the clouds are, the more they allow the sun’s rays to reach the surface of our planet. In addition to their “umbrella effect” which reduces the levels of solar radiation reaching the surface during the day, the clouds also generate a powerful “greenhouse effect” (water being the main greenhouse gas) by trapping the infrared radiation emitted from the Earth’s surface. Up to now, it was thought that the natural greenhouse effect of clouds was the dominant effect over the ice sheet and that the polar clouds actually had a warming effect, and therefore, that the acceleration of melt over the Greenland ice sheet was caused almost exclusively by the global warming phenomenon observed in recent years and amplified in Arctic (+3°C since 1980). The study, which Xavier Fettweis, a climatologist at the University of Liege (SPHERES research unit), participated in, shows that the changes in cloud levels recorded for the last twenty years, have been one of the principal drivers of this acceleration and have called into question the role played by the clouds in the Arctic.
These results were obtained by data collection from satellite observations of the Earth (American satellites AVHRR and MODIS) and simulated by the MAR model (for Modèle Atmosphérique Régional), a regional climate model, developed by Xavier Fettweis and his colleagues at the University of Liege. Thanks to the use of the MAR model, the researchers demonstrated that there had been a constant reduction in cloud cover during summer periods (June, July, August) since 1995. The researchers were able to use this model to calculate the volume of extra melt in tonnes, in terms of the reduction in cloud cover. “Since 1995, an annual reduction of 1% in cloud cover during the summer months has been recorded over the island”, explainsXavier Fettweis, which corresponds to an extra melt of around 27 gigatons (GT), every year! That is roughly equivalent to the annual domestic water supply of the USA. We can thus estimate that, since 1995, the Greenland ice sheet has lost 4,000 gigatons of ice, which means that the island can now be classed as the biggest contributor to the observed rise in sea levels”.
The team reports that, in addition to higher temperatures, global warming has probably modified the general circulation of the atmosphere above Greenland by favouring more frequent anticyclones above the island since the 1990s, thus allowing a higher level of solar radiation to reach the surface of the ice sheet. These changes in the circulation of the atmosphere in the North Atlantic and the Arctic are unprecedented since weather records began (1850). This unusual state of the atmosphere is very probably linked to the lowest observed extents of sea ice in the Arctic Ocean in summer since the 2000s.
This study demonstrates the complex and interconnected nature of the climate system and the consequences that changes in one climatic component can have on another and the way they combine to accelerate the melting of the Greenland ice sheet which has already been negatively impacted by the increase in temperature.
(1) Hofer et al., Decreasing cloud cover drives the recent mass loss on the Greenland Ice Sheet, Science Advances 28 Jun 2017: Vol. 3, no. 6, e1700584, DOI: 10.1126/sciadv.1700584 http://advances.sciencemag.org/content/3/6/e1700584(2) Van Tricht et al., Clouds enhance Greenland ice sheet meltwater runoff, Nature Communications, January 2016.
Total variation in cloud cover in summer (June-July-August) according to satellite data AVHRR and MODIS and the regional climatic model MAR. (A) Comparison between AVHRR (12) (left, top) and MAR (right, top) total JJA cloud cover change (%) during the full available data period of AVHRR between 1982 and 2009. Bottom: Trend of JJA 500-hPa geopotential height (Z500) in meters per year. The arrows show the wind trend in meters per second per year and highlight the circulation anomalies induced by the JJA Z500 changes. The arrow length of a change of 0.2 m/s per year is given in the legend for indication. (B) Comparison is the same as in (A) but for MODIS (11) (left; full observation period, 2002–2015) and MAR (right; 2002–2015) (B).