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Impact of atmospheric circulation on the melting of the Greenland ice sheet
Illustration : ©NASA
Researchers from the Laboratory of Climatology (SPHERES Research Unit) of the University of Liège used the MAR regional climate model to assess the impact of recent atmospheric circulation changes recorded over Greenland and to compare them with climate predictions. Their result is clear: if these atmospheric circulation changes continue in summer, the ice sheet will melt twice as fast as global models predict. These results are published twice in The Cryosphere (1,2).
ince the early 2000s, scientists have observed a change in the summer atmospheric circulation over the North Atlantic, favouring warmer and sunnier weather conditions in summer over the Greenland ice sheet. "These conditions, described as anticyclonic, have accentuated the melting of the ice, generating historic records as in 2012 when melting was observed on the surface of almost the whole ice sheet," explains Alison Delhasse, a researcher at the ULiège Laboratory of Climatology and first author of one of the two articles published in the journal The Cryosphere(1). However, this summer 2018 was characterised by more frequent passages of depressions bringing large quantities of snowfall." The Greenland ice sheet therefore has currently a lower melting rate compared to previous years, in sharp contrast to the record temperatures recorded in Scandinavia and the heat wave observed overall in Europe. While this year's observations tend to be reassuring, researchers are concerned about these frequent changes in atmospheric circulation that could well reverse the trend next year or later, despite what global climate models have previously predicted. In Liège, the researchers at the Climatology Laboratory preferred to adopt a different approach, using the MAR (Regional Atmospheric Model), a regional climate model with which they are familiar.
Climate projections for the coming decades are based on so-called "global" climate models that represent the global climate. These simulations are widely used by climatologists around the world to predict future climate change. However, these models do not represent the current change in atmospheric circulation responsible for the higher occurrence of high pressure systems in summer over Greenland. Nor do they plan any for the future, as explained in the second study in which ULiège participated and which is the subject of a second publication in the journal The Cryosphere(2). In contrast, so-called "regional" climate models simulate climate at the scale of a given region. "Unlike global climate models, these regional models are able to represent these changes and their consequences on the ice sheet because they use data from observations that take into account these changes in atmospheric circulation," explains Xavier Fettweis, FNRS Research associate and head of the ULiège Laboratory of Climatology. »
The objective of their study was therefore to assess the impact of a change in atmospheric circulation (as currently observed) in a warmer climate on future projections of the surface mass balance - i. e. the difference between mass gains (snowfall) and surface losses (melting, evaporation, etc.) - of the Greenland ice sheet. In order to assess the impact of the persistence of high pressure in summer in a warmer climate on Greenland, researchers in Liège used the regional climate model, the MAR, a model specially developed for the polar regions and designed by the Laboratory of Climatology from the University of Liège.
On the left, estimates of the melting of the ice sheet estimated by the IPCC with a temperature increase of 2°C. On the right, identical scenario but taking into account the change in atmospheric circulation.
"Since it is not possible to determine atmospheric circulation in the future," says Delhasse, we artificially imposed a warming of 1°C to 2°C on the current climate (2000-2016 data), which has been characterized by a change in atmospheric circulation." Under these conditions, the researchers were able to observe that the increase in the melting of the Greenland ice sheet would be at least twice as large as what was announced in the last IPCC (Intergovernmental Panel on Climate Change) AR5 report, which could obviously have disastrous consequences, resulting in a very significant rise in water levels in certain regions of the world. However, the IPCC has already sounded the alarm by explaining that if we do not comply with the COP21* agreements, the rise in sea level caused by the melting of the surface of the Greenland ice sheet alone could reach 9 cm by 2100. Now imagine the consequences if the scenario presented by the researchers at the Climatology Laboratory, which predicts persistence in atmospheric circulation changes, were to prove to be accurate. "It could therefore be that the share of sea level rise from the melting of the Greenland ice cap could be doubled, concluded Alison Delhasse, thus bringing the sea level rise to more than 18cm! »
*The COP is an international climate conference that brings together each year the signatory countries of the United Nations Framework Convention on Climate Change (UNFCCC). The 2015 edition (COP21) was organised by France. The Paris Agreement adopted there marks a turning point in the fight against global warming by committing all countries in the world to reduce their greenhouse gas emissions and keep global warming below 2°C by 2100.
(1) Delhasse, A., Fettweis, X., Kittel, C., Amory, C., and Agosta, C.: Brief communication: Impact of the recent atmospheric circulation change in summer on the future surface mass balance of the Greenland ice sheet, The Cryosphere, https://doi.org/10.5194/tc-2018-65, accepted, 2018.
(2) Hanna, E., Fettweis, X., and Hall, R. J.: Brief communication: Recent changes in summer Greenland blocking captured by none of the CMIP5 models, The Cryosphere, https://doi.org/10.5194/tc-2018-91, accepted, 2018.
Laboratory of Climatology I SPHERES Research Unit I Faculty of Sciences
Xavier FETTWEIS & Alisson DELHASSE