500,000 images. This is the harvest harvested in twenty years by the Terra satellite, the key instrument for measuring the cryosphere, the frozen areas on the planet’s surface. And it is this mass of data that an international team led by the Laboratory for Studies in Geophysics and Space Oceanography (Legos, Cnes / CNRS / IRD and University of Toulouse-Paul Sabatier) has used to obtain the most effective evaluation. details of the state of these 220,000 tongues of ice eroding mountains all over the world that has just been published Nature.
No surprise: all melt. Their contribution to sea level rise is even now major. “Their volumes of water leaving for the sea represent 267 billion tonnes per year, compared to 200 billion for the Greenland ice sheet and 130 billion for that of the Antarctic ”, summarizes Étienne Berthier, researcher at Legos and co-author of the article. These losses are all the more impressive as the total surface of the glaciers is only 700,000 km² (a little more than France, 551,000 km²) while that of Greenland is 2 million km² and that of France. ‘Antarctica of 13 million km². The 267 billion tonnes released on average every year between 2000 and 2019 could cover the surface of France every year under one meter of water. Glaciers are thus responsible for 20% of the 0.74 mm annual rise in sea level, the remaining 80% coming half from the expansion of the oceans due to higher water temperature and the contribution north and south ice caps.
The precipitation fattens them, the heat makes them thinner
Glaciers are not all in the same boat. Those on the outskirts of Antarctica and Greenland hold up better than glaciers in the Alps, Alaska and Iceland. “It is those which are located in subpolar regions and undergoing oceanic influences which lose the most mass ”, continues Étienne Berthier. The rate of melting is not, however, correlated with the contribution to sea level rise.
Thus, alpine glaciers are among those which melt the fastest, but their small surfaces and thicknesses make them small suppliers of water volume. Climatic conditions can also slow down or accelerate melting. Glaciers in Greenland, Iceland and Scandinavia lost much less mass between 2010 and 2019 due to weather anomalies that increased precipitation. Conversely, glaciers in Alaska and western Canada have seen their melting accelerate due to a locally drier decade.
The decline of the JorgeMontt glacier in Patagonia between 2002 and 2017 © Étienne Berthier
The decadal and regional variability of the mass loss of glaciers therefore depends on the snow that feeds them. But it is the rise in temperatures that explains why their melting is accelerating. The images of Terra show that the global losses went from 227 billion tons of ice lost per year between 2000 and 2004 to 298 billion tons between 2015 and 2019. To obtain a longer time series, the Legos intends to exploit the images of spy satellites which since the 1970s have carried instruments to measure variations in water masses on the Earth’s surface.
Terra dies after twenty years of loyal service
Because all the measurements obtained depend mainly on the instruments on board the satellites. In this case, Terra is a sort of “flagship” of earth observation in NASA’s own terms. The researchers were able to use the images of Aster, an instrument providing wide spectral coverage of the earth’s surface because it is equipped with two cameras that record pairs of stereo images allowing the creation of high-resolution topographic maps. This is how we obtain a precise measurement of the loss of thickness of glaciers.
Problem: It is not certain that this essential mission will continue. Launched in December 1999, Terra is now at the end of its life. After having orbited 705 kilometers above the Earth’s surface for two decades, crossing the equator at the same time every day, the satellite is gradually losing altitude and should soon end its life in the atmosphere. “There is currently no replacement project for this unique tool of its kind and we advocate that it be replaced in order to continue with crucial measurements of the impact of climate change on the cryosphere”, pleads Étienne Berthier.