At the heart of the fragile polar regions or at high altitudes, glaciers and polar caps play an integrating and revealing role in interannual variability and climate change. They are (and will be) important contributors to past, present and future sea level rise. This sensitivity to temperature and precipitation fluctuations makes them one of the icons of climate change.
Our main scientific objective is to reduce uncertainties in decadal glacier mass losses since 2000 and, where possible, further back in time using archival images. In addition, a new generation of very high-resolution satellites is complementing in situ observations to better understand the processes controlling mass gain/loss at different time scales. For the very flat Antarctic ice sheet, the preferred tool is radar altimetry, which provides very nice time series of volume change. However, the radar wave from the altimeter penetrates the snow to a greater or lesser extent. It is therefore also important to understand the spatio-temporal variations due to this error, both to improve the altimeter measurement and to obtain information on the snow cover.
Our observations of both ice caps and glaciers require a high level of methodological expertise in order to make the most of past and current satellite missions and to help prepare future satellites.
An important result is that the height variations over East Antarctica are mainly due to variations in meteorological forcing, but that some glaciers, particularly in West Antarctica, are undergoing strong mass losses. The first measurements from Altika/SARAL (French-Indian mission) show that this acceleration is increasing and spreading. We also show that the signature of the circumpolar wave detected by the team is clearly visible in the height variations. The agreement between mass and volume measurements, i.e. between gravimetry and altimetry, is excellent and offers us the possibility to map the density of losses or gains.
In the last 4-5 years, a major advance has been to move from estimates of the mass balance of one (or a few) glacier(s) to entire glaciated regions. To do this, we have exploited the vast archive built by the ASTER satellite since 2000. By exploiting the depth of the ASTER time series, we are able to construct maps of glacier elevation changes over periods of 10 years (or more) with an accuracy of 10 to 20 cm/year. Our processing chain, ASTERIX, was developed and evaluated on the Mont Blanc massif where accurate in situ data are numerous thanks to the ORE GLACIOCLIM (Berthier et al., 2016). Then this method could be deployed in 3 different geographical contexts: high mountain glaciers in Asia (Brun et al., 2017), Andean glaciers (Dussaillant et al., 2019) and glaciers in Alaska and Canada (Berthier et al., 2018), typically generating several tens of thousands of DTMs on each area. We thus measure very strong mass losses in maritime regions (Alaska, Patagonia) while losses are more moderate in continental regions, some even gaining mass (Central Andes, Karakoram, West Kunlun). This result highlights, in addition to the imprint of climate change, the importance of the sensitivity of the glacier mass balance, which differs greatly from one region to another. Our estimates also clarify the contribution of glaciers to regional hydrology (Southeast Asia, Andes) and sea level rise.
A second important development has been the more systematic use of Pleiades high-resolution stereo imagery to provide complete and accurate DTMs in snow-covered areas. We are now able to detect elevation changes of typically 0.5 to 1 m, allowing access to seasonal (summer/winter) mass balance. For example, the first satellite measurement of a winter mass balance using the geodetic method was performed on an Icelandic glacier (Belart et al., 2017). The accuracy of these high-resolution DTMs and the responsiveness of satellites also make it possible to study hazards related to glacier evolution, such as the disintegration of two glaciers on the Tibetan plateau (Kääb et al., 2018).