Guest post by Dr. Jan Wuite, Enveo, Innsbruck
A new study released by NASA scientist Jay Zwally and colleagues in the Journal of Glaciology, receiving wide coverage in the media last month, reports an 82 Gigatons per year increase of land ice in Antarctica during the period 2003-2008. The study received much skepticism from other leading scientists in the field, as there are many indications that point at the contrary: ice loss, possibly irreversible. How does this new study fit in that picture, what are the consequences for expected sea level rise and are these numbers correct? Glaciologist and polar scientist Jan Wuite, working at Enveo in Innsbruck and involved in various international studies related to Antarctica explains.
One of the adverse consequences of climate change is global sea level rise. At more than 3 mm per year, the current sea level rises twice as fast as during the 20th century. The expectations are a rise of at least 70 cm by the turn of this century. The principal causes are clear: global decline of land ice (mountain glaciers & ice sheets) and thermal expansion of ocean water (water expands as it becomes warmer). To clarify, land ice is resting on land and can reach a thicknesses of up to several kilometers, in contrast to seasonally restricted sea ice (mainly just frozen ocean water), that is typically only a few meters thick and has no direct influence on sea level. Studies have indicated an increasing contribution of the two largest ice sheets, the Greenland and Antarctic ice sheets, to sea level rise.
The largest unknown for future sea level rise is caused by uncertainty in the predicted response of the Antarctic ice sheet to global warming. As warmer air can hold more moisture, it is possible that increasing snow accumulation compensates part of the sea level rise. On the other hand it is also possible that ice drains faster to the oceans accelerating it.
There is a lot of ice in Antarctica; in some places the ice thickness reaches well over 4 km. There is enough ice to, when melted completely, raise global sea levels with roughly 58 m. But even if only a small part of that melts it could have a significant impact on coastal communities, or ocean circulation. For this reason scientists are very interested in mass changes of the ice sheets: the mass balance.
First of all, how do scientists actually measure these changes? Because of the remoteness and vastness of Antarctica, satellite observation is the only feasible way to study the ice sheet. On a smaller scale these measurements are supported by airborne and field data. Three principal methods are used: satellite altimetry, satellite gravimetry and the so-called input-output method (IOM).
In the IOM approach, as the name implies, estimates of the total ice input – the net snow accumulation or surface mass balance – are compared with estimates of the discharge of ice to the ocean (output). The discharge is calculated combining surface velocity measurements with ice thickness estimates. The independent and relative new gravimetry method determines changes in the ice mass directly by measuring tiny fluctuations in the earth’s gravity field. Satellite altimetry uses very precise repeat elevation measurements of the ice surface to determine volume changes. These volume changes can then be converted to ice mass using estimates of the density. Each of the methods has its strengths and weaknesses. In the study from Zwally and colleagues only the last so-called volumetric method is used.
In his study Zwally uses data from different satellites for two periods: the ESA radar satellites ERS-1 and ERS-2 during the period 1992-2001 and the NASA laser satellite ICESat during 2003-2008. The surface elevation measurements are used to create maps of annual elevation change. These maps must be corrected for processes that do not influence the total ice mass, such as for instance firn densification (compacting of the upper layers of the ice sheet as the air is squeezed out) and to a lesser degree vertical movement of the bedrock underneath the ice sheet in response to changing ice masses. After the correction the remainder elevation change is caused by either short-term variations in snowfall on the one hand or dynamic changes of the ice sheet on the other hand. The latter are related to ice flow velocity and reflect the ice sheets slow response to changing boundary conditions such as climatic changes. A crucial step is to convert the volumetric changes to ice mass using density estimates. In this study meteorological models are used to separate the part caused by accumulation changes (with the lower density of snow), leaving in theory the part caused by dynamic changes (with the density of pure ice). Using this approach the scientists come to the surprising finding that during the study period (1992-2008) increasing ice losses in the coastal areas of West Antarctica and the Antarctic Peninsula are entirely compensated for by ice growth in interior West Antarctica and the much larger East Antarctic Ice Sheet. In other words, according to Zwally and colleagues the total (land) ice mass of Antarctica is increasing, buffering part of the – independently determined – global sea level rise. The authors caution that under current conditions this could change within the coming decades.
The study from Zwally and colleagues drew a lot of criticism, as the claims contradict a decade of studies and measurements. One of the main arguments is that the used data and method is not accurate enough to support such firm conclusions. The signal scientists are trying to measure is only in the order of a few centimeters. Because of the vastness of the ice sheet, tiny errors or wrong assumptions could translate into huge quantities giving entirely different results. For this reason multi-disciplinary studies that try to reconcile mass balance estimates using different independent methods are deemed more reliable. Just a few years ago such a study reconciling ice sheet mass balance estimates was published in the prestigious scientific journal Science. The study, a collaboration of many research groups (including the group of Zwally) compared estimates of ice sheet mass balance derived from the three independent methods as described above. The outcome was an unambiguous ice loss in Antarctica. The slight gain in East Antarctica, in particular Dronning Maud Land since 2009, did not outweigh the tremendous loss in other parts of Antarctica. The consensus was a net ice loss of 71 Gigatons per year for Antarctica as a whole. Moreover the period of observations was longer running from 1992 to 2011.
Nevertheless, even at that time there was an unsatisfactory spread in mass balance estimates, in particular for East Antarctica. The authors recommended acquisition of additional measurements with denser spatial and temporal sampling. The CryoSat-2 satellite, launched in 2010, adhered to this recommendation and subsequential mass balance estimates, that used this new altimetric satellite but also gravimetric satellites, confirmed earlier findings of overall ice loss in Antarctica. Moreover, the ice loss even appeared to accelerate. For example a study published last year by McMillan and collegues, that used CryoSat-2 data, reported a total ice loss of 159 Gigatons per year between 2010 and 2013. It is remarkable that in this new study only older data up to 2008 is re-analyzed without incorporating this newly acquired data.
At present the newly released study stands alone in its claim of ice gain in Antarctica, contradicting the bulk of scientific literature of the last decade, including other NASA studies, using in part the same data or completely different methods. At first it remains to be seen whether the conclusions of this study will hold after more thorough scientific evaluation. In that respect it should also be noted that the conclusions are not specifically endorsed by NASA as portrayed in many of the media reports. Nevertheless, if the conclusions are right than it actually raises many new and interesting questions, such as, if not from Antarctica, where does the part of sea level rise thus far ascribed to it come from? Perhaps the part of other sea level rise contributors has been underestimated thus far. In that case we could even end up with more sea level rise if in the coming decades Antarctica does not act as a buffer anymore as the study suggests.
– Press release Nasa about Zwally et al.
– RealClimate: “So what is really happening in Antarctica?”.
– Explainer from Prof. Richard Alley on Zwally et al.
– Original Dutch version on Klimaatverandering.