Is the Antarctic ice sheet getting bigger or smaller? Is it warming or cooling?
As we’ve reported in earlier posts (here and here), getting accurate answers to these questions is non-trivial, because the available instrumental data remain sparse and generally date back only a few decades, at best. While modern satellite-based techniques such as laser altimetery and gravity anomaly measurements provide important information on very recent changes, to get at the longer term we must rely on less direct methods. In the last 5 years or so, an effort has been under way, much of it under the banner “International Trans Antarctic Scientific Expedition” (ITASE), to do this by collecting many dozens of ice cores from across the Antarctic continent. Two papers out this month represent the first major compilations of results from these efforts. The first, in Science on August 11th, provides a new estimates of Antarctic snowfall changes over the last 50 years. The second, in Geophysical Research Letters (August 30th) provides the first statistical reconstruction of Antarctic temperature change, extending about 200 years into the past.
In the Science paper, Monaghan and others show that there has been no significant change in Antarctic snowfall in the last ~50 years. This is a potentially important result because most calculations suggest that as the globe warms, polar snowfall should increase, somewhat mitigating the sea level rise that is expected to result as the margins of the ice sheets melt and thin. The new results differ from those of Davis and others, who used data from the European Remote-Sensing Satellite (ERS-1) and ERS-2 satellite altimeters to estimate that the Antarctic ice sheet had gained enough mass between 1992 and 2003 to slow sea level rise at a rate of about 1 cm/century. The two sets of results are not necessarily in conflict, but do suggest that the intepretation of Davis et al. that the observed change is due to snow accumulation increases may need revision. [See also our earlier post on the use of gravity measurements to determine mass changes in the Antarctic, here. There are also some new results on mass change in Greenland, reported by Chen et al., out last week in Science Express (here), based on measurements from the GRACE satellites, essentially confirming earlier results of major changes on the Greenland ice sheet.]
Monaghan et al.’s estimates are based on a clever combination of ice core measurements of annual snow layer thickness, and model determinations from the ERA-40 climate model Reanalyses. The reason for using both observed and model data is that, while the sixteen ice cores they use come from all over the Antarctic continent, they nevertheless remain only point sources of information. To obtain useful averages over broad geographic regions, it is necessary to interpolate between the different ice core records. Monaghan et al. use the model results to guide this interpolation. The technique is not dissimilar to other climate field reconstruction methods that we’ve discussed frequently on RealClimate. The chief difference is that Monaghan use the output of a climate model, rather than direct observations, to determine the patterns of covariance in the climate field (in this case, snow accumulation rate). This technique allows Monaghan et al. to extrapolate the model results (which are realiable only for the period 1985-2004) back in time as far as the last International Geophysical Year (IGY), 1957, with a great deal of confidence.
Extrapolated to the future, a possible interpretation of Monaghan et al.’s results is that sea level rise in the future would be even greater than otherwise expected. However, we caution that such extrapolation is probably not warranted. After all, the relationship between temperature and snowfall is based on physical arguments and atmospheric dynamics calculations, not only on data; just because we cannot yet detect an increase in snow accumulation does not mean that it will not happen in the future. Furthermore, while Southern Hemisphere temperature have increased, on average, along with the rest of the globe during the last century, the data are rather sparse at the higher latitudes. And although there is evidence for warming in the mid troposphere over Antarctica from radiosonde data over the last 30 years, it is also clear from satellite data that surface temperatures decreased during the years 1982 through 2002. In fact, one can make the case that the timing of accumulation changes agree rather well with the observed timing of surface temperature change in the Antarctic. Monaghan et al.’s results shows that while there has been no net change, snow accumulation actually increased from IGY until the mid-1980s, and decreased thereafter. Similarly, a simple average of the weather station data show the same pattern for temperature — average warming prior to the mid 1980, and average cooling thereafter. [And as we have reported previously, the recent cooling is quite well understood and not expected to continue over the long term.] It is thus premature to conclude (as the paper does) that we may need to revisit GCM assessments that show increased precipitation over Antarctica in conjunction with projected warming in the future.
The results of the Geophysical Research Letters paper by Schneider and others — on which I am second author — are relevant here. We’ll report on this at greater length once the paper is published. As Monaghan et al. did, we used ITASE ice core records. In this case, our goal was to determine temperature, rather than snow accumulation history, and we used the available Antarctic weather station data (much more complete for temperature than for accumulation) rather than model results. We found that the stable isotope composition of the ice cores mimics the observed temperature pattern — warming between the 1960 and 1980s, cooling since then. Using the stable isotope records to extrapolate farther into the past, we find that Antarctica has warmed, on average, in the last century, along with the rest of the globe. Once the accumulation records have been extended, it will be possible to revisit the relationship between temperature and accumulation trends. I suspect that there will not be any surprises here.