The media is buzzing frenziedly with tales of the impending doom of almost a quarter of the West Antarctic ice sheet, with ‘catastrophic’ rises in sea levels as a consequence. A new Antarctic ice loss death spiral study has been released and the lame stream media are loving it. I was going to comment on Geoff’s article, but the comment got too long, so I decided to post my observations here instead.
Despite being ‘new’, Shepherd’s paper tells us very little that is new. We already knew that that the Thwaites and Pine Island glaciers in the Antarctic Peninsula have been rapidly thinning since the 1990s.
The authors admit that the overall height of the Antarctic ice sheet has changed very little:
Although most of the AIS surface has changed little in elevation over the past 25 years (Figure 2), there are clear patterns of thinning and thickening in coastal sectors -especially in WAIS and the APIS.
So what do they do? They use models to supposedly discriminate between snow accumulation and ice sheet melting below the surface and demonstrate somehow that ‘vast’ areas of West Antarctic are in ‘dynamical imbalance’.
Changes in Antarctic ice sheet elevation arise predominantly due to fluctuations in accumulation and ice flow, which occur at the densities of snow and ice, respectively (Wingham, 2000). These processes can be distinguishable in altimeter records due to their coincidence with areas of rapid or changing ice flow, or due to their persistence over periods that are long in comparison to expected snowfall fluctuations. To discriminate them, we adjusted the satellite elevation changes to account for fluctuations in surface mass balance by removing model estimates of the firn layer thickness change (Ligtenberg et al., 2011; Melchior Van Wessem et al., 2018). We then classified regions exhibiting correlated patterns of sustained and significant thickening or thinning relative to the firn thickness changes as areas of ice dynamical imbalance.
Yay, guess what, they also found that Pine Island and Thwaites glaciers were retreating! Like, wow, what a surprise!
At the Totten glacier, dynamical imbalance affects just 3.6% of the drainage basin, and we found little evidence that either this or the rate of elevation change have changed over the 25-year survey. In contrast, ice drawdown within the Pine Island and Thwaites glacier drainage basins has spread rapidly inland, and now affects the majority (51% and 68%, respectively) of their catchments.
By far the largest signal of imbalance has occurred in the Pine Island and Thwaites glacier drainage basins (Figure 3), which have lost mass at average rates of -28±6 Gt/yr and -46±7 Gt/yr, respectively, since 1992 (Table 1).
But they’ve got models:
We used our classification of ice dynamical imbalance as the basis of a spatially-resolved mass balance calculation, as it allows us to separate elevation changes predominantly occurring at the densities of snow and ice. Our classification is an improvement over previous schemes because it uses model estimates of firn thickness change to locate the change in ice thickness, rather than attributing elevation changes within entire drainage sectors (Wingham et al., 1998) or otherwise defined regions (Shepherd et al., 2012)to ice. It does not, however, account for potentially coincident signals of meteorological and dynamical imbalance, and this is an acknowledged shortcoming.
And they estimated snowfall:
We instead treated snowfall as an additional source of elevation change uncertainty in mass balance calculations by summing an estimate of its variability (Table 1) in quadrature with the satellite elevation trend uncertainties.
While we’re on the subject of snowfall in Antarctica, I came across this little nugget a few years ago:
The Amundsen Sea sector of the West Antarctic ice sheet has been losing mass in recent decades; however, long records of snow accumulation are needed to place the recent changes in context.
Here we present 300 year records of snow accumulation from two ice cores drilled in Ellsworth Land, West Antarctica. The records show a dramatic increase in snow accumulation during the twentieth century, linked to a deepening of the Amundsen Sea Low (ASL), tropical sea surface temperatures, and large-scale atmospheric circulation. The observed increase in snow accumulation and interannual variability during the late twentieth century is unprecedented in the context of the past 300 years and evidence that the recent deepening of the ASL is part of a longer trend.
Oh dear. Definitely not what the Graun editor would like to see at this point in time when she is no doubt contemplating a change in the ‘style guide’ re. boring old ‘Antarctic ice loss’, following this latest catastrophic news that it’s worse, so much worse than we thought. ‘Antarctic meltdown’ is probably high on the list of options.
Then there’s this equally inconvenient study by Zwally et al at NASA:
“We’re essentially in agreement with other studies that show an increase in ice discharge in the Antarctic Peninsula and the Thwaites and Pine Island region of West Antarctica,” said Jay Zwally, a glaciologist with NASA Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the study, which was published on Oct. 30 in the Journal of Glaciology. “Our main disagreement is for East Antarctica and the interior of West Antarctica – there, we see an ice gain that exceeds the losses in the other areas.” Zwally added that his team “measured small height changes over large areas, as well as the large changes observed over smaller areas.”
All Shepherd has done really is confirm that the Thwaites and Pine island glaciers have been melting and he’s used models of firn ice melt and estimates of snowfall to conclude that 24% of West Antarctica is now ‘unstable’. Ridiculous.
Lastly, but not leastly, global warming (or should that be global heating now?) has taken time off in Antarctica, so it can’t be the dreaded CO2 Thermageddon ‘death rays’ reflecting back onto the Antarctic ice which are causing it to melt ‘deep in the interior’. Got no idea what that might be to be honest . . . . .
The Antarctic Peninsula (AP) is often described as a region with one of the largest warming trends on Earth since the 1950s, based on the temperature trend of 0.54 °C/decade during 1951–2011 recorded at Faraday/Vernadsky station. Accordingly, most works describing the evolution of the natural systems in the AP region cite this extreme trend as the underlying cause of their observed changes. However, a recent analysis (Turner et al., 2016) has shown that the regionally stacked temperature record for the last three decades has shifted from a warming trend of 0.32 °C/decade during 1979–1997 to a cooling trend of − 0.47 °C/decade during 1999–2014.