Calling Greenland's potential collapse another climate "tipping point" would be doing it fair justice -- after all, scientists have estimated that were its ice sheet (which holds one-twentieth of the world's ice) to melt completely, global sea levels would jump 7 meters. As Alexandra Witze reports in the latest issue of Nature (sub. required), Greenland's disappearance is one of the foremost concerns weighing on climate researchers' minds. It's no big secret that Greenland has been melting at an accelerated rate over the past few summers: It has lost an average of 380 - 490 billion tonnes of ice each year over the last 4 years -- or around 150 billion tonnes more than it gains in snow during the summer. Despite this worrying trend, however, cautious scientists will tell you that it is still too early to predict Greenland's untimely demise. One main reason is that the models and tools scientists use simply aren't yet up to the task.
Various resources exist, primarily satellites, that have driven most of the recent research in this field. The Gravity Recovery and Climate Experiment (or GRACE, for short), a pair of U.S.-satellites orbiting the Earth 500 kilometers above us, has proven an invaluable asset in documenting much of Greenland's changes. The satellite pair continuously measure the distance separating them -- a value that changes as massive objects on Earth pull one or the other (through gravity) -- and use it as a proxy for assessing the changes in Greenland's ice sheets.
Yet the data GRACE provides is not foolproof: one problem is that it fails to accurately take the effect of "post-glacial rebound" into account. During the last ice age, Greenland's ice sheet was much bigger and, as a result, pressed the underlying crust into the dense mantle below; even though much of this ice has now disappeared, large portions of the crust have not yet rebounded from their position. This slow, gradual bounceback has made the analysis of GRACE's data difficult.
The Greenland GPS Network (GNET), a system of GPS receivers installed around Greenland to measure the bounceback effect, could eventually help resolve these discrepancies, though scientists will first need to ramp up their measurements and improve the quality of their models. The ever-evolving nature of climate science -- and the changes in our understanding of the subject it inevitably sparks -- mean that the tools we use will need to constantly adapt to the changing circumstances and, in some cases, simply be discarded.
In addition to air temperatures, scientists are now also starting to assess the impact of changing ocean temperatures on the rate of retreat of Greenland's glaciers. Many realize that their past models were too conservative in estimating the rate of ice sheet melting -- a problem many ascribe to a lack of data. Moreover, the modelers are wary of using a single set of data from one melting season or location to generalize over the whole ice sheet.
Tackling these challenges will, at the very least, take many months, if not years. As satellite technologies improve and scientists develop new tools to obtain data and measure other variables, our understanding will improve and our predictions will become more precise. The worry is that by the time we get it just right, it might already be too late.
In her blog post on the subject, Witze provides more insight and some updates on our understanding of the science and links to several other posts dealing with Greenland's melting, including a very incisive one from the crack team at RealClimate, which is well worth the reading.
Image courtesy of NASA/GSFC via Nature
Via ::Climate Feedback: Losing Greenland (blog)
Check out an animated version of Greenland melting on Discovery's Earth Live tool.