In his second Q&A Graham Cogley, new Chief Editor of the Journal of Glaciology and Annals of Glaciology, answers questions on how climate change is affecting glaciology and the affect of glaciers on global sea levels.
How is climate change affecting glaciology?
As every schoolchild knows, if it gets warm enough then ice will melt. When we measure changes in glacier size we don’t use thermometers, so glaciology gives an independent check on what climatology is telling us. The answer is the same from both subjects, so people who don’t believe in climatic change have run out of places to hide – if they want to be taken seriously as citizens, that is. Glaciologists are making giant strides in their ability to pin down rates of change, and we can already say confidently not just that glaciers are getting shorter, thinner and less extensive, but that these changes are happening more rapidly than they would have in the absence of human interference with the climate. In other words, the answer from the glaciers is equivalent to the one we get from the weather stations and the climate models about the rate of change of air temperature. Seen from the schoolchildren’s angle this is utterly unsurprising, but it is powerful reinforcement of our conviction that we are on the right track towards understanding what is happening to the planet.
How are glaciers affecting global sea levels?
A tricky question. We know unequivocally that the glaciers are shedding mass but we have great difficulty nailing down the uncertainty in the rate. A recent estimate that Antarctica is gaining mass, made by Zwally and others, is proving controversial to put it mildly. On smaller glaciers the in-situ measurements of mass balance by field workers, when compiled regionally, tend to disagree at least as often as they agree with the gravimetric measurements by GRACE. I suspect that estimating uncertainty is a much tougher problem than we think it is, and that many of our error bars are not tall enough. But we probably have the big picture about right: glaciers contribute most of the current positive mass balance of the ocean, with a large uncertainty due to the liquid-water balance of the landmasses. I would love to see somebody “join up” glaciology to hydrology and limnology, and put a number on the amount of melting glacier ice that goes into aquifers and lakes instead of the ocean. Conventional, but at present just hand-waving, wisdom says that this amount is negligible. The glacier ice now entering the ocean accounts for roughly a half of the current rise of sea level, with the rest coming from thermal expansion.