This week’s report, by Elizabeth Thomas and colleagues from the British Antarctic Survey, that snowfall has been increasing in Antarctica is hardly surprising. What is different that it is much more comprehensive than previous studies, which were largely limited by a virtual lack of pre-1957 data. That was the “International Geophysical Year”, in which systematic observations of Antarctica’s climate began.
The new study looks at the last 200 years of snowfall trapped in 79 ice cores taken from around the continent. It supplements other recent findings that also made headlines.
Determining Antarctica’s overall ice balance has been, well, slippery. One favored method has been to look at gravitational data measured by satellite. Thicker ice means more mass, which means greater gravity. These studies usually come up with a net loss, translating to from 6/1000 of an inch of sea level rise per year to 12/1000 (both values being rather small beer). But different measurements show otherwise. Three years ago, Jay Zwally and his colleagues at NASA used satellite-based altimetry and concluded Antarctica was undergoing a net gain in ice.
Common sense dictates that it should be snowing more in Antarctica. Think of it as Buffalo on steroids when it comes to snow. In the fall, when Lake Erie isn’t frozen, cold air passing over it from the west picks up evaporated moisture and dumps it on the land in the form of snow squalls. The warmer the water and/or the colder the air is, the more is snows. Unlike a mere Great Lake, Antarctica is surrounded by a largely unfrozen ocean, and when any atmospheric disturbance sends moisture onshore, it snows too.
Around Antarctica, there’s been a slight—meaning a couple of tenths of a degree—warming of the surrounding ocean, which means that the air blowing over it picks up a bit more moisture than it used to. Unlike Lake Erie, the Southern Ocean is huge, and any atmospheric disturbance that shoves more oceanic air up onto the continent is going to be pushing a substantial stream inland with ever more moisture, even for a very slight ocean temperature rise.
The “surface mass balance” of a glacier or an ice sheet is the difference between accumulated snowfall and what either melts or evaporates. In anticipation of increased snowfall, the last (2013) scientific summary by the United Nations’ Intertgovernmental Panel on Climate Change shows that the projected 21st change in the Antarctic mass balance to be weakly positive. That’s why it’s perplexing that the new finding is so newsworthy.
But now we know that the snow has been increasing down there for the past 200 years…and that the increase started before the major emissions of atmospheric carbon dioxide.
