From the outside, the science of warming-related sea level rise doesn’t seem that complicated. Carbon enters atmosphere, planet warms up, land ice melts and runs into the oceans, sea level rises. Minus the greenhouse effect, you can pretty much see that reaction in action by heating an ice-cube over a stove.
In the real world, however, ice-sheet modeling is incredibly complex, which means it’s surprisingly difficult for scientists to predict just how quickly glaciers will melt—and sea levels will rise—as global temperatures increase. That’s one of the reasons that the UN’s Intergovernmental Panel on Climate Change (IPCC) put forward a rather conservative prediction for sea levels in its 2007 report, estimating that they would rise by just 7 to 23 inches by 2100. The models around polar ice sheets were so uncertain at the time that the IPCC decided to largely disregard the role they might play in sea-level rise over the next century—focusing instead on thermal expansion of water as the oceans themselves warm—even though melting is already underway.
We already know that Arctic ice is melting faster than expected, and that sea level rise will likely bust the IPCC predictions. Now, thanks to a new paper published yesterday in Nature Geoscience, we have a better idea of why. Researchers from the British Antarctic Survey (BAS) and Columbia University’s Lamont-Doherty Earth Observatory examined the Pine Island Glacier in West Antarctica—one of the frozen continent’s largest glaciers—and found that it was melting more than 50% faster than it had been just 15 years ago, when an earlier group of scientists visited it. The glacier is now losing 80 cu. km of ice a year, up from 50 cu. km in 1994.
(More from TIME: Unfrozen Tundra)
Though the water in Pine Island Bay has warmed by 0.2 C over the same amount of time, that heating alone isn’t enough to account for the accelerated melting. (I told you ice-sheet modeling was tricky.) The researchers sent an autonomous submarine—named, not very creatively, the Autosub—to explore beneath the ice shelf. The robot found an underwater ridge on the seafloor that they believe might have once slowed the glacier as it flowed into the sea. Once the glacier broke free of the ice shelf, however, warmer water from the depths of the ocean could flow into the underbelly of the glacier, opening up a cavity inside it. The more the cavity grew, the more warm water that could flow in, accelerating the melting and allowing the glacier to speed towards the sea. (Or at least, speed in the glacial sense.) Said study co-author Adrian Jenkins of the BAS in a statement:
The rate at which the ice shelf is melting has increased significantly, because more warm water is circulating in the cavity beneath it…The inner cavity didn’t exist at all before, so this is the most likely explanation for why a subtle change in temperature can have a huge effect.
Pay attention to the second part of Jenkins’ statement. Those who are skeptical of the scientific consensus on climate change often make use of the uncertainty inherent in climate models. If we can’t be sure of the weather a few days hence, how can we be sure of changes in the climates over decades or even longer? And who’s to say the uncertainty might flow the other way, that climate change might turn out to be much smaller than many of us thought.
(Photos from TIME: Greenland Odyssey)
It might—and let’s hope it does. But it seems more likely that uncertainty will flow the other way, that relatively small changes in the Earth’s temperature may have a large effect on the climate system, and the planet we live in. (If you really want to get scared, look at the possibility of positive feedback cycles in Arctic warming.) In his Rolling Stone essay last week—not all of which I agreed with—Al Gore summed up the state of climate science:
Scientists used to caution that we were increasing the probability of such extreme events by “loading the dice” — pumping more carbon into the atmosphere. Now the scientists go much further, warning that we are “painting more dots on the dice.” We are not only more likely to roll 12s; we are now rolling 13s and 14s. In other words, the biggest storms are not only becoming more frequent, they are getting bigger, stronger and more destructive.
Thanks to humans, polar melting may be ready to go to 11—even faster than we expected.