Antarctica. Image: Flickr.
Another reason we need to begin preparing for rising tides, asap: New science shows that Antarctica is prone to rather violent periods of accelerated melt that then lead to years of rapid sea level rise. This is contrary to the current understanding of Antarctic melt, which had long been assumed to be slow and steady when the world emerged from the last ice age. Not so: At one point, thanks to Antarctica's melting ice sheets, sea levels rose 50 feet in a period of 350 years.
A new study published in Nature shows that beginning 20,000 years ago, the polar continent shed its ice in eight "meltwater pulses," each of which contributed to sea level rise worldwide. The most rapid pulse, 1A, described above, occurred 14,500 years ago—and it meant sea levels were rising 20 times faster than they did in the 20th century.
"Conventional thinking based on past research is that the Antarctic Ice Sheet has been relatively stable since the last ice age, that it began to melt relatively late during the deglaciation process, and that its decline was slow and steady until it reached its present size," the study's lead author, the University of Cologne's Michael Weber said in a statement.
"The sediment record suggests a different pattern—one that is more episodic and suggests that parts of the ice sheet repeatedly became unstable during the last deglaciation."
This, as the authors point out, is especially important right now, at a moment when a major ice sheet has just collapsed in West Antarctica. A two-mile thick sheet had collapsed, and all but consigned the world to 10 feet of sea level rise from its melt alone. The study that relayed those findings came with a silver lining: scientists expected the rise from the sheet's melt not to occur in full for at least 200 years.
This research changes that picture. The Nature study postulates that positive feedback loops can quicken the process, and send a huge jolt of meltwater into the sea in quick succession. Their working theory is that warming waters may have changed ocean circulation, which brought the warmer water to the Antarctic subsurface. That could very well happen again, and bring more ice sheets into the warmer waters.
The results show that "small perturbations to the ice sheet can be substantially enhanced, providing a possible mechanism for rapid sea-level rise," the paper's authors write.
Peter Clark, one of the study's authors and an Oregon State paleoclimatologist, cautions that they don't fully understand what spurred the pulses. "However, our new results suggest that the Antarctic Ice Sheet is more unstable than previously considered," he said.
Instability—that's the concern. The unpredictability. It's another feedback mechanism to add to the list of looming catastrophe accelerators: thawing methane stores in tundra permafrost that could speed the flow of heat-trapping gases into the atmosphere. Shrinking ice cover that no longer reflects as much sunlight back into space—and so on.
It's another reminder that climate change has made the world a far more unpredictable place—one where the mercury or the sea levels could begin rising faster than we're prepared for.