Ice monuments such as this one, off Scoresby Sound, Greenland, are disappearing – and scientists still aren’t sure exactly what will happen to the rest of the planet when they do.
by Kristen Minogue
Oct 07, 2009
Rising sea levels act as one of the greatest and most feared indicators of global warming, as melting ice crashing into the ocean threatens coastlines with flooding.
And researchers warn that oceans are rising much more rapidly than they thought as world leaders wrack their brains for ways to stay below the 2-to-3 degree warming threshold that human society can tolerate.
“Probably the big failure in terms of climate change projections so far is sea level is going up faster than we suspected,” said Richard Alley, professor of geology at Penn State.
The International Panel on Climate Change did attempt to estimate how high it would rise by the end of the century in their most recent 2007 assessment. Their model-based range goes from just above half a foot to just under two feet. But the numbers are too low, possibly 13 feet too low, and scientists know it.
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Richard Alley talks about what makes melting ice sheets so hard to predict and how much it will cost the world’s economy to stop the trend of rising sea levels before it’s too late.
The problems lay with the models and factoring in melting ice sheets of glaciers. Scientists still don’t understand many of the fundamentals of how ice sheets behave. This makes predicting how they fast will melt difficult, said Alley. So the scientists drafting the IPCC predictions tacked a catch-all qualifier onto their estimates: “Excluding future rapid dynamical changes in ice flow.”
“When the IPCC was faced with sea level rise, they did what we in America call punting,” Alley said. “They said, ‘Well, I don’t know what to do.’ So they kicked it away.”
Since 2007, scientists have given what Alley calls “back-of-the-envelope” estimates that take ice flow into account. The projections for sea level rise by the end of the century range from less than half a foot to 15 feet, although most fall between 2 to 6 feet.This means the Everglades in Florida would flood completely and the Manhattan subway system would flood more often as severe hurricanes along the coast become more frequent.
On its most basic level, the relationship between ice sheets and sea level is fairly predictable. When the planet cools and ice sheets grow, sea level falls. When the planet warms and ice sheets shrink, sea level rises. Faster melt, faster flow (or both) makes them shrink further. When that happens, sea level rises even more.
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Richard Alley talks about what makes melting ice sheets so hard to predict and how much it will cost the world’s economy to stop the trend of rising sea levels before it’s too late.
But when it comes to understanding how ice sheets flow, scientists run into several barriers.
The first is relatively intuitive. Ice sheets are approximately 2 miles thick. Much of how they flow depends on the surface beneath, and how rough, smooth, dry or wet it is. But as Alley points out, peering beneath 2 miles of ice is no easy task.
Alley also said the satellites scientists use for many of their observations have passed their design life. But that isn’t necessarily a bad thing, according to NASA scientist Claire Parkinson. While they would like to have up-to-date technology, constantly changing instruments can pose a problem for climate scientists who often need to take long-term measurements with standardized results.
“For those of us who are in the climate field, the long consistent record just kind of overwhelms in importance the issue of maybe getting a somewhat better instrument up,” Parkinson said.
Meanwhile, Alley said the scientific community itself has shown a rather lackluster commitment to studying ice sheets.
“The major modeling groups, when they had to build their ocean-atmosphere models, they went out and bought geophysical fluid dynamicists, and they built models,” he said. “When they’re trying to do ice sheets, they go out and they buy a post-doc.”
When those groups come back from the ice sheets, they continue to bring back new discoveries about how they work. Theoretically this benefits science. But it also further confirms what scientists already suspect: They know much less about ice sheets than they would like.
They do know one thing, however. Meltwater – either above or beneath the ice – tends to speed up the process. Surface water on the ice can seep through the crevices, slicken the bed beneath the glacier and make it flow and breakup more quickly. Or when ice shelves, giant slabs of ice stretching out into the ocean, collapse due to warming, the ice on the land falls apart even faster. This can result in a drastically destructive calving event in which enormous slabs of ice crash into the sea.
And it turns out ice sheets are incredibly sensitive to even tiny climate changes. According to some rough estimates, if the ocean temperature rises even one degree Celsius, it could thin the bottom of a floating ice shelf an extra 30 feet a year.
Scientists still don’t know exactly what a figure like that could mean for the future of the planet. That fact scares them even more – and makes finding out how ice sheets work before the rising oceans invade coastal megalopolises all the more urgent.
