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Interviewee: Anders Carlson, University of Wisconsin-Madison
By Heather Mayer
Record summertime Arctic ice melts and threatened polar bears are sad signs of the warming climate. Now a new report shows that future sea level rise due to global warming may be worse than scientists thought.
After studying the last great North American ice sheet, the Laurentide, which encompassed large parts of Canada and the United States, researchers at the University of Wisconsin-Madison calculated unsettling numbers about the melting rate of today’s largest Arctic ice mass, the Greenland ice sheet.
"The Greenland ice sheet is probably more sensitive to climate warming than we previously thought, " says UW geologist Anders Carlson. "Because of this heightened sensitivity, it will probably be melting much faster than we’ve currently been thinking within the next 100 years. "
Carlson and his team of researchers looked at the conditions and climate during the time of the Laurentide’s melting — about 9,000 years ago — and noted its similarity to the Greenland ice sheet. While the climate warming 9000 years ago was due to the earth being closer to the sun, they wrote in the journal, Nature Geoscience that the conditions were similar to what the Greenland ice sheet will experience by 2100.
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"We compared how much summertime warming was there, and how much energy was going into the ice sheet from this radiation from the sun, against how much summertime warming and how much energy [will be] going into the Greenland ice sheet at the end of this centery," Carlson explains.. "We found that for these two different time periods, the energy and the temperature warming were about the same."
Reconstructing the Past and Projecting the Future
The researchers used several types of geological records from both land and seas to study the how much of the ice sheet was melting and how rapidly. On land, a method called cosmogenic surface exposure shows how long the surface of a boulder left by the ice sheet has been exposed to cosmic rays, which are "basically neutrons and electrons flying around in space,’ Carlson says..
"These neutrons and electrons hit atoms in the surface of the boulder and split the atoms and make a new atom that we can measure how much of this new atom has been accumulating in the surface of this boulder, " Carlson describes. "That tells us how long that boulder has been sitting there since the ice sheet left. So for those records, we can actually say the ice left this spot X number of years ago."
They also used radiocarbon dating of fossil plants that grew after the ice retreated.
From the ocean, the ratios of oxygen isotopes in tiny shell fossils record changes in the water chemistry during the melting. "The water chemistry is affected by the amount of freshwater runoff we’re getting from the continents, which is related to how much the sheet is melting on the continents," he says.
The researchers used what is called a general circulation model, a type of global climate simulation, to predict climates at the end of the century.
Using all of this information, the researchers focused on what’s likely going to happen to the Greenland ice sheet, which is the Arctic’s largest and world’s second largest source of ice.
"Because that climate 9000 years ago is similar to the climate we’re expecting to see at the end of this century, we would expect to see the Greenland ice sheet adding to sea level rise at about a half an inch a year," says Carlson.
If that doesn’t sound like much, consider that it’s a contribution to sea level rise of 10 inches over 20 years, and more than two feet over 50 years from the Greenland ice sheet alone. That’s more than double the previous estimates from the most recent Intergovernmental Panel on Climate Change (IPCC) report in 2007.
Carlson says whether we hit that rate in 2050 or 2080 depends on our behavior in the next couple of decades.
A Growing Problem for the Next Generation
"There isn’t much good news," Carlson say. "Until we really start changing how we behave, there really isn’t going to be much good news."
Carlson says they compared their results to a "middle-of-road" scenario in which we change our carbon economy, but not drastically. But right now we’re still near a much worse scenario– "business-as-usual."
As if to underscore his concerns, new greenhouse gas monitoring data shows global carbon emissions in 2007 exceeded even the IPCC’s worst-case-scenario estimates.
Carlson and his researchers reiterated that the only way to reverse these high rates of melting ice and rising sea levels is to modify human behavior. "Even if we get very green and start pulling carbon out of the atmosphere, we’re still going to have sea level rise over the coming century, " Carlson explains. "But we can minimize how much sea level rise that is. If we don’t change anything and we continue using carbon-based fuels, which is what seems right now to be the case, it could be much worse than even what we’re predicting based upon the past."
On the up side, Carlson notes, is that people — the government included — are recognizing there is a problem and making efforts to rectify it. "Most people are beginning to accept this is a growing problem, " he says.
However, Carlson is extremely concerned for the welfare of his own grandchildren and the generations to come, if this problem isn’t reversed.
"They’ll be out of gasoline by then," he points out. "So they’ll be dealing with a problem we made and our parents and grandparents made, but they’re not actually contributing to the problem at that point, " he says. "So we’ll be responsible but not really around to sit there and deal with the impacts and consequences of our actions."
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