WASHINGTON - The Earth is already shaking beneath melting ice as rising temperatures threaten to shrink polar glaciers and raise sea levels around the world.
By the end of this century, Arctic readings could rise to levels not seen in 130,000 years — when the oceans were several feet higher than now, according to new research appearing in Friday's issue of the journal Science.
Even now, giant glaciers lubricated by melting water have begun causing earthquakes in Greenland as they lurch toward the ocean, other scientists report in the same journal.
In principal findings:
• At the current warming rate, Earth's temperature by 2100 will probably be at least 4 degrees warmer than now, with the Arctic at least as warm as it was 130,000 years ago, reports a research group led by Jonathan T. Overpeck of the University of Arizona.
• Computer models indicate that warming could raise the average temperature in parts of Greenland above freezing for multiple months and could have a substantial impact on melting of the polar ice sheets, says a second paper by researchers led by Bette Otto-Bliesner of the National Center for Atmospheric Research. Melting could raise sea level one to three feet over the next 100 to 150 years, she said.
• And a team led by Goeran Ekstroem of Harvard University reported an increase in "glacial earthquakes," which occur when giant rivers of ice — some as big as Manhattan — move suddenly as meltwater eases their path. That sudden movement causes the ground to tremble.
Otto-Bliesner and Overpeck wrote separate papers and also worked together, studying ancient climate and whether modern computer climate models correctly reflect those earlier times. That allowed them to use the models to look at possible future conditions. The researchers studied ancient coral reefs, ice cores and other natural climate records.
"Although the focus of our work is polar, the implications are global," Otto-Bliesner said. "These ice sheets have melted before and sea levels rose. The warmth needed isn't that much above present conditions."
According to the studies, increases in greenhouse gases in the atmosphere over the next century could raise Arctic temperatures as much as 5 to 8 degrees...
So what do our coastlines look like if Greenland melts? Let's look at what's being said.
Going under? Global warming might trigger a 6-meter rise in sea level that would inundate coasts (red) worldwide. Southern Louisiana (left) and South Florida (lower right) would be hard hit.
CREDIT: IMAGE BY JEREMY L. WEISS AND JONATHAN T. OVERPECK/UNIVERSITY OF ARIZONA
...If the recent behavior of ice sheets is not fully understood, their future is largely a blank. "We don't actually understand what's driving these higher velocities," says Dowdeswell, so "it's difficult to say whether that's going to continue," or spread.
At the moment, ice loss from Greenland and West Antarctica combined is contributing less than half of the ongoing 2-millimetersper- year rise in sea level; the rest comes from melting mountain glaciers and the simple thermal expansion of seawater. If the recent surge of ice to the sea continues, sea level might reach something like half a meter higher by 2100. That would be substantial but not catastrophic. To produce really scary rises really fast (say, a meter or more per century), the air and water will have to continue warming in the right--or wrong--places. The temperature rise will have to spread northward around Greenland and in the south around West Antarctica, reaching the big ice shelves where most of that ice sheet drains. And glacier accelerations triggered near the sea must propagate far inland to draw on the bulk of an ice sheet.
Faced with uncertainty about the present, paleoclimatologists look to the past. About 130,000 years ago, between the last two ice ages, the poles may have warmed as much as they will with only a couple of degrees of global warming. But sea level was considerably higher then, something like 3 to 4 meters higher...
The Figure above is using the model discussed in this paper. The paper is reproduced here in part for educational purposes only:
Paleoclimatic Evidence for Future Ice-Sheet Instability and Rapid Sea-Level Rise
Jonathan T. Overpeck, Bette L. Otto-Bliesner, Gifford H. Miller, Daniel R. Muhs, Richard B. Alley, Jeffrey T. Kiehl
Science 24 March 2006:
Vol. 311. no. 5768, pp. 1747 - 1750
DOI: 10.1126/science.1115159
Sea-level rise from melting of polar ice sheets is one of the largest potential threats of future climate change. Polar warming by the year 2100 may reach levels similar to those of 130,000 to 127,000 years ago that were associated with sea levels several meters above modern levels; both the Greenland Ice Sheet and portions of the Antarctic Ice Sheet may be vulnerable. The record of past ice-sheet melting indicates that the rate of future melting and related sea-level rise could be faster than widely thought...
Simulated climate for each of four time periods, from left to right: present day (Modern), 130,000 years ago (anomalies from present day, {Delta} LIG), 2100 A.D. (the time atmosphere reaches three times preindustrial CO2 levels, climate anomalies from present day, {Delta} AD 2100), and 2130 A.D. (four times preindustrial CO2 levels, climate anomalies from present day, {Delta} AD 2130). Shown for each time period are peak summertime (July to August and January to February means) surface air temperature and annual snow depth. Note significant warming at north polar latitudes and the lack of any summer warming over Antarctic at 130,000 years ago. LIG stands for the Latest InterGlacial period, from about 150,000 to 116,000 years ago.
These rates of change assume the current linear accumulations of carbon dioxide in the atmosphere. If some other models are right, and the rate of change itself accelerates, the sea level rises will be faster than these models predict. They might also be more extensive, because the warmer it gets, the more likely other reservoirs of deep ice will melt. Continental Antarctica is the size of North America, and with an ice shield two miles high.
The other parameter to bear in mind is that this won't be a steady creep of the sea level upwards. Land loss will occur one hurricane at a time. Given last year, and the way this year's begun, I'd say we have a stormy ride ahead.
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