Used to be, the phrase “glacial pace” was synonymous with slow, almost imperceptible motion. Well, forget that.
Mark Meier, emeritus professor of geological sciences at the University of Colorado at Boulder, has calculated the rate at which the Earth’s glaciers are melting. He and Mark Dyurgerov, a colleague at UC-Boulder, estimate that the rate of ice loss since 1988 has more than doubled that observed in the previous 17 years. Their calculations suggest that the consequent rise of global sea level could be far higher by the end of the 21st century than estimates made by the Intergovernmental Panel on Climate Change (IPCC).
IPCC, an international agency established in 1988 to monitor research on global warming and other climate issues, has estimated that melting ice will add somewhere between 2 and 4 inches (5 to 11 centimeters) to sea level by the end of the century, and that overall sea level rise, due to all factors, will be between 6 inches and 21 inches. Meier and Dyurgerov, on the other hand, believe the rise attributable to glacial melt alone will be somewhere between 7 and 11 inches (17 to 27 cm) in that time period – “and that’s a conservative estimate,” Meier said.
Meier, former director of UC-Boulder’s Institute for Arctic and Alpine Research, has been studying the question of glacier melt since 1984. He presented recent findings at the 2002 meetings of the American Association for the Advancement of Science, held in Boston in February.
“The recent retreats of glaciers are larger than any observed in thousands of years,” he said. The glaciers of the Alps are down 50 percent by volume (and 35 percent in areas) since the 1850s, he noted, while Alaskan glaciers are losing depth at a rate of about 1 meter a year.
“The mass balance of ice is negative,” Meier said, meaning that the annual rate of ice formation is more than offset by melting ice. Since 1961, the “negativity” has been increasing, he said, with a large jump in the rate noted since 1988. From 1961 to 1998, the annual average rate of sea level rise attributable to glacier melt was .25 millimeters, according to Meier’s calculations. But in the decade from 1988 to 1998, the annual increase due to glacier melt has been .41 mm a year, an increase of 64 percent.
If the glaciers of Alaska are factored into the mix – they were not in the IPCC calculations – Meier says the average annual rate of sea level rise caused by melting ice jumps to .49 mm a year since 1988, or an additional four inches over the course of a century.
Such relatively small annual rises may not seem consequential, but if the rate of melt observed in the last decade of the 20th century continues through the 21st century, by 2100, glacier melt alone may be expected to have bounced sea levels up by half a meter, roughly a foot and a half.
Where did the IPCC go wrong? The IPCC does no research of its own but uses data published in peer-reviewed journals and other scientific literature. According to Meier, the estimates on which the IPCC relied under-estimated the volume of water locked up in glaciers and ice sheets. The IPCC figure was 680,000 square kilometers of glacier cover and did not include most of the small glaciers bordering the huge ice sheets of Greenland and the Antarctic. New estimates of ice cover (again excluding Greenland and the Antarctic) are more than double that, Meier said. With the volume of ice increasing, the volume of water available to raise sea level also increases.
What’s more, Meier said, the IPCC undercounted the smaller glaciers. Of 157,000 glaciers, fewer than half – 67,400 – have been inventoried. And a near-infinitesimal fraction of that – a mere 35 – have been subjected to mass balance measurement, which precisely gauges the annual net loss or gain of ice.
While the recent break-up of a huge iceberg off the Ross Peninsula of Antarctica may be seen as bearing out Meier’s warning, it is important to keep in mind that Meier’s predictions of sea-level rise are based on the melting of land-based glaciers. The melting of icebergs, no matter how huge, does not in itself contribute to sea-level rise, since the volume of water they displace when frozen roughly equals the volume they occupy when melted.
And for those readers who might still be wondering about the cause of that part of sea-level rise that’s not attributable to glaciers, the answer lies in the thermal expansion of oceans due to increasing atmospheric temperatures.
— Patricia Tummons
Volume 12, Number 11 May 2002
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