By Jim Erickson
Scripps Howard News Service
March 18, 2005
Even if the amount of carbon dioxide and other greenhouse gases being belched into the sky could be stabilized now, sea levels would rise 10 to 20 inches per century for 400 years or more, said Tom Wigley, a senior scientist at the National Center for Atmospheric Research in Boulder, Colo.
"We've already done so much to the atmosphere that sea-level rise is just going to keep going, and there's just not much that we can do about it," Wigley said. "We'd better learn to live with those future changes and develop strategies to reduce our vulnerability."
Wigley and NCAR senior scientist Gerald Meehl report their latest findings about "climate-change commitment" in Friday's edition of the journal Science.
The concept of climate commitment has been around for about 20 years. What's new about the NCAR reports is that some of the latest, most sophisticated computer climate models are now confirming the dire predictions of earlier, cruder simulations.
"It's additional evidence that those initial findings were sound and that we need to pay attention," said Vivien Gornitz, a sea-level-rise expert at NASA's Goddard Institute for Space Studies in New York City. "Now's the time to get working on it."
The idea behind climate commitment is simple:
Carbon dioxide gas has a lifetime of about a century, so gases pumped into the air today will continue trapping heat long into the future.
That heat will warm the air, land surfaces and the ocean. As the heat creeps deeper and deeper into the ocean, the warmed water expands, resulting in sea-level rise.
"Because the ocean takes longer to warm up than the land or the air, the sea level will continue to rise over several centuries," said Meehl, who used two of NCAR's global-coupled models, or GCMS, in his study.
Melting of ice sheets and glaciers also will contribute to the rise.
"I think people have thought that if the problem gets bad enough, OK, we'll just hit the stop button on the greenhouse-gas increases, and that'll solve the problem," Meehl said. "But it's not that easy."
A few critics question the ability of climate models to accurately predict the types of changes described by Meehl and Wigley. One outspoken critic is Colorado State University climatologist Roger Pielke Sr.
"To present them (GCM results) as predictions is overstating their capabilities," Pielke said Thursday. "They are valuable tools to study climate processes but are not skillful climate prediction tools."
But Meehl said the latest-generation models are more realistic and reliable than their predecessors. And the results they yield agree, in general, with earlier predictions about sea-level rise.
Over the past century, global temperatures increased about 1 degree and sea levels rose 6 to 8 inches.
Humans now pump the equivalent of about 7 billion tons of carbon into the air each year in the form of carbon dioxide gas. Under a "business-as-usual" scenario in which little is done to curtail the growth of emissions, 20 billion tons of carbon will enter the atmosphere each year by 2100, Wigley said.
In his study, Meehl determined that if current concentrations of carbon dioxide and other greenhouse gases could be frozen now - an impossibility, given ongoing emissions - the planet would warm 1 degree and sea levels would rise at least 8 inches by 2100.
Under various scenarios involving continued emissions increases, the sea would rise 15 to 24 inches by 2100, according to Meehl's study.
In its 2001 climate-change update, the authoritative Intergovernmental Panel on Climate Change projected a sea-level rise of 3.6 to 35.2 inches by 2100, with a best-guess estimate of 19.2 inches.
It's too late to avoid some future sea-level changes. But there's still time to avert the most drastic shifts if worldwide greenhouse emissions are cut below present levels, Meehl and Wigley said.
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