Climate Change Will Be Significant but Not Extreme, Study Predicts

By Rick Weiss
Washington Post Staff Writer
Thursday, April 20, 2006

Earth will experience significant climate change in the coming century as a result of greenhouse gas buildups, but the more extreme estimates of global warming generated by some studies are unlikely to occur, according to newly published research.

"This still commits us to quite a bit of climate change, but it leaves the door open to avoiding the largest and most devastating consequences," said Gabriele C. Hegerl, a Duke University climate expert who led the study.

The new work extends a difficult line of research that uses historical climate data and computer models to predict the impact of atmospheric carbon dioxide levels, which are increasing as a result of human activity, such as burning fossil fuels.

Specifically, the research aims to refine a value known as "climate sensitivity," which is defined as the global average temperature change that can be expected to occur in response to a doubling of carbon dioxide levels.

Climate scientists from around the world have for more than a decade concurred that climate sensitivity's most likely value is in the range of about 2.5 degrees to 8 degrees Fahrenheit. But because many factors can affect global temperatures in poorly understood ways -- including the extent to which the oceans have tempered climate trends -- scientists have not been able to rule out more extreme calculations suggesting a warm-up of 16 degrees Fahrenheit or more.

Moreover, most of the modeling done to date has been based on data gathered over just the past century, a period that has experienced a potentially confounding increase in aerosols that can blunt temperature buildups by reflecting incoming radiation from the sun.

The new work, described in today's issue of the journal Nature, reaches back 700 years. It recalculates the relationship between atmospheric composition and climate, taking into account the climate-affecting impacts of sun-blocking volcanic eruptions; carbon dioxide levels derived from air bubbles trapped in Antarctic ice; and temperature data derived from tree rings.

The result: Climate sensitivity almost certainly falls within the more conventional range of current predictions, with only a 5 percent chance that it will exceed 11 degrees Fahrenheit.

"This is the first use of several different independent data sets to come up with a constraint on climate sensitivity," said Reto Knutti of the National Center for Atmospheric Research in Boulder, Colo. "It's a very solid piece of science."

James E. Hansen, of NASA's Goddard Institute for Space Studies, said the results were consistent with his estimate, derived differently, of about 4.9 degrees Fahrenheit.

Even a few degrees increase can have significant environmental and economic impacts, but by downgrading the worst-case scenarios the new work may convince governments that it is not too late to take action, Hegerl said. Models suggest that carbon dioxide levels could reach double the pre-industrial levels between 2050 and 2100. Peak temperatures would occur decades later, as the planet's climate system settled into a new balance.

Figures released by the Environmental Protection Agency earlier this week showed that U.S. greenhouse gas emissions rose by 1.7 percent in 2004, the most recent year for which data have been compiled. The U.S. release -- equivalent to 6.3 billion tons of carbon dioxide -- amounts to about one-quarter of the world's greenhouse gas emissions.

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