Carbon dioxide doubling impact has limit - USA Today (blog)

Global warming likely won't reach extremely catastrophic temperatures even if carbon dioxide levels double, climate researchers suggest.

Today, carbon dioxide concentrations in the air are 39% higher than they were before the industrial revolution, a number that grows every year because the greenhouse gas remains for centuries in the sky. Tied to the burning of coal, oil and natural gas, the boost in the greenhouse gas has contributed to a 1.4 degree Fahrenheit increase in global average surface temperatures over the last century, according to the National Academy of Sciences.

"The most important question is the upper limit to warming if we double carbon dioxide," says climate scientist Andreas Schmittner of Oregon State University in Corvallis, lead author on the new analysis of climate "sensitivity." Economic projections from the Intergovernmental Panel on Climate Change see this doubling happening around 2060 under "business as usual" burning of fossil fuels by people and industries. The extent to which it will raise temperatures higher has been debated for more than a decade, with the IPCC forecasting a 5.4 degree rise as most likely (within a range of 2.7 to 8.1 degrees).

But some more recent studies have suggested a rise as high as 18 degrees was possible. If that were to happen, that's more than high enough to trigger worries of massive ice sheet collapses in Greenland and Antarctica, which would bring catastrophic coastal flooding worldwide, as well as continental crop failures, droughts and other abrupt climate change calamities.

In the analysis released by the journalScience, Schmittner and colleagues looked at temperature indicators from the height of the last Ice Age, more than 19,000 years ago, and the atmosphere's temperatures then. Records in their analysis include preserved pollen from lakebed cores, which indicate land temperatures, and sea floor sediments indicating sea surface temperatures.

Overall, they find that a temperature increase of 3.1 degrees in global average surface temperatures seems the most likely as a result of doubling, with a range varying from 2 to 4.7 degrees. Moreover, simulations suggest that 4.7 degree rise will be difficult to exceed -- "completely unrealistic", Schmittner says -- as a result of carbon dioxide levels doubling.

"The work of Schmittner (and colleagues) demonstrates that climates of the past can provide potentially powerful information to reduce uncertainty in future climate predictions and evaluate the likelihood of climate change that is larger than captured in present models," says climate scientists Gabriele Hegerl and Tom Russon of Scotland's University of Edinburgh, in a commentary accompanying the study. Hegerl and Russon note caveats to the past climate reconstruction, however, and are more cautious about its upper limit.

One issue is that the study assumes that the atmosphere responds to influences leading to colder temperatures, as in an Ice Age, the same way that it does to warming influences, says climate scientist James Annan of Japan's JAMSTEC Yokohama Institute for Earth Sciences in Yokohama, who was not part of the study. As a result, "their confident upper bound is probably optimistic," he says, by e-mail.

Even a 3.1 degree rise isn't a cause for celebration, Schmittner says. "Temperatures were just about that much colder during the Ice Age and the world was a completely different place, with ice sheets covering continents and plants and animals completely different from today dominant," he says. "Small changes in global temperatures mean large changes on land."