A new study suggests that the rate of global warming from doubling of atmospheric carbon dioxide may be less than the most dire estimates of some previous studies.

Authors of the study, which was funded by the National Science Foundation and published online on Thursday in the journal Science, say that global warming is real and that increases in atmospheric CO2 will have multiple serious impacts. However, the most Draconian projections of temperature increases from the doubling of CO2 are unlikely.

"Many previous climate sensitivity studies have looked at the past only from 1850 through today, and not fully integrated paleoclimate date, especially on a global scale," said Andreas Schmittner, an Oregon State University researcher and lead author on the Science article. "When you reconstruct sea and land surface temperatures from the peak of the last Ice Age 21,000 years ago -- which is referred to as the Last Glacial Maximum -- and compare it with climate model simulations of that period, you get a much different picture."

"If these paleoclimatic constraints apply to the future, as predicted by our model, the results imply less probability of extreme climatic change than previously thought," Schmittner added.

Scientists have struggled for years trying to quantify "climate sensitivity" -- which is how the Earth will respond to projected increases of atmospheric carbon dioxide. The 2007 IPCC report estimated that the air near the surface of the Earth would warm on average by two to 4.5 degrees (Celsius) with a doubling of atmospheric CO2 from preindustrial standards. The mean, or " expected value" increase in the IPCC estimates was 3.0 degrees; most climate model studies use the doubling of CO2 as a basic index.

The researchers based their study on ice age land and ocean surface temperature obtained by examining ices cores, bore holes, seafloor sediments and other factors. When they first looked at the paleoclimatic data, the researchers only found very small differences in ocean temperatures then compared to now.

"Our study implies that we still have time to prevent that from happening, if we make a concerted effort to change course soon," said Schmittner.