High levels of carbon dioxide in the atmosphere may have less of an impact on the rate of global warming than feared, a study said Thursday.

The authors of the study funded by the US National Science Foundation stressed that global warming is real, and that increases in atmospheric CO2, which has doubled from pre-industrial standards, will have multiple serious impacts.

But the more severe estimates, such as those put forth by the United Nations' Intergovernmental Panel on Climate Change, are unlikely, the researchers found in their study published in the journal Science.

The 2007 IPCC report estimated that surface temperatures could rise by as much as 2.4 to 6.4 degrees Celsius (4.3 to 11.5 Fahrenheit).

"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," said lead author Andreas Schmittner, an Oregon State University researcher.

"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."

Scientists have long struggled to quantify "climate sensitivity," or how the Earth will respond to projected increases in carbon dioxide, the main greenhouse gas.

Schmittner noted that many previous studies only looked at periods spanning from 1850 to today, thus not taking into account a fully integrated paleoclimate date on a global scale.

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.

"Yet the planet was completely different -- huge ice sheets over North America and northern Europe, more sea ice and snow, different vegetation, lower sea levels and more dust in the air," said Schmittner.

"It shows that even very small changes in the ocean's surface temperature can have an enormous impact elsewhere, particularly over land areas at mid- to high-latitudes."

He warned that continued, unabated use of fossil fuels could lead to similar warming of sea surfaces today.