One of the main mysteries scientists face with climate change is how to project it over time, particularly how to account for Earth's reaction to warmer temperatures, a phenomenon known as "feedback."

It has long been known that plants -- which use carbon dioxide, sun and water to grow through the process of photosynthesis -- are able to adapt to higher carbon dioxide levels by using nutrients more efficiently and growing bigger leaves.

"The process is called 'down-regulation.' This more efficient use of water and nutrients has been observed in experimental studies and can ultimately lead to increased leaf growth," NASA said in a statement.

But just what effect that increased plant growth would have on global warming has been difficult to predict.

NASA's new computer modeling effort, described in the December 7 issue of the journal Geophysical Research Letters, calculated the plants' cooling effect to be -0.3 degrees Celsius (-0.5 Fahrenheit) globally.

However that number falls well short of the temperature increase of two to 4.5 degrees Celsius (3.5 to 8.0 Fahrenheit) that is the standard basis for many global warming modeling simulations.

"This feedback slows but does not alleviate the projected warming," said Lahouari Bounoua, lead author of the paper and expert at the Goddard Space Flight Center outside Washington.

Bounoua's model found that warming was on the lower scale of the widely accepted range when carbon dioxide was doubled, projecting warming of 1.94 degrees C globally, not including the 0.3 C drop for feedback.

Previous climate models have not included the larger leaf growth that would come from "down-regulation," and have projected little to no cooling from vegetation growth, the US space agency said.

"This is what is completely new," said Bounoua, referring to how the model was adapted to include changed leaf growth.

"What we did is improve plants' physiological response in the model by including down-regulation. The end result is a stronger feedback than previously thought."

The latest research shows "how, over time, scientists will create more sophisticated models that will chip away at the uncertainty range of climate change and allow more accurate projections of future climate," NASA said.

Having more precise projections will help in the search for solutions, said study co-author Forrest Hall of the University of Maryland-Baltimore County and Goddard Space Flight Center.

"As we learn more about how these systems react, we can learn more about how the climate will change," said Hall.

"Each year we get better and better. It's important to get these things right just as it's important to get the track of a hurricane right. We've got to get these models right, and improve our projections, so we'll know where to most effectively concentrate mitigation efforts."

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