Some climate models suggest runaway global temperatures will be held in check -- at least a little bit -- by the uptick in green growth enabled by a warming planet. In other words, the plant life encouraged by global warming will pull excess carbon dioxide out of the atmosphere, thus slowing global warming.

But new research out of the University of Montana suggests those models are flawed, and that a finite amount of soil nutrients will limit plant growth from proliferating enough to put a dint in the climbing concentrations of global CO2.

That means, Montana scientists say, many models are underestimating global warming.

"If society stays on its current trajectory of CO2 emissions and the growth rates of plants don't increase as much as many models project, the result by the end of the century could be more extreme than we predicted," Cory Cleveland, an associate professor of biogeochemistry at Montana, said in a recent press release.

The idea that rising levels of CO2 will spur more and more plant growth is called the CO2 fertilization effect. But Cleveland and his researcher partners say the theory ignores other determinants of plant growth and CO2 absorption.

"To store that much carbon on land, plants will need more nitrogen and phosphorus," explained Will Wieder, a scientist at the National Center for Atmospheric Research. "If they can't get it, we're going to go from terrestrial ecosystems sponging up CO2 to actually having them contribute to the problem."

Plants also need nitrogen and phosphorous -- nutrients of which there are finite supplies. Most climate models ignore these factors because the biochemical processes of plant growth are hard to incorporate into broad systems. The specifics of these processes also vary from place to place, depending on the terrestrial ecosystem present. Trees, shrubs and grasses in Florida's Everglades don't use nutrients in the same way as the tall timber trees of Canada's boreal forests. Georgia's red clay isn't the same as Maine's rocky dirt.

To attack the dearth of data on the subject, Cleveland and his colleagues broke it down region by region and looked at how the changing availability of nitrogen and phosphorous would affect plant growth in different regions.

They also looked at how changing soil composition could affect not just plant growth, but carbon concentrations directly.

"We found that instead of acting as a carbon sink and drawing down CO2, the terrestrial biosphere could become a net source of the greenhouse gas to the atmosphere by the end of the century, with soil microbes releasing more carbon than growing plants could absorb," Cleveland said.

If the research is correct, there could be as much as a 10 percent increase of CO2 in the atmosphere as compared to the current most commonly cited predictions.

But Cleveland says the findings of he and his partners aren't a certainty. Researchers aren't entirely sure how microbes in the soil, which free up both nitrogen and CO2, will be affected by higher temperatures.

Still, the new research is a reminder that small factors can have major impacts on Earth's biological, chemical and climatological systems.

The findings were published this week in the journal Nature Geoscience.