When climate scientists try to estimate how much the Earth will warm due to increasing levels of greenhouse gases in the atmosphere, a key consideration is the role of plants and soils. The more carbon they absorb, the more they reduce the global warming potential.

But recent studies indicate assumptions about plants’ and soils’ capacity in the so-called “carbon cycle” may be overly optimistic. If these studies are correct, even bigger cuts in greenhouse gas emissions will be needed to prevent drastic, irreparable climate shifts.

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Not only is it possible plants won’t be able to absorb as much carbon as climate models project, but plants’ response to the carbon cycle could actually amplify global warming, Paul Higgins and John Harte write in the November edition of the Journal of Climate.

It all comes down to mobility.

Carbon dioxide is recognized as critical for photosynthesis, so the more there is in the atmosphere, the more there is available for plant growth. As Earth’s climate warms, the theory has been that trees and other plant communities would treat the added CO2 as fertilizer and grow bigger and faster.

But because climate conditions will be changing, to take advantage of the added CO2 some plant communities will have to migrate to neighboring areas that provide the necessary growing conditions. The speed at which plants can make these moves is the question.

Higgins, associate director of the American Meteorological Society’s Policy Program, said it has been very difficult to build global ecosystem models that are sensitive to the limitations of plant migration.

“If you look at the models, they had no real constraints on plant mobility,” he said. “They basically assume that any type of plant can grow in any location where the climate is the same.”

But it isn’t quite that simple.

Part of the problem is that the scientists who study plant migration and the scientists who build carbon-cycle models have tended to work separately, Higgins said.

“What we did (in the study) wasn’t something that people tended to do before,” he said. “We had to break the model.”

Scientists who have examined the fossil record by studying pollen deposits found in bogs have found that in past periods of climate change, plants had limited capacity to migrate.

The paper by Higgins and Harte, an environmental science professor at the University of California, Berkeley, describes a range of possible impacts this slower migration might have on the carbon cycle.

“Constrained plant movement would exacerbate the losses of carbon due to changes in climate, because plants wouldn’t be able to move to those locations where climate becomes favorable for them,” Higgins said. “In some locations (or for some plant types) there would be increases in carbon storage, but in other locations carbon losses from plants and soils could be large. Thus, total carbon storage could go down.

“Our results suggest that the overall effect of climate change would be to reduce carbon storage in plants and soils. CO2 fertilization could counteract the consequences of climate change. However, CO2 fertilization might or might not actually happen. If it doesn’t, then the losses of carbon due to climate change could be large.”

The United Nations’ Intergovernmental Panel on Climate Change has estimated the amount of warming by 2100 due to greenhouse gas emissions ranges from 1 to 6 degrees Celsius. Many argue that even a 1-degree rise will have devastating impacts.

Another recent study concluded that global climate models have been overestimating plants’ ability to absorb carbon by 3.4 percent. That study found that trees can’t take advantage of extended warm periods, as previously assumed, because photosynthesis basically shuts down in late summer and early fall, even when the days continue to be warm.

Another study released this fall concluded that plants won’t be the major carbon sinks many had hoped, for a different reason: Soils often don’t have enough nitrogen and phosphorus to take advantage of the added CO2 in the atmosphere.