April 20, 2015 | One of the great climate mysteries that scientists are working to solve is how trees and other plants respond to a more carbon-rich atmosphere.

Most climate scenarios, including those of the Intergovernmental Panel on Climate Change, assume that more carbon dioxide (CO₂) in the atmosphere will accelerate plant growth, thereby drawing down more of this greenhouse gas from the atmosphere. But a number of studies have indicated that plants can’t keep absorbing more CO₂ because there aren’t enough nutrients in the soil to sustain their growth.

A new study in Nature Geoscience, led by NCAR scientist Will Wieder, underscores what’s at stake.

If society stays on its current trajectory of CO₂ emissions, and the growth rates of plants aren't as robust as many models project, the result by the end of the century could mean in an additional 10 percent of the greenhouse gas in the atmosphere, the study finds. While there is uncertainty around this estimate, that amount of CO₂—an estimated 140 petagrams—would be equivalent to about 14 years of CO₂ emissions from all human-related sources worldwide at current rates, or about as much CO₂ as has been released so far this century.

New estimates indicate that plants, such as these trees in the Osa Peninsula in Costa Rica, may absorb less carbon dioxide then previously thought. This could result in more global warming. (©UCAR. Photo by Will Wieder, NCAR.)

“Humanity so far has greatly benefited from plants removing carbon dioxide from the atmosphere,” said Wieder, who also works at the Institute for Arctic and Alpine Research at the University of Colorado Boulder. “But if a lack of nutrients limits their ability to keep soaking up CO₂, then climate change becomes an even bigger problem then we thought—unless society can cut back on emissions.”

The role of nutrients

Most of the world’s leading climate models assume that plants will respond to increased atmospheric levels of CO₂ by growing more and more, which is known as the CO₂ fertilization effect. The more the plants grow, the more CO₂ they absorb from the atmosphere, thereby slowing climate change.

But CO₂ is far from the only determinant of plant growth. Nutrients in the soil, especially nitrogen and phosphorus, are also critical. Because the supply of such nutrients is limited, scientists have warned that plant growth will be less than indicated in climate models.

Most climate models so far have not included nutrients because such biogeochemical processes are difficult to simulate and vary greatly from one type of terrestrial ecosystem to another. The NCAR-based Community Earth System Model, jointly funded by the National Science Foundation and U.S. Department of Energy, is one of the first to begin considering the role of soil nutrients in the models that are used for climate change projections.

In the new study, Wieder and his co-authors turned to the world’s leading climate models that were used in an international study known as CMIP5 (the Coupled Model Intercomparison Project, Phase 5). They focused on how the 11 models represented plant growth in specific geographic regions, comparing that to changes in nitrogen and phosphorus availability caused by deposition of airborne particles and other factors.

Their results showed that nitrogen limitation could reduce plant uptake of CO₂ by 19 percent, and nitrogen and phosphorus limitation combined could reduce plant uptake by 25 percent, compared to the average results of the climate models. Instead of acting as a carbon sink and drawing down CO₂, the terrestrial biosphere would 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.

The role of uncertainty

Wieder stressed that significant uncertainties remain. One of the questions, for example, is how soil microbes—which free up nitrogen in the soil, but also release carbon dioxide into the atmosphere—will respond to warming temperatures. Similarly, scientists don’t know if plants will become more efficient at drawing up additional nutrients in the soil.

But the overall picture is that Earth’s limited amounts of nitrogen and phosphorus mean that "plants will not be able to keep up with society’s CO₂ emissions," Wieder said.

“To store that much carbon on land, plants will need more nitrogen and phosphorus,” he said. “If they can’t get it, we’re going to go from terrestrial ecosystems sponging up CO₂ to actually having them contributing to the problem.”

Writer/contact
David Hosansky

Collaborators
University of Colorado Boulder, Institute of Arctic and Alpine Research
University of Montana, Missoula
University of Oklahoma, Norman
University of Minnesota Institute on the Environment, St. Paul
Pacific Northwest National Laboratories

Funders
National Science Foundation
Andrew W. Mellon Foundation 


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