Does more atmospheric carbon mean bigger plants?

Not necessarily, according to a new study co-authored by Luc Hoffmann Institute Fellow William Kolby Smith, published this week in Nature Geoscience.

Most climate scenarios, including those of the Intergovernmental Panel on Climate Change, assume that more carbon dioxide in the atmosphere will accelerate plant growth, thereby reducing the net amount of this greenhouse gas in 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.

Plants mostly recycle nutrients but new nutrient inputs are low relative to plant demands

The study builds off previous research by co-author Bill Smith, a Luc Hoffman Institute Fellow working with the University of Minnesota Institute on the Environment Global Landscapes Initiative and the Natural Capital Project. His earlier research looked at the sources of nutrients for plant growth and found that plants mostly recycle nutrients and that new nutrient inputs are low relative to plant demands. This suggests that nutrient availability may ultimately constrain plant growth across much of the world.

Earth’s limited supply of nitrogen and phosphorus will limit plant uptake of CO2

The study team set out to test this concept using 11 of the world’s leading climate models, projected forward to the year 2100. They focused on how the 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. What they found was that plant uptake of CO2 will indeed be strongly reduced by the Earth’s limited supply of nitrogen and phosphorus.

“Humanity so far has greatly benefited from plants removing carbon dioxide from the atmosphere,” said lead author William R. Wieder of the National Center for Atmospheric Research and the Institute for Arctic and Alpine Research in Boulder, CO., in a press release “But if a lack of nutrients limits their ability to keep soaking up CO2, then climate change becomes an even bigger problem than we thought — unless society can cut back on emissions.”

The team found that nitrogen limitation could reduce plant uptake of CO2 by 19 percent, and nitrogen and phosphorous 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.

Including nitrogen and phosphorous in future global climate models is critical

The team is calling for the inclusion of nitrogen and phosphorous availability in global climate models to more accurately assess future atmospheric carbon and set greenhouse gas reduction targets.

“Our results show that if society continues on its current trajectory of CO2 emission, plants will simply not be able to keep up the way current climate models predict,” Smith said. “While we mainly considered natural systems, a next step that I am excited to explore is how these findings could impact agricultural systems and future food production.”

Blog post courtesy of the University of Minnesota Institute on the Environment.