Increases in global CO2 levels aren't increasing plant growth as predicted, dashing hopes that worldwide carbon storage increases could help slow climate change, researchers say.
Because plants depend on carbon dioxide for growth, it has been predicted that increases in the greenhouse gas would significantly boost plant growth, creating more global greenery to absorb yet more CO2 and at least partially offsetting human-produced emissions.
That hope may have been over-optimistic, researchers are saying in a study appearing in the journal Nature Climate Change.
While plant growth globally has in fact increased in the last three decades, the increase is nowhere near what would have been expected with the rise in levels of CO2 concentrations in the atmosphere, they say.
"Current Earth system models assume that global plant growth will provide the tremendous benefit of offsetting a significant portion of humanity's CO2 emissions, thus buying us much needed time to curb emissions," says study leader William Kolby Smith from the Institute on the Environment at the University of Minnesota.
Unfortunately, he says, estimates of worldwide vegetation growth based on observations indicate actual plant growth may not provide humanity as much time as expected.
That makes action aimed at curbing emissions "all the more urgent," he says.
Because current climate models that assume increased plant growth mirroring CO2 increases haven't accurately predicted such future growth, allowable emissions targets based on the models may need to be revisited, the researchers say.
Looking to explain the lower-than-expected rate of plant growth, the researchers point to the availability of water and nutrients as possible factors in the difference between what is predicted by models and what has actually been observed.
Warmer climates resulting from increasing CO2 in the atmosphere may be putting increased water stress on plants, offsetting any positive effects from rising levels of the greenhouse gas, they explain.
In addition, they suggest, limited availability of environmental nutrients such as phosphorus and nitrogen may also be limiting plants' ability to take in additional CO2.
Figuring out the ways plants may or may not respond to rising levels of CO2 in the atmosphere will require scientists from many different specialties working together to understand how ecosystems respond to global changes, says study co-author Sasha Reed of the U.S. Geological Survey.
"We have many scientific tools in our toolbox, and bringing them together is a powerful approach to asking questions and to solving problems," she says.
