Biology 101 says plants use photosynthesis to convert CO2 and water into sugar and oxygen. A simplified expansion on photosynthesis suggests that as more and more CO2 is spewed into the global system, plants would grow more: bigger and more widespread. In term,  this would help offset some of the damage, i.e. global warming, by soaking and sequestrating that carbon.  After all, the term greenhouse gas effect comes from experiments on plants which grow bigger and faster in a rich CO2 environment. This thinking is true, but only to a certain degree. Researchers from the University of Minnesota’s Institute on the Environment now claim that plants aren’t keeping up with CO2 levels — effectively, these are decoupled. What this means is plants soak up far less CO2 than most climate scientists imagine, which of course spells trouble.

Image: Jason Samfield/Flickr, CC BY-NC-SA

Image: Jason Samfield/Flickr, CC BY-NC-SA

The researchers, led by William Kolby Smith, a Luc Hoffman Institute postdoctoral fellow, used a satellite-derived global terrestrial NPP (net primary productivity) data set to see how vegetation has evolved around the globe in response to CO2. This data was compared to  widely used on-the-ground measurements and the best available models of plant responses to increasing CO2. The findings show that while plant growth has indeed increased over the past 30 years, this was not as significant as one might expect given the change in atmospheric CO2.

“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 Smith. “Unfortunately, our observation-based estimates of global vegetation growth indicate that plant growth may not buy us as much time as expected, [so] action to curb emissions is all the more urgent.”

Plants grow more from CO2, but maybe not as much as current climate models assume new research suggests. Image: CO2Science

Plants grow more from CO2, but maybe not as much as current climate models assume new research suggests. Image: CO2Science

The researchers outline two possible explanation for the decoupling effect:
1) Rising CO2 drives more climate change which stress plants, counteracting any positive effect of CO2.
2) Limited availability of nitrogen and phosphorus in the environment could also limit the ability of plants to soak up additional CO2.

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The findings published in Nature Climate Change suggest that the current models are flawed when assuming how much CO2 plants can soak. The authors recommend better integration among model, satellite and on-the-ground measurement approaches to improve our understanding of the effects of rising atmospheric CO2 on plant growth.

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