Plants use the energy in sunlight to convert carbon dioxide and water to sugar, which they use for food to nourish themselves, and oxygen, which they release into the atmosphere. So you might think that if you gave plants more CO2 to work with, the way we're doing by pumping emissions into the air, it would enhance plant growth - which, in turn, would enable plants to absorb more of the greenhouse gas. Scientists have assumed that, too. and they've been counting on that carbon-storing capacity to help slow climate change.
But, unfortunately, a new study published in Nature Climate Change shows that maybe we've been a little too optimistic.
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The study, led by William Kolby Smith, a postdoctoral fellow at the Luc Hoffmann Institute who worked with the Global Landscapes Initiative and the Natural Capital Project, found that while plant growth has increased over the last 30 years, the growth isn't as much as scientists would have expected, considering the increase in CO2 concentrations in the atmosphere.
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," Smith explained in a press release.
"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."
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The researchers identify two explanations for why scientists' previous optimistic results may be off. Satellite data show that the climate, which is warming due to increased CO2, may be increasing plant stress, counteracting any positive effects from having more of the greenhouse gas. Additionally, there may not be enough nitrogen and phosphorus in the environment to enable plants to absorb additional carbon dioxide.
If we can't depend upon plants to absorb as much carbon as we'd been counting upon them to do, that means that emissions targets based upon those numbers may need another look.