Atmospheric CO2 levels have risen by 30% in a century. A Franco-Australian study by scientists from INRA, James Cook University and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) has evaluated the global impact of rising atmospheric CO2 levels on the terrestrial vegetation that uses this gas as a source to drive its growth. Published in Trends in Plant Science on 16 May 2019, the results suggest that global photosynthesis has increased by the same proportion as atmospheric CO than ecosystems release, and this ’carbon sink’ phenomenon can stem rises in atmospheric CO2 levels. However, until now, scientists had not been able to estimate the intensity of the response by the global biosphere to this rise in CO2, nor a possible limit to it.
To make this evaluation, a Franco-Australian team focused on photosynthesis. They combined an analysis of published studies with the Community Atmosphere Biosphere Land Exchange model (CABLE) which was developed more than 10 years ago by teams from CSIRO and INRA. CABLE can calculate the fluxes of energy, water and carbon between the terrestrial surface and the atmosphere and thus model the principal biogeochemical cycles of terrestrial ecosystems. This model can notably examine how rises in CO2 levels affect plants, from the scale of a leaf to an entire ecosystem.
In view of the complexity of plant-environment relationships, the initial hypothesis advanced by the scientists was that with three times more CO2 in the air, photosynthesis would double. But against all expectations, they were able to demonstrate that at a global scale, photosynthetic activity increased entirely in proportion to the rise in atmospheric CO2. The results showed that more CO2 allows plants to increase their leaf unit area, and this larger area enables them to capture more CO2. Therefore, vegetation as a whole assimilates surplus CO2 emissions using all the resources available.
So in a context of global warming, plants are therefore trying to slow down the effects of anthropogenic climate change. However, the scientists warned that increasingly frequent and extreme climatic events (such as heatwaves, droughts, hurricanes, etc.) have the potential to cause major stress to terrestrial vegetation and thus hamper its growth.