Vegetation and soil are currently slowing down global warming by absorbing about a quarter of human emissions of carbon dioxide (CO2). This land carbon sink is believed to be in part due to increases in photosynthesis. A new study in the journal Nature shows that doubling of the carbon dioxide concentration in the atmosphere will cause global plant photosynthesis to increase by approximately one third. The research was conducted by scientists from the DLR Institute of Atmospheric Physics and the University of Exeter in the UK, in a collaboration supported by the European Union through the CRESCENDO project.
It is widely accepted that a rise of carbon dioxide levels will increase plant photosynthesis, so long as nutrients, such as nitrogen and phosphorus, are not limiting. Global Earth System Models (ESMs) predict that global photosynthesis will increase with carbon dioxide, but they differ by a factor of three in the size of this ‘CO2 fertilisation’ effect. In the new study, the scientists show that the size of the CO2 fertilization is revealed by how the seasonal cycle in carbon dioxide concentration varies in the atmosphere.
Long-term measurements made on Hawaii and in Alaska show an increasing amplitude of the seasonal cycle. The study found a correlation between the increase in carbon dioxide amplitude simulated by a model and the predicted CO2-fertilisation. This means that the observed increase in the carbon dioxide amplitude can help derive a much improved estimate of the CO2 fertilisation effect. This method is known as Emergent Constraint.
Despite nutrient limitations in some regions, the study indicates that CO2 fertilisation on photosynthesis is currently playing a major role in the global land carbon sink. This means that one needs to work even harder to reduce carbon dioxide emissions, as it should be expected the land carbon sink to decline as carbon dioxide begins to stabilize.
As well as its role in the climate system, photosynthesis also provides the primary food-source for life on Earth. The study therefore has relevance to the future health of ecosystems, as well as to the challenge of slowing climate change.
Dr. habil. Veronika Eyring
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