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 German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) and the University of Exeter in the UK, in a collaboration supported by the European Union through theCRESCENDO 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 CO2fertilisation’ effect. In the new study, the scientists show that the size of the CO2 fertilisation is revealed by how the seasonal cycle in carbon dioxide concentration varies in the atmosphere.
The carbon dioxide concentrations measured for many decades on Hawaii and in Alaska show characteristic cycles, with lower values in the summer when strong photosynthesis causes plants to absorb carbon dioxide, and higher values in the winter when photosynthesis stops. The peak-to-trough amplitude of the seasonal cycle therefore depends on the strength of the summer photosynthesis and the duration of the growing season.
The measurements made on Hawaii and in Alaska show an increasing amplitude of the seasonal cycle – but what does this mean for the future? The lead author of the study, Dr Sabrina Wenzel of the DLR Institute of Atmospheric Physics, explains: “Our study shows 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.”
Co-author Professor Peter Cox of the University of Exeter summarises the consequences of the study: “Despite nutrient limitations in some regions, our study indicates that CO2 fertilisation on photosynthesis is currently playing a major role in the global land carbon sink. This means that we will need to work even harder to reduce carbon dioxide emissions, as we should expect the land carbon sink to decline as we begin to stabilise carbon dioxide.”
In addition to its role in the climate system, photosynthesis also provides the primary food source for life on Earth. The study therefore has relevance for the future health of ecosystems, as well as to the challenge of slowing climate change.