IRACCS · Investigating Increases in Global Terrestrial Carbon Uptake During the Last Three Decades

Investigating Increases in Global Terrestrial Carbon Uptake During the Last Three Decades

The objective of the proposed work was to estimate changes in global terrestrial carbon uptake over the last decades and identify corresponding controls, processes and mechanism(s) through a combination of observation (satellite) based and modelling approaches. We specifically investigated three main hypotheses in regards to increased terrestrial carbon uptake during recent decades, including (H1) a progressive relaxation of climatic constraints on plant productivity including warming in northern temperate, boreal and tundra ecosystems and increases in rainfall rates over tropical savannas, (H2) increases in the fraction of diffuse/direct radiation associated predominantly with East Asian sulfate aerosol emissions that spurred plant photosynthetic rates, and (H3) increases in nitrogen deposition from East Asian fossil fuel burning that increased carbon plant uptake. If our main hypotheses H2 and H3 would be correct, it would provide a basis for a causal link between recent terrestrial carbon sink strength and global fossil fuel emissions which has implications for carbon climate mitigation strategies. Overall, this research is designed to improve our knowledge about carbon cycle feedbacks under global environmental change, a first-order uncertainty in climate models, with the hope that outputs lead to more credible projections of future climate. The key findings at this final stage show that increases in plant productivity since the turn of the 21st century due to increases in fossil fuel aerosols and diffuse radiation in East Asia (dimming regions) are effectively offset by reductions in diffuse radiation and plant productivity across Europe and North America (brightening regions), leading to a relatively small global effect (H2). Similarly, increases in plant productivity over this time frame due to increasing nitrogen deposition in East Asia are also offset by decreasing rates of nitrogen deposition in Europe and North America, resulting again in a small effect at global scales (H3). These results suggest therefore that the increase in the land carbon sink since the turn of the 21st century is not causally linked to increasing rates of fossil fuel emissions via the effect of aerosols and diffuse radiation (H2) and nitrogen deposition or nitrogen-carbon interactions (H3). This implies that other factors not directly linked to fossil fuel emissions such as climate variability or land use change may be the drivers underlying the recent increase in terrestrial carbon uptake. Corresponding results are already written up and are already published under open access in Geophysical Research Letters and are presently under review in the Global Biochemical Cycles. The fellow holds presently a ‘tenured’ Associate Professor position ensuring a high level of independence. In part because of the success in securing this Marie-Curie CIG grant, the fellow’s probation period of initially 2 years was shortened to 1.5 years and his position was converted into a more stable tenured position (in March 2014). The Marie-Curie CIG grant also enabled the fellow to start new collaborations and knowledge transfers within the University of Leeds, which also led to a recent success in securing a UK NERC DTP PhD scholarship, which is supporting a second PhD student in the fellow’s research group.

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