Grand Challenge Carbon feedbacks in the climate system
The land and ocean biogeochemical cycles are key components of the Earth's climate system. The surface fluxes of many greenhouse gases and aerosol precursors are controlled by biogeochemical and physical processes and are sensitive to changes in climate and atmospheric composition. Most importantly, biogeochemical processes control atmospheric concentrations of the main greenhouse gases (CO2, CH4 and N2O) Plants, soils and permafrost on the land together contain at least five times as much carbon as the atmosphere, and the global ocean contains at least fifty times more carbon than the atmosphere. Based on the 2015 Global Carbon Budget estimates, since 1870, CO2 emissions from fossil fuel combustion have released about 400+ 20 GtC (Gigatonnes carbon) to the atmosphere, while land use change is estimated to have released an additional 145+ 50 GtC. Of these cumulative anthropogenic CO2 emissions, 230+ 5 GtC have accumulated in the atmosphere, while the ocean and land ecosystems have taken up 155+ 20 GtC and 160+ 60 GtC, respectively. Land and oceans are hence absorbing around half of the carbon emitted from human activity, significantly mitigating the build up of CO2 in the atmosphere and climate change. In the absence of these carbon sinks, current atmospheric CO2 would already be about twice the pre industrial CO2 concentration with an associated global warming likely above 2degC. However, substantial uncertainty remains about the magnitude of these carbon sinks, how they currently operate, and whether they will continue to be as effective as climate changes. When climate models first included climatecarbon cycle feedbacks, they indicated the potential for these carbon sinks to be significantly weakened under global warming, accelerating atmospheric CO2 increases and hence global