Drivers and patterns of land biosphere carbon balance reversal
The carbonbalance of the landbiosphere is the result of complex interactions between land, atmosphere andoceans, including climatic change, carbondioxide fertilization and land-use change.While the land biosphere currently absorbs carbondioxide from the atmosphere, this carbonbalancemight be reversed under climate and land-use change (‘carbonbalance reversal’). A carbonbalance reversalwould render climatemitigationmuchmoredifficult, as net negative emissionswould beneeded to even stabilize atmospheric carbondioxide concentrations.We investigate the robustness of the landbiosphere carbon sinkunderdifferent socio-economicpathwaysby systematically varying climate sensitivity, spatial patterns of climate change and resulting land-use changes. For this,we employ amodelling frameworkdesigned to account for all relevant feedbackmechanismsby coupling the integrated assessmentmodel IMAGEwith theprocess-baseddynamic vegetation, hydrology and crop growthmodel LPJmL.Wefind that carbon balance reversal can occurunder a broad range of forcings and is connected to changes in tree cover and soil carbonmainly innorthern latitudes. These changes are largely a consequence of vegetation responses to varying climate andonly partially of land-use change and the rate of climate change. Spatial patterns of climate change as deduced fromdifferent climatemodels, substantially determinehowmuchpressure in termsof globalwarming and land-use change the landbiospherewill tolerate before the carbonbalance is reversed.A reversal of the landbiosphere carbonbalance canoccur as early as 2030, although at very low probability, and shouldbe considered in thedesign of so-calledpeak-and-decline strategies. Introduction: the land biosphere carbon sink The land biosphere presently absorbs substantial amounts of carbon dioxide (CO2) from the atmosphere, partially compensating CO2 emissions from fossil fuel combustion and land use change and thus slowing anthropogenic climate change. Over the last three decades, land surfaces have absorbed about 2.3±0.8 Pg carbon (C) per year. Over the same period, land use change has led to average emissions of 1.0±0.5 Pg C yr, leaving a net carbon sink of 1.3 Pg C yr (Le Quéré et al 2014). This net carbon flux from the atmosphere to the land biosphere is a prominent negative (dampening) feedback mechanism in the Earth system (Friedlingstein et al 2006) that slows the rate of increase of atmospheric CO2. The interannual variability of the land–atmosphere carbon flux reflects its sensitivity to changes in precipitation in sensitive ecosystems (Schwalm et al 2012, Gatti et al 2014, Poulter et al 2014) as well as to variations in temperature (Lucht et al 2002). Land carbon uptake is projected to increase under climate change, mainly driven by the positive effects of CO2 fertilization of photosynthesis (Sitch et al 2008, Friend et al 2014), which are subject to large uncertainties (Schimel et al 2015). However, under high emission scenarios and severe climate change, some studies have found that the OPEN ACCESS