CO2 Emissions Determined by HadGEM2-ES to be Compatible with the Representative Concentration Pathway Scenarios and Their Extensions

Abstract

his paper presents the fossil fuel?derived CO2 emissions simulated by the Hadley Centre Global Environmental Model, version 2, Earth System (HadGEM2-ES) to be compatible with four representative concentration pathways (RCPs) from 2006 to 2100. For three of the four RCPs, the analysis is extended to 2300. The compatible emissions compare well with those generated by integrated assessment models from which the RCPs were constructed. Historical compatible emissions are also presented, which closely match observation-based estimates from 1860 to 2005 (cumulatively 330 and 319 GtC, respectively). Simulated land and ocean carbon uptake, which determines the compatible emissions, is examined, with an emphasis on changes in vegetation carbon. In addition, historical land and ocean carbon uptake is compared with observations. The influences of climate change and the carbon cycle on compatible emissions are investigated individually through two additional experiments in which either aspect is decoupled from the CO2 pathway. Exposure of the biogeochemical components of the Earth system to increasing CO2 is found to be responsible for 68% of the compatible emissions of the fully coupled simulation, while increased radiative forcing from the CO2 pathway reduces its compatible emissions by 11%. The importance of dynamic vegetation to compatible emissions is investigated and discussed. Two different methods of determining emissions from land use and land-use change are compared; differencing the land?atmosphere CO2 exchange of two experiments, one with fixed land use and the other variable, results in historical land-use emissions within the uncertainty range of observed estimates, while those simulated directly by the model are well below the lower limit of the observations.