A Three-Dimensional Coupled Climate-Carbon Simulation of a Business-As-Usual Carbon Emissions Pathway to Year 2300

Eventual emissions from recoverable fossil-fuel carbon resources, if unabated, may exceed 5000 GtC over several centuries, yet most studies of climate change have focused on doubled-CO2 or century scale experiments. Here, we investigate climate change and carbon budget out to year 2300 assuming that humans will continue the current trend using fossil fuels and releasing CO2 to the atmosphere. We use emissions and non-CO2-GHG concentrations from the SRES A2 scenario for the period 2000 to 2100; this trajectory is extended with a smooth logistic curve that eventually releases 5000 GtC to the atmosphere as CO2, with non-CO2-GHG concentration fixed at year 2100 values. Our simulations are performed in a fully-coupled three-dimensional climate and carbon cycle model, the INtegrated Climate and CArbon model (INCCA). INCCA is the NCAR/DOE Parallel Coupled Model coupled to the IBIS terrestrial biosphere model and a modified-version of the OCMIP ocean biogeochemistry model. By year 2300, atmospheric CO2 reaches 1423 ppm the global climate warms by about 8 K relative to the pre-industrial control run. The climate sensitivity of this model for a doubling of atmospheric CO2 is estimated to be 2.1 K; however, an 8 K response to 1423 ppm of CO2 by year 2300 (with radiative forcing from non-CO2-GHGs) suggests that climate sensitivity may be higher on a warmer planet (i.e., climate may warm more rapidly than the log of CO2 concentration); if so, unrestrained emissions may lead to conditions that are more severe than might be expected by extrapolation of results from doubled-CO2 experiments. This work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.