Narrowing the Range of Future Climate Projections Using Historical Observations of Atmospheric CO2
Journal of Climate
Uncertainty in the behavior of the carbon cycle is important in driving the range in future projected climate change. Previous comparisons of model responses with historical CO 2 observations have suggested a strong constraint on simulated projections that could narrow the range considered plausible. This study uses a new 57-member perturbed parameter ensemble of variants of an Earth system model for three future scenarios, which 1) explores a wider range of potential climate responses than
... responses than before and 2) includes the impact of past uncertainty in carbon emissions on simulated trends. These two factors represent a more complete exploration of uncertainty, although they lead to a weaker constraint on the range of future CO 2 concentrations as compared to earlier studies. Nevertheless, CO 2 observations are shown to be effective at narrowing the distribution, excluding 30 of 57 simulations as inconsistent with historical CO 2 changes. The perturbed model variants excluded are mainly at the high end of the future projected CO 2 changes, with only 8 of the 26 variants projecting RCP8.5 2100 concentrations in excess of 1100 ppm retained. Interestingly, a minority of the highend variants were able to capture historical CO 2 trends, with the large-magnitude response emerging later in the century (owing to high climate sensitivities, strong carbon feedbacks, or both). Comparison with observed CO 2 is effective at narrowing both the range and distribution of projections out to the mid-twenty-first century for all scenarios and to 2100 for a scenario with low emissions.