Relative climatic effects of landcover change and elevated carbon dioxide combined with aerosols: A comparison of model results and observations

In this study we examine the possibility that the historical total of human landcover changes have had a comparable effect on climate to that of historical increases in CO2 and aerosols. We compared results from two coupled climate model simulations which investigated transient climate changes produced by observed historical changes of CO2 combined with sulfate aerosol forcing with two other climate model simulations that examined the equilibrium climatic effects of currently observed changes in landcover from its natural state. We found that simulated, near-surface temperature anomalies due to transient increases in atmospheric CO2 combined with aerosols at the level currently observed are of similar amplitude as simulated temperature anomalies due to the direct and remote (nonlocal) equilibrium effects of historical anthropogenic landcover change in all models. Both effects are of comparable amplitude to observed temperature trends in the past 2 decades, the period of largest global surface warming. These results provide evidence for a confounding influence on surface temperatures and may be an indication that the problem of detection of the radiative warming effect of increased CO2 in the observational record may be more complicated than previously appreciated.