Observational Constraints on Past Attributable Warming and Predictions of Future Global Warming

• Published in: Journal of climate Vol. 19; no. 13; pp. 3055 - 3069
• Main Authors: Stott, Peter A., Mitchell, John F. B., Allen, Myles R., Delworth, Thomas L., Gregory, Jonathan M., Meehl, Gerald A., Santer, Benjamin D.
• Format: Journal Article
• Language: English
• Published: Boston, MA American Meteorological Society 01.07.2006
• Subjects: Aerosols; Air pollution; Anthropogenic factors; Carbon dioxide; Climate change; Climate models; Climatology. Bioclimatology. Climate change; Cooling; Earth, ocean, space; Emissions; Exact sciences and technology; External geophysics; Fluid dynamics; Global climate models; Global temperatures; Global warming; Greenhouse effect; Greenhouse gases; Meteorology; Modeling; Predictions; Simulations; Studies; Sulfates; Temperature; Temperature control; Uncertainty; Uncertainty; Century 21st; aerosols; Climate models; digital simulation; Forecast model; troposphere; greenhouse gas; Dynamical climatology; Century 20th; climate modification; global change; warming; Air pollution; Forcing; Anthropogenic factor
• ISSN: 0894-8755; 1520-0442
• DOI: 10.1175/jcli3802.1

This paper investigates the impact of aerosol forcing uncertainty on the robustness of estimates of the twentieth-century warming attributable to anthropogenic greenhouse gas emissions. Attribution analyses on three coupled climate models with very different sensitivities and aerosol forcing are carried out. The Third Hadley Centre Coupled Ocean–Atmosphere GCM (HadCM3), Parallel Climate Model (PCM), and GFDL R30 models all provide good simulations of twentieth-century global mean temperature changes when they include both anthropogenic and natural forcings. Such good agreement could result from a fortuitous cancellation of errors, for example, by balancing too much (or too little) greenhouse warming by too much (or too little) aerosol cooling. Despite a very large uncertainty for estimates of the possible range of sulfate aerosol forcing obtained from measurement campaigns, results show that the spatial and temporal nature of observed twentiethcentury temperature change constrains the component of past warming attributable to anthropogenic greenhouse gases to be significantly greater (at the 5% level) than the observed warming over the twentieth century. The cooling effects of aerosols are detected in all three models. Both spatial and temporal aspects of observed temperature change are responsible for constraining the relative roles of greenhouse warming and sulfate cooling over the twentieth century. This is because there are distinctive temporal structures in differential warming rates between the hemispheres, between land and ocean, and between mid-and low latitudes. As a result, consistent estimates of warming attributable to greenhouse gas emissions are obtained from all three models, and predictions are relatively robust to the use of more or less sensitive models. The transient climate response following a 1% yr-1increase in CO₂ is estimated to lie between 2.2 and 4 K century-1(5–95 percentiles).