The Climatic Importance of Uncertainties in Regional Aerosol–Cloud Radiative Forcings over Recent Decades

• Published in: Journal of climate 2015-09, Vol.28 (17), p.6589-6607
• Main Authors: Regayre, Leighton A., Pringle, Kirsty J., Lee, Lindsay A., Rap, Alexandru, Browse, Jo, Mann, Graham W., Reddington, Carly L., Carslaw, Ken S., Booth, Ben B. B., Woodhouse, Matthew T.
• Format: Article
• Language: English
• Subjects: Aerosol clouds; Aerosols; Albedo; Albedo effects; Carbon dioxide emissions; Climate; Climate change; Climate effects; Climatic zones; Cloud albedo; Clouds; El Nino; El Nino phenomena; El Nino-Southern Oscillation event; Emissions; Emissions control; Energy balance; Global climate models; Hurricane development; Hurricanes; Mathematical analysis; Meteorology; Monsoons; Northern Hemisphere; Parameters; Precipitation; Process parameters; Radiation; Radiative forcing; Regions; Sea surface; Sea surface temperature; Sensitivity; Sensitivity analysis; Simulation; Southern Oscillation; Surface temperature; Time; Trends; Tropical depressions; Tropical regions; Tropical storms; Uncertainty
• ISSN: 0894-8755; 1520-0442
• DOI: 10.1175/jcli-d-15-0127.1

Regional patterns of aerosol radiative forcing are important for understanding climate change on decadal time scales. Uncertainty in aerosol forcing is likely to vary regionally and seasonally because of the short aerosol lifetime and heterogeneous emissions. Here the sensitivity of regional aerosol cloud albedo effect (CAE) forcing to 31 aerosol process parameters and emission fluxes is quantified between 1978 and 2008. The effects of parametric uncertainties on calculations of the balance of incoming and outgoing radiation are found to be spatially and temporally dependent. Regional uncertainty contributions of opposite sign cancel in global-mean forcing calculations, masking the regional importance of some parameters. Parameters that contribute little to uncertainty in Earth’s global energy balance during recent decades make significant contributions to regional forcing variance. Aerosol forcing sensitivities are quantified within 11 climatically important regions, where surface temperatures are thought to influence large-scale climate effects. Substantial simulated uncertainty in CAE forcing in the eastern Pacific leaves open the possibility that apparent shifts in the mean ENSO state may result from a forced aerosol signal on multidecadal time scales. A likely negative aerosol CAE forcing in the tropical North Atlantic calls into question the relationship between Northern Hemisphere aerosol emission reductions and CAE forcing of sea surface temperatures in the main Atlantic hurricane development region on decadal time scales. Simulated CAE forcing uncertainty is large in the North Pacific, suggesting that the role of the CAE in altering Pacific tropical storm frequency and intensity is also highly uncertain.