Abstract
Abstract 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.
Highlights
Aerosols affect Earth’s climate by absorbing and scattering solar and terrestrial radiation (Twomey 1977; Boucher et al 2013)
In this paper we aim to identify the aerosol process parameters and emission fluxes, hereafter referred to as parameters, which are the largest sources of regional-mean cloud albedo effect (CAE) forcing variance and have potential to be influential on global-mean forcing in near-future climates
Because contributions of opposing signs cancel, parameter perturbations that act to enhance CAE forcing in regions of both positive and negative forcing could make significant contributions to regional CAE forcing variance that are not detected in a sensitivity analysis of global-mean CAE forcing variance
Summary
Aerosols affect Earth’s climate by absorbing and scattering solar and terrestrial radiation (Twomey 1977; Boucher et al 2013). (Boucher et al 2013), characterized by a decrease in cloud drop effective radius that results from an increase in cloud droplet number concentration for a given amount of liquid water (Twomey 1977), is the largest component of the aerosol–cloud interaction. Uncertainty in the magnitude of CAE forcing remains the dominant source of uncertainty in net aerosol radiative forcing within current global climate models (Skeie et al 2011; IPCC 2013). CAE forcing variance in recent decades; credible ranges of global-mean CAE forcing were found to be small compared to the magnitude of forcing resulting from changes in atmospheric CO2 concentrations. Regayre et al (2014) hypothesized that a large component of multimodel CAE forcing diversity during recent decades is determined by the extent to which regional positive and negative forcings cancel in individual models when calculating global-mean CAE forcing
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