Comment on egusphere-2022-811

Abstract

The Earth’s albedo is observed to be symmetric between the hemispheres on the annual mean timescale, despite the clear-sky albedo being asymmetrically higher in the northern hemisphere due to more land area and aerosol sources; this is because the mean cloud distribution currently compensates for the clear-sky asymmetry almost exactly. We investigate the evolution of the hemispheric difference in albedo in CMIP6 coupled model simulations following an abrupt quadrupling of CO2 concentrations, to which all models respond with an initial decrease of albedo in the northern hemisphere (NH) due to loss of Arctic sea ice. After this initial NH darkening, the evolution of the hemispheric albedo difference diverges among models, with some models remaining at their new hemispheric albedo difference, and others returning towards their pre-industrial difference through either a reduction in SH clouds or an increase in NH clouds, or a combination of the two. These responses have different implications on the reduction in global albedo, and thereby the strength of the shortwave cloud feedback: if a cross-hemispheric communicating mechanism is primarily responsible for maintaining hemispheric albedo symmetry, the total shortwave radiative feedback must be more strongly positive. We also show that in these models, there is a link between the extent of reductions in SH extratropical cloud cover and Antarctic albedo decline due to increased poleward heat transport in the SH.