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Abstract

Aone-dimensional radiative–convective equilibriummodel is used to investigate the influence of clouds on the onset of a runaway greenhouse under strong solar forcing. By comparing experiments with clear-sky conditions (clouds are transparent to radiation) to experiments with full-sky conditions (clouds are radiatively active), the authors find that the critical solar irradiance that is necessary to trigger a runaway greenhouse is increased from around 1.15–1.20 times the present-day total solar irradiance (TSI) on Earth S0 for clear-sky conditions to around 1.40–1.45S0 for full-sky conditions. Cloud thickness increases with TSI, leading to a substantially higher albedo, which in turn allows the climate to remain in equilibrium for markedly higher values of TSI. The results suggest that steady states with sea surface temperatures higher than 335K exist for a large range of TSI. The thickening clouds in these states do not reduce the outgoing longwave radiation any more, implying that the thickening of clouds increases only their shortwave effect. This mechanism allows the column to remain in balance even at high sea surface temperatures. The authors find double equilibria for both clear-sky and full-sky conditions, but the range for which they occur extends to considerably higher values of TSIs for full-sky conditions. Moreover, when clouds are included in the radiative transfer calculations, climate instabilities are no longer caused by longwave effects but by the cloud albedo effect.