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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.
