Abstract
Abstract Previous work has shown that a convective cloud feedback can greatly increase high-latitude surface temperature upon the removal of sea ice and can keep sea ice from forming throughout polar night. This feedback activates at increased greenhouse gas concentrations. It may help to explain the warm “equable climates” of the late Cretaceous and early Paleogene eras (∼100 to ∼35 million years ago) and may be relevant for future climate under global warming. Here, the factors that determine the critical threshold CO2 concentration at which this feedback is active and the magnitude of the warming caused by the feedback are analyzed using both a highly idealized model and NCAR’s single-column atmospheric model (SCAM) run under Arctic-like conditions. The critical CO2 is particularly important because it helps to establish the relevance of the feedback for past and future climates. Both models agree that increased heat flux into the high latitudes at low altitudes generally decreases the critical CO2. Increases in oceanic heat transport and in solar radiation absorbed during the summer should cause a sharp decrease in the critical CO2, but the effect of increases in atmospheric heat transport depends on its vertical distribution. It is furthermore found (i) that if the onset of convection produces more clouds and moisture, the critical CO2 should decrease, and the maximum temperature increase caused by the convective cloud feedback should increase and (ii) that reducing the depth of convection reduces the critical CO2 but has little effect on the maximum temperature increase caused by the convective cloud feedback. These results should help with interpretation of the strength and onset of the convective cloud feedback as found, for example, in Intergovernmental Panel on Climate Change (IPCC) coupled ocean–atmosphere models with different cloud and convection schemes.
Highlights
Cloud feedbacks represent the most important source of uncertainty in the climate system (Cess et al 1990, 1996; Baker 1997; Murphy et al 2004; Stainforth et al 2005; Soden and Held 2006)
The critical CO2 is important because it determines whether the convective cloud feedback could have been active during periods of equable climate and whether it could be active in a future climate under global warming
To the extent that the surface heat flux in these models can be interpreted as the winter release of heat stored by the ocean during the summer, this suggests that more clouds during summer, which reflect solar radiation and reduce the amount of solar radiation absorbed by the surface, should lead to a higher critical CO2
Summary
Cloud feedbacks represent the most important source of uncertainty in the climate system (Cess et al 1990, 1996; Baker 1997; Murphy et al 2004; Stainforth et al 2005; Soden and Held 2006). A related suggestion was briefly made by Sloan et al (1999) and Huber and Sloan (1999) In this proposed feedback, an initial warming leads to destabilization of the highlatitude atmosphere to convection, causing convection, which results in convective clouds and increased atmospheric moisture, both of which trap outgoing longwave radiation and lead to further warming. An initial warming leads to destabilization of the highlatitude atmosphere to convection, causing convection, which results in convective clouds and increased atmospheric moisture, both of which trap outgoing longwave radiation and lead to further warming Over ocean, this feedback should occur preferentially during winter Hereafter AWT) because during summer marine boundary layer clouds block low-level atmospheric solar absorption, so that solar absorption occurs preferentially in the midtroposphere and stabilizes the lower atmosphere
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