Attribution Analysis of Cloud Feedback



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Uncertainty on cloud feedback is the primary contributor to the large spread of equilibrium climate sensitivity (ECS) in climate models. In this study, we compare the short-term cloud feedback in climate models with observations, and evaluate the magnitude of long-term cloud feedback predicted by models.

Observations suggest that there are more low clouds in the planetary boundary layer in response to inter-annual surface warming, contributing a strong negative cloud feedback. The overall cloud optical depth decreases, contributing a positive cloud feedback. The overall cloud height in the free atmosphere increases, contributing a positive feedback. The total short-term cloud feedback in response to global surface warming is likely positive.

Climate models generally show a positive cloud feedback in response to surface temperature trend and variability. The spatial pattern of short-term and long-term cloud response is different. However, the vertical profiles of tropical cloud responses to tropical surface temperature trend and variability are the same.

Uncertainty on low cloud amount is the primary source of the large spread in model predicted cloud feedback. Observations suggest that the tropical low cloud fraction increases in response to tropical surface temperature variability, but most climate models show a negative response. The disagreement between models and observations is induced by the poor estimated inversion strength (EIS)-low cloud fraction relationship in climate models. The observed positive short-term tropical low cloud fraction response results from EIS increase, and changes in large-scale dynamics have little impact on the short-term low cloud response.

Most climate models suggest that tropical EIS will increase under long-term global warming. If the EIS-low cloud fraction relationship holds under global warming, it is likely that the tropical low cloud fraction change is non-negative. Climate models without significant negative low cloud fraction change suggest that the cloud feedback is 0-0.5 W/m^(2)/?C, and the corresponding ECS is 1.5-3.5 ?C.