09 November 2017

Observations Indicate that the Low-Cloud Feedback is Positive

Science

Recent studies using observations to infer tropical low-cloud feedbacks were reviewed by scientists at Lawrence Livermore National Laboratory and the Universities of Colorado and California at Los Angeles and San Diego. The scientists found that the feedback was positive in all reviewed studies and formed a consensus estimate that was compared to the results of large-eddy simulations and global climate models. The authors also discussed fundamental and implementation issues with the approach of using observations to infer the low-cloud feedback and identify action items that could help reduce uncertainty in the low-cloud feedback.

Impact

The uncertainty in the response of tropical low-clouds to climate warming (i.e. the low-cloud feedback) is a major source of uncertainty for predictions of how much Earth will warm in response to an increase of greenhouse gases. This review article demonstrates that observational studies, as well as separate studies by high-resolution low-cloud resolving models, indicate that the tropical low-cloud feedback is positive. This means that the climate models that simulate negative low-cloud feedback are most likely in error and thus their predictions for relatively smaller climate warming are likely in error.  Hopefully, scientists will use this evidence to develop models that are more consistent with observations.

Summary

The uncertain response of tropical low-clouds to climate warming – the cloud feedback – is a major cause of disparate estimates of climate sensitivity – the amount of warming resulting from doubling atmospheric carbon dioxide. In this review article published in the Survey of Geophysics, the authors demonstrate that five recent observational studies all predict a positive low-cloud feedback and are consistent with independent evidence from very high-resolution cloud modeling studies. This increases our confidence in the conclusion that the tropical low-cloud feedback is positive. While many climate models simulate a tropical low-cloud feedback of similar magnitude and sign, there are some models with negative tropical low-cloud feedback. These climate models are inconsistent with observations and the very high-resolution cloud models and their predictions of negative cloud feedback and relatively smaller climate sensitivity should be discounted.

Contact
Stephen Klein
LLNL
Publications
Norris, J. "Low-Cloud Feedbacks from Cloud-Controlling Factors: A Review." Surveys in Geophysics (2017). [10.1007/s10712-017-9433-3].