21 September 2016

Will Low-level Clouds Reflect More Sunlight with Warming?

Satellite retrievals suggest that low-level clouds brighten too much with warming in many climate models

Science

LLNL scientists in the Cloud Processes Research Group compared how optical properties of clouds respond to warming in satellite observations and climate models.  The comparison reveals that compared to satellite observations, climate models over-predict the increase in the low-level clouds’ reflectivity with warming.

Impact

Most climate models predict that the reflectivity of clouds in the middle and high latitudes increases with warming, which dampens the local warming due to increased greenhouse gas concentrations. However, this study finds that the increase in the reflectivity is likely overestimated in many climate models, suggesting there would be more local warming than currently predicted in climate models if this bias did not exist.  

Summary

In most climate models, the optical depth of low-level clouds increases with warming poleward of 40°. In other words, the modeled clouds reflect back more sunlight to space with warming. Although models agree on the sign, they disagree on the magnitude of this response. Building on previous results that suggest that the cloud response to temperature is timescale invariant for low-level clouds, LLNL scientists examined how well the year-to-year response in cloud reflectivity agreed between climate models and satellite observations. They found that most climate models tended to overestimate the increase in cloud reflectivity with warming. Instead of predicting an increase in cloud reflectivity, satellite-based estimates predict that the low-level clouds become slightly less reflective with warming. Because the year-to-year cloud response in climate models matches the response due to long-term warming, this result has direct relevance to predicting how clouds will change with greenhouse gas-induced warming. The impact on changes in global temperature is likely small, but the results suggest that local warming in the middle and high latitudes from increased greenhouse gases are underestimated because of this bias in the models.

Contact
Christopher Terai
Lawrence Livermore National Laboratory (LLNL)
Publications
Terai, CR.  2016.  "Constraining the Low-Cloud Optical Depth Feedback at Middle and High Latitudes using Satellite Observations."  Journal of Geophysical Research: Atmospheres n/a–n/a, pp. n/a–n/a.  https://doi.org/10.1002/2016JD025233.