Spatial Patterns of Cloud Trends from SBUV Observations Compared with Simulated Trends of Clouds and Ssts from CMIP6 Models Since 1980

We produce a proxy Shortwave (SW) cloud albedo from UV satellite observations. Near global mean (60^oS to 60^oN) temporal trends of SW cloud+aerosol albedo from our long-term (1980-2013), well-calibrated observations are compared with individual CMIP6 realizations. Most, but not all, historical models fail to simulate our observed spatial patterns in cloud trends over the Pacific. Our observations show increasing cloudiness over the eastern equatorial Pacific and off the coast of South America, consistent with recent observed cooling SSTs over these areas. We focus on four historical models that qualitatively simulate our observed spatial patterns in cloud albedo over the Pacific. Model minus observational trends are plotted against the historical CMIP6 model’s Equilibrium Climate Sensitivity (ECS). Those four historical models estimate an ECS of 2.2-2.3 ^oC . Linear fits are within the range of the observed of uncertainty up to an ECS of 4.2 ^oC; this is our upper bound. Our proxy cloud+aerosol albedo record will not capture the expected reduction in albedo due to phase changes from ice crystals to liquid droplets as the climate warms.

The UV measurements used in this study are observed by the series of Solar Backscatter UV (SBUV) instruments onboard the Nimbus-7, NOAA-9, -11, -14, -16, -17, -18, and -19 spacecraft. The satellite radiances are calibrated over the Antarctic and Greenland ice sheets. Cloud optical depth (COD) and Black sky cloud albedo are derived from the narrowband 340nm radiance measurements. The radiances are observed at different local times of the day, so we empirically adjust the CODs to a reference local time of 1:30pm. To directly compare with the CMIP6 model runs, we produce a proxy-Shortwave broadband cloud albedo product using an empirical approach trained on correlations between NOAA-18 observations and coincident SNY1deg CERES Shortwave cloud albedos.

Latitudinal variability of zonal mean trends of our SW cloud+aerosol albedo agrees with CERES variability trends as well as the Extended Pathfinder Atmospheres (Patmos-x) observational dataset especially in the SH.