We assess evidence relevant to Earth's equilibrium climate sensitivity per doubling of atmospheric CO 2 , characterized by an effective sensitivity S. This evidence includes feedback process understanding, the historical climate record, and the paleoclimate record. An S value lower than 2 K is difficult to reconcile with any of the three lines of evidence. The amount of cooling during the Last Glacial Maximum provides strong evidence against values of S greater than 4.5 K. Other lines of evidence in combination also show that this is relatively unlikely. We use a Bayesian approach to produce a probability density function (PDF) for S given all the evidence, including tests of robustness to difficult-to-quantify uncertainties and different priors. The 66% range is 2.6-3.9 K for our Baseline calculation and remains within 2.3-4.5 K under the robustness tests; corresponding 5-95% ranges are 2.3-4.7 K, bounded by 2.0-5.7 K (although such high-confidence ranges should be regarded more cautiously). This indicates a stronger constraint on S than reported in past assessments, by lifting the low end of the range. This narrowing occurs because the three lines of evidence agree and are judged to be largely independent and because of greater confidence in understanding feedback processes and in combining evidence. We identify promising avenues for further narrowing the range in S, in particular using comprehensive models and process understanding to address limitations in the traditional forcing-feedback paradigm for interpreting past changes. Plain Language Summary Earth's global "climate sensitivity" is a fundamental quantitative measure of the susceptibility of Earth's climate to human influence. A landmark report in 1979 concluded that it probably lies between 1.5°C and 4.5°C per doubling of atmospheric carbon dioxide, assuming that other influences on climate remain unchanged. In the 40 years since, it has appeared difficult to reduce this uncertainty range. In this report we thoroughly assess all lines of evidence including some new developments. We find that a large volume of consistent evidence now points to a more confident view of a climate sensitivity near the middle or upper part of this range. In particular, it now appears extremely unlikely that the climate sensitivity could be low enough to avoid substantial climate change (well in excess of 2°C warming) under a high-emission future scenario. We remain unable to rule out that the sensitivity could be above 4.5°C per doubling of carbon dioxide levels, although this is not likely. Continued ©2020. American Geophysical Union. All Rights Reserved.

Feedbacks involving low-level clouds remain a primary cause of uncertainty in global climate model projections. This issue was addressed by examining changes in low-level clouds over the Northeast Pacific in observations and climate models. Decadal fluctuations were identified in multiple, independent cloud data sets, and changes in cloud cover appeared to be linked to changes in both local temperature structure and large-scale circulation. This observational analysis further indicated that clouds act as a positive feedback in this region on decadal time scales. The observed relationships between cloud cover and regional meteorological conditions provide a more complete way of testing the realism of the cloud simulation in current-generation climate models. The only model that passed this test simulated a reduction in cloud cover over much of the Pacific when greenhouse gases were increased, providing modeling evidence for a positive low-level cloud feedback.

[1] We examine multidecadal changes in surface downward shortwave (SW) radiation flux, total cloud cover, SW cloud effect, and related parameters over Europe during 1965Europe during -2004 using monthly gridded data from the Global Energy Balance Archive (GEBA), synoptic cloud reports, and the International Satellite Cloud Climatology Project (ISCCP). One key issue is distinguishing the effects of natural cloud variability from long-term anthropogenic aerosol influences on surface SW flux. Accordingly, we introduce the concept of cloud cover radiative effect (CCRE), defined as the change in downward SW flux produced by a change in cloud cover. The correlation between pan-European time series of CCRE anomalies and GEBA solar radiation anomalies is 0.88, indicating that cloud cover variability and associated changes in cloud albedo dominate SW radiation variability on monthly to decadal timescales. After these weather-related cloud effects are removed by subtracting CCRE anomalies from GEBA solar radiation anomalies via linear regression, a distinct decreasing trend followed by a distinct increasing trend remain in the residual time series. Depending on the method of trend calculation, pan-European residual flux declined by a statistically significant 2.7-3.5 W m À2 per decade during 1971-1986 and rose by a statistically significant 2.0-2.3 W m À2 per decade during 1987-2002. The fact that independent grid boxes exhibit mostly negative trends in the earlier period and mostly positive trends in the later period demonstrates that these longterm variations in SW flux are real and widespread over Europe. Changes in cloud cover cannot account for the trends in surface SW flux since cloud cover actually slightly decreased during 1971-1986 and slightly increased during 1987-2002. The most likely explanation is changes in anthropogenic aerosol emissions that led to more scattering and absorption of SW radiation during the earlier period of solar ''dimming'' and less scattering and absorption during the later period of solar ''brightening.'' Citation: Norris, J. R., and M. Wild (2007), Trends in aerosol radiative effects over Europe inferred from observed cloud cover, solar ''dimming,'' and solar ''brightening,''

Clouds substantially affect Earth's energy budget by reflecting solar radiation back to space and by restricting emission of thermal radiation to space. They are perhaps the largest uncertainty in our understanding of climate change, owing to disagreement among climate models and observational datasets over what cloud changes have occurred during recent decades and will occur in response to global warming. This is because observational systems originally designed for monitoring weather have lacked sufficient stability to detect cloud changes reliably over decades unless they have been corrected to remove artefacts. Here we show that several independent, empirically corrected satellite records exhibit large-scale patterns of cloud change between the 1980s and the 2000s that are similar to those produced by model simulations of climate with recent historical external radiative forcing. Observed and simulated cloud change patterns are consistent with poleward retreat of mid-latitude storm tracks, expansion of subtropical dry zones, and increasing height of the highest cloud tops at all latitudes. The primary drivers of these cloud changes appear to be increasing greenhouse gas concentrations and a recovery from volcanic radiative cooling. These results indicate that the cloud changes most consistently predicted by global climate models are currently occurring in nature.