An Assessment of Earth's Climate Sensitivity Using Multiple Lines of Evidence

Abstract

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.

Anthropogenic CO2 of High Emission Scenario Compensated After 3500 Years of Ocean Alkalinization With an Annually Constant Dissolution of 5 Pg of Olivine

“…They obtain for high cumulative emissions larger temperatures than the box model used here, and are thus more in line with a linear relationship in TCRE generally found for complex climate models. The long-term temperature response can be approximated if I assume an ECS of 2.6-3.9 according to Sherwood et al (2020) instead of the TCR of 2 K used here. This would lead to a long-term T for the full cumulative emissions of 8-12 K (instead of ∼6 K seen in Figure 7), more in agreement with SSP5-85-EXT-based warming of 8 K obtained with the simple climate model MAGICC (O'Neill et al, 2016) or of 12 K simulated with the MPI-ESM (Kleinen, personal communication).…”

Reduced global warming from CMIP6 projections when weighting models by performance and independence

“…Global mean temperature increase in particular is one of the most widely discussed effects of continuing climate change and the main focus of many public and political discussions. With the release of the new generation of CMIP6 models, this discussion has been sparked yet again, as several CMIP6 models show stronger warming than most of the earlier-generation CMIP5 models (Andrews et al, 2019;Gettelman et al, 2019;Golaz et al, 2019;Voldoire et al, 2019;Swart et al, 2019;Zelinka et al, 2020;Forster et al, 2020). This raises the question of whether these models are accurate representations of the climate system and what that means for the interpretation of the historical climate record and the expected change due to future anthropogenic emissions.…”

Multivariate Estimations of Equilibrium Climate Sensitivity From Short Transient Warming Simulations

“…On top of that, recent results of the new generation of global climate models show even higher sensitivities, possibly due to better representation of cloud formation when using finer spatial grids (Bacmeister et al, 2020;Zelinka et al, 2020;Andrews et al, 2019;Bony et al, 2015;Duffy et al, 2003;Govindasamy et al, 2003;Haarsma et al, 2016). Still, even these state-of-the-art climate models report significantly different climate sensitivities (Flynn & Mauritsen, 2020;Zelinka et al, 2020;Forster et al, 2020); moreover, estimates for a single model tend to have large uncertainties further hampering accurate pinpointing of the climate sensitivity (Rugenstein et al, 2020;Dai et al, 2020).…”