Context for interpreting equilibrium climate sensitivity and transient climate response from the CMIP6 Earth system models

Meehl, Gerald A.; Senior, C. A.; Eyring, Veronika; Flato, Gregory M.; Lamarque, Jean‐François; Stouffer, Ronald J.; Taylor, Karl E.; Schlund, Manuel

doi:10.1126/sciadv.aba1981

Cited by 467 publications

(433 citation statements)

References 66 publications

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“…In 1979, the Charney report determined an ECS range of 1.5 to 4.5 K for the Earth system (Charney et al, 1979). This range has not changed substantially over time (Meehl et al, 2020). In the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5), the reported likely range of ECS based on multiple lines of evidence is still between 1.5 and 4.5 K, whereas ECS calculated from climate and Earth system models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5, Taylor et al (2012)) supporting AR5 is ranging between 2.1 and 4.7 K (Collins et al, 2013).…”

Section: Introductionmentioning

confidence: 92%

Emergent constraints on Equilibrium Climate Sensitivity in CMIP5: do they hold for CMIP6?

Schlund¹,

Lauer²,

Gentine³

et al. 2020

Preprint

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Abstract. An important metric for temperature projections is the equilibrium climate sensitivity (ECS) which is defined as the global mean surface air temperature change caused by a doubling of the atmospheric CO2 concentration. The range for ECS assessed by the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report is between 1.5 and 4.5 K and has not decreased over the last decades. Among other methods, emergent constraints are potentially promising approaches to reduce the range of ECS by combining observations and output from Earth System Models (ESMs). In this study, we systematically analyze 11 published emergent constraints on ECS that have mostly been derived from models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) project. These emergent constraints are – except for one that is based on temperature variability – all directly or indirectly based on cloud processes, which stem the major source of uncertainty in ECS. The focus of the study is on testing if these emergent constraints hold for ESMs participating in the new Phase 6 (CMIP6). Since none of the emergent constraints considered here has been derived using the CMIP6 ensemble, CMIP6 can be used for cross-checking of the emergent constraints on a new model ensemble. The application of the emergent constraints to CMIP6 data shows a large decrease of the correlation coefficients for most emergent relationships, indicating that nearly all of the constraints are less skillful in predicting ECS than they were in CMIP5. Many do not appear sufficiently skillful to be useful in constraining ECS, and several of them do not pass a significance test. This is likely because of changes in the representation of cloud processes from CMIP5 to CMIP6, but may in some cases also be due to spurious statistical relationships or a too small number of models in the ensemble the emergent constraint was originally derived from. The emergent-constrained best estimate of ECS increased from CMIP5 to CMIP6, with a best estimate range of 2.97–3.88 K for CMIP5 and 3.41–4.36 K for CMIP6. This can be at least partly explained by the increased number of high-ECS models in CMIP6 with an ECS above 4.5 K without a corresponding change in the constraint predictors. Our results support previous studies concluding that emergent constraints should be based on an independently verifiable physical mechanism, and that emergent constraints focusing on specific processes contributing to ECS are more promising than emergent constraints based on statistical model analysis.

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Section: Introductionmentioning

confidence: 92%

Emergent constraints on Equilibrium Climate Sensitivity in CMIP5: do they hold for CMIP6?

Schlund¹,

Lauer²,

Gentine³

et al. 2020

Preprint

Self Cite

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“…The larger warming resulting from the CMIP6 experiments is a combination of different forcings and the presence among the new ensemble of models with higher climate sensitivities than the members of the previous generations. The higher climate sensitivities in CMIP6 compared to CMIP5 (Meehl et al, 2020;Zelinka et al, 2020) become more critical for higher forcings, explaining the differential in the higher warming across the range of new scenarios, with the largest difference evident for SSP5-8.5. Tokarska et al (2020) and Liang et al (2020) are at the time of writing the only published studies that sought to constrain the ensemble projections according to the evaluation of the ensemble historical behavior (Ribes et al, 2020 adopts a similar approach and is currently in revision).…”

Section: Comparison Of Climate Projections From Cmip6 and Cmip5 For Tmentioning

confidence: 99%

Climate model projections from the Scenario Model Intercomparison Project (ScenarioMIP) of CMIP6

Tebaldi¹,

Debeire²,

Eyring³

et al. 2020

Preprint

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Abstract. The Scenario Model Intercomparison Project (ScenarioMIP) defines and coordinates the primary future climate projections within the Coupled Model Intercomparison Project Phase 6 (CMIP6). This paper presents a range of its outcomes by synthesizing results from the participating global coupled Earth system models for concentration driven simulations. We limit our scope to the analysis of strictly geophysical outcomes: mainly global averages and spatial patterns of change for surface air temperature and precipitation. We also compare CMIP6 projections to CMIP5 results, especially for those scenarios that were designed to provide continuity across the CMIP phases, at the same time highlighting important differences in forcing composition, as well as in results. The range of future temperature and precipitation changes by the end of the century encompassing the Tier 1 experiments (SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5) and SSP1-1.9 spans a larger range of outcomes compared to CMIP5, due to higher warming (by 1.15 °C) reached at the upper end of the 5–95 % envelope of the highest scenario, SSP5-8.5. This is due to both the wider range of radiative forcing that the new scenarios cover and to higher climate sensitivities in some of the new models compared to their CMIP5 predecessors. Spatial patterns of change for temperature and precipitation averaged over models and scenarios have familiar features, and an analysis of their variations confirms model structural differences to be the dominant source of uncertainty. Models also differ with respect to the size and evolution of internal variability as measured by individual models' initial condition ensembles' spread, according to a set of initial condition ensemble simulations available under SSP3-7.0. The same experiments suggest a tendency for internal variability to decrease along the course of the century, a new result that will benefit from further analysis over a larger set of models. Benefits of mitigation, all else being equal in terms of societal drivers, appear clearly when comparing scenarios developed under the same SSP, but to which different degrees of mitigation have been applied. It is also found that a mild overshoot in temperature of a few decades in mid-century, as represented in SSP5-3.4OS, does not affect the end outcome in terms of temperature and precipitation changes by 2100, which return to the same level as those reached by the gradually increasing SSP4-3.4. Central estimates of the time at which the ensemble means of the different scenarios reach a given warming level show all scenarios reaching 1.5 °C of warming compared to the 1850–1900 baseline in the second half of the current decade, with the time span between slow and fast warming covering 20–28 years from present. 2 °C of warming is reached as early as the late '30s by the ensemble mean under SSP5-8.5, but as late as the late '50s under SSP1-2.6. The highest warming level considered, 5 °C, is reached only by the ensemble mean under SSP5-8.5, and not until the mid-90s.

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“…It is therefore critically important to understand the reasons for the increased span in ECS given by the latest generation of CMIP models. In addition to Meehl et al (2020), several modeling groups have already published studies confirming higher ECS values in their CMIP6 models (Andrews et al, 2019;Gettelman et al, 2019;Wyser et al, 2019).…”

Section: 1029/2019jd032321mentioning

confidence: 99%

Quantifying Progress Across Different CMIP Phases With the ESMValTool

et al. 2020

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More than 40 model groups worldwide are participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6), providing a new and rich source of information to better understand past, present, and future climate change. Here, we use the Earth System Model Evaluation Tool (ESMValTool) to assess the performance of the CMIP6 ensemble compared to the previous generations CMIP3 and CMIP5. While CMIP5 models did not capture the observed pause in the increase in global mean surface temperature between 1998 and 2013, the historical CMIP6 simulations agree well with the observed recent temperature increase, but some models have difficulties in reproducing the observed global mean surface temperature record of the second half of the twentieth century. While systematic biases in annual mean surface temperature and precipitation remain in the CMIP6 multimodel mean, individual models and high-resolution versions of the models show significant reductions in many long-standing biases. Some improvements are also found in the vertical temperature, water vapor, and zonal wind speed distributions, and root-mean-square errors for selected fields are generally smaller with reduced intermodel spread and higher average skill in the correlation patterns relative to observations. An emerging property of the CMIP6 ensemble is a higher effective climate sensitivity with an increased range between 2.3 and 5.6 K. A possible reason for this increase in some models is improvements in cloud representation resulting in stronger shortwave cloud feedbacks than in their predecessor versions.

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Context for interpreting equilibrium climate sensitivity and transient climate response from the CMIP6 Earth system models

Cited by 467 publications

References 66 publications

Emergent constraints on Equilibrium Climate Sensitivity in CMIP5: do they hold for CMIP6?

Emergent constraints on Equilibrium Climate Sensitivity in CMIP5: do they hold for CMIP6?

Climate model projections from the Scenario Model Intercomparison Project (ScenarioMIP) of CMIP6

Quantifying Progress Across Different CMIP Phases With the ESMValTool

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