Estimating Remaining Carbon Budgets Using Emulators of CMIP6 Models

Abstract A remaining carbon budget (RCB) estimates how much CO2 we can emit and still reach a specific temperature target. The RCB concept is attractive since it easily communicates to the public and policymakers, but RCBs are also subject to uncertainties. The expected warming levels for a given carbon budget has a wide uncertainty range, which we show here to increase with less ambitious targets, i.e., with higher CO2 emissions and temperatures. Leading causes of RCB uncertainty are the future non-CO2 emissions, Earth system feedbacks, and the spread in the climate sensitivity among climate models. The latter is investigated in this paper, using simple emulators of Earth System Models in the Coupled Model Intercomparison Project Phase 6 (CMIP6) ensemble. It is shown that the transient climate response to cumulative emissions of carbon (TCRE) is approximately proportional to the effective equilibrium climate sensitivity (ECS). For temperature targets between 1.5-3.0 degrees C, the models exhibiting low ECS increase RCB by a factor two compared to those with high sensitivity, suggesting that observational constraints imposed on the ECS in the model ensemble also will reduce uncertainty in the RCB estimates.

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Effective radiative forcing and adjustments in CMIP6 models

Atmospheric Chemistry and Physics ◽

10.5194/acp-20-9591-2020 ◽

2020 ◽

Vol 20 (16) ◽

pp. 9591-9618 ◽

Cited By ~ 9

Author(s):

Christopher J. Smith ◽

Ryan J. Kramer ◽

Gunnar Myhre ◽

Kari Alterskjær ◽

William Collins ◽

...

Keyword(s):

Climate Sensitivity ◽

Radiative Forcing ◽

Climate Models ◽

Coupled Model ◽

Internal Variability ◽

Cmip5 Models ◽

Model Intercomparison ◽

Aerosol Forcing ◽

Intercomparison Project ◽

Effective Radiative Forcing

Abstract. The effective radiative forcing, which includes the instantaneous forcing plus adjustments from the atmosphere and surface, has emerged as the key metric of evaluating human and natural influence on the climate. We evaluate effective radiative forcing and adjustments in 17 contemporary climate models that are participating in the Coupled Model Intercomparison Project (CMIP6) and have contributed to the Radiative Forcing Model Intercomparison Project (RFMIP). Present-day (2014) global-mean anthropogenic forcing relative to pre-industrial (1850) levels from climate models stands at 2.00 (±0.23) W m−2, comprised of 1.81 (±0.09) W m−2 from CO2, 1.08 (± 0.21) W m−2 from other well-mixed greenhouse gases, −1.01 (± 0.23) W m−2 from aerosols and −0.09 (±0.13) W m−2 from land use change. Quoted uncertainties are 1 standard deviation across model best estimates, and 90 % confidence in the reported forcings, due to internal variability, is typically within 0.1 W m−2. The majority of the remaining 0.21 W m−2 is likely to be from ozone. In most cases, the largest contributors to the spread in effective radiative forcing (ERF) is from the instantaneous radiative forcing (IRF) and from cloud responses, particularly aerosol–cloud interactions to aerosol forcing. As determined in previous studies, cancellation of tropospheric and surface adjustments means that the stratospherically adjusted radiative forcing is approximately equal to ERF for greenhouse gas forcing but not for aerosols, and consequentially, not for the anthropogenic total. The spread of aerosol forcing ranges from −0.63 to −1.37 W m−2, exhibiting a less negative mean and narrower range compared to 10 CMIP5 models. The spread in 4×CO2 forcing has also narrowed in CMIP6 compared to 13 CMIP5 models. Aerosol forcing is uncorrelated with climate sensitivity. Therefore, there is no evidence to suggest that the increasing spread in climate sensitivity in CMIP6 models, particularly related to high-sensitivity models, is a consequence of a stronger negative present-day aerosol forcing and little evidence that modelling groups are systematically tuning climate sensitivity or aerosol forcing to recreate observed historical warming.

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Statistisches Downscaling von CMIP6 Projektionen mit EPISODES

10.5194/dkt-12-45 ◽

2020 ◽

Author(s):

Philip Lorenz ◽

Frank Kreienkamp ◽

Tobias Geiger

Keyword(s):

Climate Sensitivity ◽

Coupled Model ◽

Model Intercomparison ◽

Equilibrium Climate Sensitivity ◽

Intercomparison Project

<p>Die Ergebnisse der Klimamodellierung, die im Rahmen des j&#252;ngsten Coupled Model Intercomparison Project (CMIP6) durchgef&#252;hrt wurden, zeigen signifikante Ver&#228;nderungen der modellspezifischen Gleichgewichtsklimaempfindlichkeit (Equilibrium Climate Sensitivity, ECS) im Vergleich zum Vorg&#228;ngerprojekt CMIP5. Die neueren Versionen vieler globaler Klimamodelle (GCMs) weisen h&#246;here ECS-Werte auf, die zu einer st&#228;rkeren globalen Erw&#228;rmung f&#252;hren als zuvor berechnet. Gleichzeitig ist die Multi-GCM-Streuung von ECS unter CMIP6 deutlich gr&#246;&#223;er als unter CMIP5.</p> <p>Ein Teil der im Rahmen von CMIP6 durchgef&#252;hrten Klimaprojektionen wurden mittels der am DWD entwickelten statistisch-empirischen Downscaling-Methode EPISODES f&#252;r das Gebiet von Deutschland regionalisiert. Diese Ergebnisse wurden mit vergleichbaren Datens&#228;tzen der CMIP5-L&#228;ufe verglichen. Die Ergebnisse dieser Analysen werden vorgestellt.</p>

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Could the Pliocene constrain the equilibrium climate sensitivity?

Climate of the Past ◽

10.5194/cp-12-1591-2016 ◽

2016 ◽

Vol 12 (8) ◽

pp. 1591-1599 ◽

Cited By ~ 8

Author(s):

J. C. Hargreaves ◽

J. D. Annan

Keyword(s):

Climate Sensitivity ◽

Atmospheric Carbon Dioxide ◽

Climate Models ◽

Climate Model ◽

Temperature Anomaly ◽

Model Intercomparison ◽

Equilibrium Climate Sensitivity ◽

Intercomparison Project ◽

First Time ◽

Industrial State

Abstract. The mid-Pliocene Warm Period (mPWP) is the most recent interval in which atmospheric carbon dioxide was substantially higher than in modern pre-industrial times. It is, therefore, a potentially valuable target for testing the ability of climate models to simulate climates warmer than the pre-industrial state. The recent Pliocene Model Intercomparison Project (PlioMIP) presented boundary conditions for the mPWP and a protocol for climate model experiments. Here we analyse results from the PlioMIP and, for the first time, discuss the potential for this interval to usefully constrain the equilibrium climate sensitivity. We observe a correlation in the ensemble between their tropical temperature anomalies at the mPWP and their equilibrium sensitivities. If the real world is assumed to also obey this relationship, then the reconstructed tropical temperature anomaly at the mPWP can in principle generate a constraint on the true sensitivity. Directly applying this methodology using available data yields a range for the equilibrium sensitivity of 1.9–3.7 °C, but there are considerable additional uncertainties surrounding the analysis which are not included in this estimate. We consider the extent to which these uncertainties may be better quantified and perhaps lessened in the next few years.

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Experimental protocol for sealevel projections from ISMIP6 standalone ice sheet models

10.5194/tc-2019-322 ◽

2020 ◽

Cited By ~ 2

Author(s):

Sophie Nowicki ◽

Antony J. Payne ◽

Heiko Goelzer ◽

Helene Seroussi ◽

William H. Lipscomb ◽

...

Keyword(s):

Sea Level ◽

Climate Models ◽

Ice Sheet ◽

Coupled Model ◽

Sea Level Change ◽

Model Intercomparison ◽

Level Change ◽

Ice Sheet Modeling ◽

Emissions Scenarios ◽

Intercomparison Project

Abstract. Projection of the contribution of ice sheets to sea-level change as part of the Coupled Model Intercomparison Project – phase 6 (CMIP6) takes the form of simulations from coupled ice-sheet-climate models and standalone ice sheet models, overseen by the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). This paper describes the experimental setup for process-based sea-level change projections to be performed with standalone Greenland and Antarctic ice sheet models in the context of ISMIP6. The ISMIP6 protocol relies on a suite of polar atmospheric and oceanic CMIP-based forcing for ice sheet models, in order to explore the uncertainty in projected sea-level change due to future emissions scenarios, CMIP models, ice sheet models, and parameterizations for ice-ocean interactions. We describe here the approach taken for defining the suite of ISMIP6 standalone ice sheet simulations, document the experimental framework and implementation, as well as present an overview of the ISMIP6 forcing to be used by participating ice sheet modeling groups.

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AerChemMIP: quantifying the effects of chemistry and aerosols in CMIP6

Geoscientific Model Development ◽

10.5194/gmd-10-585-2017 ◽

2017 ◽

Vol 10 (2) ◽

pp. 585-607 ◽

Cited By ~ 66

Author(s):

William J. Collins ◽

Jean-François Lamarque ◽

Michael Schulz ◽

Olivier Boucher ◽

Veronika Eyring ◽

...

Keyword(s):

Air Quality ◽

Atmospheric Chemistry ◽

Radiative Forcing ◽

Climate Models ◽

Coupled Model ◽

Climate Impacts ◽

Atmospheric Composition ◽

Historical Period ◽

Model Intercomparison ◽

Intercomparison Project

Abstract. The Aerosol Chemistry Model Intercomparison Project (AerChemMIP) is endorsed by the Coupled-Model Intercomparison Project 6 (CMIP6) and is designed to quantify the climate and air quality impacts of aerosols and chemically reactive gases. These are specifically near-term climate forcers (NTCFs: methane, tropospheric ozone and aerosols, and their precursors), nitrous oxide and ozone-depleting halocarbons. The aim of AerChemMIP is to answer four scientific questions. 1. How have anthropogenic emissions contributed to global radiative forcing and affected regional climate over the historical period? 2. How might future policies (on climate, air quality and land use) affect the abundances of NTCFs and their climate impacts? 3.How do uncertainties in historical NTCF emissions affect radiative forcing estimates? 4. How important are climate feedbacks to natural NTCF emissions, atmospheric composition, and radiative effects? These questions will be addressed through targeted simulations with CMIP6 climate models that include an interactive representation of tropospheric aerosols and atmospheric chemistry. These simulations build on the CMIP6 Diagnostic, Evaluation and Characterization of Klima (DECK) experiments, the CMIP6 historical simulations, and future projections performed elsewhere in CMIP6, allowing the contributions from aerosols and/or chemistry to be quantified. Specific diagnostics are requested as part of the CMIP6 data request to highlight the chemical composition of the atmosphere, to evaluate the performance of the models, and to understand differences in behaviour between them.

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Reduced complexity model intercomparison project phase 1: Protocol, results and initial observations

10.5194/gmd-2019-375 ◽

2020 ◽

Cited By ~ 1

Author(s):

Zebedee R. J. Nicholls ◽

Malte Meinshausen ◽

Jared Lewis ◽

Robert Gieseke ◽

Dietmar Dommenget ◽

...

Keyword(s):

Radiative Forcing ◽

Climate Models ◽

Phase 1 ◽

Coupled Model ◽

Surface Air Temperature ◽

Internal Variability ◽

Second Phase ◽

Model Intercomparison ◽

Intercomparison Project ◽

Reduced Complexity

Abstract. Here we present results from the first phase of the Reduced Complexity Model Intercomparison Project (RCMIP). RCMIP is a systematic examination of reduced complexity climate models (RCMs), which are used to complement and extend the insights from more complex Earth System Models (ESMs), in particular those participating in the Sixth Coupled Model Intercomparison Project (CMIP6). In Phase 1 of RCMIP, with 14 participating models namely ACC2, AR5IR (2 and 3 box versions), CICERO-SCM, ESCIMO, FaIR, GIR, GREB, Hector, Held et al. two layer model, MAGICC, MCE, OSCAR and WASP, we highlight the structural differences across various RCMs and show that RCMs are capable of reproducing global-mean surface air temperature (GSAT) changes of ESMs and historical observations. We find that some RCMs are capable of emulating the GSAT response of CMIP6 models to within a root-mean square error of 0.2 °C (of the same order of magnitude as ESM internal variability) over a range of scenarios. Running the same model configurations for both RCP and SSP scenarios, we see that the SSPs exhibit higher effective radiative forcing throughout the second half of the 21st Century. Comparing our results to the difference between CMIP5 and CMIP6 output, we find that the change in scenario explains approximately 46 % of the increase in higher end projected warming between CMIP5 and CMIP6. This suggests that changes in ESMs from CMIP5 to CMIP6 explain the rest of the increase, hence the higher climate sensitivities of available CMIP6 models may not be having as large an impact on GSAT projections as first anticipated. A second phase of RCMIP will complement RCMIP Phase 1 by exploring probabilistic results and emulation in more depth to provide results available for the IPCC's Sixth Assessment Report author teams.

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On the Climate Sensitivity and Historical Warming Evolution in Recent Coupled Model Ensembles

10.5194/acp-2019-1175 ◽

2020 ◽

Author(s):

Clare Marie Flynn ◽

Thorsten Mauritsen

Keyword(s):

Climate Sensitivity ◽

Climate Models ◽

Coupled Model ◽

Cmip5 Models ◽

Antarctic Sea Ice ◽

Intercomparison Project ◽

Future Warming ◽

Model Treatment ◽

Systematic Shift ◽

Model Ensembles

Abstract. The Earth's equilibrium climate sensitivity (ECS) to a doubling of atmospheric CO2, along with the transient 35 climate response (TCR) and greenhouse gas emissions pathways, determines the amount of future warming. Coupled climate models have in the past been important tools to estimate and understand ECS. ECS estimated from Coupled Model Intercomparison Project Phase 5 (CMIP5) models lies between 2.0 and 4.7 K (mean of 3.2 K), whereas in the latest CMIP6 the spread has increased: 1.8–5.5 K (mean of 3.7 K), with 5 out of 25 models exceeding 5 K. It is thus pertinent to understand the causes underlying this shift. Here we compare the CMIP5 and CMIP6 model ensembles, and find a systematic shift between CMIP eras to be unexplained as a process of random sampling from modeled forcing and feedback distributions. Instead, shortwave feedbacks shift towards more positive values, in particular over the Southern Ocean, driving the shift towards larger ECS values in many of the models. These results suggest that changes in model treatment of mixed-phase cloud processes and changes to Antarctic sea ice representation are likely causes of the shift towards larger ECS. Somewhat surprisingly, CMIP6 models exhibit less historical warming than CMIP5 models; the evolution of the warming suggests, however, that several of the models apply too strong aerosol cooling resulting in too weak mid 20th Century warming compared to the instrumental record.

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Experimental protocol for sea level projections from ISMIP6 stand-alone ice sheet models

The Cryosphere ◽

10.5194/tc-14-2331-2020 ◽

2020 ◽

Vol 14 (7) ◽

pp. 2331-2368 ◽

Cited By ~ 9

Author(s):

Sophie Nowicki ◽

Heiko Goelzer ◽

Hélène Seroussi ◽

Anthony J. Payne ◽

William H. Lipscomb ◽

...

Keyword(s):

Sea Level ◽

Climate Models ◽

Ice Sheet ◽

Coupled Model ◽

Sea Level Change ◽

Model Intercomparison ◽

Level Change ◽

Ice Sheet Modeling ◽

Emissions Scenarios ◽

Intercomparison Project

Abstract. Projection of the contribution of ice sheets to sea level change as part of the Coupled Model Intercomparison Project Phase 6 (CMIP6) takes the form of simulations from coupled ice sheet–climate models and stand-alone ice sheet models, overseen by the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). This paper describes the experimental setup for process-based sea level change projections to be performed with stand-alone Greenland and Antarctic ice sheet models in the context of ISMIP6. The ISMIP6 protocol relies on a suite of polar atmospheric and oceanic CMIP-based forcing for ice sheet models, in order to explore the uncertainty in projected sea level change due to future emissions scenarios, CMIP models, ice sheet models, and parameterizations for ice–ocean interactions. We describe here the approach taken for defining the suite of ISMIP6 stand-alone ice sheet simulations, document the experimental framework and implementation, and present an overview of the ISMIP6 forcing to be used by participating ice sheet modeling groups.

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The Pliocene Model Intercomparison Project Phase 2: large-scale climate features and climate sensitivity

Climate of the Past ◽

10.5194/cp-16-2095-2020 ◽

2020 ◽

Vol 16 (6) ◽

pp. 2095-2123 ◽

Cited By ~ 4

Author(s):

Alan M. Haywood ◽

Julia C. Tindall ◽

Harry J. Dowsett ◽

Aisling M. Dolan ◽

Kevin M. Foley ◽

...

Keyword(s):

Climate Sensitivity ◽

Large Scale ◽

Climate Models ◽

Temperature Response ◽

Surface Air Temperature ◽

System Response ◽

Phase 2 ◽

Model Intercomparison ◽

Intergovernmental Panel ◽

Intercomparison Project

Abstract. The Pliocene epoch has great potential to improve our understanding of the long-term climatic and environmental consequences of an atmospheric CO2 concentration near ∼400 parts per million by volume. Here we present the large-scale features of Pliocene climate as simulated by a new ensemble of climate models of varying complexity and spatial resolution based on new reconstructions of boundary conditions (the Pliocene Model Intercomparison Project Phase 2; PlioMIP2). As a global annual average, modelled surface air temperatures increase by between 1.7 and 5.2 ∘C relative to the pre-industrial era with a multi-model mean value of 3.2 ∘C. Annual mean total precipitation rates increase by 7 % (range: 2 %–13 %). On average, surface air temperature (SAT) increases by 4.3 ∘C over land and 2.8 ∘C over the oceans. There is a clear pattern of polar amplification with warming polewards of 60∘ N and 60∘ S exceeding the global mean warming by a factor of 2.3. In the Atlantic and Pacific oceans, meridional temperature gradients are reduced, while tropical zonal gradients remain largely unchanged. There is a statistically significant relationship between a model's climate response associated with a doubling in CO2 (equilibrium climate sensitivity; ECS) and its simulated Pliocene surface temperature response. The mean ensemble Earth system response to a doubling of CO2 (including ice sheet feedbacks) is 67 % greater than ECS; this is larger than the increase of 47 % obtained from the PlioMIP1 ensemble. Proxy-derived estimates of Pliocene sea surface temperatures are used to assess model estimates of ECS and give an ECS range of 2.6–4.8 ∘C. This result is in general accord with the ECS range presented by previous Intergovernmental Panel on Climate Change (IPCC) Assessment Reports.

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