Equilibrium Climate Sensitivity Estimated by Equilibrating Climate Models

Abstract. The mid-PlioceneWarm 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 inter-comparison 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 present an estimate of 1.8–3.6 °C, but there are considerable uncertainties surrounding the analysis. We consider the extent to which these uncertainties may be lessened in the next few years.

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Constraints on Climate Sensitivity from Space-Based Measurements of Low-Cloud Reflection

Journal of Climate ◽

10.1175/jcli-d-15-0897.1 ◽

2016 ◽

Vol 29 (16) ◽

pp. 5821-5835 ◽

Cited By ~ 51

Author(s):

Florent Brient ◽

Tapio Schneider

Keyword(s):

Climate Sensitivity ◽

Climate Models ◽

Temporal Variations ◽

Shortwave Radiation ◽

Information Theoretic ◽

Equilibrium Climate Sensitivity ◽

Tropical Oceans ◽

Low Clouds ◽

Low Cloud ◽

Well Models

Abstract Physical uncertainties in global-warming projections are dominated by uncertainties about how the fraction of incoming shortwave radiation that clouds reflect will change as greenhouse gas concentrations rise. Differences in the shortwave reflection by low clouds over tropical oceans alone account for more than half of the variance of the equilibrium climate sensitivity (ECS) among climate models, which ranges from 2.1 to 4.7 K. Space-based measurements now provide an opportunity to assess how well models reproduce temporal variations of this shortwave reflection on seasonal to interannual time scales. Here such space-based measurements are used to show that shortwave reflection by low clouds over tropical oceans decreases robustly when the underlying surface warms, for example, by −(0.96 ± 0.22)% K−1 (90% confidence level) for deseasonalized variations. Additionally, the temporal covariance of low-cloud reflection with temperature in historical simulations with current climate models correlates strongly (r = −0.67) with the models’ ECS. Therefore, measurements of temporal low-cloud variations can be used to constrain ECS estimates based on climate models. An information-theoretic weighting of climate models by how well they reproduce the measured deseasonalized covariance of shortwave cloud reflection with temperature yields a most likely ECS estimate around 4.0 K; an ECS below 2.3 K becomes very unlikely (90% confidence).

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Revisiting the relationship between dynamical sensitivity and climate sensitivity in the Southern hemisphere

10.5194/egusphere-egu2020-16108 ◽

2020 ◽

Author(s):

Thomas Wood ◽

Amanda Maycock ◽

Christine McKenna ◽

Andreas Chrysanthou ◽

John Fyfe ◽

...

Keyword(s):

Southern Hemisphere ◽

Climate Sensitivity ◽

Climate Models ◽

Stratospheric Ozone ◽

Storm Track ◽

Temperature Gradients ◽

Cmip5 Models ◽

Positive Trend ◽

Equilibrium Climate Sensitivity ◽

The Relationship

The Southern Annular Mode (SAM) is the dominant mode of midlatitude atmospheric circulation variability in the Southern hemisphere. In the future, the SAM trend is expected to be the net result of opposing effects from increasing greenhouse gases (GHG) and ozone recovery. Different greenhouse gas scenarios, which induce different rates of surface and atmospheric temperature change, are therefore associated with different future SAM trends (Barnes et al., 2014). Since the magnitude of warming due to GHGs is an important component of this response, one might expect to find a relationship between equilibrium climate sensitivity (ECS) and future Southern hemisphere circulation trends. In CMIP5, the relationship between the SAM and the level of tropospheric warming across models was found to be strongest in the summer and autumn and could explain around 20% of the intermodel spread (Grise and Polvani, 2014). The spread is more strongly correlated with differences in meridional temperature gradients (Harvey et al., 2014).Many of the latest CMIP6 models show a larger equilibrium climate sensitivity (ECS) of up to ~5.5 K (Forster et al., 2019) compared to a maximum of ~4.7 K in CMIP5. This raises the important question of how a higher level of warming affects projections of the SH midlatitude circulation. In this study, we examine the response of the SAM in CMIP6 models and quantify its relationship to ECS and temperature gradients. Our starting hypothesis is that stronger surface warming will induce a larger increase in tropical free tropospheric temperatures, and hence all being equal, a larger tropics-to-pole temperature gradient and a more positive SAM trend. However, results show that despite the higher level of warming in the CMIP6 models, there is a smaller positive trend in SAM index than in CMIP5 indicating a different relationship between warming and midlatitude circulation trends in CMIP6. We attempt to explain potential reasons for these differences.References:Barnes, E.A., N.W. Barnes, and L.M. Polvani, 2014: Delayed Southern Hemisphere Climate Change Induced by Stratospheric Ozone Recovery, as Projected by the CMIP5 Models. J. Climate, 27, 852&#8211;867, https://doi.org/10.1175/JCLI-D-13-00246.1Forster, P.M., Maycock, A.C., McKenna, C.M. et al. (2019), Latest climate models confirm need for urgent mitigation. Nat. Clim. Chang. (2019) doi:10.1038/s41558-019-0660-0Grise, K. M., and Polvani, L. M. (2014),&#160;Is climate sensitivity related to dynamical sensitivity? A Southern Hemisphere perspective, Geophys. Res. Lett.,&#160;41, 534&#8211; 540, doi:10.1002/2013GL058466.Harvey, B.J., Shaffrey, L.C. & Woollings, T.J. (2014) Equator&#173;-to-&#173;pole temperature differences and the extra&#173;tropical storm track responses of the CMIP5 climate models, Clim Dyn, 43: 1171. https://doi.org/10.1007/s00382-013-1883-9

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Equilibrium Climate Sensitivity Obtained From Multimillennial Runs of Two GFDL Climate Models

Journal of Geophysical Research Atmospheres ◽

10.1002/2017jd027885 ◽

2018 ◽

Vol 123 (4) ◽

pp. 1921-1941 ◽

Cited By ~ 15

Author(s):

D. Paynter ◽

T. L. Frölicher ◽

L. W. Horowitz ◽

L. G. Silvers

Keyword(s):

Climate Sensitivity ◽

Climate Models ◽

Equilibrium Climate Sensitivity

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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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A Positive Iris Feedback: Insights from Climate Simulations with Temperature-Sensitive Cloud–Rain Conversion

Journal of Climate ◽

10.1175/jcli-d-18-0845.1 ◽

2019 ◽

Vol 32 (16) ◽

pp. 5305-5324 ◽

Cited By ~ 3

Author(s):

R. L. Li ◽

T. Storelvmo ◽

A. V. Fedorov ◽

Y.-S. Choi

Keyword(s):

Carbon Dioxide ◽

Surface Temperature ◽

Climate Sensitivity ◽

Climate Models ◽

Cirrus Clouds ◽

Climate Feedback ◽

Convective Precipitation ◽

Temperature Sensitive ◽

Equilibrium Climate Sensitivity ◽

Climate Simulations

AbstractEstimates for equilibrium climate sensitivity from current climate models continue to exhibit a large spread, from 2.1 to 4.7 K per carbon dioxide doubling. Recent studies have found that the treatment of precipitation efficiency in deep convective clouds—specifically the conversion rate from cloud condensate to rain Cp—may contribute to the large intermodel spread. It is common for convective parameterization in climate models to carry a constant Cp, although its values are model and resolution dependent. In this study, we investigate how introducing a potential iris feedback, the cloud–climate feedback introduced by parameterizing Cp to increase with surface temperature, affects future climate simulations within a slab ocean configuration of the Community Earth System Model. Progressively stronger dependencies of Cp on temperature unexpectedly increase the equilibrium climate sensitivity monotonically from 3.8 to up to 4.6 K. This positive iris feedback puzzle, in which a reduction in cirrus clouds increases surface temperature, is attributed to changes in the opacity of convectively detrained cirrus. Cirrus clouds reduced largely in ice content and marginally in horizontal coverage, and thus the positive shortwave cloud radiative feedback dominates. The sign of the iris feedback is robust across different cloud macrophysics schemes, which control horizontal cloud cover associated with detrained ice. These results suggest a potentially strong but highly uncertain connection among convective precipitation, detrained anvil cirrus, and the high cloud feedback in a climate forced by increased atmospheric carbon dioxide concentrations.

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Multivariate Estimations of Equilibrium Climate Sensitivity from Short Transient Warming Simulations

10.5194/egusphere-egu21-187 ◽

2021 ◽

Author(s):

Robbin Bastiaansen ◽

Henk Dijkstra ◽

Anna von der Heydt

Keyword(s):

Time Series ◽

Climate Sensitivity ◽

Climate Models ◽

Global Climate ◽

Equilibrium Temperature ◽

Global Climate Models ◽

Equilibrium Climate Sensitivity ◽

Wide Range ◽

Short Time

One of the most used metrics to gauge the effects of climate change is the equilibrium climate sensitivity, defined as the long-term (equilibrium) temperature increase resulting from instantaneous doubling of atmospheric CO2. Since global climate models cannot be fully equilibrated in practice, extrapolation techniques are used to estimate the equilibrium state from transient warming simulations. Because of the abundance of climate feedbacks &#8211; spanning a wide range of temporal scales &#8211; it is hard to extract long-term behaviour from short-time series; predominantly used techniques are only capable of detecting the single most dominant eigenmode, thus hampering their ability to give accurate long-term estimates. Here, we present an extension to those methods by incorporating data from multiple observables in a multi-component linear regression model. This way, not only the dominant but also the next-dominant eigenmodes of the climate system are captured, leading to better long-term estimates from short, non-equilibrated time series.

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Estimating Remaining Carbon Budgets Using Emulators of CMIP6 Models

10.21203/rs.3.rs-104000/v1 ◽

2020 ◽

Author(s):

Martin Rypdal ◽

Niklas Boers ◽

Hege-Beate Fredriksen ◽

Kai-Uwe Eiselt ◽

Andreas Johansen ◽

...

Keyword(s):

Climate Sensitivity ◽

Carbon Budget ◽

Climate Models ◽

Coupled Model ◽

Model Intercomparison ◽

Equilibrium Climate Sensitivity ◽

Uncertainty Range ◽

The Public ◽

Intercomparison Project ◽

Specific Temperature

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. We demonstrate that the leading cause of the revealed RCB uncertainty is the spread in the equilibrium climate sensitivity (ECS) among climate models. In the Coupled Model Intercomparison Project Phase 6 (CMIP6) ensemble, the models with the lower ECS predict an RCB that is twice as high as that of models with the higher ECS, for temperature targets between 1.5-3.0°C.

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Extreme sensitivity and climate tipping points

10.5194/egusphere-egu2020-4671 ◽

2020 ◽

Author(s):

Anna von der Heydt ◽

Peter Ashwin

Keyword(s):

Climate Sensitivity ◽

Climate Models ◽

Tipping Point ◽

Climate System ◽

Tipping Points ◽

Equilibrium Climate Sensitivity ◽

Future Global Warming ◽

Wide Range ◽

Extreme Sensitivity ◽

Global Mean

The equilibrium climate sensitivity (ECS) is widely used as a measure for possible future global warming. It has been determined from a wide range of climate models, observations and palaeoclimate records, however, it still remains relatively unconstrained. In particular, large values of warming as a consequence of atmospheric greenhouse gas increase cannot be excluded, with some of the most recent state-of-the-art climate models (CMIP6) supporting (much) more warming than previous generations of climate models. Moreover, a number of tipping elements have been identified within the climate system, some of which may affect the global mean temperature. Therefore, it is interesting to explore how the climate systems response (e.g. ECS) behaves when the system is close to a tipping point.&#160; A climate state close to a tipping point will have a degenerate linear response to perturbations, which can be associated with extreme values of the equilibrium climate sensitivity (ECS). In this talk we contrast linearized ('instantaneous') with fully nonlinear geometric ('two-point') notions of ECS, in both presence and absence of tipping points. For a stochastic energy balance model of the global mean surface temperature with two stable regimes, we confirm that tipping events cause the appearance of extremes in both notions of ECS. Moreover, multiple regimes with different mean sensitivities are visible in the two-point ECS. We confirm some of our findings in a physics-based multi-box model of the climate system.Reference P. Ashwin and A. S. von der Heydt (2019), Extreme Sensitivity and Climate Tipping Points, J. Stat. Phys.&#160; 370, 1166&#8211;24. http://doi.org/10.1007/s10955-019-02425-x.

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