Slow climate mode reconciles historical and model-based estimates of climate sensitivity

Proistosescu, Cristian; Huybers, Peter

doi:10.1126/sciadv.1602821

Cited by 91 publications

(132 citation statements)

References 49 publications

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“…the Paris agreement). The United Nations Framework Convention on Climate Change (UNFCCC) has not adopted a pre-defined definition of GMST and the stronger long-term feedbacks found in analysis of CMIP5 simulations (Proistosescu and Huybers, 2017) operates on a time scale longer than the timescale for reaching 2°C. 25…”

Section: Discussionmentioning

confidence: 99%

“…First, recent analysis of time-varying feedbacks in AOGCMs simulations from Coupled Model Intercomparison Project Phase 5 (CMIP5) (Proistosescu and Huybers, 2017;Armour, 2017;Andrews et al, 2015) have indicated that in most AOGCMs the 10 net feedbacks become more positive over time as a new equilibrium is approached. This is most likely due to evolution of the pattern of sea surface temperature increase in the Pacific and Southern Ocean and associated cloud feedbacks.…”

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confidence: 99%

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Climate sensitivity estimates – sensitivity to radiative forcing time series and observational data

Skeie¹,

Berntsen²,

Aldrin³

et al. 2017

Preprint

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Abstract. Inferred Effective Climate Sensitivity (ECSinf) is estimated using a method combining radiative forcing (RF) time series and several series of observed ocean heat content (OHC) and near-surface temperature change in a Bayesian-15 framework using a simple energy balance model and a stochastic model. The model is updated compared to our previous analysis by using recent forcing estimates from IPCC, including OHC data for the deep ocean, and extending the time series to 2014. The mean value of the estimated ECSinf is 2.0°C, with a 90% credible interval of 1.2-3.1°C. The mean estimate has recently been shown to be consistent with the higher values for the equilibrium climate sensitivity estimated by climate models. We show a strong sensitivity of the estimated ECSinf to the choice of a priori RF time series, excluding pre-1950 20 data and the treatment of OHC data. Sensitivity analysis performed by merging the upper (0-700m) and the deep ocean OHC or using only one OHC data set (instead of four in the main analysis), both give an enhancement the mean ECSinf by about 50% from our best estimate.

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Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 99%

Climate sensitivity estimates – sensitivity to radiative forcing time series and observational data

Skeie¹,

Berntsen²,

Aldrin³

et al. 2017

Preprint

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“…A linearized forcing-feedback framework has often been used to study the global mean energy budget and assess despite its known temporal variations (e.g., Andrews et al, 2015;Armour, 2017;Armour et al, 2013;Forster & Gregory, 2006;GA16;Gregory et al, 2004;Mauritsen et al, 2013;Proistosescu & Huybers, 2017;Rose et al, 2014;Senior & Mitchell, 2000;Winton et al, 2010). Decadal variability of the energy budget has been shown to be particularly pronounced in both , and the cloud radiative effect (CRE;Figures 1a,1c,1e,and 1f,GA16;Z16).…”

Section: Introductionmentioning

confidence: 99%

The Diversity of Cloud Responses to Twentieth Century Sea Surface Temperatures

Silvers

Paynter

Zhao

2018

Geophysical Research Letters

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Low‐level clouds are shown to be the conduit between the observed sea surface temperatures (SST) and large decadal fluctuations of the top of the atmosphere radiative imbalance. The influence of low‐level clouds on the climate feedback is shown for global mean time series as well as particular geographic regions. The changes of clouds are found to be important for a midcentury period of high sensitivity and a late century period of low sensitivity. These conclusions are drawn from analysis of amip‐piForcing simulations using three atmospheric general circulation models (AM2.1, AM3, and AM4.0). All three models confirm the importance of the relationship between the global climate sensitivity and the eastern Pacific trends of SST and low‐level clouds. However, this work argues that the variability of the climate feedback parameter is not driven by stratocumulus‐dominated regions in the eastern ocean basins, but rather by the cloudy response in the rest of the tropics.

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“…The delayed Eastern tropical Pacific and Southern ocean warming in the observed historical period, combined with feedbacks changing as the warming pattern changes, plus the composition of the historical forcing and an underestimation of the observed warming (see Methods), imply that ECS estimates assuming constant λ are likely underestimated 3,4,54,[59][60][61][62] . In principle, climate models can be used to study how feedbacks vary, but different models show different changes in λ, so the bias in the ECS estimated from historical data when assuming constant λ may be anywhere from near zero to about a factor of two 54,59,60,65,68,92 . Accounting for changes in feedbacks and the observation issues largely resolve the apparent discrepancies between the estimates from the observed warming and those from comprehensive models.…”

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confidence: 99%

“…The product here results in a combined constraint that is very narrow and likely overconfident. When the individual PDFs of GCMs and paleoclimate are, just for illustration (Figure 5b), inflated in their lower and upper bounds to account for potential structural problems, state dependent feedbacks and dependency across the lines of evidence, and in addition the mismatch between the historically inferred and future sensitivity 4,59,60 is accounted for by extending the historical PDF upward (see Methods), the combined evidence from the three PDFs would still yield a rather narrow range constraining ECS to 2-7 4°C with a most likely value near 3°C. However, this toy model does not replace a full assessment.…”

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confidence: 99%

Beyond equilibrium climate sensitivity

2017

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Equilibrium climate sensitivity characterizes the Earth' long-term global temperature response to an increased atmospheric carbon dioxide concentration. It has reached almost iconic status as the single number that describes how severe climate change will be. The consensus 'likely' range for climate sensitivity of 1.5-4.5 o C today is the same as given by Jule Charney in 1979, but is now based on quantitative evidence from across the climate system, and through climate history. The quest to constrain climate sensitivity has revealed important insights into the timescales of the climate system response, natural variability, limitations in observations and climate models, but also concerns about the simple concepts underlying climate sensitivity and radiative forcing, which open avenues to better understand and constrain the climate response to forcing. Estimates of the transient climate response are better constrained by observed warming and are more relevant for predicting warming over the next decades. Newer metrics relating global warming directly to the total emitted CO2 show that in order to keep warming to within 2 o C, future CO2 emissions have to remain strongly limited, irrespective of climate sensitivity being at the high or low end.If we increase the amount of CO2 in the Earth's atmosphere and wait for climate to respond, how much warmer would surface temperatures eventually get? What seems like a simple but important question to ask given current and projected human-induced CO2 emissions, is an issue that scientists have struggled with since the first rough estimates were made more than a century ago 1,2 . To answer that question, starting in the 1960s, scientists have used energy balance arguments combined with observed changes in the global energy budget, evaluated comprehensive climate models against observations, and analyzed the relationship between external forcing and climate change over different climate states in the past (see Methods for a list of early publications). The idea of using those different lines of evidence has not changed, but progress in simulating climate, a longer and more accurate observed record of past warming, and better constrained paleoclimate reconstructions now offer more possibilities to evaluate and constrain models. However, recent research has pointed out previously unknown limitations in some of the concepts and assumptions underlying a single constant climate sensitivity 3 . While publications have appeared using various methods, arguably the most important conceptual recent insights is that feedbacks change with equilibration time, an insight that is based on studies in comprehensive climate models. Other recent insights show that the treatment of observations is important 4 . Knowing, to first order, how the global climate will warm in response to increased CO2 is critical: for unabated emissions, it is the difference between a hot and an extremely hot future. The value of halving the uncertainty in that projection may be in the trillions of dollars 5 . Here w...

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Slow climate mode reconciles historical and model-based estimates of climate sensitivity

Abstract: Estimates of climate sensitivity from models and observations are reconciled by accounting for slowly responding climate mode.

Cited by 91 publications

References 49 publications

Climate sensitivity estimates – sensitivity to radiative forcing time series and observational data

Climate sensitivity estimates – sensitivity to radiative forcing time series and observational data

The Diversity of Cloud Responses to Twentieth Century Sea Surface Temperatures

Beyond equilibrium climate sensitivity

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