Lessons on Climate Sensitivity From Past Climate Changes

Heydt, Anna S. von der; Dijkstra, Henk A.; Wal, R. S. W. van de; Caballero, Rodrigo; Crucifix, Michel; Foster, Gavin L.; Huber, Matthew; Köhler, Peter; Rohling, Eelco J.; Valdes, Paul J.; Ashwin, Peter; Bathiany, Sebastian; Berends, Constantijn J.; Bree, L.G.J. van; Ditlevsen, Peter; Ghil, Michael; Haywood, Alan M.; Katzav, Joel; Lohmann, Gerrit; Lohmann, Johannes; Lucarini, Valerio; Marzocchi, Alice; Pälike, Heiko; Baroni, Itzel Ruvalcaba; Simon, Dirk; Sluijs, Appy; Stap, Lennert B.; Tantet, Alexis; Viebahn, Jan; Ziegler, Martin

doi:10.1007/s40641-016-0049-3

Cited by 55 publications

(45 citation statements)

References 106 publications

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“…Nevertheless, this paleodata‐based analysis suggests that the equilibrium climate sensitivity for present‐day is more at the high end with respect to reported values in the IPCC AR5 report (e.g., Thematic Focus Element 6 in Stocker et al, ). Other paleostudies applying climate models of different complexity to various different climate background states, which have been up to 16 K warmer than the preindustrial climate, also found a state dependency in

S_{{[CO}_{2}, LI]}

with mean numbers ranging from 0.6 to 1.6 K W −1 m 2 (von der Heydt et al, ).…”

Section: Discussion Conclusion and Implicationsmentioning

confidence: 95%

A State‐Dependent Quantification of Climate Sensitivity Based on Paleodata of the Last 2.1 Million Years

et al. 2017

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The evidence from both data and models indicates that specific equilibrium climate sensitivity S[X]—the global annual mean surface temperature change (ΔTg) as a response to a change in radiative forcing X (ΔR[X])—is state dependent. Such a state dependency implies that the best fit in the scatterplot of ΔTg versus ΔR[X] is not a linear regression but can be some nonlinear or even nonsmooth function. While for the conventional linear case the slope (gradient) of the regression is correctly interpreted as the specific equilibrium climate sensitivity S[X], the interpretation is not straightforward in the nonlinear case. We here explain how such a state‐dependent scatterplot needs to be interpreted and provide a theoretical understanding—or generalization—how to quantify S[X] in the nonlinear case. Finally, from data covering the last 2.1 Myr we show that—due to state dependency—the specific equilibrium climate sensitivity which considers radiative forcing of CO2 and land ice sheet (LI) albedo, S[CO2,LI], is larger during interglacial states than during glacial conditions by more than a factor 2.

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S_{{[CO}_{2}, LI]}

with mean numbers ranging from 0.6 to 1.6 K W −1 m 2 (von der Heydt et al, ).…”

Section: Discussion Conclusion and Implicationsmentioning

confidence: 95%

A State‐Dependent Quantification of Climate Sensitivity Based on Paleodata of the Last 2.1 Million Years

et al. 2017

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“…Analyses of paleoreconstructions (Köhler et al, 2015; K2015 in the following) and paleoclimate simulations (Friedrich et al, 2016; F2016 in the following), covering the late Pleistocene, have suggested that climate sensitivity might not be a constant parameter of the climate system but a state-dependent variable that increases toward warmer climates. Most other studies on this topic indicate a similar behavior, including a review that covers a wide range of colder and warmer climate states (von der Heydt et al, 2016). However, there have also been studies using general circulation models (GCMs) or Earth system models of intermediate complexity (EMICs) which simulate an increase in climate sensitivity for colder than present-day climate (e.g., Colman & McAvaney, 2009;Kutzbach et al, 2013;Pfister & Stocker, 2017).…”

Section: Introductionmentioning

confidence: 86%

The Effect of Obliquity‐Driven Changes on Paleoclimate Sensitivity During the Late Pleistocene

Köhler

Knorr

Stap

et al. 2018

Geophysical Research Letters

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We reanalyze existing paleodata of global mean surface temperature ΔTg and radiative forcing ΔR of CO2 and land ice albedo for the last 800,000 years to show that a state‐dependency in paleoclimate sensitivity S, as previously suggested, is only found if ΔTg is based on reconstructions, and not when ΔTg is based on model simulations. Furthermore, during times of decreasing obliquity (periods of land ice sheet growth and sea level fall) the multimillennial component of reconstructed ΔTg diverges from CO2, while in simulations both variables vary more synchronously, suggesting that the differences during these times are due to relatively low rates of simulated land ice growth and associated cooling. To produce a reconstruction‐based extrapolation of S for the future, we exclude intervals with strong ΔTg‐CO2 divergence and find that S is less state‐dependent, or even constant state‐independent), yielding a mean equilibrium warming of 2–4 K for a doubling of CO2.

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“…This implies that ECS based on atmosphere-ocean climate models will underestimate warming on millennial timescales. If Earth system feedbacks are treated as additional forcings, as they generally are for example in ECS estimates from the Last Glacial Maximum, then the overall assessed paleoclimate constraints 7,8,446 also support the consensus range of ECS, including its upper bound, but do not constrain it further.…”

Section: Paleoclimatementioning

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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Lessons on Climate Sensitivity From Past Climate Changes

Cited by 55 publications

References 106 publications

A State‐Dependent Quantification of Climate Sensitivity Based on Paleodata of the Last 2.1 Million Years

A State‐Dependent Quantification of Climate Sensitivity Based on Paleodata of the Last 2.1 Million Years

The Effect of Obliquity‐Driven Changes on Paleoclimate Sensitivity During the Late Pleistocene

Beyond equilibrium climate sensitivity

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