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  • 1
    Publication Date: 2014-05-05
    Description: The equilibrium (Charney) climate sensitivity, here indicated by Sa, is the equilibrium change in Earth’s global mean surface temperature due to a radiative forcing associated with a doubling of pCO2, the atmospheric CO2 concentration. Although known for decades, little progress has been made in constraining upper and lower limits for climate sensitivity. Originally, Sa was derived from climate models where the atmospheric CO2 concentration is doubled in typically about 100 years. Also palaeo data have been frequently used to determine Sa, and — if slow feedback processes are adequately taken into account — indicate a similar range as those based on climate models used in the IPCC. However, palaeo data usually span a much larger time than the 100 year model experiments. Here, we focus on the last 800 kyr, where climate variability has occurred on time scales ranging from the 100.000- year ice-age cycles to millennial-scale climate variations. The traditional linear and equilibrium concept of climate sensitivity as is applied in typical (short time scale) climate model simulations might not apply to the climate system’s non-stationary and non-linear response to changing forcing. One example is the background state dependency of the fast feedback processes. In this presentation, we assess the dependency of the fast feedback processes on the background climate state using data of the last 800 kyr and a conceptual climate model. Though still (locally) linear, we propose a different approach to estimate climate sensi- tivity which better accounts for a possible state dependency of the fast feedbacks. This approach uses local slopes of temperature versus radiative perturbation and is most suitable for palaeo-data spanning a range of background climate states. We find the specific climate sensitivities generally lower during cold (glacial) than during warm periods. Within the conceptual climate model we further estimate how the background state-dependency of the fast feed- back processes might affect the distributions of feedback factors and projected temperature change when noise is included in the forcing of the model. In particular, we investigate the appearance of small but finite probabili- ties of a very large temperature response and how the shape of the response distribution might be related to state dependency.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 2
    Publication Date: 2015-12-21
    Repository Name: EPIC Alfred Wegener Institut
    Type: PANGAEA Documentation , NonPeerReviewed
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  • 3
    Publication Date: 2015-04-20
    Description: A still open question is how equilibrium warming in response to increasing radiative forcing (equilibrium climate sensitivity S) is depending on background climate. We here bring paleo-data based evidence on the state-dependency of S by using CO2 proxy data together with model-based reconstruction of land ice albedo over the last 5 million years. We find that the land-ice albedo forcing depends non-linearly on the background climate, while any non-linearity of CO2 radiative forcings depends on the CO2 data set used. Over the last 2 million years the combined S_[CO2,LI] from CO2 and land-ice albedo forcing is state-dependent and during interglacials at least twice as high as during glacials, thus CO2 doubling leads to an interglacial warming of 5 K. In the Pliocene data uncertainties prevents a well-supported calculation, but our analysis suggests that S_[CO2,LI] during a land-ice free northern hemisphere was smaller than during interglacials of the Pleistocene.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 4
    Publication Date: 2018-03-15
    Description: A still open question is how equilibrium warming in response to increasing radiative forcing – the specific equilibrium climate sensitivity S – is depending on background climate. We here present paleo-data based evidence on the state-dependency of S, by using CO2 proxy data together with 3-D ice-sheet model-based reconstruction of land ice albedo over the last 5 million years (Myr). We find that the land-ice albedo forcing depends non-linearly on the background climate, while any non-linearity of CO2 radiative forcing depends on the CO2 data set used. This non-linearity was in similar approaches not accounted for due to previously more simplistic approximations of land-ice albedo radiative forcing being a linear function of sea level change. Important for the non-linearity between land-ice albedo and sea level is a latitudinal dependency in ice sheet area changes.In our setup, in which the radiative forcing of CO2 and of the land-ice albedo (LI) is combined, we find a state-dependency in the calculated specific equilibrium climate sensitivity S[CO2 ,LI] for most of the Pleistocene (last 2.1 Myr). During Pleistocene intermediate glaciated climates and interglacial periods S_[CO2,LI] is on average ∼45% larger than during Pleistocene full glacial conditions. In the Pliocene part of our analysis (2.6–5 Myr BP) the CO2 data uncertainties prevents a well-supported calculation for S_[CO2,LI], but our analysis suggests that during times without a large land-ice area in the Northern Hemisphere (e.g. before 2.82MyrBP) the specific equilibrium climate sensitivity S_[CO2,LI] was smaller than during interglacials of the Pleistocene. We thus find support for a previously proposed state-change in the climate system with the wide appearance of northern hemispheric ice sheets. This study points for the first time to a so far overlooked non-linearity in the land-ice albedo radiative forcing, which is important for similar paleo data-based approaches to calculate climate sensitivity. However, the implications of this study for a suggested warming under CO2 doubling are not yet entirely clear since the necessary corrections for other slow feedbacks are in detail unknown and the still existing uncertainties in the ice sheet simulations and global temperature reconstructions are large.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 5
    Publication Date: 2015-12-21
    Description: It is still an open question how equilibrium warming in response to increasing radiative forcing – the specific equilibrium climate sensitivity S – depends on background climate. We here present palaeodata-based evidence on the state dependency of S, by using CO2 proxy data together with a 3-D ice-sheet-model-based reconstruction of land ice albedo over the last 5 million years (Myr). We find that the land ice albedo forcing depends non-linearly on the background climate, while any non-linearity of CO2 radiative forcing depends on the CO2 data set used. This nonlinearity has not, so far, been accounted for in similar approaches due to previously more simplistic approximations, in which land ice albedo radiative forcing was a linear function of sea level change. The latitudinal dependency of icesheet area changes is important for the non-linearity between land ice albedo and sea level. In our set-up, in which the radiative forcing of CO2 and of the land ice albedo (LI) is combined, we find a state dependence in the calculated specific equilibrium climate sensitivity, STCO2,LIU, for most of the Pleistocene (last 2.1 Myr). During Pleistocene intermediate glaciated climates and interglacial periods, STCO2,LIU is on average � 45% larger than during Pleistocene full glacial conditions. In the Pliocene part of our analysis (2.6–5 MyrBP) the CO2 data uncertainties prevent a well-supported calculation for STCO2,LIU, but our analysis suggests that during times without a large land ice area in the Northern Hemisphere (e.g. before 2.82 MyrBP), the specific equilibrium climate sensitivity, STCO2,LIU, was smaller than during interglacials of the Pleistocene. We thus find support for a previously proposed state change in the climate system with the widespread appearance of northern hemispheric ice sheets. This study points for the first time to a so far overlooked non-linearity in the land ice albedo radiative forcing, which is important for similar palaeodata-based approaches to calculate climate sensitivity. However, the implications of this study for a suggested warming under CO2 doubling are not yet entirely clear since the details of necessary corrections for other slow feedbacks are not fully known and the uncertainties that exist in the ice-sheet simulations and global temperature reconstructions are large.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 6
    Publication Date: 2017-12-19
    Description: 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, math formula, is larger during interglacial states than during glacial conditions by more than a factor 2.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 7
    Publication Date: 2018-05-09
    Description: Climate sensitivity represents the global mean temperature change caused by changes in the radiative balance of climate; it is studied for both present/future (actuo) and past (paleo) climate variations, with the former based on instrumental records and/or various types of model simulations. Paleo-estimates are often considered informative for assessments of actuo-climate change caused by anthropogenic greenhouse forcing, but this utility remains debated because of concerns about the impacts of uncertainties, assumptions, and incomplete knowledge about controlling mechanisms in the dynamic climate system, with its multiple interacting feedbacks and their potential dependence on the climate background state. This is exacerbated by the need to assess actuo- and paleoclimate sensitivity over different timescales, with different drivers, and with different (data and/or model) limitations. Here, we visualize these impacts with idealized representations that graphically illustrate the nature of time-dependent actuo- and paleoclimate sensitivity estimates, evaluating the strengths, weaknesses, agreements, and differences of the two approaches. We also highlight priorities for future research to improve the use of paleo-estimates in evaluations of current climate change.
    Repository Name: EPIC Alfred Wegener Institut
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  • 8
    Publication Date: 2013-01-24
    Type: paper
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  • 9
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    In:  Geological Society Special Publication 355: 305-318.
    Publication Date: 2011-06-27
    Description: The existence of El Niño/Southern Oscillation (ENSO) variability in past climates is still debated. Based on evidence from geological records indicating a different long-term mean climate in the tropical Pacific, a permanent El Niño state has been hyothesized to exist prior to the Plio-Pleistocene transition. However, model studies of past climate and geological records suggest that ENSO variability has existed on Earth as far back as in the Eocene and the Miocene. In the early-to-middle Miocene, climate was not only warmer than today, but oceanic gateways such as the Indonesian Passage and the Central American Seaway established deep connections between the main ocean basins. Here, we analyse the effect of increased levels of atmospheric greenhouse gases and open tropical gateways on the amplitude, period and pattern of ENSO variability using results of fully coupled climate model simulations. While our model shows only small changes in ENSO variability under increased greenhouse gas levels, it suggests a significantly stronger and less frequent ENSO due to altered oceanic gateways. In particular, a deeper and more open Indonesian Passage does not prevent a Western Pacific warm pool from developing, but it allows the warm pool to shift into the Indian Ocean.
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  • 10
    Publication Date: 2018-02-20
    Description: We examine ocean changes in response to changes in paleogeography from the Cretaceous to present in an intermediate complexity model and in the fully coupled CCSM3 model. Greenhouse gas concentrations are kept constant to allow a focus on effects arising from changing continental configurations. We find consistent and significant geography-related Cenozoic cooling arising from the opening of Southern Ocean (SO) gateways. Both models show significant deep ocean cooling arising from tectonic evolution alone. Simulations employing continental configurations associated with greenhouse climates, namely the Turonian and the Eocene simulations, systematically exhibit warm deep ocean temperatures at elevated pCO2 close to 10 °C. In contrast, continental configurations associated with (later) icehouse climates are associated with cooler deep ocean temperatures at identical pCO2, arising from a progressive strengthening of the Antarctic Circumpolar Current. This suggests that a component of the Cenozoic benthic cooling trend recorded in oxygen isotopes could arise directly from changes in continental configuration, and so be partially decoupled from the Cenozoic greenhouse gas history. In this paper we will present our model results against the background of an extensive review of previous work on ocean gateways and additional modelling results from several other global climate models.
    Type: Article , PeerReviewed
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