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  • 1
    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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  • 2
    Publication Date: 2018-06-26
    Description: Polar amplification and specific equilibrium climate sensitivity (S) have been the subject of many paleoclimate studies. While earlier studies inferred them as single constant parameters of the climate system, there are now indications that both are conditioned by the type of forcing. Moreover, they might be affected by fast feedback mechanisms that have different strengths depending on the background climate. Here, we use the intermediate complexity climate model CLIMBER-2 to study the influence of land ice and CO2 on polar amplification and S. We perform transient five-million-year simulations, forced by different combinations of insolation, land ice and CO2. We find that land ice and CO2 changes have separate effects on temperature, both on the global mean and the meridional distribution. Land ice changes are mainly manifested in the high latitudes of the Northern Hemisphere. They lead to 77% higher northern polar amplification, 38% lower southern polar amplification, and 42% lower S than homogeneously distributed CO2 changes. Furthermore, towards colder climates northern polar amplification increases, and consequently southern polar amplification decreases, due to the albedo-temperature feedback. As an effect, a global average temperature change calculated from high-latitude temperatures by using a constant polar amplification would lead to errors of up to 0.6 K in our model set-up. We conclude that to constrain feedback strengths and climate sensitivity in climate models by paleoclimate data, the underlying forcing mechanisms and background climate states have to be taken into consideration.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 3
    Publication Date: 2018-04-16
    Description: Climate change projections for the future are uncertain, also due to inter-model differences. The application of these models to paleo times, which can be constrained by reconstructions, is therefore essential, not only to gain a better understanding of past climate changes, but also for model validation purposes. In this respect both data- and model-based approaches have been used to generate time series of global temperature changes, ∆Tg. The ratio of ∆Tg over radiative forcing, ∆R, defines the specific equilibrium climate sensitivity S, and has been suggested to be state-dependent, potentially increasing towards warming climates, and therefore suggesting climate sensitivity for the future to be at the upper end of the range of published results (Köhler et al., 2015, 2017). Here we reanalyse existing time series of ∆Tg and ∆R for the last 800,000 years and show that this proposed state-dependency of S is only found if ∆Tg is based on data (reconstructions), and not if ∆Tg is based on models (simulations). We furthermore identify that in data-based reconstructions ∆Tg is decoupled from atmospheric CO2 predominantely during times of decreasing obliquity (identical to periods of land-ice sheet growth and sea level fall), while in model simulations ∆Tg and CO2 vary in phase throughout. This multi-millennial decoupling of CO2 and temperature has been suggested to be partially caused by a sea level-induced surge in magma and CO2 fluxes from oceanic hotspot volcanoes and mid ocean ridges (Hasenclever et al., 2017). The neglection of these feedbacks between the solid Earth and the climate system in recent Earth system models is partly responsible for the data/model misfit, and illustrates our current limitation in the model-based interpretation of the paleo records. Paleo-based estimates of S might be restricted to data without this ∆Tg-CO2-decoupling leading to a 20% smaller quantification of S for interglacial conditions of the late Pleistocene.
    Repository Name: EPIC Alfred Wegener Institut
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  • 4
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    Publication Date: 2016-03-24
    Repository Name: EPIC Alfred Wegener Institut
    Type: PANGAEA Documentation , NonPeerReviewed
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  • 5
    Publication Date: 2018-04-16
    Description: Some studies suggest that specific equilibrium climate sensitivity S might be state-dependent. Reanalyzing existing paleodata of global mean surface temperature ∆Tg and radiative forcing ∆R of CO2 and land ice albedo for the last 800,000 years we show that this state-dependency of S is only found if ∆Tg is based on reconstructions, and not when ∆Tg is based on model simulations. Furthermore, during times of decreasing obliquity (periods of land-ice sheet growth and sea level fall) the multi-millennial component of reconstructed ∆Tg is diverging from atmospheric CO2, while in simulations both variables vary more synchronously. For a reconstruction-based extrapolation of S to the future we eliminate these periods due to an expected sea level rise. Consequently, S determined from proxy-based reconstructions without these data with strong ∆Tg-CO2 divergence is less state-dependent or even constant (state-independent), and yields into an equilibrium warming for 2 × CO2 of 1.9–3.8 K.
    Repository Name: EPIC Alfred Wegener Institut
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  • 6
    Publication Date: 2017-10-04
    Description: Since the inception of the Antarctic ice sheet at the Eocene–Oligocene transition (∼ 34 Myr ago), land ice has played a crucial role in Earth's climate. Through feedbacks in the climate system, land ice variability modifies atmospheric temperature changes induced by orbital, topographical, and greenhouse gas variations. Quantification of these feedbacks on long timescales has hitherto scarcely been undertaken. In this study, we use a zonally averaged energy balance climate model bidirectionally coupled to a one-dimensional ice sheet model, capturing the ice–albedo and surface–height–temperature feedbacks. Potentially important transient changes in topographic boundary conditions by tectonics and erosion are not taken into account but are briefly discussed. The relative simplicity of the coupled model allows us to perform integrations over the past 38 Myr in a fully transient fashion using a benthic oxygen isotope record as forcing to inversely simulate CO2. Firstly, we find that the results of the simulations over the past 5 Myr are dependent on whether the model run is started at 5 or 38 Myr ago. This is because the relation between CO2 and temperature is subject to hysteresis. When the climate cools from very high CO2 levels, as in the longer transient 38 Myr run, temperatures in the lower CO2 range of the past 5 Myr are higher than when the climate is initialised at low temperatures. Consequently, the modelled CO2 concentrations depend on the initial state. Taking the realistic warm initialisation into account, we come to a best estimate of CO2, temperature, ice-volume-equivalent sea level, and benthic δ18O over the past 38 Myr. Secondly, we study the influence of ice sheets on the evolution of global temperature and polar amplification by comparing runs with ice sheet–climate interaction switched on and off. By passing only albedo or surface height changes to the climate model, we can distinguish the separate effects of the ice–albedo and surface–height–temperature feedbacks. We find that ice volume variability has a strong enhancing effect on atmospheric temperature changes, particularly in the regions where the ice sheets are located. As a result, polar amplification in the Northern Hemisphere decreases towards warmer climates as there is little land ice left to melt. Conversely, decay of the Antarctic ice sheet increases polar amplification in the Southern Hemisphere in the high-CO2 regime. Our results also show that in cooler climates than the pre-industrial, the ice–albedo feedback predominates the surface–height–temperature feedback, while in warmer climates they are more equal in strength.
    Repository Name: EPIC Alfred Wegener Institut
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  • 7
    Publication Date: 2019-01-02
    Description: 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.
    Repository Name: EPIC Alfred Wegener Institut
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  • 8
    Publication Date: 2019-08-13
    Description: The equilibrium climate sensitivity (ECS) of climate models is calculated as the equilibrium global mean surface air warming resulting from a simulated doubling of the atmospheric CO2 concentration. In these simulations, long-term processes in the climate system, such as land ice changes, are not incorporated. Hence, climate sensitivity derived from paleodata has to be compensated for these processes, when comparing it to the ECS of climate models. Several recent studies found that the impact these long-term processes have on global temperature cannot be quantified directly through the global radiative forcing they induce. This renders the prevailing approach of deconvoluting paleotemperatures through a partitioning based on radiative forcings inaccurate. Here, we therefore implement an efficacy factor ε[LI] that relates the impact of land ice changes on global temperature to that of CO2 changes in our calculation of climate sensitivity from paleodata. We apply our refined approach to a proxy-inferred paleoclimate dataset, using ε[LI]=0.45+0.34−0.20 based on a multi-model assemblage of simulated relative influences of land ice changes on the Last Glacial Maximum temperature anomaly. The implemented ε[LI] is smaller than unity, meaning that per unit of radiative, forcing the impact on global temperature is less strong for land ice changes than for CO2 changes. Consequently, our obtained ECS estimate of 5.8±1.3 K, where the uncertainty reflects the implemented range in ε[LI], is ∼50 % higher than when differences in efficacy are not considered.
    Repository Name: EPIC Alfred Wegener Institut
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  • 9
    Publication Date: 2019-10-01
    Description: Over the last decade, our understanding of cli- mate sensitivity has improved considerably. The climate system shows variability on many timescales, is subject to non-stationary forcing and it is most likely out of equi- librium with the changes in the radiative forcing. Slow and fast feedbacks complicate the interpretation of geolog- ical records as feedback strengths vary over time. In the geological past, the forcing timescales were different than at present, suggesting that the response may have behaved differently. Do these insights constrain the climate sensitiv- ity relevant for the present day? In this paper, we review the progress made in theoretical understanding of climate sensitivity and on the estimation of climate sensitivity from proxy records. Particular focus lies on the background state dependence of feedback processes and on the impact of tipping points on the climate system. We suggest how to further use palaeo data to advance our understanding of the currently ongoing climate change.
    Repository Name: EPIC Alfred Wegener Institut
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  • 10
    Publication Date: 2020-03-20
    Description: Following Milankovitch's theory the incoming insolation or summer energy at 65°N is typically analysed to predict the waxing or waning of land ice. We here use a model-based deconvolution of the LR04 benthic-d18O stack into land ice distribution (de Boer et al., 2014, Köhler et al., 2015) to verify if the latitudinal focal point of land ice dynamics has changed over the last 2 Myr or whether this choice of 65°N in orbital data is indeed well justified. We find that the 5°-latitudinal band which contributes most to land ice albedo radiative forcing (ΔR_[LI]) is 70-75°N between 2.0-1.5 Myr, which is then until 1.0 Myr gradually substituted by 65-70°N. During the last 1 Myr both 60-65°N and 65-70°N dominate ΔR_[LI] and contribute approximately the same amount, while the relative importance of 70-75°N is shrinking. Our analyses illustrates that the choice of 65°N seems for the last 1 Myr to be well justified, while for earlier parts of the last 2 Myr the dominant land ice changes seems to happen up to 10° further to the north. Focusing on the last 800 kyr (the time for which precise data on atmospheric CO2 concentration exists) we furthermore find that the multi-millennial land ice growth and proxy-based reconstruction of global cooling (= the glaciation) appear synchronously to each other and to decreasing obliquity, but diverge from CO2. This suggests that the global cooling associated with Earth's way into an ice age as deduced in the reconstructions has to be mainly caused by the land ice albedo feedback, and is not dominated by the CO2 greenhouse forcing. One way of perceiving this CO2-glaciation divergence in reconstructions is that the reduced incoming insolation at high latitudes causes land ice growth and cooling, while there is a coexisting process that keeps CO2 at a relatively constant level. Solid Earth modeling experiments have indicated that falling sea level might lead to enhanced magma and CO2 production at mid-ocean ridges. Hasenclever et al. (2017) suggested that the combination of marine volcanism at mid-ocean ridges and at hot spot island volcanoes might react to decreasing sea level and be a potential cause for this CO2-glaciation divergence. This CO2-glaciation divergence needs to be considered, when using paleo data to quantify paleoclimate sensitivity: periods with diverging CO2 and global temperature change should be filtered out when approximating the relationship between global temperature rise and CO2 concentrations (Köhler et al., 2018). References: de Boer et al. (2014). https://doi.org/10.1038/ncomms3999. Köhler et al. (2015). https://doi.org/10.5194/cp-11-1801-2015. Hasenclever et al. (2017). https://doi.org/10.1038/ncomms15867. Köhler et al. (2018). https://doi.org/10.1029/2018GL077717.
    Repository Name: EPIC Alfred Wegener Institut
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