Feedbacks, climate sensitivity and the limits of linear models

Knutti, Reto; Rugenstein, Maria

doi:10.1098/rsta.2015.0146

Cited by 128 publications

(130 citation statements)

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“…Consequently, the T intercept (N = 0) for the first 20 years of the simulation underestimates the actual warming of the model. Over longer timescales, the slope of the regression line becomes less negative, implying that the sensitivity of the climate system to the forcing increases Gregory et al, 2004;Knutti and Rugenstein, 2015). This non-linearity has been found to be particularly apparent in cloud feedback parameters, in particular shortwave cloud feedback processes .…”

Section: Gregory Plotsmentioning

confidence: 97%

Emulation of long-term changes in global climate: application to the late Pliocene and future

et al. 2017

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Abstract. Multi-millennial transient simulations of climate changes have a range of important applications, such as for investigating key geologic events and transitions for which high-resolution palaeoenvironmental proxy data are available, or for projecting the long-term impacts of future climate evolution on the performance of geological repositories for the disposal of radioactive wastes. However, due to the high computational requirements of current fully coupled general circulation models (GCMs), long-term simulations can generally only be performed with less complex models and/or at lower spatial resolution. In this study, we present novel longterm "continuous" projections of climate evolution based on the output from GCMs, via the use of a statistical emulator. The emulator is calibrated using ensembles of GCM simulations, which have varying orbital configurations and atmospheric CO 2 concentrations and enables a variety of investigations of long-term climate change to be conducted, which would not be possible with other modelling techniques on the same temporal and spatial scales. To illustrate the potential applications, we apply the emulator to the late Pliocene (by modelling surface air temperature -SAT), comparing its results with palaeo-proxy data for a number of global sites, and to the next 200 kyr (thousand years) (by modelling SAT and precipitation). A range of CO 2 scenarios are prescribed for each period. During the late Pliocene, we find that emulated SAT varies on an approximately precessional timescale, with evidence of increased obliquity response at times. A comparison of atmospheric CO 2 concentration for this period, estimated using the proxy sea surface temperature (SST) data from different sites and emulator results, finds that relatively similar CO 2 concentrations are estimated based on sites at lower latitudes, whereas higher-latitude sites show larger discrepancies. In our second illustrative application, spanning the next 200 kyr into the future, we find that SAT oscillations appear to be primarily influenced by obliquity for the first ∼ 120 kyr, whilst eccentricity is relatively low, after which precession plays a more dominant role. Conversely, variations in precipitation over the entire period demonstrate a strong precessional signal. Overall, we find that the emulator provides a useful and powerful tool for rapidly simulating the long-term evolution of climate, both past and future, due to its relatively high spatial resolution and relatively low computational cost. However, there are uncertainties associated with the approach used, including the inability of the emulaPublished by Copernicus Publications on behalf of the European Geosciences Union. 1540 N. S. Lord et al.: Emulation of long-term changes in global climate tor to capture deviations from a quasi-stationary response to the forcing, such as transient adjustments of the deep-ocean temperature and circulation, in addition to its limited range of fixed ice sheet configurations and its requirement for prescribed atmos...

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Section: Gregory Plotsmentioning

confidence: 97%

Emulation of long-term changes in global climate: application to the late Pliocene and future

et al. 2017

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“…R and T s change over time. In addition, some studies suggest that the heat uptake by the ocean is also time dependent and even λ is not a constant parameter (e.g., Knutti and Rugenstein, 2015).…”

Section: Introductionmentioning

confidence: 99%

The importance of mixed-phase and ice clouds for climate sensitivity in the global aerosol–climate model ECHAM6-HAM2

Lohmann

Neubauer

2018

Atmos. Chem. Phys.

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Abstract. How clouds change in a warmer climate remains one of the largest uncertainties for the equilibrium climate sensitivity (ECS). While a large spread in the cloud feedback arises from low-level clouds, it was recently shown that mixed-phase clouds are also important for ECS. If mixedphase clouds in the current climate contain too few supercooled cloud droplets, too much ice will change to liquid water in a warmer climate. As shown by Tan et al. (2016), this overestimates the negative cloud-phase feedback and underestimates ECS in the CAM global climate model (GCM). Here we use the newest version of the ECHAM6-HAM2 GCM to investigate the importance of mixed-phase and ice clouds for ECS.Although we also considerably underestimate the fraction of supercooled liquid water globally in the reference version of the ECHAM6-HAM2 GCM, we do not obtain increases in ECS in simulations with more supercooled liquid water in the present-day climate, different from the findings by Tan et al. (2016). We hypothesize that it is not the global supercooled liquid water fraction that matters, but only how well low-and mid-level mixed-phase clouds with cloud-top temperatures in the mixed-phase temperature range between 0 and −35 • C that are not shielded by higher-lying ice clouds are simulated. These occur most frequently in midlatitudes, in particular over the Southern Ocean where they determine the amount of absorbed shortwave radiation. In ECHAM6-HAM2 the amount of absorbed shortwave radiation over the Southern Ocean is only significantly overestimated if all clouds below 0 • C consist exclusively of ice. Only in this simulation is ECS significantly smaller than in all other simulations and the cloud optical depth feedback is the dominant cloud feedback. In all other simulations, the cloud optical depth feedback is weak and changes in cloud feedbacks associated with cloud amount and cloud-top pressure dominate the overall cloud feedback. However, apart from the simulation with only ice below 0 • C, differences in the overall cloud feedback are not translated into differences in ECS in our model. This insensitivity to the cloud feedback in our model is explained with compensating effects in the clear sky.

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“…Crucifix, 2006;Hargreaves et al, 2007;Yoshimori et al, 2011;Yin and Berger, 2012;Caballero and Huber, 2013;Goldner et al, 2013;Kutzbach et al, 2013;Meraner et al, 2013) and palaeodata-based (PALAEOSENS-Project Members, 2012;von der Heydt et al, 2014) approaches have already indicated that S varies for different background climates; see also a recent review of Knutti and Rugenstein (2015) on the limits of linear models to constrain climate sensitivity. The majority of simulation studies shows a rise in climate sensitivity for a warmer background climate.…”

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confidence: 99%

On the state dependency of the equilibrium climate sensitivity during the last 5 million years

et al. 2015

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Abstract.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 CO 2 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 CO 2 radiative forcing depends on the CO 2 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 CO 2 and of the land ice albedo (LI) is combined, we find a state dependence in the calculated specific equilibrium climate sensitivity, S [CO 2 ,LI] , for most of the Pleistocene (last 2.1 Myr). During Pleistocene intermediate glaciated climates and interglacial periods, S [CO 2 ,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 CO 2 data uncertainties prevent a well-supported calculation for S [CO 2 ,LI] , but our analysis suggests that during times without a large land ice area in the Northern Hemisphere (e.g. before 2.82 Myr BP), the specific equilibrium climate sensitivity, S [CO 2 ,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 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 CO 2 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.

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Feedbacks, climate sensitivity and the limits of linear models

Cited by 128 publications

References 125 publications

Emulation of long-term changes in global climate: application to the late Pliocene and future

Emulation of long-term changes in global climate: application to the late Pliocene and future

The importance of mixed-phase and ice clouds for climate sensitivity in the global aerosol–climate model ECHAM6-HAM2

On the state dependency of the equilibrium climate sensitivity during the last 5 million years

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