Can feedback analysis be used to uncover the physical origin of climate sensitivity and efficacy differences?

Rieger, Vanessa; Dietmüller, Simone; Ponater, Michael

doi:10.1007/s00382-016-3476-x

Cited by 18 publications

(28 citation statements)

References 51 publications

(86 reference statements)

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“…Feedback analysis has proven to be a powerful tool in global climate dynamics for unraveling processes that cause climate sensitivity and efficacy differences between different forcings. Most frequently applied to explain the climate sensitivity variation among climate models with respect to a reference CO 2 perturbation (e.g., Bony et al 2006), the method can as well be used to identify feedback differences occurring between different forcing mechanisms (e.g., Yoshimori and Broccoli 2008;Rieger et al 2017). Most important for the present study, the method is not only suitable to quantify feedbacks developing in response to surface temperature changes (then usually given in W m 22 K 21 ) but also for calculating rapid radiative adjustments (Vial et al 2013;Geoffroy et al 2014;Smith et al 2018) that contribute to the ERF of some perturbation.…”

Section: Analysis Of Rapid Radiative Adjustmentsmentioning

confidence: 99%

Estimating the Effective Radiative Forcing of Contrail Cirrus

et al. 2020

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Evidence from previous climate model simulations has suggested a potentially low efficacy of contrails to force global mean surface temperature changes. In this paper, a climate model with a state-of-the-art contrail cirrus representation is used for fixed sea surface temperature simulations in order to determine the effective radiative forcing (ERF) from contrail cirrus. ERF is expected to be a good metric for intercomparing the quantitative importance of different contributions to surface temperature and climate impact. Substantial upscaling of aviation density is necessary to ensure statistically significant results from our simulations. The contrail cirrus ERF is found to be less than 50% of the respective instantaneous or stratosphere adjusted radiative forcings, with a best estimate of roughly 35%. The reduction of ERF is much more substantial for contrail cirrus than it is for a CO2 increase when both stratosphere adjusted forcings are of similar magnitude. Analysis of all rapid radiative adjustments contributing to the ERF indicates that the reduction is mainly induced by a compensating effect of natural clouds that provide a negative feedback. Compared to the CO2 reference case, a less positive combined water vapor and lapse rate adjustment also contributes to a more distinct reduction of contrail cirrus ERF, but not as much as the natural cloud adjustment. Based on the experience gained in this paper, respective contrail cirrus simulations with interactive ocean will be performed as the next step toward establishing contrail cirrus efficacy. ERF results of contrail cirrus from other climate models equipped with suitable parameterizations are regarded as highly desirable.

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Section: Analysis Of Rapid Radiative Adjustmentsmentioning

confidence: 99%

Estimating the Effective Radiative Forcing of Contrail Cirrus

et al. 2020

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“…This classical feedback analysis has been frequently applied to outputs from numerical cli-T. Vaillant de Guélis et al: Link between OLR and the lidar full attenuation altitude mate system simulations in order to estimate the effects of changes in water vapor, temperature lapse rate, clouds, and surface albedo on the overall climate radiative response (e.g., Cess et al, 1990;Le Treut et al, 1994;Watterson et al, 1999;Colman, 2003;Bony et al, 2006;Bates, 2007;Soden et al, 2008;Boucher et al, 2013;Sherwood et al, 2015;Rieger et al, 2016). Focusing only on the cloud feedback mechanisms, Zelinka et al (2012a) and others used this approach to isolate the role of each of the fundamental cloud variables that contribute to the cloud radiative response: cloud cover, cloud optical depth or water phase (liquid or ice), and cloud altitude (or cloud temperature).…”

Section: Introductionmentioning

confidence: 99%

The link between outgoing longwave radiation and the altitude at which a spaceborne lidar beam is fully attenuated

et al. 2017

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Abstract. According to climate model simulations, the changing altitude of middle and high clouds is the dominant contributor to the positive global mean longwave cloud feedback. Nevertheless, the mechanisms of this longwave cloud altitude feedback and its magnitude have not yet been verified by observations. Accurate, stable, and long-term observations of a metric-characterizing cloud vertical distribution that are related to the longwave cloud radiative effect are needed to achieve a better understanding of the mechanism of longwave cloud altitude feedback. This study shows that the direct measurement of the altitude of atmospheric lidar opacity is a good candidate for the necessary observational metric. The opacity altitude is the level at which a spaceborne lidar beam is fully attenuated when probing an opaque cloud. By combining this altitude with the direct lidar measurement of the cloud-top altitude, we derive the effective radiative temperature of opaque clouds which linearly drives (as we will show) the outgoing longwave radiation. We find that, for an opaque cloud, a cloud temperature change of 1 K modifies its cloud radiative effect by 2 W m −2 . Similarly, the longwave cloud radiative effect of optically thin clouds can be derived from their top and base altitudes and an estimate of their emissivity. We show with radiative transfer simulations that these relationships hold true at single atmospheric column scale, on the scale of the Clouds and the Earth's Radiant Energy System (CERES) instantaneous footprint, and at monthly mean 2 • × 2 • scale. Opaque clouds cover 35 % of the ice-free ocean and contribute to 73 % of the global mean cloud radiative effect. Thin-cloud coverage is 36 % and contributes 27 % of the global mean cloud radiative effect. The link between outgoing longwave radiation and the altitude at which a spaceborne lidar beam is fully attenuated provides a simple formulation of the cloud radiative effect in the longwave domain and so helps us to understand the longwave cloud altitude feedback mechanism.

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“…Additional difficulties arise in separating forcings and feedbacks 27,80,81 , and defining appropriate forcings that account for short term atmospheric and oceanic adjustments [82][83][84][85][86][87][88][89][90][91] . The slope of the regression as a measure of the feedback may further depend on the climate base state, and the particular observed realization of natural variability in the real world.…”

Section: Limitations and Future Research Avenuesmentioning

confidence: 99%

“…The evolution in CESM clearly deviates from a straight line implied by a constant feedback. The slope of the regression λ is also not constant for most other CMIP5 models due to both shortterm atmospheric and oceanic adjustments, and due to feedbacks and warming patterns changing over time.Other concerns are that feedbacks may not be additive, and the climate response depends on the type and magnitude of the forcing 37,61,[63][64][65][66][67][68][69][70][71][72][73][74][75][76][77][78][79][80] . Additional difficulties arise in separating forcings and feedbacks 27,80,81 , and defining appropriate forcings that account for short term atmospheric and oceanic adjustments [82][83][84][85][86][87][88][89][90][91] .…”

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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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Can feedback analysis be used to uncover the physical origin of climate sensitivity and efficacy differences?

Cited by 18 publications

References 51 publications

Estimating the Effective Radiative Forcing of Contrail Cirrus

Estimating the Effective Radiative Forcing of Contrail Cirrus

The link between outgoing longwave radiation and the altitude at which a spaceborne lidar beam is fully attenuated

Beyond equilibrium climate sensitivity

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