Quantifying the Sources of Intermodel Spread in Equilibrium Climate Sensitivity

Caldwell, Peter; Zelinka, Mark D.; Taylor, Karl E.; Marvel, Kate

doi:10.1175/jcli-d-15-0352.1

Cited by 119 publications

(146 citation statements)

References 36 publications

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“…The same may be said about intermodel differences in OHU. This is a tantalizing proposition, as cloud feedbacks continue to be the largest source of spread in estimates of climate sensitivity [10,21,59]. If part of that spread is driven in systematic ways by patterns of sea surface heat fluxes, it may be more reducible and falsifiable than typically acknowledged.…”

Section: Resultsmentioning

confidence: 99%

The Effects of Ocean Heat Uptake on Transient Climate Sensitivity

Rose

Rayborn

2016

Curr Clim Change Rep

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Transient climate sensitivity tends to increase on multiple timescales in climate models subject to an abrupt CO 2 increase. The interdependence of radiative and ocean heat uptake processes governing this increase are reviewed. Heat uptake tends to be spatially localized to the subpolar oceans, and this pattern emerges rapidly from an initially uniform distribution. Global climatic impact of heat uptake is studied through the lens of the efficacy concept and a linear systems perspective in which responses to individual climate forcing agents are additive. Heat uptake can be treated as a slowly varying forcing on the atmosphere and surface, whose efficacy is strongly determined by its geographical pattern. An illustrative linear model driven by simple prescribed uptake patterns demonstrates the emergence of increasing climate sensitivity as a consequence of the slow decay of high-efficacy subpolar heat uptake. Evidence is reviewed for the key role of shortwave cloud feedbacks in setting the high efficacy of ocean heat uptake and thus in increasing climate sensitivity. A causal physical mechanism is proposed, linking subpolar heat uptake to a global-scale increase in lower-tropospheric stability. It is shown that the rate of increase in estimated inversion strength systematically slows as heat uptake decays. Variations in heat uptake should therefore manifest themselves as differences in low cloud feedbacks.

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Section: Resultsmentioning

confidence: 99%

The Effects of Ocean Heat Uptake on Transient Climate Sensitivity

Rose

Rayborn

2016

Curr Clim Change Rep

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“…The climate feedback parameter is equal to the sum of the Planck response and feedbacks from water vapor, lapse, surface albedo and clouds. We use average climate model values for the forcing and non-cloud feedbacks as reported in Caldwell et al (2016) (Table 3). Further assuming a high-cloud altitude feedback ) of ?…”

Section: Implications For Climate Sensitivitymentioning

confidence: 99%

“…The differing model experiments means that the rapid cloud adjustments to CO 2 are included in some studies. In addition to these four estimates, we also consider the average cloud feedbacks in tropical subsidence regions calculated from the abrupt CO 2 quadrupling simulations analyzed in Caldwell et al (2016). These feedbacks primarily reflect the shortwave feedbacks from low clouds due to the absence of upper-level clouds.…”

Section: Large-eddy Simulation Cloud Feedbacksmentioning

confidence: 99%

Low-Cloud Feedbacks from Cloud-Controlling Factors: A Review

et al. 2017

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The response to warming of tropical low-level clouds including both marine stratocumulus and trade cumulus is a major source of uncertainty in projections of future climate. Climate model simulations of the response vary widely, reflecting the difficulty the models have in simulating these clouds. These inadequacies have led to alternative approaches to predict low-cloud feedbacks. Here, we review an observational approach that relies on the assumption that observed relationships between low clouds and the ''cloud-controlling factors'' of the large-scale environment are invariant across time-scales. With this assumption, and given predictions of how the cloud-controlling factors change with climate warming, one can predict low-cloud feedbacks without using any model simulation of low clouds. We discuss both fundamental and implementation issues with this approach and suggest steps that could reduce uncertainty in the predicted low-cloud feedback. Recent studies using this approach predict that the tropical low-cloud feedback is positive mainly due to the observation that reflection of solar radiation by low clouds decreases as temperature increases, holding all other cloud-controlling factors fixed. The positive feedback from temperature is partially offset by a negative feedback from the tendency for the inversion strength to increase in a warming world, with other cloudcontrolling factors playing a smaller role. A consensus estimate from these studies for the contribution of tropical low clouds to the global mean cloud feedback is

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“…Cloud feedbacks remain the main source of uncertainty in predictions of climate sensitivity (e.g., Dufresne and Bony, 2008;Vial et al, 2013;Webb et al, 2013;Caldwell et al, 2016). One reason for this uncertainty is that clouds simulated by climate models in the current climate exhibit large biases compared to observations (e.g., Zhang et al, 2005;Haynes et al, 2007;Chepfer et al, 2008;Williams and Webb, 2009;Marchand and Ackerman, 2010;Chepfer, 2012, 2013;Kay et al, 2012;Nam et al, 2012;Klein et al, 2013), leading to low confidence in the cloud feedbacks predicted by the models.…”

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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Quantifying the Sources of Intermodel Spread in Equilibrium Climate Sensitivity

Cited by 119 publications

References 36 publications

The Effects of Ocean Heat Uptake on Transient Climate Sensitivity

The Effects of Ocean Heat Uptake on Transient Climate Sensitivity

Low-Cloud Feedbacks from Cloud-Controlling Factors: A Review

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

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