scholarly journals Constraints on Climate Sensitivity from Space-Based Measurements of Low-Cloud Reflection

2016 ◽  
Vol 29 (16) ◽  
pp. 5821-5835 ◽  
Author(s):  
Florent Brient ◽  
Tapio Schneider
Keyword(s):  
Climate Sensitivity ◽  
Climate Models ◽  
Temporal Variations ◽  
Shortwave Radiation ◽  
Information Theoretic ◽  
Equilibrium Climate Sensitivity ◽  
Tropical Oceans ◽  
Low Clouds ◽  
Low Cloud ◽  
Well Models

Abstract Physical uncertainties in global-warming projections are dominated by uncertainties about how the fraction of incoming shortwave radiation that clouds reflect will change as greenhouse gas concentrations rise. Differences in the shortwave reflection by low clouds over tropical oceans alone account for more than half of the variance of the equilibrium climate sensitivity (ECS) among climate models, which ranges from 2.1 to 4.7 K. Space-based measurements now provide an opportunity to assess how well models reproduce temporal variations of this shortwave reflection on seasonal to interannual time scales. Here such space-based measurements are used to show that shortwave reflection by low clouds over tropical oceans decreases robustly when the underlying surface warms, for example, by −(0.96 ± 0.22)% K−1 (90% confidence level) for deseasonalized variations. Additionally, the temporal covariance of low-cloud reflection with temperature in historical simulations with current climate models correlates strongly (r = −0.67) with the models’ ECS. Therefore, measurements of temporal low-cloud variations can be used to constrain ECS estimates based on climate models. An information-theoretic weighting of climate models by how well they reproduce the measured deseasonalized covariance of shortwave cloud reflection with temperature yields a most likely ECS estimate around 4.0 K; an ECS below 2.3 K becomes very unlikely (90% confidence).

scholarly journals The Impact of Low Clouds on Surface Shortwave Radiation in the ECMWF Model

2012 ◽  
Vol 140 (11) ◽  
pp. 3783-3794 ◽  
Author(s):  
Maike Ahlgrimm ◽  
Richard Forbes
Keyword(s):  
Great Plains ◽  
Liquid Water ◽  
Shortwave Radiation ◽  
Fair Weather ◽  
Liquid Water Path ◽  
Water Path ◽  
Surface Irradiance ◽  
Low Clouds ◽  
Low Cloud ◽  
The Impact

Abstract The long-term measurement records from the Atmospheric Radiation Measurement site on the Southern Great Plains show evidence of a bias in the ECMWF model’s surface irradiance. Based on previous studies, which have suggested that summertime shallow clouds may contribute to the bias, an evaluation of 146 days with observed nonprecipitating fair-weather cumulus clouds is performed. In-cloud liquid water path and effective radius are both overestimated in the model with liquid water path dominating to produce clouds that are too reflective. These are compensated by occasional cloud-free days in the model such that the fair-weather cumulus regime overall does not contribute significantly to the multiyear daytime mean surface irradiance bias of 23 W m−2. To further explore the origin of the bias, observed and modeled cloud fraction profiles over 6 years are classified and sorted based on the surface irradiance bias associated with each sample pair. Overcast low cloud conditions during the spring and fall seasons are identified as a major contributor. For samples with low cloud present in both observations and model, opposing surface irradiance biases are found for overcast and broken cloud cover conditions. A reduction of cloud liquid to a third for broken low clouds and an increase by a factor of 1.5 in overcast situations improves agreement with the observed liquid water path distribution. This approach of combining the model shortwave bias with a cloud classification helps to identify compensating errors in the model, providing guidance for a targeted improvement of cloud parameterizations.

scholarly journals Could the Pliocene constrain the Equilibrium Climate Sensitivity?

2016 ◽  
Author(s):  
J. C. Hargreaves ◽  
J. D. Annan
Keyword(s):  
Carbon Dioxide ◽  
Boundary Conditions ◽  
Climate Sensitivity ◽  
Atmospheric Carbon Dioxide ◽  
Climate Models ◽  
Climate Model ◽  
Equilibrium Climate Sensitivity ◽  
Model Experiments ◽  
First Time ◽  
Industrial State

Abstract. The mid-PlioceneWarm Period (mPWP) is the most recent interval in which atmospheric carbon dioxide was substantially higher than in modern pre-industrial times. It is, therefore, a potentially valuable target for testing the ability of climate models to simulate climates warmer than the pre-industrial state. The recent Pliocene model inter-comparison Project (PlioMIP) presented boundary conditions for the mPWP, and a protocol for climate model experiments. Here we analyse results from the PlioMIP and, for the first time, discuss the potential for this interval to usefully constrain the equilibrium climate sensitivity. We present an estimate of 1.8–3.6 °C, but there are considerable uncertainties surrounding the analysis. We consider the extent to which these uncertainties may be lessened in the next few years.

Low clouds link equilibrium climate sensitivity to hydrological sensitivity

2018 ◽  
Vol 8 (10) ◽  
pp. 901-906 ◽  
Author(s):  
Masahiro Watanabe ◽  
Youichi Kamae ◽  
Hideo Shiogama ◽  
Anthony M. DeAngelis ◽  
Kentaroh Suzuki
Keyword(s):  
Climate Sensitivity ◽  
Equilibrium Climate Sensitivity ◽  
Low Clouds ◽  
Hydrological Sensitivity

scholarly journals Equilibrium Climate Sensitivity Estimated by Equilibrating Climate Models

2020 ◽  
Vol 47 (4) ◽  
Author(s):  
Maria Rugenstein ◽  
Jonah Bloch‐Johnson ◽  
Jonathan Gregory ◽  
Timothy Andrews ◽  
Thorsten Mauritsen ◽  
...  
Keyword(s):  
Climate Sensitivity ◽  
Climate Models ◽  
Equilibrium Climate Sensitivity

scholarly journals Cloud, precipitation and radiation responses to large perturbations in global dimethyl sulfide

2018 ◽  
Vol 18 (14) ◽  
pp. 10177-10198 ◽  
Author(s):  
Sonya L. Fiddes ◽  
Matthew T. Woodhouse ◽  
Zebedee Nicholls ◽  
Todd P. Lane ◽  
Robyn Schofield
Keyword(s):  
Large Scale ◽  
Climate Models ◽  
Dimethyl Sulfide ◽  
Cloud Fraction ◽  
Global Climate Models ◽  
Radiation Budget ◽  
Shortwave Radiation ◽  
Stratiform Cloud ◽  
Aerosol Emission ◽  
Low Cloud

Abstract. Natural aerosol emission represents one of the largest uncertainties in our understanding of the radiation budget. Sulfur emitted by marine organisms, as dimethyl sulfide (DMS), constitutes one-fifth of the global sulfur budget and yet the distribution, fluxes and fate of DMS remain poorly constrained. This study evaluates the Australian Community Climate and Earth System Simulator (ACCESS) United Kingdom Chemistry and Aerosol (UKCA) model in terms of cloud fraction, radiation and precipitation, and then quantifies the role of DMS in the chemistry–climate system. We find that ACCESS-UKCA has similar cloud and radiation biases to other global climate models. By removing all DMS, or alternatively significantly enhancing marine DMS, we find a top of the atmosphere radiative effect of 1.7 and −1.4 W m−2 respectively. The largest responses to these DMS perturbations (removal/enhancement) are in stratiform cloud decks in the Southern Hemisphere's eastern ocean basins. These regions show significant differences in low cloud (-9/+6 %), surface incoming shortwave radiation (+7/-5 W m−2) and large-scale rainfall (+15/-10 %). We demonstrate a precipitation suppression effect of DMS-derived aerosol in stratiform cloud deck regions due to DMS, coupled with an increase in low cloud fraction. The difference in low cloud fraction is an example of the aerosol lifetime effect. Globally, we find a sensitivity of temperature to annual DMS flux of 0.027 and 0.019 K per Tg yr−1 of sulfur, respectively. Other areas of low cloud formation, such as the Southern Ocean and stratiform cloud decks in the Northern Hemisphere, have a relatively weak response to DMS perturbations. We highlight the need for greater understanding of the DMS–climate cycle within the context of uncertainties and biases of climate models as well as those of DMS–climate observations.

scholarly journals On the Emergent Constraints of Climate Sensitivity

2018 ◽  
Vol 31 (2) ◽  
pp. 863-875 ◽  
Author(s):  
Xin Qu ◽  
Alex Hall ◽  
Anthony M. DeAngelis ◽  
Mark D. Zelinka ◽  
Stephen A. Klein ◽  
...  
Keyword(s):  
Climate Sensitivity ◽  
Climate Models ◽  
Selection Process ◽  
Temperature Response ◽  
Cloud Feedback ◽  
Emergent Constraints ◽  
Low Cloud ◽  
Model Ensembles ◽  
Statistical Connection ◽  
Emergent Constraint

Differences among climate models in equilibrium climate sensitivity (ECS; the equilibrium surface temperature response to a doubling of atmospheric CO2) remain a significant barrier to the accurate assessment of societally important impacts of climate change. Relationships between ECS and observable metrics of the current climate in model ensembles, so-called emergent constraints, have been used to constrain ECS. Here a statistical method (including a backward selection process) is employed to achieve a better statistical understanding of the connections between four recently proposed emergent constraint metrics and individual feedbacks influencing ECS. The relationship between each metric and ECS is largely attributable to a statistical connection with shortwave low cloud feedback, the leading cause of intermodel ECS spread. This result bolsters confidence in some of the metrics, which had assumed such a connection in the first place. Additional analysis is conducted with a few thousand artificial metrics that are randomly generated but are well correlated with ECS. The relationships between the contrived metrics and ECS can also be linked statistically to shortwave cloud feedback. Thus, any proposed or forthcoming ECS constraint based on the current generation of climate models should be viewed as a potential constraint on shortwave cloud feedback, and physical links with that feedback should be investigated to verify that the constraint is real. In addition, any proposed ECS constraint should not be taken at face value since other factors influencing ECS besides shortwave cloud feedback could be systematically biased in the models.

Observed Southern Ocean Cloud Properties and Shortwave Reflection. Part II: Phase Changes and Low Cloud Feedback*

2014 ◽  
Vol 27 (23) ◽  
pp. 8858-8868 ◽  
Author(s):  
Daniel T. McCoy ◽  
Dennis L. Hartmann ◽  
Daniel P. Grosvenor
Keyword(s):  
Solar Radiation ◽  
Optical Depth ◽  
Climate Models ◽  
Phase Changes ◽  
Shortwave Radiation ◽  
Cloud Optical Depth ◽  
Cloud Properties ◽  
Low Cloud ◽  
Model Transition ◽  
Comparable Magnitude

Abstract Climate models produce an increase in cloud optical depth in midlatitudes associated with climate warming, but the magnitude of this increase and its impact on reflected solar radiation vary from model to model. Transition from ice to liquid in midlatitude clouds is thought to be one mechanism for producing increased cloud optical depth. Here observations of cloud properties are used from a suite of remote sensing instruments to estimate the effect of conversion of ice to liquid associated with warming on reflected solar radiation in the latitude band from 40° to 60°S. The calculated increase in upwelling shortwave radiation (SW↑) is found to be important and of comparable magnitude to the increase in SW↑ associated with warming-induced increases of optical depth in climate models. The region where the authors' estimate increases SW↑ extends farther equatorward than the region where optical depth increases with warming in models. This difference is likely caused by other mechanisms at work in the models but is also sensitive to the amount of ice present in climate models and its susceptibility to warming.

Revisiting the relationship between dynamical sensitivity and climate sensitivity in the Southern hemisphere

2020 ◽  
Author(s):  
Thomas Wood ◽  
Amanda Maycock ◽  
Christine McKenna ◽  
Andreas Chrysanthou ◽  
John Fyfe ◽  
...  
Keyword(s):  
Southern Hemisphere ◽  
Climate Sensitivity ◽  
Climate Models ◽  
Stratospheric Ozone ◽  
Storm Track ◽  
Temperature Gradients ◽  
Cmip5 Models ◽  
Positive Trend ◽  
Equilibrium Climate Sensitivity ◽  
The Relationship

<p>The Southern Annular Mode (SAM) is the dominant mode of midlatitude atmospheric circulation variability in the Southern hemisphere. In the future, the SAM trend is expected to be the net result of opposing effects from increasing greenhouse gases (GHG) and ozone recovery. Different greenhouse gas scenarios, which induce different rates of surface and atmospheric temperature change, are therefore associated with different future SAM trends (Barnes et al., 2014). Since the magnitude of warming due to GHGs is an important component of this response, one might expect to find a relationship between equilibrium climate sensitivity (ECS) and future Southern hemisphere circulation trends. In CMIP5, the relationship between the SAM and the level of tropospheric warming across models was found to be strongest in the summer and autumn and could explain around 20% of the intermodel spread (Grise and Polvani, 2014). The spread is more strongly correlated with differences in meridional temperature gradients (Harvey et al., 2014).</p><p>Many of the latest CMIP6 models show a larger equilibrium climate sensitivity (ECS) of up to ~5.5 K (Forster et al., 2019) compared to a maximum of ~4.7 K in CMIP5. This raises the important question of how a higher level of warming affects projections of the SH midlatitude circulation. In this study, we examine the response of the SAM in CMIP6 models and quantify its relationship to ECS and temperature gradients. Our starting hypothesis is that stronger surface warming will induce a larger increase in tropical free tropospheric temperatures, and hence all being equal, a larger tropics-to-pole temperature gradient and a more positive SAM trend. However, results show that despite the higher level of warming in the CMIP6 models, there is a smaller positive trend in SAM index than in CMIP5 indicating a different relationship between warming and midlatitude circulation trends in CMIP6. We attempt to explain potential reasons for these differences.</p><p><strong>References:</strong></p><p>Barnes, E.A., N.W. Barnes, and L.M. Polvani, 2014: Delayed Southern Hemisphere Climate Change Induced by Stratospheric Ozone Recovery, as Projected by the CMIP5 Models. J. Climate, 27, 852–867, https://doi.org/10.1175/JCLI-D-13-00246.1</p><p>Forster, P.M., Maycock, A.C., McKenna, C.M. et al. (2019), Latest climate models confirm need for urgent mitigation. Nat. Clim. Chang. (2019) doi:10.1038/s41558-019-0660-0</p><p>Grise, K. M., and Polvani, L. M. (2014), Is climate sensitivity related to dynamical sensitivity? A Southern Hemisphere perspective, Geophys. Res. Lett., 41, 534– 540, doi:10.1002/2013GL058466.</p><p>Harvey, B.J., Shaffrey, L.C. & Woollings, T.J. (2014) Equator­-to-­pole temperature differences and the extra­tropical storm track responses of the CMIP5 climate models, Clim Dyn, 43: 1171. https://doi.org/10.1007/s00382-013-1883-9</p>

scholarly journals Equilibrium Climate Sensitivity Obtained From Multimillennial Runs of Two GFDL Climate Models

2018 ◽  
Vol 123 (4) ◽  
pp. 1921-1941 ◽  
Author(s):  
D. Paynter ◽  
T. L. Frölicher ◽  
L. W. Horowitz ◽  
L. G. Silvers
Keyword(s):  
Climate Sensitivity ◽  
Climate Models ◽  
Equilibrium Climate Sensitivity
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