Cloud feedback remains a leading source of uncertainty in climate model projections under increasing atmospheric carbon dioxide. Cloud-controlling factor (CCF) analysis is a method used to observationally constrain cloud feedback and, subsequently, the climate sensitivity. Although high clouds contribute significantly to this uncertainty, they have historically received comparatively little attention in CCF studies. Here, we apply CCF analysis to observationally constrain high-cloud feedback, focusing on feedback associated with changes in cloud amount due to its dominant contribution to uncertainty. Our observational constraints reveal larger decreases in high cloud amount with warming than climate models predict, yet the net high-cloud radiative feedback remains near-neutral due to compensating shortwave and longwave effects. We also show that including upper-tropospheric static stability as a predictor effectively captures the stability iris mechanism and associated changes in cloud amount. This work highlights the importance of using physically relevant CCFs for robust observational constraints on high-cloud feedback and improving mechanistic understanding of its underlying drivers.
How to cite: Wilson Kemsley, S. J., Nowack, P., and Ceppi, P.: Observational high-cloud feedback constraints indicate climate models underestimate global reductions in high-cloud amount with warming, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2499, https://doi.org/10.5194/egusphere-egu25-2499, 2025.
The climatological atmospheric circulation is key to establishing the tropical 'pattern effect', whereby cloud feedbacks induced by sea surface temperature (SST) warming depend on the spatial structure of that warming. But how patterned warming-induced circulation changes affect cloud responses is less clear. Here we use idealized simulations with prescribed SST perturbations to understand the contributions to changes in tropical-mean cloud radiative effects (CRE) from different circulation regimes. We develop a novel framework based on moist static energy to understand the circulation response, targeting in particular the bulk circulation metric of ascent fraction. Warming concentrated in regions of ascent leads to a strong 'upped-ante' effect and spatial contraction of the ascending region. Our framework reveals substantial contributions to tropical-mean CRE changes not only from traditional 'pattern effect' regimes, but also from the intensification of convection in ascent regions as well as a smaller contribution from cloud changes in convective margins.
How to cite: Byrne, M., Mackie, A., Van de Koot, E., and Williams, A.: Circulation and cloud responses to patterned SST warming, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1658, https://doi.org/10.5194/egusphere-egu25-1658, 2025.
How to cite: McCulloch, D., Lambert, H., Webb, M., and Vallis, G.: Why weakening the overturning Walker circulation in the tropical ascent region leads to a reduction in subtropical low clouds, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10273, https://doi.org/10.5194/egusphere-egu25-10273, 2025.
Tropical high cloud feedbacks exhibit considerable spread across climate models. This study applies the cloud radiative kernel technique of Zelinka et al. (2012a; 2013) to 22 models across the Coupled Model Intercomparison Project CMIP5 and CMIP6 ensembles to survey tropical high cloud feedbacks and analyze their relationships to climate sensitivity, changes to the tropical overturning circulation, and changes to deep convective organization across scales. First, the inter-model spread in tropical high cloud net, altitude, and optical depth feedbacks exhibit significant correlations to climate sensitivity in the tropical mean and on convective margins. Additionally, we find that inter-model variability in deep convective organization – at both the mesoscale and planetary scales – relates to the inter-model spread in high cloud feedbacks along convective margins. More specifically, decreases in tropical ascent area and increases in mesoscale organization of deep convection relate to more positive high cloud feedbacks, particularly within weak ascent and weak descent regimes. Increases in mesoscale organization also coincide with a greater weakening of the Pacific Walker circulation. Finally, relationships between the inter-model spread in tropical high cloud feedbacks, convective organization across scales, and sea surface temperature patterns will be discussed.
How to cite: Schiro, K. and Dawson, E.: Spread in high cloud feedback along tropical convective margins linked to changes in convective organization across scales, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14904, https://doi.org/10.5194/egusphere-egu25-14904, 2025.
The Southern Ocean is known to be one of the cloudiest places on Earth, and the important contribution of Southern Ocean clouds to Earth’s energy budget is undisputed. By changing their composition in response to warming, clouds in this region currently limit the rate of warming, as they become brighter with increasing temperature and thus exert a stabilising feedback on the climate system. Here, based on multiple lines of evidence, we show that in the current state of the Southern Ocean climate, this negative feedback happens to be maximised. Moving away from the present climate state in either direction (cooling or warming) will thus reduce the feedback, such that the climate sensitivity to any perturbation can be expected to grow rapidly with each degree of temperature change. This finding adds urgency to the implementation of effective climate mitigation to limit warming and thus preserve the stabilising climate effect of Southern Ocean clouds.
How to cite: Storelvmo, T., Che, H., Bjordal, J., Carlsen, T., David, R., Gjermundsen, A., Whitehead, L., and McFarquhar, G.: Strong pattern effect evident in Southern Ocean cloud feedback based on multiple lines of evidence, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15351, https://doi.org/10.5194/egusphere-egu25-15351, 2025.
Tropical high cloud feedback remains a key uncertainty in estimating Equilibrium Climate Sensitivity, particularly the optical depth feedback. The Pacific Intertropical Convergence Zone is a major contributor to tropical cloud radiative effect (CRE). The Tropical Pacific is also projected to see shifts in convection from West to East. In this study, we analyse the key differences in the observed high cloud radiative effect, optical depth and feedback between the East and West Pacific. Notably, we find that the strongest climatological high cloud optical depths and net radiative effects in the tropical region are found in the East Pacific, despite greater high cloud amounts in the West Pacific.
We further estimate the high cloud feedback from the observed variability, using 20 years of CERES Flux By Cloud Type data from MODIS satellite (Sun et al. 2022), following Raghuraman et al [2024] for the regions. We find significant, opposite total high cloud feedbacks between the East and West Pacific, driven primarily by the high cloud amount feedback, with smaller contributions from the optical depth and altitude feedbacks. The shortwave and longwave cloud amount feedbacks are significant in both regions, greater in the West and opposite in sign to the East Pacific. However, the net amount feedback is negative in both regions and twice as strong in the East Pacific than in the West. As expected, the cloud altitude feedback is positive in every region analysed, primarily driven by the longwave component. Only the West Pacific shows a significant optical depth feedback, driven by a positive shortwave feedback. The distinct high cloud amount and optical depth feedbacks estimated in the regions are not apparent when analysing the entire tropics.
We find that the estimated cloud feedbacks in the tropical Pacific strongly depend on the inclusion of ENSO events in the record. Since climate projections suggest an El Nino-like warming in response to CO2 forcing, understanding the potential for changes in high cloud properties in the Pacific, as suggested by our observational evidence, is vital.
References:
Raghuraman, S.P. et al. (2024) ‘Observational Quantification of Tropical High Cloud Changes and Feedbacks’, Journal of Geophysical Research: Atmospheres, 129(7), p. e2023JD039364. Available at: https://doi.org/10.1029/2023JD039364.
Sun, M. et al. (2022) ‘Clouds and the Earth’s Radiant Energy System(CERES) FluxByCldTyp Edition 4 Data Product’, Journal of Atmospheric and Oceanic Technology, 39(3), pp. 303–318. Available at: https://doi.org/10.1175/JTECH-D-21-0029.1.
How to cite: Romero Jure, P. V., Finney, D., Maycock, A., Blyth, A., Mackie, A., and Lambert, H.: Observational quantification of high cloud radiative effect and feedback: An Analysis of differences across the tropical Pacific, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4227, https://doi.org/10.5194/egusphere-egu25-4227, 2025.
Changes in cloud scattering properties and emissivity that arise from atmospheric warming cause substantial radiative feedbacks in model projections of anthropogenic climate change, and the relative importance of the underlying mechanisms is poorly understood. One leading hypothesis is that ice-to-liquid conversions cause clouds to optically thicken, producing a major negative feedback. We test this hypothesis by developing a method to decompose cloud radiative feedbacks by cloud-top phase. The method is applied to an ensemble of six state-of-the-art global climate models run with prescribed sea-surface temperature. In these simulations, the global mean of the net cloud scattering and emissivity feedback from cloud-phase conversions ranges from -0.17 to -0.01 W m-2 K-1, while the overall net cloud feedback ranges from 0.02 to 0.91 W m-2 K-1. The multi-model mean of the cloud scattering and emissivity feedback from cloud-phase conversions is approximately 18% of the magnitude of the multi-model mean of the overall cloud feedback (-0.10 W m-2 K-1 vs. 0.52 W m-2 K-1). These results indicate that cloud-phase conversions cause a robust negative feedback by changing cloud scattering and emissivity, but this mechanism makes a modest contribution to the overall cloud feedback at the global scale.
How to cite: Wall, C., Paynter, D., Qin, Y., Debolskiy, M., Duffy, M., Michibata, T., Duran, B., Lutsko, N., Ma, P.-L., Medeiros, B., Storelvmo, T., and Zhao, M.: Decomposing Cloud Radiative Feedbacks by Cloud-Top Phase, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8415, https://doi.org/10.5194/egusphere-egu25-8415, 2025.
We include idealised clouds in a single column model to estimate the all-sky climate sensitivity. Our results show that the cloud radiative effects observed from satellites can be accurately reproduced by combining high and low/mid-level clouds. We introduce a "fixed cloud albedo" null hypothesis, which assumes a fixed cloud albedo but allows for changes in cloud temperature as the surface warms. By analysing cloud distributions consistent with present-day observations, we estimate a mean fixed-albedo climate sensitivity of 2.2 K, slightly less than the clear-sky value. Our results highlight the importance of cloud masking effects, especially by mid-level clouds, and the reduction of radiative forcing by high clouds. Giving more weight to low-level clouds, which are assumed to change temperature with warming, results in a reduced estimate of 2.0 K. This provides a baseline to which changes in surface albedo, and a believed reduction in cloud albedo, would add to.
How to cite: Kluft, L., Stevens, B., Brath, M., and Buehler, S. A.: Quantifying all-sky climate sensitivity with idealized clouds, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11046, https://doi.org/10.5194/egusphere-egu25-11046, 2025.
Despite decades of developments, the inter-model spread in climate sensitivity in the latest CMIP ensemble remains substantial, with relevant implications for mid- to long-term climate projections. The inter-model differences are driven by model biases and structural deficiencies, mostly linked to cloud feedbacks, but the specific processes that dominate this issue remain unclear. Physical parametrizations are of primary importance for the performance of climate models, in particular those regarding microphysics and convection - especially at current model resolutions. Indeed, a fundamental yet often overlooked aspect of coupled model development is the tuning of parameters involved in these parametrizations to align with some specific constraints (e.g. radiative balance and global mean temperature in the pre-industrial state).
Here, we propose a methodology to evaluate the uncertainty in equilibrium climate sensitivity (ECS) arising from parameter tuning and apply it to the EC-Earth3 climate model. Our approach consists in systematically perturbing a set of tuning parameters - primarily those affecting tropical convection and precipitation - aiming to maximize their impact on climate sensitivity while ensuring the parameters remain within a plausible range. We obtain a low and a high sensitivity configuration of the model, resulting in a moderate change in climate sensitivity of approximately ±0.3 K. Finally, the results are discussed in the context of the CMIP6 ensemble, suggesting that the inter-model spread is likely driven by deeper structural differences within the models rather than uncertainties arising from the tuning process.
How to cite: Fabiano, F., Ventrucci, C., Davini, P., von Hardenberg, J., and Corti, S.: Quantifying the tuning uncertainty on the climate sensitivity of the EC-Earth climate model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17908, https://doi.org/10.5194/egusphere-egu25-17908, 2025.
How to cite: Hwang, Y.-T., Xie, S.-P., Chen, P.-J., Tseng, H.-Y., Deser, C., Yeh, H.-C., Chen, Y.-J., Dong, Y., Watanabe, M., Kang, S. M., and Stuecker, M. F.: Decoding the Anthropogenic Influences on Pacific Warming Patterns, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13127, https://doi.org/10.5194/egusphere-egu25-13127, 2025.
The majority of climate models predict the development of an enhanced eastern equatorial Pacific (EEP) warming pattern (“El Niño-like”) by century-end, characterized by greater mean warming over the Pacific cold tongue compared to the western Pacific warm pool and the corresponding weakening of the Walker circulation. A number of plausible mechanisms have been proposed to explain this pattern; however, it remains unclear which proposed mechanism is dominant in this response. Moreover, the magnitude of the EEP pattern varies greatly across climate models. To understand these differences, we conduct partially coupled experiments with an abrupt 4xCO2 increase, wherein surface wind stress and shortwave fluxes are overridden to values prescribed from the preindustrial control simulations, using two climate models – CESM1 and CESM2. Although both models were developed at NCAR, their behaviors are very different. In the former model, changes in the east-west SST gradient along the equator are relatively small. In contrast, the latter model, known to have a high climate sensitivity, develops a very strong EEP pattern. We find that the key factors that explains these differences are the different strengths of the Bjerknes (wind stress-SST) and shortwave (low clouds-SST) feedbacks critical in reducing the Pacific zonal SST gradient, whereas differential evaporative cooling in the equatorial region appears to be similar between the two models. We discuss the implications of these results to the ongoing and future changes in the tropical Pacific.
How to cite: Fedorov, A. and Fu, M.: The role of the Bjerknes and low-cloud feedbacks in the formation of the eastern equatorial Pacific warming pattern: contrasting two climate models, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14461, https://doi.org/10.5194/egusphere-egu25-14461, 2025.
The rise in anthropogenic greenhouse gases since the 20th century has led to regionally varying warming rates. Observations over recent decades reveal subdued warming—or even cooling—in the eastern and southeastern tropical Pacific, linked to intensified equatorial trade winds. Understanding this warming pattern is crucial due to its wide-reaching impacts: it alters atmospheric stability, driving rainfall changes through the warmer-get-wetter mechanism; affects El Niño variability and tropical cyclone intensity; and influences cloud cover, planetary albedo, and transient climate sensitivity.
While models from the Coupled Model Intercomparison Project (CMIP), including IPSL-CM6A-LR, generally capture the subdued warming in the Southeast Pacific, they project enhanced warming in the eastern equatorial Pacific (El Niño-like response) that contradicts observations. A major factor behind this discrepancy could be the persistent cold and dry equatorial Pacific bias in these models, particularly pronounced in IPSL-CM6A-LR
To test this hypothesis, we analyze coupled flux-corrected simulations designed to reduce mean state biases. Corrections to momentum and heat fluxes mitigate cold tongue and western Pacific dry biases, as well as easterly wind errors. However, the double Intertropical Convergence Zone (ITCZ) bias remains substantial. We examine how these biases influence the tropical Pacific warming pattern during the historical period and under 21st-century climate projections, while addressing the limitations of stationary flux correction. Finally, we outline planned sensitivity experiments to explore the key physical processes driving the tropical warming pattern in response to climate change.
How to cite: Stoppelli, A., Éthé, C., Mignot, J., and Vialard, J.: Influence of Mean State Biases on Projections of the Tropical Warming Pattern, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5900, https://doi.org/10.5194/egusphere-egu25-5900, 2025.
The notion of climate sensitivity has become synonymous with Equilibrium Climate Sensitivity (ECS). But, 21st century warming is affected at zeroth order by ocean heat uptake, which isn't accounted for by ECS but is accounted for by the Transient Climate Response (TCR). In this talk, we highlight some potentially underappreciated aspects of TCR and ocean heat uptake, using the two-box ocean model as a common theoretical framework. We emphasize that i) TCR can be scaled by the forcing to estimate transient temperature change across a variety of scenarios, ii) this scaling can be used to estimate the time to cross a given temperature target in a given forcing scenario, using only a model's TCR, and iii) the two-box model predicts a linear relationship between ocean heat content and surface temperature which is inconsistent with most models. This talk is based on an forthcoming article in Annual Reviews of Earth and Planetary Sciences.
How to cite: Jeevanjee, N., Paynter, D., Dunne, J., Krasting, J., and Sentman, L.: Perspectives on Climate Sensitivity and Ocean Heat Uptake, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7361, https://doi.org/10.5194/egusphere-egu25-7361, 2025.
Understanding the relationships between internal variability and forced climate feedbacks is key for using observations to constrain future climate change. Here we probe and interpret the differences in these relationships between the idealised climate change projections provided by the CMIP5 and CMIP6 experiment ensembles. We find that internal variability feedbacks better predict forced feedbacks in CMIP6 relative to CMIP5 by over 50%, and that the increased predictability derives primarily from the slow (>20 year) response to greenhouse gas forcing. A key novel result is that the increased predictability is consistent with the greater resemblance between patterns of internal and forced temperature change in CMIP6, which suggests temperature pattern effects play a key role in predicting forced climate feedbacks. In general, we find that forced feedbacks are more predictable when the response more closely resembles El Niño, with amplified East Pacific warming and cloud changes reflecting a weakened Walker circulation. Despite the increased predictability, emergent constraints provided by observed internal variability are weak and largely unchanged from CMIP5 to CMIP6 due to the relative shortness of the observational record.
How to cite: Davis, L., Thompson, D. W. J., Rugenstein, M., and Birner, T.: Links between internal variability and forced climate feedbacks: The importance of patterns of temperature variability and change, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20967, https://doi.org/10.5194/egusphere-egu25-20967, 2025.
We estimate the contributions of the spatially varying temperature field to internal climate feedbacks through the statistical relationships between the observed global-mean radiative response R and the spatially-varying temperature field Ti. The results indicate regions where surface temperature covaries with R and thus provide a statistical analogue to the causal response functions derived from simulations forced with surface temperature “patches”. Notably, the results of the statistical analyses yield patterns in temperature that explain roughly the same fraction of the variability in R as that explained by patch experiments. Consistent with the results of those experiments, the observational analyses indicate large negative internal feedbacks due to temperature variability over the western Pacific. Unlike the results inferred from such experiments, the analyses indicate equally large positive internal feedbacks over the southeastern tropical Pacific and negative internal feedbacks over land areas. When estimated from observations, temperature variability over the land areas accounts for roughly 80% of the global-mean, negative internal feedback; and temperature variability over the southeastern tropical Pacific acts to attenuate the global-mean negative internal feedback by nearly 10%.
How to cite: Thompson, D. W., Rugenstein, M., Forster, P. M., and Fredericks, L.: An Observational Estimate of the Pattern Effect on Climate Sensitivity, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7524, https://doi.org/10.5194/egusphere-egu25-7524, 2025.
Recent studies have provided analytical descriptions of Earth’s longwave feedback λLW; to expand on this, we propose an analytical model for the shortwave water vapor feedback λSW. In this model, λSW is proportional to the change in the square of atmopsheric transmissivity tatm with temperature T and thus mainly originates from spectral regions that ”transition” from optically thin to optically thick. Following Jeevanjee (2023, DOI: 10.1119/5.0135727), we approximate tatm based on the column water vapor MH2O and the water vapor mass absorption cross-section κH2O. We show that in order to capture the weak T dependence of λSW, it is crucial to account for spectral variations in κH2O, which can already be achieved by a simple exponential fit.
We further demonstrate that the T dependence of λSW can be explained by the opposing effects of two main processes: At low T, more optically thin parts of the spectrum ”start” their transition than optically thick parts ”complete” their transition, leading to an increase in λSW with T. At high T, the inverse T dependence of the Clausius-Clapeyron relation leads to a decrease in λSW.
We can also extend our model to incorporate second-order effects such as spectral variations in solar irradiance and deviations of κH2O from the idealized exponential fit. This version of the model is in good agreement with full radiative transfer simulations. The remaining discrepancies can be attributed to non-linear absorption by the water vapor continuum, deviations in MH2O from the approximated Clausius-Clapeyron scaling, and the effects of molecular Rayleigh scattering.
In conclusion, we demonstrate that the shortwave water vapor feedback λSW can be understood using a simple analytical model. This model also demonstrates the merits of a spectral approach to understand λSW and illuminates the two key processes that drive its T dependence.
How to cite: Roemer, F. E., Buehler, S. A., and Menang, K. P.: How to think about the shortwave water vapor feedback, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1486, https://doi.org/10.5194/egusphere-egu25-1486, 2025.
CO2 is widely appreciated as a radiative forcing agent, but recent work showed that CO2 also acts as a climate feedback (Seeley & Jeevanjee 2020). CO2’s ability to emit longwave radiation allows the atmosphere to shed more energy in response to surface warming, and gives rise to a “radiator fin” effect which dominates Earth’s climate sensitivity in hot-and-high-CO2 climates. However, the general CO2-dependence of the longwave feedback is still poorly understood.
Here we explore the CO2-dependence of Earth’s longwave clear-sky feedback using a line-by-line model. We report a dividing surface temperature (Ts) of ~290 K for typical relative humidities. Above 290K, CO2 increases the feedback; below 290K, CO2 decreases the feedback; around 290K, the feedback is CO2-independent. We explain our results via a spectral competition between CO2 radiator fins, which enhance the feedback, and CO2 blocking the surface’s emission, which decreases the feedback. Only at high Ts, once H2O shuts down all window regions, does CO2 enhance the feedback.
Given that Earth’s global-mean temperature is close to ~290K, our results explain why feedback CO2-dependence is weak in our current climate but could have been important for paleoclimates. Finally, because feedback CO2-dependence is identical to forcing Ts-dependence, our results also explain the temperature-dependence of the CO2 forcing. Analogous to the clear-sky feedback, CO2 forcing also changes its behavior above versus below ~290K.
How to cite: Xu, Y. and Koll, D.: CO2-dependence of Longwave Clear-sky Feedback is sensitive to Temperature, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20708, https://doi.org/10.5194/egusphere-egu25-20708, 2025.
We use atmospheric profiles from ERA5, JRA55 and MERRA2 between 1993 and 2023 to estimate Earth’s global clear-sky longwave feedback strength on the seasonal and interannual timescale. Differences in the relationship of relative humidity with skin temperature prior to 2008 lead to interannual feedback strengths between 1.34 W m−2 K−1 (JRA55) and 1.89 W m−2 K−1 (MERRA2). Restricting the analysis to the last 16 years
yields more consistent interannual estimates of 2.05 W m−2 K−1 on average, which is larger than the overall seasonal estimate of 1.91 W m−2 K−1. The mid-tropospheric drying causing this difference suggest a substantial influence of ENSO variability on the interannual timescale. This indicates a long-term feedback strength smaller than 2.0 W m−2 K−1, which is already at the lower end of previous estimates; emphasizing the importance of accurate long-term RH measurements to reliably project Earth’s clear-sky feedback strength.
How to cite: Gloeckner, H., Kluft, L., Schmidt, H., and Stevens, B.: Estimates of the Global Clear-Sky Longwave Radiative Feedback Strength from Reanalysis Data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2583, https://doi.org/10.5194/egusphere-egu25-2583, 2025.
Understanding how radiative feedbacks respond to different historical forcing agents (e.g. aerosols or greenhouse gases) improves our ability to relate historical changes (1850-2014) to future projections. This is often investigated using historical single forcing experiments, where only one forcing agent is allowed to vary, to decompose the total (all-forcing) response. However, using a 45-member ensemble, we demonstrate that there are non-linearities in this decomposition which challenge its utility in HadGEM3-GC31-LL. Specifically, strong warming in the Southern Ocean and sea ice loss are seen in the aerosol single forcing experiment despite global cooling, which is found to be a feature that does not combine linearly with other climate drivers. Instead, we calculate the aerosol response as the difference between the all-forcing experiment and an “all-but-aerosol” experiment, where all forcing agents apart from aerosols are included. This method does not show strong Southern Ocean warming and sea ice loss in response to anthropogenic aerosols. We instead see a positive surface albedo feedback in this region, which is more consistent with the feedbacks seen in the all-forcing response. This allows for a more accurate comparison between feedbacks caused by different forcing agents.
How to cite: Wright, M., Mutton, H., Ringer, M., and Andrews, T.: Addressing non-linearities when estimating radiative feedbacks associated with different historical forcing agents, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4417, https://doi.org/10.5194/egusphere-egu25-4417, 2025.
The clear-sky response to surface warming is generally the result of increases in tropospheric temperature and water vapour, assuming constant relative humidity (RH). While this purely thermodynamic response is fairly well understood, there has been less focus on whether the response of the large-scale circulation to surface warming can alter the clear-sky response. Therefore, in this study, we investigate how the large-scale circulation on Earth-like planets would respond to warming, whether the constant RH assumption holds for different circulation responses, and how deviations from this assumption can affect the clear-sky response. We use the ECHAM6 general circulation model in an aquaplanet configuration and modify the large-scale circulation by changing the planet's rotation rate from 1/32 to 8 times the current Earth's rotation rate. We run two sets of experiments, one with a fixed SST as a control scenario and the other with a +4K warming scenario. We analyse the radiative flux-circulation response as the difference between the warming and control scenarios.
From faster to slower rotation, the Hadley cell expands and strengthens, increasing the dryness of the atmosphere and decreasing the water vapour masking effect. Therefore, at first order, when RH is assumed to be constant, the clear-sky response increases from faster to slower rotation. However, there are second order effects at rates slower than 1/4 of the Earth's current rotation rate, which we associate with the large changes ( > 10%) in RH. At such slow rotation rates, the Hadley cell becomes global. Meanwhile, a secondary circulation develops, characterised by convergence at the equator in the lower troposphere and divergence in the mid-troposphere. We refer to this as the congestus circulation. Changes in RH correlate well with changes in the response of the congestus circulation to warming. The deep Hadley circulation weakens with surface warming like on Earth. But the congestus circulation strengthens, increasing mid-tropospheric RH, which in turn reduces the clear-sky response. We discuss to what extent this effect is due to increased upper-tropospheric radiative cooling that is not compensated by the deep circulation. Alternatively, we discuss whether this effect is due to increased convective self-aggregation with surface warming that increases the congestus outflow.
How to cite: Gnanaraj, A. M., Schmidt, H., and Bao, J.: Changes in the large-scale circulation and the clear-sky response to warming at very slow rotation rates, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20772, https://doi.org/10.5194/egusphere-egu25-20772, 2025.
Uncertainty in climate sensitivity remains a critical challenge for effective mitigation and adaptation strategies. While cloud radiative feedbacks are often highlighted as a major source of this uncertainty, here we explore the impact of clear-sky shortwave radiation absorption by water vapor (SWA). Using abrupt 4xCO2 simulations with altered SWA, we show that higher SWA conditions lead to a larger increase in climate sensitivity over time due to the faster and stronger recovery of the initially weakened Atlantic Meridional Overturning Circulation (AMOC). Enhanced SWA reduces surface shortwave radiation, leading to global cooling, particularly in the Arctic, where increased salinity creates conditions favorable for AMOC recovery. This accelerated recovery amplifies warming in the subpolar North Atlantic, intensifying positive lapse rate and cloud feedbacks, ultimately leading to a larger increase in net climate feedback and climate sensitivity. This underscores the need to constrain clear-sky SWA uncertainties to improve projections of climate sensitivity and associated feedback mechanisms.
How to cite: Kang, S. and Lee, D.: Effective climate sensitivity increases with enhanced shortwave absorption by water vapor due to its impact on AMOC recovery, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11416, https://doi.org/10.5194/egusphere-egu25-11416, 2025.
Throughout Earth's history and its potential future, surface temperatures (Ts) have fluctuated across a far broader range than those of the present-day climate. However, the characteristics of extremely cold or warm climates remain less explored compared to modern climates. This study investigates the hydrological trends and atmospheric stratification in hothouse climates (Ts>330 K). Our results show that in climate models, precipitation decreases as surface temperature rises in hothouse climates, in contrast to the behavior observed in modern climates. This reversal trend results from the upper limit of outgoing longwave radiation and the continuously increasing shortwave absorption by H2O and aligns with a pronounced increase in atmospheric stratification. One remarkable feature of such a highly stable atmosphere is the occurrence of a large-scale “atmospheric temperature inversion”, where the upper atmosphere is warmer than the lower’s. Although this inversion has been noted in previous studies, its formation mechanisms have remained unclear. Our work demonstrates that while radiative heating in the lower troposphere is necessary, it is not independently sufficient to form this atmospheric inversion. Instead, large-scale subsidence-induced dynamic heating plays an essential role in forming this inversion. Hothouse climates, as characterized by these findings, are feeble worlds rather than vibrant worlds.
How to cite: Liu, J., Yang, J., Ding, F., Chen, G., and Hu, Y.: Reversal of Precipitation Trend and Large-Scale Atmospheric Temperature Inversion in Hothouse Climates, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4990, https://doi.org/10.5194/egusphere-egu25-4990, 2025.
As Earth warms, the tropopause is expected to rise, but predictions of its temperature change are less certain. Longstanding theories tie the tropopause temperature to outgoing longwave radiation (OLR), but this contradicts recent work in which simulations exhibit a Fixed Tropopause Temperature (FiTT) even as OLR increases. The FiTT is thought to result from the interaction between upper tropospheric moisture and radiation, but a predictive theory for FiTT has not yet been formulated. Here, we build on a recent explanation for the temperature of anvil clouds and argue that tropopause temperature, defined by where radiative cooling becomes negligible, is set by water vapor's maximum spectroscopic absorption and Clausius-Clapeyron scaling. This "thermospectric constraint'' makes quantitative predictions for tropopause temperature that are borne out in single column and general circulation model experiments where the spectroscopy is modified and both the radiative and lapse-rate tropopause change in response. This constraint provides a theoretical foundation for the FiTT hypothesis, shows how tropopause temperature can decouple from OLR, and suggests a way to relate the temperatures of anvil clouds and the tropopause.
How to cite: McKim, B., Jeevanjee, N., Vallis, G., and Lewis, N.: Water Vapor Spectroscopy and Thermodynamics Constrain Earth’s Tropopause Temperature, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17356, https://doi.org/10.5194/egusphere-egu25-17356, 2025.
Posters on site: Tue, 29 Apr, 16:15–18:00 | Hall X5
Methane (CH4) and carbon dioxide (CO2) are two major greenhouse gases with distinct radiative properties and climate responses. Using the National Centre for Atmospheric Research (NCAR) Community Atmosphere Model (CAM5) in two configurations (prescribed sea surface temperature and slab ocean) to estimate radiative forcing and climate response and radiative kernel analyses, we compare their slow feedback mechanisms and implications for climate sensitivity. We perform simulations with a 10X increase in CH4 and 1.35X CO2 concentration to simulate global mean warming of about 1.5 K in both cases. We find that CH4 requires a larger effective radiative forcing, indicating a lower efficacy relative to CO2.
We attribute CH4's lower efficacy to differences in slow feedback processes. CH4 exhibits more negative lapse rate feedback (difference of -0.10 Wm-2K-1) and more positive water vapor feedback (difference of 0.06 Wm-2K-1) due to equatorially concentrated radiative forcing and stronger upper-tropospheric warming. Feedback differences also include weaker positive shortwave cloud (difference of -0.05 Wm-2K-1) and smaller albedo (difference of -0.04 Wm-2K-1) feedback responses for CH4, resulting in a net feedback difference of -0.12 Wm-2K-1. These findings underscore the role of spatial forcing patterns, including CH4’s near-infrared shortwave absorption bands and low-latitude warming, in shaping feedback processes.
We find that CO2 forcing results in relatively stronger polar warming, enhancing albedo feedback, and induces larger mid-latitude cloud reductions, amplifying shortwave cloud feedback. Both gases have comparable positive longwave cloud feedback, broadly consistent with fixed anvil temperatures. All individual feedback differences are statistically significant. Our results highlight that distinct feedback responses arise from basic physical mechanisms, such as differing meridional warming patterns and small but distinct relative humidity changes.
Our study advances the understanding of radiative forcing structure and feedbacks in determining greenhouse gas impacts on climate sensitivity. It also highlights the need for multi-model assessments and Earth system modeling to evaluate feedback uncertainties and refine projections of long-term climate responses as relative contributions to radiative forcing evolve.
How to cite: Kumar, S., Seshadri, A. K., and Bala, G.: An Estimation of the Efficacy of Methane Radiative Forcing Using Radiative Kernels, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5950, https://doi.org/10.5194/egusphere-egu25-5950, 2025.
We set out to disentangle the impacts of forcing over land vs. forcing over ocean on the sea surface temperature (SST) pattern. Based on previous research showing that forcings over land and ocean have distinct impacts on the circulation, we hypothesize that they would also affect the pattern of sea surface temperatures in different ways. We investigate the research question by quadrupling the CO2 concentration either only over ocean or only over land in the coupled global climate model MPI-ESM-1.2.
Our main results are:
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the climate response to 4 x CO2 forcing only over land surface and forcing only over ocean adds up surprisingly linearly to the climate response to forcing everywhere.
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4 x CO2 forcing over land causes a cooling of up to 1.4 K in the equatorial, Eastern, and Southeastern Pacific Ocean within two years. In contrast, positive forcing over the ocean does not produce such a cooling on any time scale
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Two main mechanisms contribute to the Pacific cooling in response to positive forcing over land:
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(a) a northward ITCZ shift originating from the fact that there is more land in the Northern than the Southern hemisphere, enhancing equatorial upwelling and cooling from strengthened trade winds
- (b) the monsoon-desert mechanism (Rodwell & Hoskins 1996), which strengthens the subtropical highs in response to atmospheric heating over the Americas, increasing the equatorward advection of cold air and initiating a wind-evaporation-SST feedback.
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We find an equatorial and Eastern Pacific cooling not only in the abrupt land-forced simulation, but also in a transient simulation forced with a 1% / year CO2 increase over land, on a time scale of 20 years. The surprising finding that a positive forcing can cause a cooling in the Eastern Pacific, along with the mechanisms we describe, may contribute to better understanding the recent cooling of the Eastern Pacific Ocean as well as the long-standing model bias in simulating Eastern Pacific sea surface temperature patterns.
How to cite: Günther, M., Kang, S., and Kaspi, Y.: Positive forcing over land cools the Eastern Pacific Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3714, https://doi.org/10.5194/egusphere-egu25-3714, 2025.
The interaction of cloud droplets and ice crystals with radiation, known as cloud radiative heating, alters temperature gradients in the atmosphere, affecting both cloud evolution as well as circulation and precipitation. Despite its climatic relevance, the response of cloud radiative heating to global warming remains largely unknown.
We study changes to cloud radiative heating profiles in a warmer climate, identify physical mechanisms responsible for these changes, and develop a theory based on well-understood physics to predict them. Our approach involves a stepwise procedure that starts with a simple hypothesis of an upward shift in cloud radiative heating at constant temperature, and gradually incorporates additional physical effects.
We find that cloud radiative heating intensifies as high clouds move upward, despite minimal changes in cloud properties and temperatures. We attribute this intensification to a decrease in air density, which often overcompensates for the decrease in high cloud fraction with warming in idealized multi-model simulations of radiative-convective equilibrium. Furthermore, the density-mediated changes in cloud radiative heating are also observed in satellite-derived retrievals of cloud radiative heating in the tropics.
The density-mediated increment in cloud radiative heating may increase the role of high clouds in controlling atmospheric flows in a warmer climate. Moreover, our results suggest that the uncertainty in model‐predicted changes in atmospheric circulations and hence regional climate could be reduced by narrowing the spread in model‐simulated cloud radiative heating in the present‐day climate.
How to cite: Gasparini, B., Mandorli, G., Stubenrauch, C., and Voigt, A.: Basic physic predicts stronger high cloud radiative heating with warming, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8545, https://doi.org/10.5194/egusphere-egu25-8545, 2025.
Climate forcers perturb the energy amount inside the Earth, and atmospheric interactions in the troposphere sequentially vary to pursue the new stable state in the given energy budget. The varied energy amount of longwave, shortwave, and sensible heat flux in the atmosphere is balanced with latent heat flux, equivalent to the changes in precipitation in the global mean sense. For example, rising temperature emits more longwave radiation from the atmosphere (longwave cooling, LWC), and it allows more energy budget room for latent heat flux (LHF) heating, which explains enhanced precipitation.
Although previous studies argued hydrological sensitivity as the linearized scale of precipitation change per the global mean temperature change, this study confirms that tropical tropospheric stability has additionally affected hydrological sensitivity over the decades. Our results reveal that tropical ocean temperature patterns correlate statistically with the stability index. The numerically simplified term of this stability effect improves the prediction skills of the theoretical equation for the global mean precipitation change under scenarios with various forcing conditions. Lastly, we discuss the possible impacts of recent ocean patterns and the tropical tropospheric stability phase on precipitation by comparing the observed data and climate models’ simulations, which are forced by the observed sea surface temperature.
How to cite: Lee, D. and Ceppi, P.: Hydrological sensitivity affected by tropical tropospheric stability, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14568, https://doi.org/10.5194/egusphere-egu25-14568, 2025.
We construct a radiative-advective model to investigate the drivers of Arctic amplification. The Rapid Radiative Transfer Model for GCMs (RRTMG) is utilized to calculate radiative heating rates, while the atmospheric horizontal energy transport (AHT) from JRA-55 reanalysis data is used as boundary conditions. We perturb individual factors in the model to assess the warming contributions from radiative forcing by different greenhouse gases, poleward energy transport at different vertical levels, and clouds.
We first examine the Arctic climate sensitivity to CO2, CH4, and O3. The climate sensitivity is defined as the surface temperature change per unit of TOA flux perturbation. The Arctic climate sensitivity to CO2 is 3.46 K/W/m². When CO2 is doubled, the instantaneous radiative forcing at TOA is 1.68 W/m², resulting in 5.7 K surface warming. For CH4, the Arctic climate sensitivity is 1.65 K/W/m², and doubling CH4 leads to a TOA perturbation of 0.46 W/m², leading to merely 0.76 K surface warming. The sensitivity to O3 is 0.21 K/W/m², with a doubling of O3 causing a 3.51 W/m² perturbation and 0.75 K surface warming.
The sensitivity to AHT is strongly dependent on its vertical structure, with greater sensitivity at lower levels. At 975 hPa level, the climate sensitivity reaches its peak value of 3.15 K/W/m², comparable to that of CO2. At the 900 hPa level where climatological AHT peaks, the climate sensitivity dramatically drops to 0.73 K/W/m². At higher altitude, the sensitivity continues to decrease: 0.64 K/W/m² at 850 hPa, 0.62 K/W/m² at 700 hPa, and 0.30 K/W/m² at 500 hPa. The climate sensitivity to clouds is 1.62 K/W/m². The climatological cloud fraction in the Arctic is 15%, with radiative effect of 2.09 W/m² at the TOA, resulting 3.41 K surface warming.
In summary, the Arctic region shows highest climate sensitive to CO2, followed by sensitivity to AHT at 975 hPa and CH4, although CH4 increase does not induce significant flux perturbations at the TOA. Clouds also play an important role. The sensitivities to AHT above 900 hPa and O3 are relatively smaller.
How to cite: Zhang, H. and Wang, Y.: Understanding the drivers of Arctic amplification through an idealized radiative-advective equilibrium model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15286, https://doi.org/10.5194/egusphere-egu25-15286, 2025.
It is well-known that Earth's planetary albedo is about 0.3. Less clear is how this value might change in different climates. Here we propose a simple conceptual model for Earth's albedo. Our main insight is that, for a clear-sky N2-H2O atmosphere, the atmosphere can be approximated in the shortwave spectrum as either perfectly absorbing (due to water vapor absorption) or perfectly scattering (due to Rayleigh scattering); in contrast, clouds are approximately perfect scatterers throughout the shortwave spectrum. We use these approximations to derive analytic albedo expressions from the two-stream equations, which we validate against line-by-line model calculations.
Our results indicate that, for a clear-sky atmosphere, as surface temperature rises from 200 K to 500 K, Earth’s planetary albedo initially decreases with warming until around 350 K due to enhanced water vapor absorption, and then increases due to intensified Rayleigh scattering. Turning to idealized high and low cloud scenarios, cloudy atmospheres have significantly higher albedos than a clear-sky atmosphere at low temperatures. However, for an atmosphere with low clouds, albedo generally decreases with warming due to increased water vapor absorption above the clouds. In contrast, an atmosphere with high clouds exhibits nearly constant albedo with temperature, as high clouds mask the influence of the underlying atmosphere. These findings suggest that Earth’s clear-sky shortwave feedback is positive below 350 K and negative above 350 K. As for cloudy scenarios, low clouds induce a strong positive shortwave feedback at low temperatures, while high clouds don’t. Our simple model improves understanding of Earth’s planetary albedo and the role of shortwave feedback for the runaway greenhouse. Furthermore, our work suggests low clouds generally tend to destabilize Earth’s climate, which has potential implications for future climate change adaptation.
How to cite: Zhang, Z., Koll, D., and Cronin, T.: A Simple Spectral Model for Earth’s Albedo, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13431, https://doi.org/10.5194/egusphere-egu25-13431, 2025.
Cloud responses to warming are a known uncertainty for temperature projections, yet how these same responses affect precipitation has been little evaluated. Here we explore how cloud radiative feedbacks influence the global mean precipitation change per degree of warming (hydrological sensitivity). With radiative kernels, we examine how warming-induced changes in cloud amount, altitude, and optical depth alter the atmosphere’s ability to radiatively cool and form precipitation. Our results suggest that high cloud responses are the single largest cause of spread in hydrological sensitivity across climate models. Applying the cloud locking methodology to one model, we find that cloud radiative responses reduce hydrological sensitivity by 14% and investigate the controls on this value.
How to cite: McGraw, Z., Polvani, L., Gasparini, B., Van de Koot, E., and Voigt, A.: Cloud Feedbacks Affect Hydrological Sensitivity, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12505, https://doi.org/10.5194/egusphere-egu25-12505, 2025.
