Tropospheric Adjustment Induces a Cloud Component in CO2 Forcing

2008 ◽  
Vol 21 (1) ◽  
pp. 58-71 ◽  
Author(s):  
Jonathan Gregory ◽  
Mark Webb
Keyword(s):  
Radiative Forcing ◽  
Climate Models ◽  
Surface Air Temperature ◽  
Co2 Concentration ◽  
Climate Feedback ◽  
Climate Projections ◽  
Feedback Parameter ◽  
Tropospheric Adjustment ◽  
Effective Radiative Forcing ◽  
Global Mean

Abstract The radiative forcing of CO2 and the climate feedback parameter are evaluated in several climate models with slab oceans by regressing the annual-mean global-mean top-of-atmosphere radiative flux against the annual-mean global-mean surface air temperature change ΔT following a doubling of atmospheric CO2 concentration. The method indicates that in many models there is a significant rapid tropospheric adjustment to CO2 leading to changes in cloud, and reducing the effective radiative forcing, in a way analogous to the indirect and semidirect effects of aerosol. By contrast, in most models the cloud feedback is small, defined as the part of the change that evolves with ΔT. Comparison with forcing evaluated by fixing sea surface conditions gives qualitatively similar results for the cloud components of forcing, both globally and locally. Tropospheric adjustment to CO2 may be responsible for some of the model spread in equilibrium climate sensitivity and could affect time-dependent climate projections.

scholarly journals The inconstancy of the transient climate response parameter under increasing CO 2

2015 ◽  
Vol 373 (2054) ◽  
pp. 20140417 ◽  
Author(s):  
J. M. Gregory ◽  
T. Andrews ◽  
P. Good
Keyword(s):  
Radiative Forcing ◽  
Coupled Model ◽  
Surface Air Temperature ◽  
Deep Ocean ◽  
Climate Feedback ◽  
Climate Response ◽  
Ocean Heat Uptake ◽  
Transient Climate Response ◽  
Feedback Parameter ◽  
Response Parameter

In the Coupled Model Intercomparison Project Phase 5 (CMIP5), the model-mean increase in global mean surface air temperature T under the 1pctCO2 scenario (atmospheric CO 2 increasing at 1% yr −1 ) during the second doubling of CO 2 is 40% larger than the transient climate response (TCR), i.e. the increase in T during the first doubling. We identify four possible contributory effects. First, the surface climate system loses heat less readily into the ocean beneath as the latter warms. The model spread in the thermal coupling between the upper and deep ocean largely explains the model spread in ocean heat uptake efficiency. Second, CO 2 radiative forcing may rise more rapidly than logarithmically with CO 2 concentration. Third, the climate feedback parameter may decline as the CO 2 concentration rises. With CMIP5 data, we cannot distinguish the second and third possibilities. Fourth, the climate feedback parameter declines as time passes or T rises; in 1pctCO2, this effect is less important than the others. We find that T projected for the end of the twenty-first century correlates more highly with T at the time of quadrupled CO 2 in 1pctCO2 than with the TCR, and we suggest that the TCR may be underestimated from observed climate change.

Warmer climate projections in CMIP6: the role of changes in the greenhouse gas concentrations from CMIP5 to CMIP6

2020 ◽  
Author(s):  
Klaus Wyser ◽  
Erik Kjellström ◽  
Torben Koenigk ◽  
Helena Martins ◽  
Ralf Döscher
Keyword(s):  
Radiative Forcing ◽  
Climate Models ◽  
Climate Impact ◽  
Earth Model ◽  
Minor Role ◽  
Climate Projections ◽  
First Results ◽  
Effective Radiative Forcing ◽  
A Minor ◽  
New Generation

<p>Many modelling groups have contributed with CMIP6 scenario experiments to the CMIP6 archive. The analysis of CMIP6 future projections has started and first results indicate that CMIP6 projections are warmer than their counterparts from CMIP5. To some extent this is explained with the higher climate sensitivity of many of the new generation of climate models. However, not only have models been updated since CMIP5 but also the forcings have changed from RCPs to SSPs. The new SSPs have been designed to have the same instantaneous radiative forcing at the end of the 21st century. However, we find that in the EC-Earth3 model the effective radiative forcing differs substantially when the GHG concentrations from the SSP are replaced by those from the corresponding RCP with the same nameplate RF. We estimate that for the SSP5-8.5 and SSP2-4.5 scenarios 50% or more of the stronger warming in CMIP6 than CMIP5 for the EC-Earth model can be explained by changes in GHG gas concentrations. Other changes in the forcing datasets such as aerosols only play a minor role for the additional warming. The discrepancy between RCP and SSP forcing datasets needs to be accounted for when comparing CMIP5 and CMIP6 climate projections and should be properly conveyed to the climate impact, adaptation and mitigation communities.</p>

scholarly journals Explaining the Spread in Global Mean Thermosteric Sea Level Rise in CMIP5 Climate Models*

2015 ◽  
Vol 28 (24) ◽  
pp. 9918-9940 ◽  
Author(s):  
Angélique Melet ◽  
Benoit Meyssignac
Keyword(s):  
Twentieth Century ◽  
Sea Level ◽  
Radiative Forcing ◽  
Climate Models ◽  
First Century ◽  
Model Ensemble ◽  
Ocean Heat Uptake ◽  
Feedback Parameter ◽  
Twenty First Century ◽  
Global Mean

Abstract The ocean stores more than 90% of the energy excess associated with anthropogenic climate change. The resulting ocean warming and thermal expansion are leading contributors to global mean sea level (GMSL) rise. Confidence in projections of GMSL rise therefore depends on the ability of climate models to reproduce global mean thermosteric sea level (GMTSL) rise over the twentieth century. This study first compares the GMTSL of the climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) to observations over 1961–2005. Although the model ensemble mean is within the uncertainty of observations, the model ensemble exhibits a large spread. The authors then aim to explain the spread in CMIP5 climate model GMTSL over the twentieth and twenty-first centuries. It is shown that the climate models’ GMTSL rise depends linearly on the time-integrated radiative forcing F (under continuously increasing radiative forcing). The constant of proportionality μ expresses the transient thermosteric sea level response of the climate system, and it depends on the fraction of excess heat stored in the ocean, the expansion efficiency of heat, the climate feedback parameter, and the ocean heat uptake efficiency. The across-model spread in μ explains most (>70%) of the across-model spread in GMTSL rise over the twentieth and twenty-first centuries, while the across-model spread in time-integrated F explains the rest. The time-integrated F explains less variance in the across-model GMTSL rise in twenty-first-century than in twentieth-century simulations, as the spread in F is reduced over the twenty-first century because the anthropogenic aerosol forcing, which is a large source of uncertainty in F, becomes relatively smaller.

scholarly journals The Dependence of Radiative Forcing and Feedback on Evolving Patterns of Surface Temperature Change in Climate Models

2015 ◽  
Vol 28 (4) ◽  
pp. 1630-1648 ◽  
Author(s):  
Timothy Andrews ◽  
Jonathan M. Gregory ◽  
Mark J. Webb
Keyword(s):  
Surface Temperature ◽  
Temperature Change ◽  
Radiative Forcing ◽  
General Circulation ◽  
Climate Feedback ◽  
Climate Feedbacks ◽  
Surface Temperature Change ◽  
Feedback Parameter ◽  
Surface Warming ◽  
Global Mean

Abstract Experiments with CO2 instantaneously quadrupled and then held constant are used to show that the relationship between the global-mean net heat input to the climate system and the global-mean surface air temperature change is nonlinear in phase 5 of the Coupled Model Intercomparison Project (CMIP5) atmosphere–ocean general circulation models (AOGCMs). The nonlinearity is shown to arise from a change in strength of climate feedbacks driven by an evolving pattern of surface warming. In 23 out of the 27 AOGCMs examined, the climate feedback parameter becomes significantly (95% confidence) less negative (i.e., the effective climate sensitivity increases) as time passes. Cloud feedback parameters show the largest changes. In the AOGCM mean, approximately 60% of the change in feedback parameter comes from the tropics (30°N–30°S). An important region involved is the tropical Pacific, where the surface warming intensifies in the east after a few decades. The dependence of climate feedbacks on an evolving pattern of surface warming is confirmed using the HadGEM2 and HadCM3 atmosphere GCMs (AGCMs). With monthly evolving sea surface temperatures and sea ice prescribed from its AOGCM counterpart, each AGCM reproduces the time-varying feedbacks, but when a fixed pattern of warming is prescribed the radiative response is linear with global temperature change or nearly so. It is also demonstrated that the regression and fixed-SST methods for evaluating effective radiative forcing are in principle different, because rapid SST adjustment when CO2 is changed can produce a pattern of surface temperature change with zero global mean but nonzero change in net radiation at the top of the atmosphere (~−0.5 W m−2 in HadCM3).

scholarly journals The Greenhouse Effect Calculations by an Iteration Method and the Issue of Stratospheric Cooling

2020 ◽  
pp. 1-18
Author(s):  
Antero Ollila
Keyword(s):  
Greenhouse Effect ◽  
Iteration Method ◽  
Radiative Forcing ◽  
Climate Models ◽  
Co2 Concentration ◽  
Transient Climate Response ◽  
Effective Radiative Forcing ◽  
Stratospheric Cooling ◽  
Temperature Impacts ◽  
Increased Co2 Concentration

The anthropogenic global warming theory is based on the greenhouse effect (GH), which is due to the longwave (LW) absorption by GH gases and clouds according to the IPCC. This LW radiation downward is the imminent cause for the GH effect increasing the surface temperature by 33°C. It has been shown that latent and sensible heating are essential parts of downward LW radiation and the total GH effect. In this study, an iteration method utilizing this basic GH effect mechanism has been applied to simulate the warming impacts of enhanced GH effect changes. The results are compatible with the Transient Climate Response (TCR) of 0.6°C. The issue of stratospheric cooling due to increased CO2 concentration has been calculated and analyzed. The stratospheric cooling effect is real but its impact on the Effective Radiative Forcing (ERF) has been shown to be negative and not positive as generally implied. The reason is that the decreased absorption of LW radiation in the atmosphere always decreases the GH effect. This result challenges the new concept of the ERF that is the sum of Instantaneous RF (IRF) and rapid adjustments as applied in General Climate Models (GCMs). If the stratospheric adjustment has the opposite effect, then the IRF values would be also wrongly calculated in these models. Two independent validation methods were applied to test the temperature impacts of CO2 concentration increases.

scholarly journals Radiative Forcing of Quadrupling CO2

2014 ◽  
Vol 27 (7) ◽  
pp. 2496-2508 ◽  
Author(s):  
Minghong Zhang ◽  
Yi Huang
Keyword(s):  
Radiative Forcing ◽  
Coupled Model ◽  
Climate Feedback ◽  
Ensemble Mean ◽  
Intercomparison Project ◽  
Radiation Variation ◽  
Tropospheric Adjustment ◽  
Validation Tests ◽  
Global Mean ◽  
Maximum Minimum

Abstract An analysis method proposed by Huang is improved and used to dissect the radiative forcing in the instantaneous quadrupling CO2 experiment from phase 5 of the Coupled Model Intercomparison Project (CMIP5). Multiple validation tests show that the errors in the forcing estimates are generally within 10%. The results show that quadrupling CO2, on average, induces a global-mean all-sky instantaneous top-of-the-atmosphere forcing of 5.4 W m−2, which is amended by a stratospheric adjustment of 1.9 W m−2 and a tropospheric adjustment of −0.1 W m−2. The resulting fully adjusted radiative forcing has an ensemble mean of 7.2 W m−2 and a substantial intermodel spread (maximum–minimum) of 2.4 W m−2, which results from all the forcing components, especially the instantaneous forcing and tropospheric adjustment. The fidelity of the linear decomposition of the overall radiation variation is improved when forcing is explicitly estimated for each model. A significant contribution by forcing uncertainty to the intermodel spread of the surface temperature projection is verified. The results reaffirm the importance of evaluating the radiative forcing components in climate feedback analyses.

scholarly journals Observational constraints reduce estimates of the global mean climate relevance of black carbon

2021 ◽  
Author(s):  
Gunnar Myhre ◽  
Bjørn Samset ◽  
Camilla Weum Stjern ◽  
Øivind Hodnebrog ◽  
Ryan Kramer ◽  
...  
Keyword(s):  
Black Carbon ◽  
Radiative Forcing ◽  
Climate Models ◽  
Absorption Efficiency ◽  
Surface Temperature Change ◽  
A Value ◽  
Rapid Adjustment ◽  
Effective Radiative Forcing ◽  
Mean Climate ◽  
Global Mean

Abstract How emissions of black carbon (BC) aerosols affect the climate is still uncertain, due to incomplete knowledge of its sources, optical properties and atmospheric processes such as transport, removal and impact on clouds. Here we constrain simulations from four climate models with observations of atmospheric BC concentrations and absorption efficiency, and the most recent emission inventories, to show that the current global mean surface temperature change from anthropogenic BC emissions is likely to be weak at +0.03 ±0.02K. Atmospheric rapid adjustment processes are found to reduce the top of atmosphere radiative imbalance relative to instantaneous radiative forcing (direct aerosol effect) by almost 50% as a multi-model mean. Furthermore, constraining the models to reproduce observational estimates of the atmospheric vertical profile reduces BC effective radiative forcing to 0.08 W m-2, a value more than 50% lower than in unconstrained simulations. Our results imply a need to revisit commonly used climate metrics such as the global warming potential of BC. This value (for a 100-year time horizon) reduces from 680 when neglecting rapid adjustments and using an unconstrained BC profile to our best estimate of 160 ±120.

Effective Radiative Forcing and Adjustments in CMIP6

2020 ◽  
Author(s):  
Christopher Smith ◽  
Ryan Kramer ◽  
Gunnar Myhre ◽  
Kari Alterskjær ◽  
Bill Collins ◽  
...  
Keyword(s):  
Climate Sensitivity ◽  
Radiative Forcing ◽  
Climate Models ◽  
Internal Variability ◽  
Cmip5 Models ◽  
Aerosol Forcing ◽  
Intercomparison Project ◽  
Effective Radiative Forcing ◽  
Contemporary Climate ◽  
Global Mean

<p>The effective radiative forcing, which includes the instantaneous forcing plus adjustments from the atmsophere and surface, as emerged as the key metric of evaluating human and natural influence on the climate. We evaluate effective radiative forcing and atmospheric adjustments in 13 contemporary climate models that are participating in CMIP6 and have contributed to the Radiative Forcing Model Intercomparison Project (RFMIP). Present-day (2014) global mean anthropogenic forcing relative to pre-industrial (1850) from climate models stands at 1.97 (± 0.26) W m<sup>-2</sup>, comprised of 1.80 (± 0.11) W m<sup>-2</sup> from CO<sub>2</sub>, 1.07 (± 0.21) W m<sup>-2</sup> from other well-mixed greenhouse gases, -1.04 (± 0.23) W m<sup>-2</sup> from aerosols and -0.08 (± 0.14) W m<sup>-2</sup> from land use change. Quoted ranges are one standard deviation across model best estimates, and 90% confidence in the reported forcings, due to internal variability, is typically within 0.1 W m<sup>-2</sup>. The majority of the remaining 0.17 W m<sup>-2</sup> is likely to be from ozone. As determined in previous studies, cancellation of tropospheric and surface adjustments means that the "traditional" stratospherically adjusted radiative forcing is approximately equal to ERF for greenhouse gas forcing, but not for aerosols, and consequentially, not for the anthropogenic total forcing. The spread of present-day aerosol forcing has narrowed compared to CMIP5 models to the range of -0.63 to -1.37 W m<sup>-2</sup>, with a less negative mean. The spread in CO<sub>2</sub> forcing has also narrowed in CMIP6 compared to CMIP5, which may be a consequence of improving radiative transfer parameterisations. We also find that present-day aerosol forcing is uncorrelated with equilibrium climate sensitivity. Therefore, there is no evidence to suggest that the higher climate sensitivity in many CMIP6 models is a consequence of stronger negative present-day aerosol forcing.</p>

scholarly journals An energy budget framework to understand mechanisms of land–ocean warming contrast induced by increasing greenhouse gases Part I: Near-equilibrium state

2021 ◽  
pp. 1-63
Author(s):  
Masaki Toda ◽  
Masahiro Watanabe ◽  
Masakazu Yoshimori
Keyword(s):  
Heat Capacity ◽  
Equilibrium State ◽  
Radiative Forcing ◽  
Energy Transport ◽  
Coupled Model ◽  
Surface Air Temperature ◽  
Ocean Warming ◽  
Climate Feedback ◽  
Energy Budgets ◽  
Effective Radiative Forcing

AbstractModeling studies have shown that surface air temperature (SAT) increase in response to an increase in the atmospheric CO2 concentration is larger over land than over ocean. This so-called land–ocean warming contrast, φ, defined as the land–mean SAT change divided by the ocean-mean SAT change, is a striking feature of global warming. Small heat capacity over land is unlikely the sole cause because the land-ocean warming contrast is found in the equilibrium state of CO2 doubling experiments.Several different mechanisms have been proposed to explain the land–ocean warming contrast, but the comprehensive understanding has not yet been obtained. In Part I of this study, we propose a framework to diagnose φ based on energy budgets at the top of atmosphere and for the atmosphere, which enables the decomposition of contributions from effective radiative forcing (ERF), climate feedback, heat capacity, and atmospheric energy transport anomaly to φ. Using this framework, we analyzed the SAT response to an abrupt CO2 quadrupling using 15 Coupled Model Intercomparison Project Phase 6 (CMIP6) Earth system models. In the near-equilibrium state (years 121-150), φ is 1.49 ± 0.11, which is primarily induced by the land–ocean difference in ERF and heat capacity. We found that contributions from ERF, feedback, and energy transport anomaly tend to cancel each other, leading to a small inter-model spread of φ compared to the large spread of individual components. In the equilibrium state without heat capacity contribution, ERF and energy transport anomaly are the major contributors to φ, which shows a weak negative correlation with the equilibrium climate sensitivity.

scholarly journals Does Antarctic glaciation cool the world?

2013 ◽  
Vol 9 (1) ◽  
pp. 173-189 ◽  
Author(s):  
A. Goldner ◽  
M. Huber ◽  
R. Caballero
Keyword(s):  
Surface Temperature ◽  
Climate Sensitivity ◽  
Radiative Forcing ◽  
Climate Models ◽  
Ice Sheet ◽  
Cloud Feedback ◽  
Modern World ◽  
Global Mean Surface Temperature ◽  
Effective Radiative Forcing ◽  
Global Mean

Abstract. In this study, we compare the simulated climatic impact of adding an Antarctic ice sheet (AIS) to the "greenhouse world" of the Eocene and removing the AIS from the modern world. The modern global mean surface temperature anomaly (ΔT) induced by Antarctic Glaciation depends on the background CO2 levels and ranges from −1.22 to −0.18 K. The Eocene ΔT is nearly constant at ~−0.25 K. We calculate an climate sensitivity parameter S[Antarctica] which we define as ΔT divided by the change in effective radiative forcing (ΔQAntarctica) which includes some fast feedbacks imposed by prescribing the glacial properties of Antarctica. The main difference between the modern and Eocene responses is that a negative cloud feedback warms much of the Earth's surface as a large AIS is introduced in the Eocene, whereas this cloud feedback is weakly positive and acts in combination with positive sea-ice feedbacks to enhance cooling introduced by adding an ice sheet in the modern. Because of the importance of cloud feedbacks in determining the final temperature sensitivity of the AIS, our results are likely to be model dependent. Nevertheless, these model results suggest that the effective radiative forcing and feedbacks induced by the AIS did not significantly decrease global mean surface temperature across the Eocene–Oligocene transition (EOT −34.1 to 33.6 Ma) and that other factors like declining atmospheric CO2 are more important for cooling across the EOT. The results illustrate that the efficacy of AIS forcing in the Eocene is not necessarily close to one and is likely to be model and state dependent. This implies that using EOT paleoclimate proxy data by itself to estimate climate sensitivity for future climate prediction requires climate models and consequently these estimates will have large uncertainty, largely due to uncertainties in modelling low clouds.

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