Stratospheric Adjustment Research Articles

How Well do We Understand the Planck Feedback?

AbstractA reference or “no‐feedback” radiative response to warming is fundamental to understanding how much global warming will occur for a given change in greenhouse gases or solar radiation incident on the Earth. The simplest estimate of this radiative response is given by the Stefan‐Boltzmann law as W m−2 K−1 for Earth's present climate, where is a global effective emission temperature. The comparable radiative response in climate models, widely called the “Planck feedback,” averages −3.3 W m−2 K−1. This difference of 0.5 W m−2 K−1 is large compared to the uncertainty in the net climate feedback, yet it has not been studied carefully. We use radiative transfer models to analyze these two radiative feedbacks to warming, and find that the difference arises primarily from the lack of stratospheric warming assumed in calculations of the Planck feedback (traditionally justified by differing constraints on and time scales of stratospheric adjustment relative to surface and tropospheric warming). The Planck feedback is thus masked for wavelengths with non‐negligible stratospheric opacity, and this effect implicitly acts to amplify warming in current feedback analysis of climate change. Other differences between Planck and Stefan‐Boltzmann feedbacks arise from temperature‐dependent gas opacities, and several artifacts of nonlinear averaging across wavelengths, heights, and different locations; these effects partly cancel but as a whole slightly destabilize the Planck feedback. Our results point to an important role played by stratospheric opacity in Earth's climate sensitivity, and clarify a long‐overlooked but notable gap in our understanding of Earth's reference radiative response to warming.

The Climate and Ozone Impacts of Black Carbon Emissions From Global Rocket Launches

AbstractAerosol emissions from spaceflight activities play a small but increasing role in the background stratospheric aerosol population. Rockets used by the global launch industry emit black carbon (BC) particles directly into the stratosphere where they accumulate, absorb solar radiation, and warm the surrounding air. We model the chemical and dynamical response of the atmosphere to northern mid‐latitude rocket BC emissions. We initially examine emissions at a rate of 10 Gg per year, which is an order of magnitude larger than current emissions, but consistent with extrapolations of space traffic growth several decades into the future. We also perform runs at 30 and 100 Gg per year in order to better delineate the atmosphere's response to rocket BC emissions. We show that a 10 Gg/yr rocket BC emission increases stratospheric temperatures by as much as 1.5 K in the stratosphere. Changes in global circulation also occur. For example, the annual subtropical jet wind speeds slow down by as much as 5 m/s, while a 10%–20% weakening of the overturning circulation occurs in the northern hemisphere during multiple seasons. Warming temperatures lead to a ozone reduction in the northern hemisphere by as much as 16 DU in some months. The climate response increases in a near linear fashion when looking at larger 30 and 100 Gg emission scenarios. Comparing the amplitude of the atmospheric response using different emission rates provides insight into stratospheric adjustment and feedback mechanisms. Our results show that the stratosphere is sensitive to relatively modest BC injections.

Separating the shortwave and longwave components of greenhouse gas radiative forcing

AbstractMany important greenhouse gases (including water vapour, carbon dioxide, methane and ozone) absorb solar radiation. When gas concentrations change, this absorption exerts a radiative forcing that modifies the thermal infrared (‘longwave’) radiative forcing which is predominant for most gases (ozone being a major exception). The nature of the solar forcing differs from the longwave forcing in several ways. For example, the sign of the instantaneous solar forcing can differ between the tropopause and top‐of‐atmosphere, and the sign can differ between gases. In addition, a significant part of the solar forcing can be manifested in the longwave, following stratospheric temperature adjustment, which can counteract or enhance the instantaneous solar forcing. Here the nature of solar forcing is examined via a mixture of idealised and more realistic calculations, which consider the effect of perturbations in carbon dioxide, methane and ozone. An apparent contradiction in the sign of the solar forcing of carbon dioxide is resolved; it is shown to be negative, reducing the net carbon dioxide forcing by about 2.3%. The relevance of this work to the effective radiative forcing concept is also discussed.

The effect of rapid adjustments to halocarbons and N2O on radiative forcing

Rapid adjustments occur after initial perturbation of an external climate driver (e.g., CO2) and involve changes in, e.g. atmospheric temperature, water vapour and clouds, independent of sea surface temperature changes. Knowledge of such adjustments is necessary to estimate effective radiative forcing (ERF), a useful indicator of surface temperature change, and to understand global precipitation changes due to different drivers. Yet, rapid adjustments have not previously been analysed in any detail for certain compounds, including halocarbons and N2O. Here we use several global climate models combined with radiative kernel calculations to show that individual rapid adjustment terms due to CFC-11, CFC-12 and N2O are substantial, but that the resulting flux changes approximately cancel at the top-of-atmosphere due to compensating effects. Our results further indicate that radiative forcing (which includes stratospheric temperature adjustment) is a reasonable approximation for ERF. These CFCs lead to a larger increase in precipitation per kelvin surface temperature change (2.2 ± 0.3% K−1) compared to other well-mixed greenhouse gases (1.4 ± 0.3% K−1 for CO2). This is largely due to rapid upper tropospheric warming and cloud adjustments, which lead to enhanced atmospheric radiative cooling (and hence a precipitation increase) and partly compensate increased atmospheric radiative heating (i.e. which is associated with a precipitation decrease) from the instantaneous perturbation.

The HadGEM3-GA7.1 radiative kernel: the importance of a well-resolved stratosphere

Abstract. We present top-of-atmosphere and surface radiative kernels based on the atmospheric component (GA7.1) of the HadGEM3 general circulation model developed by the UK Met Office. We show that the utility of radiative kernels for forcing adjustments in idealised CO2 perturbation experiments is greatest where there is sufficiently high resolution in the stratosphere in both the target climate model and the radiative kernel. This is because stratospheric cooling to a CO2 perturbation continues to increase with height, and low-resolution or low-top kernels or climate model output are unable to fully resolve the full stratospheric temperature adjustment. In the sixth phase of the Coupled Model Intercomparison Project (CMIP6), standard atmospheric model data are available up to 1 hPa on 19 pressure levels, which is a substantial advantage compared to CMIP5. We show in the IPSL-CM6A-LR model where a full set of climate diagnostics are available that the HadGEM3-GA7.1 kernel exhibits linear behaviour and the residual error term is small, as well as from a survey of kernels available in the literature that in general low-top radiative kernels underestimate the stratospheric temperature response. The HadGEM3-GA7.1 radiative kernels are available at https://doi.org/10.5281/zenodo.3594673 (Smith, 2019).

Updated Global Warming Potentials and Radiative Efficiencies of Halocarbons and Other Weak Atmospheric Absorbers.

Human activity has led to increased atmospheric concentrations of many gases, including halocarbons, and may lead to emissions of many more gases. Many of these gases are, on a per molecule basis, powerful greenhouse gases, although at present‐day concentrations their climate effect is in the so‐called weak limit (i.e., their effect scales linearly with concentration). We published a comprehensive review of the radiative efficiencies (RE) and global warming potentials (GWP) for around 200 such compounds in 2013 (Hodnebrog et al., 2013, https://doi.org/10.1002/rog.20013). Here we present updated RE and GWP values for compounds where experimental infrared absorption spectra are available. Updated numbers are based on a revised “Pinnock curve”, which gives RE as a function of wave number, and now also accounts for stratospheric temperature adjustment (Shine & Myhre, 2020, https://doi.org/10.1029/2019MS001951). Further updates include the implementation of around 500 absorption spectra additional to those in the 2013 review and new atmospheric lifetimes from the literature (mainly from WMO (2019)). In total, values for 60 of the compounds previously assessed are based on additional absorption spectra, and 42 compounds have REs which differ by >10% from our previous assessment. New RE calculations are presented for more than 400 compounds in addition to the previously assessed compounds, and GWP calculations are presented for a total of around 250 compounds. Present‐day radiative forcing due to halocarbons and other weak absorbers is 0.38 [0.33–0.43] W m−2, compared to 0.36 [0.32–0.40] W m−2 in IPCC AR5 (Myhre et al., 2013, https://doi.org/10.1017/CBO9781107415324.018), which is about 18% of the current CO2 forcing.

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

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.

Stratospheric water vapor: an important climate feedback

The role of stratospheric water vapor (SWV) changes, in response to increasing $$\hbox {CO}_2$$ , as a feedback component of quantitative significance for climate sensitivity has remained controversial. Here, we calculate the SWV climate feedback under abrupt $$\hbox {CO}_2$$ quadrupling in the CMIP5 ensemble of models. All models robustly show a moistening of the stratosphere, causing a global mean net stratosphere adjusted radiative perturbation of $$0.89\pm 0.27\,\hbox {Wm}^{-2}$$ at the reference tropopause. The stratospheric temperature adjustment is a crucial component of this radiative perturbation. The associated climate feedback is $$0.17\pm 0.05\,\hbox {Wm}^{-2}\,\hbox{K}^{-1}$$ , with a considerable inter-model range of 0.12–0.28 $$\hbox {Wm}^{-2}\,\hbox {K}^{-1}$$ . Taking into account the rise in tropopause height under $$4\times \hbox {CO}_2$$ slightly reduces the feedback to $$0.15\pm 0.04\,\hbox {Wm}^{-2}\,\hbox {K}^{-1}$$ , with a range of 0.10– $$0.26\,\hbox {Wm}^{-2} \,\hbox {K}^{-1}$$ . The SWV radiative perturbation peaks in the midlatitudes and not the tropics: this is due primarily to increases in SWV in the extratropical lowermost stratosphere, which cause the majority (over three quarters) of the global mean feedback. Based on these results, we suggest an increased focus on understanding drivers of water vapor trends in the extratropical lowermost stratosphere. We conclude that the SWV feedback is important, being on the same order of magnitude as the global mean surface albedo and cloud feedbacks in the multi-model mean.

Conformational analysis and global warming potentials of 1,1,1,2,3,3-hexafluoropropane and 1,1,2,2,3-pentafluoropropane from absorption spectroscopy

Absorption cross-sections of 1,1,1,2,3,3-hexafluoropropane (HFC-236ea) and 1,1,2,2, 3-pentafluoropropane (HFC-245ca) were derived from Fourier transform infrared spectra recorded from 550 to 3400 cm−1 with a resolution of 0.1 cm−1 over a temperature range of 300–365 K. Good agreement was found between the experimentally derived results, theoretical spectra from density functional theory (DFT) calculations, and previously published experimental measurements. The only temperature dependence observed was in the amplitude of some of the absorption peaks due to the changing ratio of the stable conformations of both molecules studied. This temperature dependence does not result in a significant trend in integrated band strength as a function of temperature. The average value for the integrated band strength of HFC-236ea was found to be (1.982 ± 0.012)x10−16 cm molecule−1 over the spectral range of 550 to 3026 cm−1. The average value for the integrated band strength of HFC-245ca was found to be (1.348 ± 0.35)x10−16 cm molecule−1 over the spectral range of 570 to 3035 cm−1.Radiative efficiency (RE) and the global warming potential (GWP) for both molecules were also derived, accounting for corrections due to atmospheric lifetimes and stratospheric temperature adjustment. A RE of 0.256 ± 0.034 Wm−2ppbv−1 was obtained for HFC-236ea, which leads to a GWP100 of 1141 ± 402 for 550–3000 cm−1. For HFC-245ca, a RE of 0.197 ± 0.023 Wm−2ppbv−1 was derived, which leads to a GWP100 of 588 ± 205 for 550–3000 cm−1. The DFT calculation was linearly adjusted to match the experimental spectrum. Using this adjusted DFT spectrum to expand the range below 550 to 0 cm−1 increases the RE to 0.265 ± 0.035 Wm−2ppbv−1 and the GWP100 to 1181 ± 417 for HFC-236ea; while for HFC-245ca, the RE increases to 0.202 ± 0.027 Wm−2ppbv−1 and the GWP100 to 602 ± 210.

Simultaneous upward and downward propagating inertia‐gravity waves in the MLT observed at Andes Lidar Observatory

AbstractBased on the temperature and zonal and meridional winds observed with an Na lidar at Andes Lidar Observatory (30.3°S, 70.7°W) on the night of 20–21 July 2015, we report simultaneous upward and downward propagating inertia‐gravity waves (IGWs) in the mesosphere/lower stratosphere (MLT). The ground‐based periods of the upward and downward IGWs are about 5.4 h and 4.8 h, respectively. The horizontal and vertical wavelengths are about 935 km and 10.9 km for the 5.4 h IGW and about 1248 km and 22 km for the 4.8 h IGW, respectively. Hodograph analyses indicate that the 5.4 h IGW propagates in the direction of about 23° west of north, while the 4.8 h IGW travels northward with an azimuth of 20°clockwise from north. These wave parameters are in the typical IGW wavelength and period ranges; nevertheless, the downward propagating IGWs in the MLT are rarely reported in previous observations. The ray‐tracing analysis suggests that the 5.4 h IGW is likely to originate from the stratospheric jet adjustment over the Antarctic, while the 4.8 h IGW source may be above the MLT because it is unlikely to propagate downward through a reflection in the realistic atmospheric wind field. Although both IGWs do not reach their amplitude thresholds of instability, the Richard number reveals that the dynamical and convective instabilities occur intermittently, which indicates that the instability arising from the multiple‐perturbation superposition may have a significant influence on wave saturation and amplitude constraint in the MLT.

Atmospheric lifetimes, infrared absorption spectra, radiative forcings and global warming potentials of NF<sub>3</sub> and CF<sub>3</sub>CF<sub>2</sub>Cl (CFC-115)

Abstract. Fluorinated compounds such as NF3 and C2F5Cl (CFC-115) are characterised by very large global warming potentials (GWPs), which result from extremely long atmospheric lifetimes and strong infrared absorptions in the atmospheric window. In this study we have experimentally determined the infrared absorption cross sections of NF3 and CFC-115, calculated the radiative forcing and efficiency using two radiative transfer models and identified the effect of clouds and stratospheric adjustment. The infrared cross sections are within 10 % of previous measurements for CFC-115 but are found to be somewhat larger than previous estimates for NF3, leading to a radiative efficiency for NF3 that is 25 % larger than that quoted in the Intergovernmental Panel on Climate Change Fifth Assessment Report. A whole atmosphere chemistry–climate model was used to determine the atmospheric lifetimes of NF3 and CFC-115 to be (509 ± 21) years and (492 ± 22) years, respectively. The GWPs for NF3 are estimated to be 15 600, 19 700 and 19 700 over 20, 100 and 500 years, respectively. Similarly, the GWPs for CFC-115 are 6030, 7570 and 7480 over 20, 100 and 500 years, respectively.

On the link between Barents‐Kara sea ice variability and European blocking

AbstractThis study examines the connection between the variability of sea ice concentration in the Barents and Kara (B‐K) seas and winter European weather on an intraseasonal time scale. Low sea ice regimes in autumn and early winter over the B‐K seas are shown to affect the strength and position of the polar vortex, and increase the frequency of blocking regimes over the Euro‐Atlantic sector in late winter. A hypothesis is presented on the mechanism that links sea ice over the B‐K seas and circulation regimes in the North Atlantic, and is investigated considering 34 years of European Centre for Medium‐Range Weather Forecasts reanalysis data. Four key steps have been identified, starting from a local response of the near‐surface fluxes and modification of the upper tropospheric wave pattern, to the stratospheric adjustment and the tropospheric response in the North Atlantic. The proposed mechanism explains the delayed, late winter response of the North Atlantic Oscillation to the late autumn sea ice reduction, which has been found both in observations and model experiments. It also provides valuable insights on how the reduction of Arctic sea ice can influence the position of the tropospheric jet in the Euro‐Atlantic sector.

An assessment of methods for computing radiative forcing in climate models

Because the radiative forcing is rarely computed separately when performing climate model simulations, several alternative methods have been developed to estimate both the instantaneous (or direct) forcing and the adjusted forcing. The adjusted forcing accounts for the radiative impact arising from the adjustment of climate variables to the instantaneous forcing, independent of any surface warming. Using climate model experiments performed for CMIP5, we find the adjusted forcing for 4 × CO2 ranges from roughly 5.5–9 W m−2 in current models. This range is shown to be consistent between different methods of estimating the adjusted forcing. Decomposition using radiative kernels and offline double-call radiative transfer calculations indicates that the spread receives a substantial contribution (roughly 50%) from intermodel differences in the instantaneous component of the radiative forcing. Moreover, nearly all of the spread in adjusted forcing can be accounted for by differences in the instantaneous forcing and stratospheric adjustment, implying that tropospheric adjustments to CO2 play only a secondary role. This suggests that differences in modeling radiative transfer are responsible for substantial differences in the projected climate response and underscores the need to archive double-call radiative transfer calculations of the instantaneous forcing as a routine diagnostic.

Adjustments in the Forcing-Feedback Framework for Understanding Climate Change

Abstract The traditional forcing–feedback framework has provided an indispensable basis for discussing global climate changes. However, as analysis of model behavior has become more detailed, shortcomings and ambiguities in the framework have become more evident, and physical effects unaccounted for by the traditional framework have become interesting. In particular, the new concept of adjustments, which are responses to forcings that are not mediated by the global-mean temperature, has emerged. This concept, related to the older ones of climate efficacy and stratospheric adjustment, is a more physical way of capturing unique responses to specific forcings. We present a pedagogical review of the adjustment concept, why it is important, and how it can be used. The concept is particularly useful for aerosols, where it helps to organize what has become a complex array of forcing mechanisms. It also helps clarify issues around cloud and hydrological response, transient versus equilibrium climate change, and geoengineering.

Radiative Forcing of Quadrupling CO2

AbstractAn 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.

On the Longwave Climate Feedbacks

Abstract This paper mainly addresses two issues that concern the longwave climate feedbacks. First, it is recognized that the radiative forcing of greenhouse gases, as measured by their impact on the outgoing longwave radiation (OLR), may vary across different climate models even when the concentrations of these gases are identically prescribed. This forcing variation contributes to the discrepancy in these models' projections of surface warming. A method is proposed to account for this effect in diagnosing the sensitivity and feedbacks in the models. Second, it is shown that the stratosphere is an important factor that affects the OLR in transient climate change. Stratospheric water vapor and temperature changes may both act as a positive feedback mechanism during global warming and cannot be fully accounted as a “stratospheric adjustment” of radiative forcing. Neglecting these two issues may cause a bias in the longwave cloud feedback diagnosed as a residual term in the decomposition of OLR variations. There is no consensus among the climate models on the sign of the longwave cloud feedback after accounting for both issues.

On the Northern Annular Mode Surface Signal Associated with Stratospheric Variability

Abstract The wintertime northern annular mode (NAM) at the surface is known to undergo slow intraseasonal variations in association with stratospheric variability, which leads the surface signal by up to several weeks. The relative contributions, however, of potentially relevant stratosphere–troposphere coupling mechanisms are not yet fully understood. In this study the relative roles of (i) the downward effect of the zonal-mean secondary circulation induced by quasigeostrophic (QG) adjustment to stratospheric wave drag and radiative damping and (ii) wave drag local to the troposphere are estimated. For this purpose, a spectral tendency equation of the QG zonal-mean zonal wind is derived and used, in a first step, to obtain the external mechanical forcing that, in the QG framework, drives exactly the observed stratospheric and tropospheric daily NAM. In a second step, the equation is then integrated in time to reconstruct the daily NAM, but with the forcing restricted to either stratospheric or tropospheric levels, each case leaving a characteristic NAM surface signal. The relative roles of the above-mentioned mechanisms are found to be of similar quantitative importance, but to differ in a qualitative sense. The downward effect of stratospheric QG adjustment is responsible for the initiation of the NAM surface signal, whereas subsequently local tropospheric wave drag actively maintains and persists the signal over several weeks. Furthermore, the downward effect of QG adjustment to stratospheric radiative damping is shown to have only a minor impact, compared to that from stratospheric wave drag. The robustness of these conclusions is demonstrated by a sensitivity study with respect to various model parameters.

PORT, a CESM tool for the diagnosis of radiative forcing

Abstract. The Parallel Offline Radiative Transfer (PORT) model is a stand-alone tool, driven by model-generated datasets, that can be used for any radiation calculation that the underlying radiative transfer schemes can perform, such as diagnosing radiative forcing. In its present distribution, PORT isolates the radiation code from the Community Atmosphere Model (CAM4) in the Community Earth System Model (CESM1). The current configuration focuses on CAM4 radiation with the constituents as represented in present-day conditions in CESM1, along with their optical properties. PORT includes an implementation of stratospheric temperature adjustment under the assumption of fixed dynamical heating, which is necessary to compute radiative forcing in addition to the more straightforward instantaneous radiative forcing. PORT can be extended to use radiative constituent distributions from other models or model simulations. Ultimately, PORT can be used with various radiative transfer models. As illustrations of the use of PORT, we perform the computation of radiative forcing from doubling of carbon dioxide, from the change of tropospheric ozone concentration from the year 1850 to 2000, and from present-day aerosols. The radiative forcing from tropospheric ozone (with respect to 1850) generated by a collection of model simulations under the Atmospheric Chemistry and Climate Model Intercomparison Project is found to be 0.34 (with an intermodel standard deviation of 0.07) W m−2. Present-day aerosol direct forcing (relative to no aerosols) is found to be −1.3 W m−2.

Time Variation of Effective Climate Sensitivity in GCMs

Abstract Effective climate sensitivity is often assumed to be constant (if uncertain), but some previous studies of general circulation model (GCM) simulations have found it varying as the simulation progresses. This complicates the fitting of simple models to such simulations, as well as having implications for the estimation of climate sensitivity from observations. This study examines the evolution of the feedbacks determining the climate sensitivity in GCMs submitted to the Coupled Model Intercomparison Project. Apparent centennial-time-scale variations of effective climate sensitivity during stabilization to a forcing can be considered an artifact of using conventional forcings, which only allow for instantaneous effects and stratospheric adjustment. If the forcing is adjusted for processes occurring on time scales that are short compared to the climate stabilization time scale, then there is little centennial-time-scale evolution of effective climate sensitivity in any of the GCMs. Here it is suggested that much of the apparent variation in effective climate sensitivity identified in previous studies is actually due to the comparatively fast forcing adjustment. Persistent differences are found in the strength of the feedbacks between the coupled atmosphere–ocean (AO) versions and their atmosphere–mixed layer ocean (AML) counterparts (the latter are often assumed to give the equilibrium climate sensitivity of the AOGCM). The AML model can typically only estimate the equilibrium climate sensitivity of the parallel AO version to within about 0.5 K. The adjustment to the forcing to account for comparatively fast processes varies in magnitude and sign between GCMs, as well as differing between AO and AML versions of the same model. There is evidence from one AOGCM that the forcing adjustment may take a couple of decades, with implications for observationally based estimates of equilibrium climate sensitivity. It is suggested that at least some of the spread in twenty-first-century global temperature predictions between GCMs is due to differing adjustment processes, hence work to understand these differences should be a priority.

Does Model Sensitivity to Changes in CO2 Provide a Measure of Sensitivity to Other Forcings?

Abstract Simulation of both the climate of the twentieth century and a future climate change requires taking into account numerous forcings, while climate sensitivities of general circulation models are defined as the equilibrium surface warming due to a doubling of atmospheric CO2 concentration. A number of simulations with the Massachusetts Institute of Technology (MIT) climate model of intermediate complexity with different forcings have been carried out to study to what extent sensitivity to changes in CO2 concentration (SCO2) represent sensitivities to other forcings. The MIT model, similar to other models, shows a strong dependency of the simulated surface warming on the vertical structure of the imposed forcing. This dependency is a result of “semidirect” effects in the simulations with localized tropospheric heating. A method for estimating semidirect effects associated with different feedback mechanisms is presented. It is shown that forcing that includes these effects is a better measure of expected surface warming than a forcing that accounts for stratospheric adjustment only. Simulations with the versions of the MIT model with different strengths of cloud feedback show that, for the range of sensitivities produced by existing GCMs, SCO2 provides a good measure of the model sensitivity to other forcings. In the case of strong cloud feedback, sensitivity to the increase in CO2 concentration overestimates model sensitivity to both negative forcings, leading to the cooling of the surface and “black carbon”–like forcings with elevated heating. This is explained by the cloud feedback being less efficient in the case of increasing sea ice extent and snow cover or by the above-mentioned semidirect effects, which are absent in the CO2 simulations, respectively.