Radiative feedback mechanisms associated with temperature, water vapor, cloud, and surface albedo change determine climate sensitivity to radiative forcing. Here we use the linearized radiative ...kernel‐technique in combination with a Gregory analysis to determine the strength and structure of feedbacks, as well as direct and adjusted CO2 forcings in the coupled Max Planck Institute Earth System Model at base resolution (MPI‐ESM‐LR). We show that the combined Kernel‐Gregory approach yields an elegant separation of surface temperature‐dependent feedbacks from contributions to radiative forcing by fast adjustments. MPI‐ESM‐LR exhibits a relatively large cloud adjustment of nearly 2 W m−2 in direct response to quadrupled CO2, with positive cloud adjustment evident throughout the tropics, subtropics and over most landmasses whereas midlatitude storm tracks contribute negatively. The model features a nonlinear regression of radiation imbalance to global mean surface temperature change, resulting in a significantly increasing effective climate sensitivity after about 20 years which is approximately at temperatures 4–5 K above preindustrial. This feature is not uncommon among climate models and is relevant for future climate projections. We analyze the contribution of the individual feedback processes to this behavior and discuss possible origins such as differential ocean warming patterns associated with deep‐ocean heat uptake or state dependencies of the feedback processes.
Key Points
Effective climate sensitivity could be considered a function of climate state
Fast cloud and temperature adjustments contribute to forcing
Feedbacks and fast adjustments in MPI‐ESM‐LR are documented
We assess evidence relevant to Earth's equilibrium climate sensitivity per doubling of atmospheric CO2, characterized by an effective sensitivity S . This evidence includes feedback process ...understanding, the historical climate record, and the paleoclimate record. An S value lower than 2 K is difficult to reconcile with any of the three lines of evidence. The amount of cooling during the Last Glacial Maximum provides strong evidence against values of S greater than 4.5 K. Other lines of evidence in combination also show that this is relatively unlikely. We use a Bayesian approach to produce a probability density (PDF) for S given all the evidence, including tests of robustness to difficult‐to‐quantify uncertainties and different priors. The 66% range is 2.6‐3.9 K for our Baseline calculation, and remains within 2.3‐4.5 K under the robustness tests; corresponding 5‐95% ranges are 2.3‐4.7 K, bounded by 2.0‐5.7 K (although such high‐confidence ranges should be regarded more cautiously). This indicates a stronger constraint on S than reported in past assessments, by lifting the low end of the range. This narrowing occurs because the three lines of evidence agree and are judged to be largely independent, and because of greater confidence in understanding feedback processes and in combining evidence. We identify promising avenues for further narrowing the range in S , in particular using comprehensive models and process understanding to address limitations in the traditional forcing‐feedback paradigm for interpreting past changes.
Extratropical cyclones, major contributors to precipitation in the midlatitudes, comprise mesoscale fronts and fine‐scale convective storms. Intense oceanic cyclones pose natural hazards, making ...reliable projections of their changes with global warming of great interest. Here, we analyze the first ever global climate simulations to resolve such mesoscale dynamics of extratropical cyclones. The present‐day structure, frequency, and precipitation of the oceanic extratropical cyclones compare well with reanalyses and new satellite datasets that resolve the multiscale cloud‐precipitation system. Simulated precipitation from intense oceanic cyclones increases at a rate of 7%/K1, following Clausius‐Clapeyron, with warming. The same scaling is apparent also in the interhemispheric contrast, suggesting that the latter could serve as a predictor of the former. Projected changes in precipitation from intense oceanic cyclones with warming may thus be testable using a reliable global observation network of precipitation in the present day.
Plain Language Summary
Precipitation in midlatitudes is strongly governed by extratropical cyclones. Its future change is of great societal concern for adaptation to a warmer world. In this study, we use a new kind of global model and a new satellite dataset to resolve fine‐scale and frontal features of cyclones and their changes with warming. The simulation shows that the amount of precipitation around the intense oceanic cyclones will increase due to global warming, and furthermore, the rate of precipitation change around the intense oceanic cyclones is closely related to the precipitation contrast between the northern and southern hemispheres in the present climate. This suggests that the future precipitation change associated with intense oceanic cyclones could be inferred from the difference between the northern and southern hemispheres in the present climate.
Key Points
The first global climate simulations to explicitly represent the mesoscale dynamics of extratropical cyclones are analyzed
Precipitation from intense cyclones scales with temperatures across hemispheres and with global warming following Clausius‐Clapeyron
Projected change in precipitation from intense cyclones may thus be testable using global observations of precipitation in the present day
The MPI‐ESM1.2 is the latest version of the Max Planck Institute Earth System Model and is the baseline for the Coupled Model Intercomparison Project Phase 6 and current seasonal and decadal climate ...predictions. This paper evaluates a coupled higher‐resolution version (MPI‐ESM1.2‐HR) in comparison with its lower‐resolved version (MPI‐ESM1.2‐LR). We focus on basic oceanic and atmospheric mean states and selected modes of variability, the El Niño/Southern Oscillation and the North Atlantic Oscillation. The increase in atmospheric resolution in MPI‐ESM1.2‐HR reduces the biases of upper‐level zonal wind and atmospheric jet stream position in the northern extratropics. This results in a decrease of the storm track bias over the northern North Atlantic, for both winter and summer season. The blocking frequency over the European region is improved in summer, and North Atlantic Oscillation and related storm track variations improve in winter. Stable Atlantic meridional overturning circulations are found with magnitudes of ~16 Sv for MPI‐ESM1.2‐HR and ~20 Sv for MPI‐ESM1.2‐LR at 26°N. A strong sea surface temperature bias of ~5°C along with a too zonal North Atlantic current is present in both versions. The sea surface temperature bias in the eastern tropical Atlantic is reduced by ~1°C due to higher‐resolved orography in MPI‐ESM‐HR, and the region of the cold‐tongue bias is reduced in the tropical Pacific. MPI‐ESM1.2‐HR has a well‐balanced radiation budget and its climate sensitivity is explicitly tuned to 3 K. Although the obtained reductions in long‐standing biases are modest, the improvements in atmospheric dynamics make this model well suited for prediction and impact studies.
Key Points
A higher‐resolution version of MPI‐ESM1.2 is presented, which has a well‐balanced radiation budget and stable ocean circulation
The higher atmospheric resolution improves North Atlantic storm tracks, blocking frequency, and NAO representation
The higher computational costs remain manageable and enable studies of seasonal to decadal predictions and climate impacts
Aerosols interact with radiation and clouds. Substantial progress made over the past 40 years in observing, understanding, and modeling these processes helped quantify the imbalance in the Earth's ...radiation budget caused by anthropogenic aerosols, called aerosol radiative forcing, but uncertainties remain large. This review provides a new range of aerosol radiative forcing over the industrial era based on multiple, traceable, and arguable lines of evidence, including modeling approaches, theoretical considerations, and observations. Improved understanding of aerosol absorption and the causes of trends in surface radiative fluxes constrain the forcing from aerosol‐radiation interactions. A robust theoretical foundation and convincing evidence constrain the forcing caused by aerosol‐driven increases in liquid cloud droplet number concentration. However, the influence of anthropogenic aerosols on cloud liquid water content and cloud fraction is less clear, and the influence on mixed‐phase and ice clouds remains poorly constrained. Observed changes in surface temperature and radiative fluxes provide additional constraints. These multiple lines of evidence lead to a 68% confidence interval for the total aerosol effective radiative forcing of ‐1.6 to ‐0.6 W m−2, or ‐2.0 to ‐0.4 W m−2 with a 90% likelihood. Those intervals are of similar width to the last Intergovernmental Panel on Climate Change assessment but shifted toward more negative values. The uncertainty will narrow in the future by continuing to critically combine multiple lines of evidence, especially those addressing industrial‐era changes in aerosol sources and aerosol effects on liquid cloud amount and on ice clouds.
Plain Language Summary
Human activities emit into the atmosphere small liquid and solid particles called aerosols. Those aerosols change the energy budget of the Earth and trigger climate changes, by scattering and absorbing solar and terrestrial radiation and playing important roles in the formation of cloud droplets and ice crystals. But because aerosols are much more varied in their chemical composition and much more heterogeneous in their spatial and temporal distributions than greenhouse gases, their perturbation to the energy budget, called radiative forcing, is much more uncertain. This review uses traceable and arguable lines of evidence, supported by aerosol studies published over the past 40 years, to quantify that uncertainty. It finds that there are two chances out of three that aerosols from human activities have increased scattering and absorption of solar radiation by 14% to 29% and cloud droplet number concentration by 5 to 17% in the period 2005–2015 compared to the year 1850. Those increases exert a radiative forcing that offsets between a fifth and a half of the radiative forcing by greenhouse gases. The degree to which human activities affect natural aerosol levels, and the response of clouds, and especially ice clouds, to aerosol perturbations remain particularly uncertain.
Key Points
An assessment of multiple lines of evidence supported by a conceptual model provides ranges for aerosol radiative forcing of climate change
Aerosol effective radiative forcing is assessed to be between ‐1.6 and ‐0.6 W m−2 at the 16–84% confidence level
Although key uncertainties remain, new ways of using observations provide stronger constraints for models
Observations from the Arctic Summer Cloud Ocean Study (ASCOS), in the central Arctic sea-ice pack in late summer 2008, provide a detailed view of cloud–atmosphere–surface interactions and vertical ...mixing processes over the sea-ice environment. Measurements from a suite of ground-based remote sensors, near-surface meteorological and aerosol instruments, and profiles from radiosondes and a helicopter are combined to characterize a week-long period dominated by low-level, mixed-phase, stratocumulus clouds. Detailed case studies and statistical analyses are used to develop a conceptual model for the cloud and atmosphere structure and their interactions in this environment. Clouds were persistent during the period of study, having qualities that suggest they were sustained through a combination of advective influences and in-cloud processes, with little contribution from the surface. Radiative cooling near cloud top produced buoyancy-driven, turbulent eddies that contributed to cloud formation and created a cloud-driven mixed layer. The depth of this mixed layer was related to the amount of turbulence and condensed cloud water. Coupling of this cloud-driven mixed layer to the surface boundary layer was primarily determined by proximity. For 75% of the period of study, the primary stratocumulus cloud-driven mixed layer was decoupled from the surface and typically at a warmer potential temperature. Since the near-surface temperature was constrained by the ocean–ice mixture, warm temperatures aloft suggest that these air masses had not significantly interacted with the sea-ice surface. Instead, back-trajectory analyses suggest that these warm air masses advected into the central Arctic Basin from lower latitudes. Moisture and aerosol particles likely accompanied these air masses, providing necessary support for cloud formation. On the occasions when cloud–surface coupling did occur, back trajectories indicated that these air masses advected at low levels, while mixing processes kept the mixed layer in equilibrium with the near-surface environment. Rather than contributing buoyancy forcing for the mixed-layer dynamics, the surface instead simply appeared to respond to the mixed-layer processes aloft. Clouds in these cases often contained slightly higher condensed water amounts, potentially due to additional moisture sources from below.
In both the observational record and atmosphere-ocean general circulation model (AOGCM) simulations of the last
∼
150 years, short-lived negative radiative forcing due to volcanic aerosol, following ...explosive eruptions, causes sudden global-mean cooling of up to
∼
0.3 K. This is about five times smaller than expected from the transient climate response parameter (TCRP, K of global-mean surface air temperature change per W m
−2
of radiative forcing increase) evaluated under atmospheric CO
2
concentration increasing at 1 % yr
−1
. Using the step model (Good et al. in Geophys Res Lett 38:L01703,
2011
. doi:
10.1029/2010GL045208
), we confirm the previous finding (Held et al. in J Clim 23:2418–2427,
2010
. doi:
10.1175/2009JCLI3466.1
) that the main reason for the discrepancy is the damping of the response to short-lived forcing by the thermal inertia of the upper ocean. Although the step model includes this effect, it still overestimates the volcanic cooling simulated by AOGCMs by about 60 %. We show that this remaining discrepancy can be explained by the magnitude of the volcanic forcing, which may be smaller in AOGCMs (by 30 % for the HadCM3 AOGCM) than in off-line calculations that do not account for rapid cloud adjustment, and the climate sensitivity parameter, which may be smaller than for increasing CO
2
(40 % smaller than for 4 × CO
2
in HadCM3).
Clouds regulate the Earth's radiation budget, both by reflecting part of the incoming sunlight leading to cooling and by absorbing and emitting infrared radiation which tends to have a warming ...effect. Globally averaged, at the top of the atmosphere the cloud radiative effect is to cool the climate, while at the Arctic surface, clouds are thought to be warming. Here we compare a passive instrument, the AVHRR-based retrieval from CM-SAF, with recently launched active instruments onboard CloudSat and CALIPSO and the widely used ERA-Interim reanalysis. We find that in particular in winter months the three data sets differ significantly. While passive satellite instruments have serious difficulties, detecting only half the cloudiness of the modeled clouds in the reanalysis, the active instruments are in between. In summer, the two satellite products agree having monthly means of 70-80 percent, but the reanalysis are approximately ten percent higher. The monthly mean long- and shortwave components of the surface cloud radiative effect obtained from the ERA-Interim reanalysis are about twice that calculated on the basis of CloudSat's radar-only retrievals, while ground based measurements from SHEBA are in between. We discuss these differences in terms of instrument-, retrieval- and reanalysis characteristics, which differ substantially between the analyzed datasets.
ICON‐A is the new icosahedral nonhydrostatic (ICON) atmospheric general circulation model in a configuration using the Max Planck Institute physics package, which originates from the ECHAM6 general ...circulation model, and has been adapted to account for the changed dynamical core framework. The coupling scheme between dynamics and physics employs a sequential updating by dynamics and physics, and a fixed sequence of the physical processes similar to ECHAM6. To allow a meaningful initial comparison between ICON‐A and the established ECHAM6‐LR model, a setup with similar, low resolution in terms of number of grid points and levels is chosen. The ICON‐A model is tuned on the base of the Atmospheric Model Intercomparison Project (AMIP) experiment aiming primarily at a well balanced top‐of atmosphere energy budget to make the model suitable for coupled climate and Earth system modeling. The tuning addresses first the moisture and cloud distribution to achieve the top‐of‐atmosphere energy balance, followed by the tuning of the parameterized dynamic drag aiming at reduced wind errors in the troposphere. The resulting version of ICON‐A has overall biases, which are comparable to those of ECHAM6. Problematic specific biases remain in the vertical distribution of clouds and in the stratospheric circulation, where the winter vortices are too weak. Biases in precipitable water and tropospheric temperature are, however, reduced compared to the ECHAM6. ICON‐A will serve as the basis of further development and as the atmosphere component to the coupled model, ICON‐Earth system model (ESM).
Plain Language Summary
ICON‐A is a new atmospheric model as needed for research on the general circulation of the atmosphere, or as atmospheric component in an Earth system model, as used in climate research. This article describes the construction of the atmospheric model, in particular how two major parts are coupled to each other: “dynamics” and “physics.” Dynamics is the part that solves the equations for the atmospheric motion, temperature, density, and concentrations of water vapor, cloud water, and cloud ice. Physics is the part that computes the changes in these fields related to processes like radiation, cloud condensation, or turbulence. These physical changes depend on the state of the atmosphere as computed by the dynamics, and the changes computed by physics force change in the dynamics. The article documents the details of this construction. Further, the article describes how the physics is tuned to obtain a good representation of the general circulation of the period 1979 to 1988 in comparison to observations. A more detailed evaluation of such simulations is presented in a companion article by Crueger et al. (2018, https://doi.org/10.1029/2017MS001233).
Key Points
Physics package for climate modeling is coupled to a nonhydrostatic dynamical core
Tuning in five steps to obtain a balanced net radiation at top of atmosphere
Overall biases of ICON‐A are comparable to ECHAM6.3, but circulation biases remain due to problems with parameterized drag
Despite efforts to accurately quantify the effective radiative forcing (ERF) of anthropogenic aerosol, the historical evolution of ERF remains uncertain. As a further step toward a better ...understanding of ERF uncertainty, the present study systematically investigates the sensitivity of the shortwave ERF at the top of the atmosphere to model‐internal variability and spatial distributions of the monthly mean radiative effects of anthropogenic aerosol. For this, ensembles are generated with the atmospheric model ECHAM6.3 that uses monthly prescribed optical properties and changes in cloud‐droplet number concentrations designed to mimic that associated with the anthropogenic aerosol using the new parameterization MACv2‐SP. The results foremost highlight the small change in our best estimate of the global averaged all‐sky ERF associated with a substantially different pattern of anthropogenic aerosol radiative effects from the mid‐1970s (–0.51 Wm–2) and present day (–0.50 Wm–2). Such a small change in ERF is difficult to detect when model‐internal year‐to‐year variability (0.32 Wm–2 standard deviation) is considered. A stable estimate of all‐sky ERF requires ensemble simulations, the size of which depends on the targeted precision, confidence level, and the magnitude of model‐internal variability. A larger effect of the pattern of the anthropogenic aerosol radiative effects on the globally averaged all‐sky ERF (15%) occurs with a strong Twomey effect through lowering the background aerosol optical depth in regions downstream of major pollution sources. It suggests that models with strong aerosol‐cloud interactions could show a moderate difference in the global mean ERF associated with the mid‐1970s to present‐day change in the anthropogenic aerosol pattern.
Key Points
Ensembles of atmosphere‐only experiments with MACv2‐SP allow a sensitivity assessment of instantaneous and effective radiative forcing (ERF)
Global mean all‐sky ERFs with aerosol patterns of mid‐1970s and today difficult to distinguish, when atmospheric variability considered
A moderate pattern effect on ERF could occur in models with presumably strong aerosol‐cloud interaction
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