This study investigates how climate sensitivity depends upon the spatial pattern of radiative forcing. Sensitivity experiments using a coupled ocean-atmosphere model were conducted by adding anomalous incoming solar radiation over the entire globe, Northern Hemisphere mid-latitudes, Southern Ocean, and tropics, respectively, with both positive and negative perturbation considered. The varied forcing patterns led to highly divergent climate sensitivities, with extratropical forcing inducing significantly more global-mean temperature change compared to tropical forcing. This dependence is particularly strong over the Southern Hemisphere, where the climate is nearly twice as sensitive to Southern Ocean forcing as tropical forcing. This dependence of climate sensitivity on the location of radiative forcing stems from covariations between lapse rate feedback, cloud feedback and tropospheric stability. These results contrast with the conventional SST-pattern effect in which tropical surface temperature changes regulate the climate sensitivity, and has important implications for geoengineering and understanding the mechanisms of paleoclimate change.

A ~ 50 km resolution atmospheric general circulation model (GCM) is used to investigate the impact of radiative interactions on spatial organization of convection, the model’s mean state, and extreme precipitation events in the presence of realistic boundary conditions. Mechanism-denial experiments are performed in which synoptic-scale feedbacks between radiation and dynamics are suppressed by overwriting the model-generated atmospheric radiative cooling rates with its monthly-varying climatological values. When synoptic-scale radiative interactions are disabled, the annual mean circulation and precipitation remain almost unchanged, however tropical convection becomes less aggregated, with an increase in cloud fraction and relative humidity in the free troposphere but a decrease in both variables in the boundary layer. Changes in cloud fraction and relative humidity in the boundary layer exhibit more sensitivity to the presence of radiative interactions than variations in the degree of aggregation. The less aggregated state is associated with a decrease in the frequency of extreme precipitation events, coincident with a decrease in the dynamical contribution to the magnitude of extreme precipitation. At regional scales, the spatial contrast in radiative cooling between dry and moist regions diminishes when radiative interactions are suppressed, reducing the upgradient transport of energy, degree of aggregation and frequency of extreme precipitation events. However, the mean width of the tropical rain belt remains almost unaffected when radiative interactions are disabled. These results offer insights into how radiation-circulation coupling affects the spatial organization of convection, distributions of clouds and humidity, and weather extremes.

Uncertainty in climate feedbacks is the primary source of spread in model projections of surface temperature response to anthropogenic forcing. Cloud feedback persistently appears as the main source of disagreement in future projections while the combined lapse-rate plus water vapor (LR+WV) feedback is a smaller (~30%), but non-trivial source of uncertainty in climate sensitivity. Here observation-based emergent constraints are adopted to evaluate the intermodel spread in these feedbacks. The observed interannual variation provides a useful constraint on the long-term cloud feedback as evidenced by the consistency between their global-mean values as well as their similar regional contributions to the intermodel spread. However, internal variability does not serve to constrain the long-term LR+WV feedback spread, which we find is mostly associated with the relative humidity response over the tropics. Model differences in hemispheric warming asymmetries, induced primarily by ocean heat uptake differences, also contribute to the spread in water vapor feedback.

The most recent generation of climate models exhibits an alarming increase in high climate sensitivity models compared to previous generations. Because the calculation of equilibrium climate sensitivity (ECS) requires simulations of a thousand years or more, most studies estimate ECS using shorter model integrations. However, the most widely used method for estimating ECS from shorter simulations underestimates ECS. Previous studies attributed this underestimate to the time-dependence of climate feedbacks. Here we demonstrate that it actually arises from an underestimate of the radiative forcing. We present a modified method that corrects for this underestimate and is shown to better agree with ECS calculated from “long run”, millennium-scale simulations. This method reveals that the actual number of “too hot” models is roughly double that previously diagnosed, with one out of every three CMIP6 climate models having an ECS greater than 5K - the “very likely” upper bound on ECS.

Changes in atmospheric composition, such as increasing greenhouse gases, cause an initial radiative imbalance to the climate system, quantified as the instantaneous radiative forcing. This fundamental metric has not been directly observed globally and previous estimates have come from models. In part, this is because current space‐based instruments cannot distinguish the instantaneous radiative forcing from the climate’s radiative response. We apply radiative kernels to satellite observations to disentangle these components and find all‐sky instantaneous radiative forcing has increased 0.53±0.11 W/m2 from 2003 through 2018, accounting for positive trends in the total planetary radiative imbalance. This increase has been due to a combination of rising concentrations of well‐mixed greenhouse gases and recent reductions in aerosol emissions. These results highlight distinct fingerprints of anthropogenic activity in Earth’s changing energy budget, which we find observations can detect within 4 years.

A vertically resolved analysis of the budget equation for the spatial variance of moist static energy (MSE) is used to diagnose processes associated with the development of tropical cyclones (TCs) in a high-resolution general circulation model (GCM) under realistic boundary conditions. Previous studies have shown that radiation provides an important feedback which enhances TC development. Here we examine the vertical contributions to this feedback by performing a series of mechanism-denial experiments in which synoptic-scale radiative interactions are suppressed either in the boundary layer or in the free troposphere. Although the boundary layer makes up a much smaller proportion of the atmospheric column than the free troposphere, the two experiments result in similar magnitude of reduction in global TC frequency, indicating that radiative interactions in the boundary layer and those in the free troposphere are of comparable importance in modulating TC frequency. Using instantaneous 6-houly outputs, an explicit computation reveals spatial patterns of the advection term during different TC stages. Instead of damping the spatial variance of MSE as noted in previous idealized studies, the advection term is found to promote the development of TCs. We attribute this result primarily to the explicit calculation of the advection term, however the influence of SST gradients cannot be ruled out. While the vertical component of the advection term is prominent in the middle troposphere, the horizontal component dominates in the boundary layer. These results provide additional insight of how different physical processes contribute to TC development in GCMs under realistic boundary conditions.