The response of precipitation minus evaporation (P-E) to global warming is investigated using a moist energy balance model (MEBM) with a simple Hadley-Cell parameterization. The MEBM accurately emulates P-E changes simulated by a suite of global climate models (GCMs) under greenhouse-gas forcing. The MEBM also accounts for most of the intermodel differences in GCM P-E changes and better emulates GCM P-E changes when compared to the “wet-gets-wetter, dry-gets-drier” thermodynamic mechanism. The intermodel spread in P-E changes are attributed to intermodel differences in radiative feedbacks, which account for 60-70% of the intermodel variance, with smaller contributions from radiative forcing and ocean heat uptake. Isolating the intermodel spread of feedbacks to specific regions shows that tropical feedbacks are the primary source of intermodel spread in P-E changes. The ability of the MEBM to emulate GCM P-E changes is further investigated using idealized feedback patterns. A less negative and narrowly peaked feedback pattern near the equator results in more atmospheric heating, which strengthens the Hadley Cell circulation in the deep tropics through an enhanced poleward heat flux. This pattern also increases gross moist stability, which weakens the subtropical Hadley Cell circulation. These two processes in unison increase P-E in the deep tropics, decrease P-E in the subtropics, and narrow the Intertropical Convergence Zone. Additionally, a feedback pattern that produces polar-amplified warming reduces the poleward moisture flux by weakening the meridional temperature gradient and the Clausius-Clapeyron relation. It is shown that changes to the Hadley Cell circulation and the poleward moisture flux are crucial for understanding the pattern of GCM P-E changes under warming.

This study assesses the effective climate sensitivity (EffCS) and transient climate response (TCR) derived from global energy budget constraints within historical simulations of 8 CMIP6 global climate models (GCMs). These calculations are enabled by use of the Radiative Forcing Model Intercomparison Project (RFMIP) simulations, which permit accurate quantification of the historical effective radiative forcing. We find that long-term historical energy budget constraints generally underestimate EffCS from CO2 quadrupling and TCR from CO2 ramping, both by 12%, owing to changes in radiative feedbacks and changes in ocean heat uptake efficiency. Atmospheric GCMs forced by observed warming patterns produce lower values of EffCS that are more in line with those inferred from observed historical energy budget constraints. Understanding the discrepancies between modeled and observed historical surface warming patterns remains critical for constraining EffCS and TCR from the historical record.

Coupled global climate models (GCMs) generally fail to reproduce the observed sea-surface temperature (SST) trend pattern since the 1980s. The model-observation discrepancies may arise in part from the lack of realistic Antarctic ice-sheet meltwater imbalance in GCMs. Here we employ two sets of CESM1-CAM5 simulations forced by anomalous Antarctic meltwater fluxes over 1980–2013 and into the 21st century. Both show a reduced global warming rate and an SST trend pattern that better resembles observations. The meltwater drives surface cooling in the Southern Ocean and the tropical southeast Pacific, in turn increasing low-cloud cover and driving radiative feedbacks to become more stabilizing (corresponding to a lower effective climate sensitivity). These feedback changes contribute more than ocean heat uptake efficiency changes in reducing the global warming rate. Accurately projecting historical and future warming thus requires improved representation of Antarctic meltwater and its impacts in models.

Observed surface temperature trends over recent decades are characterized by (i) intensified warming in the Indo-Pacific Warm Pool and slight cooling in the eastern equatorial Pacific, consistent with strengthening of the Walker circulation, and (ii) cooling in the Southern Ocean. In contrast, state-of-the-art coupled climate models generally project Walker circulation weakening, enhanced warming in the eastern equatorial Pacific, and warming in the Southern Ocean. Here we investigate the ability of 16 climate model large ensembles to reproduce observed sea-surface temperature and sea-level pressure trends over 1979-2020 through a combination of externally forced climate change and internal variability. We find large-scale differences between observed and modeled trends that are very unlikely (<5% probability) to occur due to internal variability as represented in models. Disparate trends are found even in regions with weak multi-decadal variability, suggesting that model biases in the transient response to anthropogenic forcing constitute part of the discrepancy.