Lily Caroline Hahn

Climate models project a weakening of the Atlantic Meridional Overturning Circulation (AMOC) that will reduce Arctic warming. However, there is substantial intermodel spread in the projected AMOC decline, and its contribution to uncertainty in projected Arctic warming has not yet been quantified. To investigate this, we perform model experiments that increase CO ₂ concentrations while imposing the intermodel spread in the AMOC decline. We find that this intermodel spread in the AMOC decline reproduces 45% of the total intermodel spread in Arctic warming. A smaller AMOC decline increases Arctic warming via increased northward ocean heat transport, atmospheric moisture transport, and positive Arctic feedbacks. Arctic warming is more sensitive to AMOC differences that produce larger sea‐ice changes. Combined with a strong correlation between a small AMOC decline and Arctic warming across models, these results indicate that ocean circulation is a key source of model uncertainty in projections of Arctic warming.

Climate models vary widely in their projections of twenty-first century global warming and changes in the Atlantic meridional overturning circulation (AMOC). However, the extent to which this uncertainty in AMOC contributes to the uncertainty in global warming has not yet been quantified. To investigate this, we perform climate model experiments in which we increase CO ₂ concentrations while imposing the range of AMOC declines found in the Coupled Model Intercomparison Project phase 6 (CMIP6). We find that intermodel spread in the AMOC decline, imposed within a single model, reproduces 20% of the total intermodel spread in global warming. An idealized energy-balance model indicates that changes in ocean heat uptake and climate feedbacks contribute approximately equally to enhanced warming in our experiment with a smaller AMOC decline. A smaller AMOC decline produces greater ocean-to-atmosphere heating in the North Atlantic, which increases near-surface warming and reduces the lower-tropospheric stability to produce less-negative lapse-rate and shortwave cloud feedbacks. In the northern tropics, surface warming is enhanced by the wind–evaporation–SST feedback and by more-positive longwave cloud and water vapor feedbacks due to a northward shift of the intertropical convergence zone. A Green’s function analysis confirms that a less-negative global feedback with a smaller AMOC decline is predominantly driven by sea surface warming in the extratropical North Atlantic. Spread in the AMOC decline is correlated with similar regional changes in ocean heat uptake and climate feedbacks across CMIP6 models as in our experiments. These results suggest that constraining projections of AMOC may substantially impact projections of near-future warming.

Climate models project a weakening of the Atlantic Meridional Overturning Circulation (AMOC) that will reduce Arctic warming. However, AMOC projections vary widely among different models, and the impact of this uncertainty in AMOC on uncertainty in Arctic warming has not been quantified. To investigate this, we perform model experiments that increase CO2 concentrations while imposing the intermodel spread in the AMOC decline. We find that this intermodel spread in the AMOC decline reproduces 45% of the total intermodel spread in Arctic warming. A smaller AMOC decline increases Arctic warming via increased northward ocean heat transport, atmospheric moisture transport, and positive Arctic feedbacks. Arctic warming is more sensitive to AMOC differences that produce larger sea-ice changes. Combined with a strong correlation between a small AMOC decline and Arctic warming across models, these results indicate that ocean circulation is a key source of model uncertainty in projections of Arctic warming.

Climate models vary widely in projections of 21st century global warming and projections of the Atlantic Meridional Overturning Circulation (AMOC). However, the extent to which this uncertainty in AMOC contributes to uncertainty in warming has not yet been quantified. To investigate this, we perform climate model experiments that increase CO2 concentrations while imposing the range of AMOC declines found in the Coupled Model Intercomparison Project Phase 6 (CMIP6). We find that intermodel spread in the AMOC decline, imposed within a single model, reproduces 20% of the total intermodel spread in global warming. An idealized energy-balance model indicates that changes in ocean heat uptake and climate feedbacks contribute approximately equally to enhanced warming in our experiment with a smaller AMOC decline. A smaller AMOC decline produces greater ocean-to-atmosphere heating in the North Atlantic, which increases near-surface warming and reduces the lower-tropospheric stability to produce less-negative lapse-rate and shortwave cloud feedbacks. In the northern tropics, surface warming is enhanced by the wind-evaporation-SST feedback and by more-positive longwave cloud and water vapor feedbacks due to a northward shift of the Intertropical Convergence Zone. A Green’s function analysis confirms that a less-negative global feedback with a smaller AMOC decline is predominantly driven by sea-surface warming in the extratropical North Atlantic. Spread in the AMOC decline is correlated with similar differences in ocean heat uptake and climate feedbacks across CMIP6 models as in our experiments. These results suggest that model uncertainty in global warming may be substantially reduced by constraining projections of AMOC.

Clouds are the main source of uncertainties when projecting climate change. Mixed-phase clouds that contain ice and supercooled-liquid particles are especially hard to constrain, and climate models neither agree on their phase nor their spatial extent. This is problematic, as models that underestimate contemporary supercooled-liquid in mixed-phase clouds will underestimate future warming. Furthermore, it has recently been shown that supercooled-liquid water in mixed-phase clouds is not homogeneously-mixed, neither vertically nor horizontally. However, while there have been attempts at observationally constraining mixed-phase clouds to constrain uncertainties in future warming, all studies only use the phase of the interior of mixed-phase clouds. Here we show, using novel satellite observations that distinguish between cloud-top and interior phase in mixed-phase clouds, that mixed-phase clouds are more liquid at the cloud top globally. We use these observations to constrain the cloud top phase in addition to the interior in a global climate model, leading to +1 °C more 21^st century warming in NorESM2 SSP5-8.5 climate projections. We anticipate that the difference between cloud top and interior phase in mixed-phase clouds is an important new target metric for future climate model development, because similar mixed-phase clouds related biases in future warming are likely present in many climate models.

The observed rate of global warming since the 1970s has been proposed as a strong constraint on equilibrium climate sensitivity (ECS) and transient climate response (TCR)—key metrics of the global climate response to greenhouse-gas forcing. Using CMIP5/6 models, we show that the inter-model relationship between warming and these climate sensitivity metrics (the basis for the constraint) arises from a similarity in transient and equilibrium warming patterns within the models, producing an effective climate sensitivity (EffCS) governing recent warming that is comparable to the value of ECS governing long-term warming under CO 2 forcing. However, CMIP5/6 historical simulations do not reproduce observed warming patterns. When driven by observed patterns, even high ECS models produce low EffCS values consistent with the observed global warming rate. The inability of CMIP5/6 models to reproduce observed warming patterns thus results in a bias in the modeled relationship between recent global warming and climate sensitivity. Correcting for this bias means that observed warming is consistent with wide ranges of ECS and TCR extending to higher values than previously recognized. These findings are corroborated by energy balance model simulations and coupled model (CESM1-CAM5) simulations that better replicate observed patterns via tropospheric wind nudging or Antarctic meltwater fluxes. Because CMIP5/6 models fail to simulate observed warming patterns, proposed warming-based constraints on ECS, TCR, and projected global warming are biased low. The results reinforce recent findings that the unique pattern of observed warming has slowed global-mean warming over recent decades and that how the pattern will evolve in the future represents a major source of uncertainty in climate projections.

Arctic warming under increased CO₂ peaks in winter, but is influenced by summer forcing via seasonal ocean heat storage. Yet changes in atmospheric heat transport into the Arctic have mainly been investigated in the annual mean or winter, with limited focus on other seasons. We investigate the full seasonal cycle of poleward heat transport modeled with increased CO₂ or with individually applied Arctic sea‐ice loss and global sea‐surface warming. We find that a winter reduction in dry heat transport is driven by Arctic sea‐ice loss and warming, while a summer increase in moist heat transport is driven by sub‐Arctic warming and moistening. Intermodel spread in Arctic warming controls spread in seasonal poleward heat transport. These seasonal changes and their intermodel spread are well‐captured by down‐gradient diffusive heat transport. While changes in moist and dry heat transport compensate in the annual‐mean, their opposite seasonality may support non‐compensating effects on Arctic warming.

The polar regions are predicted to experience the largest relative change in precipitation in response to increased greenhouse-gas concentrations, where a substantial absolute increase in precipitation coincides with small precipitation rates in the present-day climate. The reasons for this amplification, however, are still debated. Here, we use an atmospheric energy budget to decompose regional precipitation change from climate models under greenhouse-gas forcing into contributions from atmospheric radiative feedbacks, dry-static energy flux divergence changes, and surface sensible heat flux changes. The polar-amplified relative precipitation change is shown to be a consequence of the Planck feedback, which, when combined with larger polar warming, favors substantial atmospheric radiative cooling that balances increases in latent heat release from precipitation. Changes in the dry-static energy flux divergence contribute modestly to the polar-amplified pattern. Additional contributions to the polar-amplified response come, in the Arctic, from the cloud feedback and, in the Antarctic, from both the cloud and water vapor feedbacks. The primary contributor to the intermodel spread in the relative precipitation change in the polar region is also the Planck feedback, with the lapse rate feedback and dry-static energy flux divergence changes playing secondary roles. For all regions, there are strong covariances between radiative feedbacks and changes in the dry-static energy flux divergence that impact the intermodel spread. These results imply that constraining regional precipitation change, particularly in the polar regions, will require constraining not only individual feedbacks but also the covariances between radiative feedbacks and atmospheric energy transport.

Arctic warming under increased CO2 peaks in winter, but is influenced by summer forcing via seasonal ocean heat storage. Yet changes in atmospheric heat transport into the Arctic have mainly been investigated in the annual mean or winter, with limited focus on other seasons. We investigate the full seasonal cycle of poleward heat transport modelled with increased CO2 or with individually applied Arctic sea-ice loss and global sea-surface warming. We find that a winter reduction in dry heat transport is driven by Arctic sea-ice loss and warming, while a summer increase in moist heat transport is driven by sub-Arctic warming and moistening. Intermodel spread in Arctic warming controls spread in seasonal poleward heat transport. These seasonal changes and their intermodel spread are well-captured by down-gradient diffusive heat transport. While changes in moist and dry heat transport compensate in the annual-mean, their opposite seasonality may support non-compensating effects on Arctic warming.

Clouds are the main source of uncertainties when projecting climate change. Mixed-phase clouds (MPCs) that contain ice and supercooled-liquid particles are especially hard to constrain, and climate models neither agree on their phase nor their spatial extent. This is problematic, as models that underestimate contemporary supercooled-liquid in MPCs will underestimate future warming. Furthermore, it has recently been shown that supercooled-liquid water in MPCs is not homogeneously-mixed, neither vertically nor horizontally. However, while there have been attempts at observationally constraining MPCs to constrain uncertainties in future warming, all studies only use the phase of the interior of MPCs. Using novel satellite observations, and contrary to current knowledge, we show that MPCs are more liquid at the cloud top globally. We use these observations to constrain, for the first time, the cloud top phase in addition to the interior of MPCs in a global climate model, leading to +1C more 21st century warming in NorESM2 SSP5-8.5 climate projections. We anticipate that the difference between cloud top and interior phase in MPCs is an important new target metric for future climate model development, because similar MPC-related biases in future warming are likely present in many climate models.