Ian Eisenman

Cloud-based climate interventions seek to reduce climate change impacts by altering Earth's radiation budget through targeted cloud modifications. Mixed-phase regime cloud thinning (MCT) is a recently proposed intervention that aims to reduce cloud longwave trapping by seeding mixed-phase regime clouds with ice-nucleating particles, thereby promoting liquid-to-ice conversion and cloud thinning. Although the microphysical basis of MCT is well known, its large-scale climatic potential is poorly known. Here we address this question by evaluating the climatic response to removing all target clouds for MCT in the wintertime Arctic in a climate model. We find that, even under this intentionally extreme scenario, the response remains both localized to the Arctic Ocean and limited in magnitude, producing -0.47±0.08°C of Arctic annual-mean cooling and 0.23±0.04 million km ² of annual-mean sea ice expansion. This offsets 4.3% of Arctic warming and 3.5% of sea ice loss under doubled CO ₂ , or approximately 18% and 13%, respectively, relative to observed changes over the past four decades. These findings suggest limited large-scale climatic leverage of MCT but potential value for locally targeted applications, particularly near susceptible marginal ice zones.

Most studies of the equatorial Pacific response to anthropogenic forcing have focused on patterns of sea surface temperature (SST) change. However, similar SST patterns can be consistent with a range of different subsurface responses, each with differing physical and biogeochemical implications. While historical observation and climate model mismatches have been suggested in the literature, we show that model simulations can largely capture the observed 1958–2020 subsurface temperature trend in the equatorial thermocline. We then analyze a hierarchy of idealized model simulations, consisting of fully coupled, mechanically decoupled, ocean‐only, and reduced gravity models, to understand which ocean dynamics contribute to this response. We show that the response of the thermocline to idealized climate change can be explained by a combination of decadal momentum dynamics and radiatively forced buoyancy‐driven dynamics. We further decompose the buoyancy‐driven pattern into a pattern driven by remote, subtropical SST forcing and a pattern driven by local, equatorial SST forcing. The remote‐SST‐forced pattern of thermocline warming shows the signature of dynamic and thermodynamic subtropical cell adjustments. Meanwhile, increased stratification in the local‐SST‐forced pattern both coherently shoals the thermocline and relaxes thermocline tilt to largely cool the thermocline. Considered together, we recreate the long‐term subsurface equatorial Pacific response to idealized greenhouse gas forcing as a linear combination of (a) wind‐stress‐driven changes, (b) remote buoyancy‐driven changes, and (c) local buoyancy‐driven changes. To conclude we discuss implications for recent temperature trends, revisit canonical theories of the ocean dynamical thermostat, and show the insensitivity of forced responses to forcing geography.

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.

Comprehensive climate model simulations with perturbed sea ice covers have been extensively used to assess the impact of future sea ice loss, suggesting substantial climate changes both in the high latitudes and beyond. However, previous work using an idealized energy balance model calls into question the methods that are used to perturb sea ice cover, demonstrating a consistent overestimate of the surface warming due to sea ice loss, while the large complexity gap between the idealized and comprehensive models makes the implications of this result unclear. To bridge this gap we have performed simulations with a new implementation of the CESM2 model in a slab ocean aquaplanet configuration coupled with thermodynamic sea ice, which is able to capture the realistic seasonal characteristics of polar climate change. Using this model setup, we perform a suite of experiments to systematically quantify the spurious climate responses associated with melting sea ice without a CO₂ forcing. We find that using the sea ice ghost flux method overestimates many aspects of the climate response by 10%–20%, including the polar warming, the mini global warming signal and the increase in both precipitation and evaporation. The location of the latitudinal band of heating applied to melt the sea ice relative to the midlatitude jet is important for determining where the midlatitude circulation response is overestimated. This work advances our ability to isolate the true climate response to sea ice loss, and provides a framework for conducting coupled sea ice loss simulations absent the spurious impacts from the addition of artificial heating.

Regional patterns of sea level rise are affected by a range of factors including glacial melting, which has occurred in recent decades and is projected to increase in the future, perhaps dramatically. Previous modeling studies have typically included fluxes from melting glacial ice only as a surface forcing of the ocean or as an offline addition to the sea surface height fields produced by climate models. However, observational estimates suggest that the majority of the meltwater from the Antarctic Ice Sheet actually enters the ocean at depth through ice shelf basal melt. Here we use simulations with an ocean general circulation model in an idealized configuration. The results show that the simulated global sea level change pattern is sensitive to the depth at which Antarctic meltwater enters the ocean. Further analysis suggests that the response is dictated primarily by the steric response to the depth of the meltwater flux.

Paleoclimate records have been used to estimate the modern equilibrium climate sensitivity. However, this requires understanding how the feedbacks governing the climate response vary with the climate itself. Here we warm and cool a state-of-the-art climate model to simulate a continuum of climates ranging from a nearly ice-covered Snowball Earth to a nearly ice-free hothouse. We find that the pre-industrial (PI) climate is near a stability optimum: warming leads to a less-stable (more-sensitive) climate, as does cooling of more than 2K. Physically interpreting the results, we find that the decrease in stability for climates colder than the PI occurs mainly due to the albedo and lapse-rate feedbacks, and the decrease in stability for warmer climates occurs mainly due to the cloud feedback. These results imply that paleoclimate records provide a stronger constraint than has been calculated in previous studies, suggesting a reduction in the uncertainty range of the climate sensitivity.

Regional patterns of sea level rise are affected by a range of factors including glacial melting, which has occurred in recent decades and is projected to increase in the future, perhaps dramatically. Previous modeling studies have typically included fluxes from melting glacial ice only as a surface forcing of the ocean or as an offline addition to the sea surface height fields produced by climate models. However, observational estimates suggest that the majority of the meltwater from the Antarctic Ice Sheet actually enters the ocean at depth through ice shelf basal melt. Here we use simulations with an ocean general circulation model in an idealized configuration. The results show that the simulated global sea level rise pattern is sensitive to the depth at which Antarctic meltwater enters the ocean. Further analysis suggests that the response is dictated primarily by the steric response to the depth of the meltwater flux.