Kyle C. Armour

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.

Paleoclimates provide examples of past climate change that inform estimates of modern warming from greenhouse-gas emissions, known as Earth’s climate sensitivity. However, differences between past and present climate change must be accounted for when inferring climate sensitivity from paleoclimate evidence. The closest paleoclimate analog to near-term warming from greenhouse-gas emissions is the Pliocene (5.3 to 2.6 Ma), a warm epoch with atmospheric CO ₂ concentrations similar to today. Recent reconstructions indicate the Pliocene was 1 °C warmer than previously thought, implying higher climate sensitivity, which is also supported by recent reconstructions showing more cooling with reduced CO ₂ at the Last Glacial Maximum (LGM; 19 to 23 thousand years ago). However, large-scale patterns of paleoclimate temperature change differ strongly from modern projections. Climate feedbacks and sensitivity depend on temperature patterns, and such “pattern effects” must be accounted for when using paleoclimates to constrain modern climate sensitivity. Here we combine data-assimilation reconstructions with atmospheric general circulation models to show Earth’s climate is more sensitive to Pliocene forcing than modern CO ₂ forcing. Pliocene ice sheets, topography, and vegetation alter patterns of ocean warming and excite destabilizing cloud feedbacks, and LGM feedbacks are similarly amplified by the North American ice sheets. Accounting for paleoclimate pattern effects produces a best estimate (median) for modern climate sensitivity of 2.8 °C and 66% CI of 2.4 to 3.4 °C (90% CI: 2.1 to 4.0 °C), substantially reducing uncertainty in projections of 21st-century warming.

Climate model emulators were extensively used in the IPCC’s Sixth Assessment Report due to their computational efficiency and consistency with key climate metrics. The emulators were calibrated to historical observations and used for future climate projections without systematic evaluation of their robustness. Here, we develop a framework to evaluate emulator performance against global climate model (GCM) large ensembles. This is demonstrated by constraining a two-layer energy balance model (EBM) to historical simulations of four GCMs and comparing their 21st century warming projections. The EBM matches projected warming in three of the four GCMs but exhibits substantial spread across ensemble members. It fails to reproduce the time-evolving global climate feedback in GCMs, with compensating biases between feedbacks and ocean heat uptake efficiency allowing seemingly accurate projections for incorrect reasons. Our results underscore the importance of evaluating the accuracy and physical realism of climate model emulators before using them for warming projections.

The planet is warming due to the burning of fossil fuels, but the geographical pattern of the observed temperature change over the Pacific Ocean over the past ~40 years is profoundly different from our expectations based on the CMIP5/6 climate model simulations of both historical and future warming. Here we will present an argument that the observed pattern of warming is consistent with a forced response to increasing atmospheric carbon dioxide shaped by two-way atmospheric teleconnections between the Southern Ocean and tropical Pacific Ocean. The same two-way atmospheric teleconnections might also be capable of yielding low-frequency natural variability in sea surface temperature and sea level pressure anomalies resembling the observed trend patterns. We will offer reasons why the observed pattern of warming is not simulated by the climate models. The observed pattern of warming in the Pacific has first-order implications for climate sensitivity as well as for the projected changes in global-scale precipitation.

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.

Climate models simulate a large spread in the projected weakening of the Atlantic meridional overturning circulation (AMOC) over the 21st century. Here, we demonstrate that this uncertainty can be substantially reduced by using a thermal-wind expression that relates the AMOC strength to the meridional density difference and the overturning depth in the Atlantic basin. This expression captures the intermodel spread in AMOC weakening across climate models, with the majority of the intermodel spread arising from overturning depth changes. The overturning depth also establishes a crucial link between the present-day and future AMOC strength. Climate models with a deeper present-day overturning tend to predict greater shoaling under warming. This occurs because their present-day North Atlantic is less stratified, allowing for a deeper penetration of surface buoyancy flux changes, greater density changes at depth, and, consequently, greater AMOC weakening. By integrating observational constraints, we conclude that, regardless of the emission scenario, the AMOC will only experience modest weakening of about 4 Sv by the end of this century. These results indicate that the uncertainty in 21st-century AMOC weakening, and a propensity to predict strong AMOC weakening, can be primarily attributed to climate model biases in accurately simulating the present-day ocean stratification.

We compare abrupt CO₂‐quadrupling (abrupt‐4xCO2) and ‐doubling (abrupt‐2xCO2) simulations across 10 CMIP6 models. Two models (CESM2 and MRI‐ESM2‐0) warm substantially more than twice as much under abrupt‐4xCO2 than abrupt‐2xCO2, which cannot be explained by the non‐logarithmic scaling of CO₂ forcing. Using an energy balance model, we show that increased warming rates within these two models are driven by both less‐negative radiative feedbacks and smaller global effective heat capacity under abrupt‐4xCO2. These differences are caused by a decrease in low cloud cover and shallower ocean heat storage, respectively; both are linked to smaller fractional declines in the Atlantic Meridional Overturning Circulation (AMOC) under abrupt‐4xCO2 (relative to abrupt‐2xCO2). On a global scale, higher climate sensitivity under larger forcing can be explained by a feedback‐temperature dependence; however, we find that forcing‐dependent spatial warming patterns due to AMOC decline are an important physical mechanism which reduces warming in a way that is not captured by a global‐mean framework.