John Patrick Dunne

Climate stabilization pathways limiting global warming to 1.5° to 2°C require rapid emission reductions alongside additional carbon dioxide removal (CDR) of ∼2 to 3 PgC year ⁻¹ . Our simulations show that sustained century-scale ocean iron fertilization (OIF) in the southern sector of the Southern Ocean (SSO) provides a contribution of ∼30 PgC, equivalent to ∼0.3 PgC year ⁻¹ . This uptake nearly matches the additional carbon uptake from global fertilization. In other iron-limited regions, fertilization-induced uptake is largely offset (70 to 100%) by anomalous outgassing in nonfertilized regions. After terminating OIF following 30 years of fertilization, the ocean retains ∼50% of the additional carbon in the SSO, whereas retention is negligible elsewhere. These results identify the SSO as the most favorable region for durable carbon removal through large-scale OIF, particularly in terms of additionality and durability, although its carbon uptake is smaller than earlier estimates. OIF-induced CDR is constrained by local and remote nutrient depletion, carbon reentrainment, incomplete macronutrient drawdown, and reduced phytoplankton carbon-to-phosphorus ratios. Incomplete macronutrient drawdown reflects the uptake controls by light, top-down controls, and rates of macronutrient resupply. Further insight into OIF potential will be enabled by standardized multimodel comparisons and targeted analysis of nutrient drawdown controls.

Marine extreme events, such as marine heatwaves (MHWs), low‐oxygen (LOXs), and ocean acidification extremes, are short‐lived phenomena with potentially severe impacts on ecosystems and the services they provide. These events are expected to intensify and become more frequent due to climate change. While resolving mesoscale activity is important for accurately simulating MHWs, the sensitivity of biogeochemical extremes to model resolution remains unclear. We evaluated three global climate models with varying ocean resolutions (1, 0.25, and 0.1) under preindustrial and increasing C forcing. The high‐resolution model (0.1) exhibits more frequent but shorter extreme events, particularly in eddy‐rich regions, when compared to the 0.25 and 1 resolution models. For example, the global frequency of LOX events increased between coarse and eddy‐rich model configurations by 136% while event duration decreased by 62%. These results highlight the importance of mesoscale dynamics in shaping biogeochemical extremes and the need for improved parameterizations of eddy effects, thereby advancing climate extreme predictions.

The Transient Climate Response (TCR) is a metric of climate sensitivity relevant to present‐day climate change, but has been less studied than the Equilibrium Climate Sensitivity. Here, we emphasize the utility of the TCR by demonstrating that temperature and radiative forcing in transient scenarios are linearly related, where the proportionality constant is essentially the TCR. This relationship has been invoked in previous studies but not extensively validated; here it is shown to hold across models as well as forcing scenarios. This linear scaling furthermore allows for a simple estimate of the time to cross a given temperature target, given only a forcing scenario and a value for TCR. Deviations from a strictly linear relationship between temperature and forcing are governed by the “slow” component of global warming driven by declining deep ocean heat uptake, but can only be fully accounted for by incorporating changing patterns of surface warming via ocean heat uptake “efficacy.”

Equilibrium climate sensitivity (ECS) quantifies surface warming in response to doubled pre‐industrial CO ₂ (2xCO ₂ ). Uncertainty in estimates arises from diverse model representations and climate‐chemistry feedbacks. We quantify ECS with five atmospheric chemistry‐composition model representations (sea salt, dust, organics, ozone, sea ice masking) with slab ocean configurations of the GFDL‐CM4.0 physical climate (4.1 ± 0.1 K) and GFDL‐ESM4.1 coupled climate‐carbon‐chemistry Earth system (3.4 ± 0.1 K) models. Estimated ECS is 0.7 K lower with GFDL‐ESM4.1, attributed to additive 0.1–0.3 K decreases as each GFDL‐ESM4.1 driver is included and a 0.4–0.6 K decrease with the addition of all five GFDL‐ESM4.1 model drivers in combination, relative to estimated ECS with GFDL‐CM4.0 neglecting these negative climate‐chemistry feedbacks. Interactive stratospheric ozone included in GFDL‐ESM4.1 contributes to the largest ECS decrease in response to 2xCO ₂ of all five drivers, characterized by reduced polar amplification. We demonstrate strong climate sensitivity to complex but often ignored climate‐chemistry feedbacks.

The Transient Climate Response (TCR) is a metric of climate sensitivity relevant to present-day climate change, but has been less well studied than the Equilibrium Climate Sensitivity. Here, we emphasize the utility of the TCR by demonstrating that temperature and radiative forcing in transient scenarios are linearly related (to a good approximation), where the proportionality constant is essentially the TCR. This previously known linear relationship is shown here to hold across forcing scenarios as well as across models. This relationship furthermore allows for a simple estimate of the time to cross a given temperature target, given only a forcing scenario and a value for TCR. Deviations from a strictly linear relationship between temperature and forcing are influenced by the ‘slow’ component of global warming driven by declining deep ocean heat uptake, especially when changing patterns of surface warming are accounted for via ocean heat uptake ‘efficacy’.

The chemical nature of river water significantly influences the coastal carbonate system, contributing to coastal acidification and creating suboptimal conditions for marine calcifiers. While several regional efforts have assessed observationally based riverine concentrations and fluxes of total alkalinity (TA) and dissolved inorganic carbon (DIC), these values in global ocean biogeochemical models have generally been simplified, often set to zero or balanced against global sediment calcium carbonate burial. To enhance our understanding of rivers' role in the coastal carbonate system, we applied multiple linear regression (MLR) to develop global empirical relationships for estimating river TA and DIC from watershed properties. We find that river TA values are primarily controlled by forest, carbonate rock coverage, and annual mean precipitation, explaining 74% of the spatial variability in TA. The variability explained improves to 77% with the inclusion of permafrost and glacial coverage, especially in high latitude and altitude regions. Additionally, nearly 30% of the spatial variability in the river DIC‐to‐TA ratio can be explained by terrestrial gross primary production and carbonate rock coverage. Applying these MLR‐derived TA and DIC concentrations to a 1/4° resolution global ocean model reduces the high bias in model estimates of global coastal uptake by 69% (equivalent to 0.11 Pg C yr ⁻¹ less uptake) compared to the case with zero river TA and DIC. This study elucidates key drivers of the river carbonate system and underscores the importance of accurately representing riverine inputs to improve predictions of global coastal carbon dynamics and ecosystem responses to environmental changes.

The Coupled Model Intercomparison Project (CMIP) coordinates community-based efforts to answer key and timely climate science questions, facilitate delivery of relevant multi-model simulations through shared infrastructure, and support national and international climate assessments. Generations of CMIP have evolved through extensive community engagement from punctuated phasing into more continuous support for the design of experimental protocols, infrastructure for data publication and access, and public delivery of climate information. We identify four fundamental research questions motivating a seventh phase of coupled model intercomparison relating to patterns of sea surface temperature change, changing weather, the water–carbon–climate nexus, and tipping points. Key CMIP7 advances include an expansion of baseline experiments, a focus on CO₂-emissions-driven experiments, sustained support for community MIPs, periodic updating of historical forcings and diagnostics requests, and a collection of prioritized experiments, or the “Assessment Fast Track”, drawn from community MIPs to support climate research, assessment, and service goals across prediction and projection, characterization, attribution, and process understanding.

We analyze the frequency and amplitude of projected warming and ocean acidification extremes under high CO ₂ and strongly mitigating scenarios. We find interpretational differences in projections arising from methodological choices associated with specification of stressor thresholds. Use of absolute versus distribution‐based thresholds, and, in the distribution‐based case, the inclusion or exclusion of seasonal variability, can lead to very different regional patterns in projected stress. The choice of fixed versus adaptive baseline, for example, determines whether future stress frequency in the low‐CO ₂ scenario most closely resembles that in the high‐emissions scenario or historical period. We find that mitigation through emissions reductions, in combination with representation of rates of adaptation that are realistic for some marine organisms, has the potential to dampen end of century threshold exceedance to frequencies of occurrence closer to the recent historical period than to the high‐emissions scenario.

Chlorophyll underpins ocean productivity yet simulating chlorophyll across biomes, seasons and depths remains challenging for earth system models. Inconsistencies are often attributed to misrepresentation of the myriad nutrient supply, growth and loss processes that govern phytoplankton biomass. They may also arise, however, from unresolved or misspecified photoacclimation or photoadaptation responses. A series of global ocean ecosystem simulations were conducted to assess these latter sensitivities: alternative photoacclimation schemes implicitly modulated investments in light harvesting versus photodamage avoidance and other cellular functions. Photoadaptation experiments probed the impact of adding low‐ and high‐light adapted phytoplankton ecotypes. Results showed that photoacclimation and photoadaptation alternatives generate chlorophyll differences exceeding a factor of 2 in some regions and seasons. In stratified waters, photoadaptation and acclimation to light levels over mixing depths consistent with the timescale of photoadaptation (days) benefitted model performance. In regions and seasons with deep mixed layers, surface‐skewed photoacclimation yielded improved fidelity across satellite chlorophyll products. Large photoacclimation‐driven differences in chlorophyll concentration had small impacts on primary productivity and carbon export, unlike those arising from changes in the nutrient supply. Improved photoacclimation and photoadaption constraints are thus needed to reduce ambiguities in the drivers of chlorophyll change and their biogeochemical implications.