Charles D. Koven

To limit global warming at well-below 2°C above pre-industrial levels, large and sustained CO2 emission reductions are needed and it may be possible to use alongside carbon dioxide removal (CDR) methods to return atmospheric CO2 concentrations to lower levels, with the assumption that lower CO2 will rapidly lead to lower temperatures and reverse or limit other climate change impacts. Despite such assumption, a comprehensive understanding of the reversibility of the Earth system components is a persistent knowledge gap, which hinders our ability to assess future outcomes of decreasing anthropogenic forcing. Here we evaluate the reversibility of climate change through a multi-model analysis of how the Earth system and climate respond in the CMIP6 Carbon Dioxide Removal Model Intercomparison Project (CDRMIP) cdr-reversibility experiment. cdr-reversibility is an idealized CO2 concentration peak and decline experiment where atmospheric CO2 increases at 1% yr-1 from a pre-industrial level until 4 times its initial value, then decreases at 1% yr-1 until pre-industrial levels are again reached, at which point CO2 is held constant. A few modelled quantities reverse or overshoot initial levels, on the same timescale as the changes in CO2 forcing, such as the land and ocean net primary productivity or the Atlantic Meridional Overturning Circulation – at least when viewed at the global annual mean level. However, when responding to decreasing CO2 concentration many properties exhibit time lags and hysteresis, e.g. changes in ocean and land carbon stocks or sea level. While some variables respond to mainly one specific driver, most show a hysteresis behaviour relative to all the considered driving forcers, namely surface air temperature, atmospheric CO2 or cumulative emissions. Due to such, the majority of the considered environmental variables would take long, many times longer than a human lifetime, to return to their unperturbed values (if this ever occurs). Therefore, exceeding and then reversing a climate target would have a clear legacy in the climate system even if CO2 concentrations return to pre-industrial levels.

As the window to limiting global warming to 1.5 °C closes, mitigation pathways consistent with this limit increasingly rely on temporary temperature overshoot. Despite recent advances in understanding how the Earth system responds to climate overshoot, uncertainty remains about the extent to which overshoot-induced changes can be reversed. Here, we synthesize the key features that determine climate reversibility and the asymmetry between responses to positive and negative emissions, challenging assumptions that net-negative emissions can undo overshoot-induced changes. We propose a typology of temperature responses in overshoot, peak and decline pathways that goes beyond established notions of reversible and irreversible climatic changes, incorporating underexplored climate feedbacks that may arise specifically during negative-emission phases. On this basis, we argue that an irreversible response, as evident from current-generation climate model simulations, cannot be ruled out. A more nuanced characterization and quantification of overshoot dynamics is critical for assessing the feasibility and risks of overshoot-based climate mitigation strategies.

Terrestrial vegetation regulates global carbon cycling and climate through responses to water availability. Yet, it remains unclear whether soil moisture (SM) or atmospheric dryness (vapour pressure deficit, VPD) dominates vegetation responses and, more critically, how their relative influence changes over time, limiting predictions of ecosystem–climate feedback under warming. Here we analyse satellite-derived vegetation variability over the past four decades using an explainable artificial intelligence framework that disentangles nonlinear hydroclimatic driver contributions. We show SM dominates vegetation variability in most regions. However, VPD influence has strengthened while SM influence has weakened over time. These changes highlight an asymmetric transition with more SM-dominated ecosystems shifting towards increasing VPD control than vice versa. Changes in the mean and variability of SM and VPD drive this transition by altering plant water-stress responses. Continued warming will probably intensify this pattern as rising VPD and more frequent extreme events strengthen atmospheric constraints on vegetation.

Model intercomparison projects (MIPs) have become an essential tool for systematically comparing Earth system models (ESMs) by identifying, characterizing, and quantifying common model behaviour through standardized experiments and metrics under idealized, historical, and scenario-based forcing. Over the past few decades, community MIPs have become a cornerstone of the broader climate modelling community and the Coupled Model Intercomparison Project (CMIP). Here, we describe the scientific objectives of the Coupled Climate-Carbon Cycle Model Intercomparison Project (C4MIP) in the context of the seventh phase of CMIP (CMIP7) and present the complete suite of C4MIP experiments, including those contributing to CMIP7's Assessment Fast Track (AFT) and those proposed for subsequent analyses beyond the AFT. The proposed experimental protocol maintains backward compatibility with previous CMIP phases while introducing new experiments. In addition to the traditional biogeochemically (BGC) and radiatively (RAD) coupled versions of the 1pctCO2 experiment, the new experiments include concentration-driven BGC and RAD variants of the abrupt 2×CO2 experiment, a suite of emissions-driven simulations that investigate carbon-cycle behaviour under positive, zero, and negative emissions, and a large pulse experiment to reassess the impulse-response of newer generation ESMs and update the widely-used function from earlier model generations. Whereas earlier phases of C4MIP primarily focused on quantifying the feedback metrics that describe interactions between the coupled carbon cycle and the physical climate system, the scope of C4MIP has expanded to include the estimation of the transient climate response to cumulative emissions (TCRE) and the zero-emissions commitment (ZEC). These policy-relevant metrics are most naturally quantified using emissions-driven ESM simulations. The increased emphasis on emissions-driven simulations reflects both the growing policy relevance of these metrics and the ability of emissions-driven simulations to represent carbon-climate interactions in a bi-directional manner. In particular, changes in carbon sinks and evolution of feedback effects influence the atmospheric CO₂ concentration in emission-driven simulations, whereas these interactions are absent from concentration-driven simulations, in which atmospheric CO₂ concentrations are prescribed. The proposed experimental design attempts to balance scientific ambition with the computational and storage resources required of participating climate modelling centres. The final suite of experiments and associated protocols has been developed through extensive community consultation and guidance from the C4MIP Scientific Steering Committee. As in previous CMIP phases, the experiments are expected to enable a broad range of community-led investigations of the coupled carbon-climate system.

We examine the behavior of ten Earth system models under a set of idealized CO2 emissions-driven simulations to look at regional temperature and precipitation changes in response to either an abrupt cessation of emissions or an idealized exceedance-and-return pathway, in which emissions linearly ramp down from positive to negative emissions. This allows a comparison of the regional climate at three different time points, all of which have the same level of cumulative CO2 emissions and therefore the same expected global warming amount. I.e., this lets us ask how regional climates would differ if we first emit, and then remove, a given amount of CO2, as compared to a case where the CO2 past a certain amount hadn't been emitted, and the coupled carbon-climate system had been left to freely evolve over the same length of time. We find consistently higher global precipitation for a given global temperature after zero emissions in both pathways, with a larger increase under the exceedance-and-return pathway than the zero-emissions pathway. Averaging over land and ocean domains, we find cooling over land relative to ocean, which is also stronger under exceedance-and-return than zero-emissions, and for about half of the models a difference in the relationship between land-ocean temperature contrast and global mean temperature change than occurs under positive emissions. We find low agreement across different models in the regional patterns of both temperature and precipitation change in all scenarios. For most of the models, there is a strong gridcell-level correlation between the temperature changes in the exceedance-and-return and zero-emissions responses, such that roughly half of the regional variance in temperature change at a given location that occurs under the exceedance-and-return pathway also occurs under zero emissions. Because only a single realisation of each experiment is available, this shared fraction is a lower bound on the common forced response: internal variability, which is uncorrelated between the two experiments, acts to reduce the apparent correspondence rather than inflate it. However, for some models there is low or negative correlation between the regional climate changes in the two cases, indicating a potential for fundamentally different regional climate change under exceedance-and-return than under zero emissions. Regional precipitation changes are more different between the exceedance-and-return and zero-emissions responses for all models.

As the window to limiting global warming to 1.5 °C closes, mitigation pathways consistent with this limit increasingly rely on temporary temperature overshoot. Despite recent advances in understanding how the Earth system responds to climate overshoot, uncertainty remains about the extent to which overshoot-induced changes can be reversed. Here, we synthesize the key features that determine climate reversibility and the asymmetry between responses to positive and negative emissions, challenging assumptions that net-negative emissions can undo overshoot-induced changes. We propose a typology of temperature responses in overshoot, peak and decline pathways that goes beyond established notions of reversible and irreversible climatic changes, incorporating underexplored climate feedbacks that may arise specifically during negative-emission phases. On this basis, we argue that an irreversible response, as evident from current-generation climate model simulations, cannot be ruled out. A more nuanced characterization and quantification of overshoot dynamics is critical for assessing the feasibility and risks of overshoot-based climate mitigation strategies.

Land carbon sinks are responsible for removing about a quarter of anthropogenic CO ₂ emissions, and make up approximately half of total global carbon sinks. Uncertainty in the response of land carbon sinks to climate and changing atmospheric CO ₂ are large, and dominate the uncertainty in total carbon sinks under future climate. Understanding the carbon cycle response to net-zero and net-negative emissions has important implications for projecting future climate. Here we characterize the response of land carbon pools and fluxes from ten emissions-driven Earth system models (ESMs) under positive, net-zero, and net-negative CO ₂ emissions using experiments from the “flat10” model intercomparison. Although there are many differences in simulated land carbon pools and fluxes across models, we find some consistent behavior across ESMs. (1) During the positive emissions phase, carbon is gained on land primarily in vegetation pools. (2) Following net-negative emissions to the point of cumulative zero emissions, carbon is lost from land in tropical latitudes, primarily from vegetation pools, but in mid- and high-latitudes most models show net land carbon gain, primarily in soil pools. (3) Following an extended period of net-zero emissions, a majority of models again show carbon gain in mid- and high-latitudes and vegetation carbon loss in the tropics. Under net-negative emissions the timing of vegetation carbon response relative to peak emissions is relatively consistent across ESMs, but timing of soil carbon response varies widely, implying larger intermodel disagreement associated with responses of soil carbon which tends to have longer timescales relative to vegetation carbon. Our findings highlight that tropical carbon is most likely to be both gained and subsequently lost under positive, zero, declining, and negative emissions, with possible implications for carbon dioxide removal efforts.

This paper outlines the scope, development and publication of data requirements for a set of reference climate simulations, and describes the methodology used to gather and synthesise them into a cohesive “Data Request” usable by data producers. The simulations supported by this Data Request comprise the initial phase of the next upgrade of research activities under the World Climate Research Programme (WCRP). Dunne et al. (2025) set out the scientific scope and objectives of the Assessment Fast Track and its role in initiating the current phase of the Coupled Model Intercomparison Project, CMIP7 – a key part of the next WCRP activity upgrade. Building on the successes of past CMIP phases which have supported an ever-expanding scope of work, the Data Request Task Team reached out to new communities to enhance engagement in CMIP. A transparent and community-led approach was adopted, where domain experts from the CMIP community were recruited into five teams by domain called “Thematic Author Teams” to co-create data requirements through wide consultation. This paper describes the process of gathering data requirements in the initial phase of CMIP7, along with the structure of the CMIP7 Data Request itself. Version 1.2.2.5 of the CMIP7 Data Request (DR7) covers the data production requirements for (i) control simulations of the past climate (both distant and recent), (ii) key sensitivity experiments focusing on critical aspects of climate and model behaviour, and (iii) a range of future climate scenarios. It consists of a relational database that maps climate variables to reference experiments according to scientific objectives, with associated metadata to enable data production and tools to allow interoperability and content exploration. The usage of climate models and CMIP data is broadening from its origins in scientific study of the physical environment to support the analysis of climate impacts, and planning for an ever-increasing portfolio of adaptation and mitigation measures. To support this growing scope, DR7 introduces a new organising component, Opportunities, to support transparent mapping between variables and experiments. The 46 Opportunities in DR7 represent the key community-driven use cases across CMIP data users – each describing why its combination of variables and experiments is important and how they contribute to impact, providing both scientific justification and technical requirements. It is challenging to represent the needs of the rapidly expanding CMIP community and user base while respecting the capacity limitations of CMIP data production. DR7 addresses this through a process of wide stakeholder engagement centred around an open consultation and community co-creation, while prioritising stakeholder representation and diversity. Innovations in interactive web tools and enhanced WCRP support through the CMIP IPO and Task Teams were also critical to the process of developing data requirements in collaboration with the community.

Achieving net-zero emissions over the coming decades requires unprecedented reductions in anthropogenic emissions of greenhouse gases (GHGs) complemented by a rapid ramp-up in the magnitude of global carbon dioxide removal (CDR). The carbon credit market (CCM) is emerging as a means to finance both emissions reductions and carbon dioxide removal from the atmosphere. To achieve necessary growth on these fronts, the total scope and diversity of projects that are candidates for inclusion in the CCM must expand, necessitating a means of comprehensively assessing the quality of carbon credit projects (CCPs) based on their ability to make quantifiable reductions to GHG concentrations in the atmosphere. Toward a comprehensive quality assessment, we propose a framework to assess and differentiate CCPs based on their estimated impact on atmospheric GHG composition. In parallel, we propose a path towards verification of the aggregated atmospheric impact of CCM actions, since a detectable and attributable signal in atmospheric GHG composition can be viewed as the clearest measure of their climate forcing and, therefore, effectiveness.