We describe a new Earth system model (ESM) experiment protocol, as part of the international Tipping Points Modelling Intercomparison Project (TIPMIP) project. We propose this as a protocol for the Coupled Model Intercomparison Project 7 (CMIP7). The protocol requires ESMs to run in CO2-emission mode, with atmospheric CO2 a predicted variable. Forcing for the protocol consists solely of a constant emission of CO2, based on each model's transient climate response to cumulative emissions of carbon dioxide (TCRE) value, to give a common global mean surface warming rate of 2 °C per century. This positive emission (ramp-up) experiment is started from the pre-industrial state of a given model. When the ramp-up run first exceeds a specified level of global warming (2 and 4 °C) relative to the model's pre-industrial global mean surface air temperature (GMSAT), CO2 emissions are set to zero and the positive emission run is branched into a zero-emission run. The zero-emission runs continue for 300 years. 50 years into each zero-emission run, CO2 emissions are set to the negative of the positive emission rate and the model run until GMSAT cools below the original pre-industrial value. Additionally, when the negative emission run started from the global warming level (GWL)=4 °C first drops below GWL=2 °C, a zero-emission run is branched off this, completing the set of experiments. Using this protocol, we are able to control the rate of global warming, and potentially also the rate of cooling, across participating models. TIPMIP experiments will support a range of analyses, including; an assessment of abrupt/rapid Earth system change under net zero CO2 emissions at a range of global warming levels, the long-term Earth system response to net zero CO2 emissions at these warming levels, the response to net negative CO2 emissions and the efficacy of negative emissions to drive cooling, and the reversibility of Earth system change under a pathway of positive (warming), zero, and negative (cooling) CO2 emissions.

Abstract. The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the climate system, yet the climate and carbon cycle responses to a collapse under emission pathways consistent with the Paris Agreement remain poorly understood. Using the comprehensive GFDL ESM2M Earth System Model with the Adaptive Emissions Reduction Approach, we impose a freshwater-induced strong AMOC weakening to 20 % of its preindustrial strength, initiated in year 2026 and achieved within 60 years. The counterfactual simulations without freshwater hosing otherwise follow a pathway in which global warming stabilizes at 2 °C and the AMOC weakens only modestly and partially recovers. Relative to the 2 °C scenario without AMOC collapse, a strong AMOC weakening cools global mean surface air temperature by −0.8 °C (5-member ensemble range: −0.7 to −0.9) by 2171–2200, offsetting 40 % of global warming. Pronounced cooling is simulated in the North Atlantic region, reaching up to -5.4 °C (-8.0 to -3.3) in winter over Iceland relative to 1861–1900 conditions. The global cooling is primarily driven by larger negative feedback from clouds, driven by an increase in low-level clouds in the North Atlantic region, with smaller contributions from enhanced global ocean heat storage and reduced atmospheric CO2. The total ocean heat content increases by an additional 488 ZJ (442–531), primarily south of 20° N, associated with reduced northward heat transport and enhanced heat uptake in the North Atlantic. The additional heat increases global thermosteric sea level rise by an additional 10 % (8–12), with enhanced rise in the western and tropical North Atlantic and northern Indian Ocean, but pronounced reductions in the eastern North Atlantic. Atmospheric CO2 declines by 13 ppm due to anomalous land carbon uptake of 44 GtC (33–53), dominated by enhanced carbon storage in the Amazon region under cooler and wetter conditions. In contrast, global ocean carbon storage decreases by 14 GtC, mainly north of 20° N. The AMOC-induced cooling temporarily breaks the near-linear relationship between cumulative CO2 emissions and warming, increasing the remaining emission budget for limiting warming to 2 °C by 63 % (54–72). Compared to identical freshwater forcing under preindustrial conditions, the surface temperature, ocean heat content, and sea-level responses to an AMOC collapse are substantially damped in a 2 °C world, indicating reduced climate sensitivity to AMOC collapse in a warmer world. These results demonstrate that a strong AMOC weakening would profoundly alter the climate–carbon cycle system and underscore the importance of explicitly accounting for AMOC risks in long-term climate assessments.

The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the climate system, yet the consequences of a pronounced weakening under emission pathways consistent with the Paris Agreement remain poorly understood. Using the comprehensive GFDL ESM2M Earth System Model with the Adaptive Emissions Reduction Approach, we impose a freshwater-induced strong AMOC weakening to 20% of its preindustrial strength starting in year 2026. These simulations otherwise follow a pathway in which global warming stabilizes at 2°C and the AMOC weakens only modestly and partially recovers. Relative to the modest-weakening scenario, a strong AMOC weakening cools global mean surface air temperature by −0.8°C (5-member ensemble range: −0.7 to −0.9) by 2171-2200, with pronounced regional cooling in the North Atlantic, reaching up to −6.8 °C (−4.1 to −9.7) in winter over Iceland. The ocean stores an additional 385 ZJ (331–428) of heat, primarily south of 20°N, associated with reduced northward heat transport and enhanced heat uptake in the North Atlantic. The additional heat increases global thermosteric sea level rise by 10% (8–12). Atmospheric CO2 declines by 13 ppm due to anomalous land carbon uptake of 44 GtC (33–53), dominated by enhanced carbon storage in the Amazon under cooler and wetter conditions. In contrast, global ocean carbon storage decreases by 14 GtC, mainly north of 20°N, although carbon uptake increases in the northern North Atlantic. The AMOC-induced cooling breaks the near-linear relationship between cumulative CO2 emissions and warming, increasing the remaining carbon budget for limiting warming to 2°C by 63% (54–72). Compared to identical freshwater forcing under preindustrial conditions, the surface temperature, ocean heat content, and sea-level responses are substantially damped, indicating reduced climate sensitivity to AMOC collapse in a warmer world. These results demonstrate that a strong AMOC weakening would profoundly alter future climate–carbon cycle interactions and underscore the importance of explicitly accounting for AMOC risks in long-term climate assessments.

Our understanding of impacts and feedbacks associated with temporarily overshooting the Paris Agreement temperature goal - where the 1.5 °C global warming target is exceeded and retraced at a later time period - is currently limited. Such overshoot scenarios are of increasing likelihood and have the potential to be devasting in terms of both their peak impacts and irreversibility, affecting natural and human systems.Here, we apply the Earth System Model GFDL-ESM2M coupled to the Adaptive Emission Reduction Approach (AERA) in order to perform novel policy-relevant simulations over the 1861 to 2500 period that temporarily overshoot the global warming target of 1.5 °C at various levels of peak global warming (2.0, 2.5 and 3.0 °C), and compare these to a reference scenario that stabilizes at 1.5 °C. We use this framework to isolate features arising from the overshoots, and investigate (1) negative emissions needed to reverse an overshoot and their impacts for cumulative emissions, (2) spatial differences in surface warming and oceanic heat content between overshoot and 1.5°C stabilization case, and (3) impacts that these spatial differences have for precipitation, sea level rise and ocean ecosystem stressors.Our framework suggests levels of negative carbon emissions of up to 9 Pg C yr-1 to revert the global temperature the most extreme overshoot of 3.0 °C back to 1.5 °C, with less cumulative emissions allowed in the long-term than in the 1.5 °C simulation to maintain global temperature at 1.5°C. We detect long-term high latitude warming of up to 2.1 °C averaged over the North Atlantic and 0.5 °C over the Southern Ocean that persists after the overshoot. We attribute the persistent warming in the high latitudes to the recovery of both Atlantic Meridional Overturning Circulation and Antarctic abyssal overturning, which retrace to even higher levels in the overshoots than in the 1.5 °C stabilization case. These impact the distribution of precipitation, for instance stronger precipitation found in the high latitudes in the overshoots, as wells as the Pacific Walker Cell. The model also shows that due to excess heat storage in the subsurface of low latitudinal oceans, sea level rise does not recover back to 1.5 °C stabilization levels in overshoot scenarios, remaining up to 20 % higher in the strongest overshoot. The persistent long-term changes that the overshoots that we detect imply consequences for regional climates, cryosphere and marine ecosystems lasting for decades or even centuries after the overshoot reversal.

High-latitude frozen soils contain a vast store of organic matter, a potential source of greenhouse gases due to permafrost thaw. Understanding natural carbon cycle responses to climate change is crucial for emission reduction strategies. We use the Max Planck Institute Earth System Model, driven by the Adaptive Emission Reduction Approach (AERA), to assess emission pathways for limiting global warming to 2 degrees C and 3 degrees C relative to preindustrial levels, while accounting for frozen soil carbon (FSC). We found that thawing FSC makes 122 PgC under 2 degrees C and 229 PgC under 3 degrees C warming, available for decomposition with about 75% reaching the atmosphere as carbon-dioxide by 2298. Emission pathways that include the release of FSC diverge from their respective reference simulations without permafrost between the middle (2 degrees C) and end (3 degrees C) of the current century. By 2298, remaining carbon budgets are reduced by similar to 13% (115 PgC) for 2 degrees C and similar to 11% (156 PgC) for 3 degrees C stabilization levels. Annual permafrost emissions average similar to 0.7 PgC/yr for 3 degrees C and similar to 0.3 PgC/yr for 2 degrees C during the simulation period (2025-2298). However, temporary emission peaks reaching half of present-day annual fossil fuel emissions (similar to 5 PgC) are possible. Surprisingly, while negative emissions are required for both reference simulations, only the simulation for the 3 degrees C warming, accounting for FSC, requires negative fossil fuel emissions. This occurs because the FSC release causes an earlier initiation of emission reduction by AERA, resulting in a smoother emission curve. These findings underscore the importance of factoring in carbon released from permafrost thaw in mitigation action.

With global warming, increased heat stress will substantially impact the rural labor force. Understanding and quantifying this impact is difficult, especially due to regional differences: Does the temperature increase? Is there more solar exposure? Does humidity respond non-linearly with respect to temperature changes? Furthermore, humans are resourceful, and local environments could provide adaptation methods to decrease heat impacts. A policy-relevant assessment in the context of the Paris Agreement is even more difficult with existing CMIP-type simulations with prescribed greenhouse gas trajectories that lead to a different and often non-stable warming for each model. To resolve the impacts climate mitigation and adaptation on heat stress on warming levels with specific relevance for the Paris Agreement, we use the Community Earth System Model (CESM2) driven by emissions from the Adaptive Emissions Reduction Approach (AERA) to generate climate mitigation scenarios stabilized at 1.5°C, 2.0°C and 3.0°C of global warming. One form of adaptation to heat stress impacts is to use the local environment for cooling. Within CESM2, we compare the direct and indirect exposure to solar radiation within the vegetated canopy as an inexpensive form adaptation. To diagnose the heat stress conditions we use the International Organization for Standardization (ISO) 7243, the Wet Bulb Globe Temperature (WBGT), realized by first principles representation of the globe, dry bulb, and natural wet bulb thermometers utilizing CESM2’s temperature, humidity, winds, and radiation. The WBGT values are transformed into labor capacity using standardized algorithms (e.g. NIOSH or Lancet) and the above canopy (no adaptation) and below canopy (with adaptation) labor capacity are directly compared to each other. We show that the potential to adapt by using the local environment for cooling is not uniform across regions. For example, evaluating the hottest seasonal period (defined as a local summer), at the 3.0°C mitigation scenario in equatorial Southeast Asia, adaptation can save up to 50% of total labor capacity losses. However, in northern South Asia, adaptation saves only 10% of the seasonal labor capacity losses. These results demonstrate that rural laborers in some locations may have limited capacity to adapt to differing global mitigation strategies and may require mechanical cooling or other expensive forms of adaptation.

As the likelihood of temporarily exceeding 1.5 °C of global warming rises, understanding the response of the ocean-climate system to overshooting this warming level is of increasing importance. Here, we apply the Adaptive Emissions Reduction Approach to the Earth System Model GFDL-ESM2M to conduct novel overshoot scenarios which temporarily exceed 1.5 °C of global warming to 2.0, 2.5 and 3.0 °C, alongside a complementary scenario that stabilizes global temperature at 1.5 °C. The simulation framework allows to isolate impacts attributable to the temperature overshoots alone, both during their peaks and after their reversals, in simulation timeframes spanning from 1861 to 2500. Our results reveal that, while global sea surface temperatures eventually retrace to 1.5 °C stabilization levels, substantial residual ocean surface warming persists regionally, particularly in the North Atlantic (regional average of up to +3.1 °C in the 3°C overshoot scenario) and the Southern Ocean (+1.2 °C). The residual warming is primarily attributed to the recoveries of the Atlantic and Southern Ocean meridional overturning circulation, resulting in a reversed pattern of disproportionate surface warming in low-latitude oceans found during the transient peak of the overshoot. Excess subsurface heat storage in low and mid-latitudes furthermore prevents steric sea level rise from reverting to 1.5 °C stabilization levels in any overshoot scenario, with sea level remaining up to 32 % higher in the 3 °C overshoot scenario. Both peak overshoot impacts and persistent changes following overshoot reversal bear significant implications for future assessments of coastlines, regional climates, marine ecosystems, and ice sheets.

Different human activities and associated emissions of CO _2 and non-CO _2 radiative forcing agents and feedbacks determine the final state of Earth’s climate. To understand and explain contributions to global temperature changes, many emission-based metrics have been employed, such as CO _2 -equivalent or -forcing equivalent. None of these metrics, however, include dynamic responses from Earth system feedbacks in terms of carbon and heat redistribution, known to play an increasingly important role in ambitious mitigation scenarios. Here we introduce a framework that allows for an assessment of such feedbacks in addition to CO _2 , non-CO _2 anthropogenic forcing and natural external variability contributions. FROT (Framework for Radiative cOntributions to Temperature response) allows for an assessment of components of direct radiative impact to the system (climate forcing), as well as Earth system feedbacks concerning heat and carbon. The framework is versatile in terms of applications and allows for exploring individual components contributions to, for example, temperature stabilisation simulations, or comparisons in different models and scenarios, as it can reasonably explain their simulated temperature variability. Here, we apply FROT to both an intermediate complexity and a fully coupled Earth system model, as we simulate highly ambitious mitigation scenarios. Comparing temperature stabilisation scenarios, we can show that both net-zero CO _2 emissions and small amounts of positive CO _2 emissions could lead to a stable global temperature trajectory. Our assessment reveals that the effects of non-CO _2 climate forcings, especially the development of sulphate aerosols in the atmosphere, and the dynamics of the carbon cycle, play a pivotal role in the final level of warming and in enabling a temperature stabilisation. Under highly ambitious climate mitigation scenarios it becomes crucial to include Earth system feedbacks, specifically ocean heat uptake, to understand interannual to decadal temperature development, since previously secondary processes now become increasingly dominant. Our framework offers the opportunity to do so.

While international climate policies now focus on limiting global warming to well below 2 degrees C or pursuing a 1.5 degrees C level of global warming, the climate modelling community has not provided an experimental design in which all Earth system models (ESMs) converge and stabilize at the same prescribed global warming levels. This gap hampers accurate estimations based on comprehensive ESMs of the carbon emission pathways and budgets needed to meet such agreed warming levels and of the associated climate impacts under temperature stabilization. Here, we apply the Adaptive Emission Reduction Approach (AERA) with ESMs to provide such simulations in which all models converge at 1.5 and 2.0 degrees C warming levels by adjusting their emissions over time. These emission-driven simulations provide a wide range of emission pathways and resulting atmospheric CO2 projections for a given warming level, uncovering uncertainty ranges that were previously missing in the traditional Coupled Model Intercomparison Project (CMIP) scenarios with prescribed greenhouse gas concentration pathways. Meeting the 1.5 degrees C warming level requires a 40 % (full model range: 7 % to 76 %) reduction in multi-model mean CO2-forcing-equivalent (CO2-fe) emissions from 2025 to 2030, a 98 % (57 % to 127 %) reduction from 2025 to 2050, and a stabilization at 1.0 (-1.7 to 2.9) PgC yr-1 from 2100 onward after the 1.5 degrees C global warming level is reached. Meeting the 2.0 degrees C warming level requires a 47 % (8 % to 92 %) reduction in multi-model mean CO2-fe emissions until 2050 and a stabilization at 1.7 (-1.5 to 2.7) PgC yr-1 from 2100 onward. The on-average positive emissions under stabilized global temperatures are the result of a decreasing transient climate response to cumulative CO2-fe emissions over time under stabilized global warming. This evolution is consistent with a slightly negative zero emissions commitment - initially assumed to be zero - and leads to an increase in the post-2025 CO2-fe emission budget by a factor of 2.2 (-0.8 to 6.9) by 2150 for the 1.5 degrees C warming level and a factor of 1.4 (0.9 to 2.4) for the 2.0 degrees C warming level compared to its first estimate in 2025. The median CO2-only carbon budget by 2150, relative to 2020, is 800 GtCO2 for the 1.5 degrees C warming level and 2250 GtCO2 for the 2.0 degrees C warming level. These median values exceed the median IPCC AR6 estimates by 60 % for the 1.5 degrees C warming level and 67 % for 2.0 degrees C. Some of the differences may be explained by the choice of the mitigation scenario for non-CO2 radiative agents. Our simulations highlight shifts in carbon uptake dynamics under stabilized temperature, such as a cessation of the carbon sinks in the North Atlantic and in tropical forests. On the other hand, the Southern Ocean remains a carbon sink centuries after temperatures stabilize. Overall, this new type of warming-level-based emission-driven simulation offers a more coherent assessment across climate models and opens up a wide range of possibilities for studying both the carbon cycle and climate impacts, such as extreme events, under climate stabilization.

AbstractAs exceeding the 1.5°C level of global warming is likely to happen in the near future, understanding the response of the ocean‐climate system to temporarily overshooting this warming level is of critical importance. Here, we apply the Adaptive Emissions Reduction Approach to the Earth System Model GFDL‐ESM2M to conduct novel overshoot scenarios that reach 2.0, 2.5 and 3.0°C of global warming before returning to 1.5°C over the time period of 1861–2500. We also perform a complementary scenario that stabilizes global temperature at 1.5°C, allowing to isolate impacts caused by the temperature overshoots alone, both during their peaks and after their reversals. The simulations indicate that substantial residual ocean surface warming persists in the high latitudes after the overshoots, with most notable regional anomalies occurring in the North Atlantic (up to +3.1°C in the 3°C overshoot scenario compared to the 1.5°C stabilization scenario) and the Southern Ocean (+1.2°C). The residual warming is primarily driven by the recoveries of the Atlantic and Southern Ocean meridional overturning circulation and associated increases in ocean heat transport. Excess subsurface heat storage in low and mid‐latitudes prevents steric sea level rise (SLR) from reverting to 1.5°C stabilization levels in any overshoot scenario, with steric sea level remaining up to 32% higher in the 3°C overshoot scenario on centennial time scales. Both peak impacts and persistent changes after overshoot reversal bear significant implications for future assessments of coastlines, regional climates, marine ecosystems, and ice sheets.

Climate policies such as the Paris Agreement are framed in terms of global warming levels. Based on past warming and past CO2 emissions, the amount of future cumulative CO2 emissions allowed to keep global warming at or below a global warming level can be estimated. Yet, global warming scenarios in the successive Coupled Model Intercomparison Projects are framed in terms of prescribed atmospheric CO2 concentration or emissions, yielding a wide range of warming levels per CO2 pathway in response to the different transient climate responses to cumulative emissions in the coupled climate models. Based on these scenarios and the latest model projections, the IPCC Sixth Assessment Report assessed climatic impacts of different warming levels. These impacts are thus evaluated in simulations where the warming targets are passed transiently, at different points in time, and not stabilized, as opposed to how climate agreements are framed.Here, we propose a new Model Intercomparison Project AERA-MIP building on an adaptive approach - the Adaptive Emissions Reduction Approach - that successively calculates the compatible emissions to stabilize global warming at the required temperature target. Earth System Models (ESMs) are run forward in emission-driven mode, with prescribed, model-specific emissions successively calculated every five years, so that all models reach the same warming target and thereafter stabilize at this warming level. The warming uncertainty is thus side-stepped, while different emissions pathways emerge out of the variety of participating ESMs. The approach is based on the TCRE framework and successively adapting for any changes in the Earth System that might affect global mean surface temperature, including the zero emissions commitment as emissions approach zero.Simulations of the first participating modelling centers already reveal a panel of emissions pathways that successfully stabilize global warming at 1.5ºC and 2ºC. This includes the decline rate from peak emissions, the timing of having to reach net-zero emissions, and the magnitude of negative emissions needed to stabilize the climate. These different emissions pathways result in a range of atmospheric CO2 concentration evolution (350 to 450 ppm at year 2100 in the 1.5°C stabilization scenario) and distribution of anthropogenic carbon in the Earth System components. Unlike concentration-driven projections, these AERA simulations provide an uncertainty range for impacts that are directly affected by atmospheric CO2 concentration such as ocean acidification. The project also includes temporary temperature overshoot simulations using the AERA approach.

The Southern Ocean greatly contributes to the regulation of the global climate by controlling important heat and carbon exchanges between the atmosphere and the ocean. Rates of climate change on decadal timescales are therefore impacted by oceanic processes taking place in the Southern Ocean, yet too little is known about these processes. Limitations come both from the lack of observations in this extreme environment and its inherent sensitivity to intermittent processes at scales that are not well captured in current Earth system models. The Southern Ocean Carbon and Heat Impact on Climate programme was launched to address this knowledge gap, with the overall objective to understand and quantify variability of heat and carbon budgets in the Southern Ocean through an investigation of the key physical processes controlling exchanges between the atmosphere, ocean and sea ice using a combination of observational and modelling approaches. Here, we provide a brief overview of the programme, as well as a summary of some of the scientific progress achieved during its first half. Advances range from new evidence of the importance of specific processes in Southern Ocean ventilation rate (e.g. storm-induced turbulence, sea-ice meltwater fronts, wind-induced gyre circulation, dense shelf water formation and abyssal mixing) to refined descriptions of the physical changes currently ongoing in the Southern Ocean and of their link with global climate.This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'.

Under current mitigation implementations, it is of increasing likelihood that global warming will exceed the target set by the Paris Agreement (PA) of “well below 2°C”. Correcting for this overshoot through intense carbon dioxide removal could reverse global warming back to safe levels, but the biophysical impacts associated with pathways exposing the planet to dangerous warming levels is essentially unknown. This is particularly the case for the ocean ecosystem, where peak warming could lead to ecosystem threshold exceedance and non-reversible changes. Here, we investigate spatial asymmetries in the response of surface ocean ecosystem stressors to temporarily overshooting the PA target using a novel model framework.To advance the knowledge on temporary overshoots, we utilized the Adaptive Emission Reduction Approach to design first-order emission pathways to reach given stabilization and overshoot peak temperature targets. With the help of this framework, we performed simulations with the Earth System Model GFDL-ESM2M that overshoot by 0.5°C and 1.5°C, and thereafter returns to the quasi-stabilized warming level of a simulation that respects the PA target. Our preliminary analysis shows important differences in regional ocean characteristics between simulations that overshoot the PA target and a simulation that stabilizes at the PA target, despite ultimately reaching the same global surface temperature. For instance, regional sea surface temperatures can differ by over 0.5°C in the extreme overshoot of 1.5°C in comparison to the PA stabilization simulation, even following the overshoot. This spatial heterogeneity is illustrated through the divergent oceanic response of the polar oceans; In northern latitudes, cooler temperatures are simulated through the expansion of the North Atlantic cold spot during the overshoot, which arises through decrease of heat transport of the Gulf Stream owing to the weakening of the Atlantic Meridional Overturning Circulation (AMOC) of around 6 Sv or 30 %. In turn, the Southern Ocean is substantially warmed regionally in the overshoot simulations versus the stabilization, likely originating from increasing cross-ocean transport of heat during the overshoots, an implication that is ongoing even after the temporary overshoots return to the PA warming level. Similar spatial heterogeneities are also found for other ecosystem stressors such as O2 and pH, hinting at potential disruption of regional ecosystems. Our analysis indicates that increased assessment of regional ocean responses to temporary warming exceedance levels and their impacts for regional ecosystems are urgently needed in the scope of fully evaluating trade-offs associated with delaying climate action and overshooting the PA.

The ocean responds to climate change through modifications of heat, freshwater and momentum fluxes at its boundaries. Disentangling the specific role of each of these contributors in shaping the changes of the thermohaline structure of the ocean is central for our process understanding of climate change and requires the design of specific numerical experiments. While it has been partly addressed by modeling studies using idealized CO2 forcings, the time evolution of these individual contributions during historical and projected climate change is however lacking. Here, we propose a novel modeling framework to isolate these contributions in coupled climate models for which large ensembles of historical and scenario simulations are available. The first step consists in reproducing a coupled pre-industrial control simulation with an ocean-only configuration, forced by prescribed fluxes at its interface, diagnosed from the coupled model. In a second step, we extract the external forcing perturbations from the historical+scenario ensemble of coupled simulations, and we add them to the prescribed fluxes of the ocean-only configuration. We then successfully replicate the ocean's response to historical and projected climate change in the coupled model during 1850–2100. In a third step, this full response is decomposed in sensitivity experiments in which the forcing perturbations are applied individually to the heat, freshwater and momentum fluxes. Passive tracers of temperature and salinity are implemented to discriminate the addition of heat and freshwater flux anomalies from the redistribution of pre-industrial heat and salt content in response to ocean circulation changes. Here, we first present this general framework and then apply it to the IPSL-CM6A-LR model and its ocean component NEMO3.6. This framework brings new opportunities to precisely explore the mechanisms driving historical and projected ocean changes within single climate models.

Emergence des changements de température et de salinité dans l’océan intérieur en réponse au changement climatique : échelles de temps et mécanismes Le changement climatique d’origine humaine impacte déjà toutes les régions habitées de la planète. 90% de l’excès de chaleur associé aux activités humaines a été absorbé par l’océan depuis les années 1970, atténuant en grande partie le réchauffement atmosphérique, mais impactant fortement les sociétés humaines et la vie marine. Dans cette thèse, j’explore à l’aide d’ensembles de modèles de climat et de simulations numériques dédiées, où et quand les changements de température et de salinité dans l’océan intérieur deviennent assez grands pour être différenciés de la variabilité interne, ainsi que les mécanismes physiques associés. Nous trouvons ainsi que le signal climatique dans les masses d’eau de l’océan supérieur émerge entre la fin du XXème et les premières décennies du XXIème siècle. Les eaux modales des moyennes latitudes de l’hémisphère Sud émergent plus tôt que leurs homologues de l’hémisphère Nord. Le réchauffement associé à ces échelles de temps est principalement du à une absorption de chaleur transportée passivement dans l’océan intérieur. Dans les profondeurs de l’océan, les changements de circulation jouent un rôle plus important aux échelles de temps d’émergence du signal climatique. Le gain de flottabilité en surface dans les régions subpolaires provoque un ralentissement de la circulation méridienne de retournement. Cela réchauffe les eaux intérieures et abyssales de l’Océan Austral dès le milieu du XXème, venant s’ajouter au faible transport passif de chaleur, alors que cela le contre dans les profondeurs de l’Atlantique Nord et retarde l’émergence. Bien que les modèles de climat passent à côté de certains aspects importants de la réponse océanique au changement climatique, ils permettent d’apporter des éléments sur l’équilibre de processus en jeu, et suggèrent que l’influence humaine impacte déjà de grandes parties de l’océan.

In response to increasing human emissions, the global ocean is continually warming. The spatial distribution of this warming can result from several mechanisms, difficult to disentangle in observations. Idealized modeling studies have successfully separated the contribution of additional heat passively entering the ocean from the contribution of the changing circulation redistributing the pre-existing heat in response to perturbations in air-sea fluxes. However, the time scales of these different contributions have been largely unexplored so far. Here, we revisit this decomposition with a novel numerical framework to investigate the mechanisms driving regional ocean warming and its emergence from internal variability. Based on the IPSL-CM6A-LR coupled model and its large ensemble of transient climate change simulations, we extract both the internal fluctuations and the externally forced signal in each component of the surface fluxes. With a stand-alone configuration of the ocean, we then test the response to perturbations applied on all surface fluxes together or individually. We find that the contribution of the different processes can largely vary in time, reinforcing or counteracting each other, causing the time of emergence of subsurface temperature changes to be advanced or delayed. Anthropogenic warming in the upper ocean water masses is generally driven by the uptake of excess heat passively stored by the ocean circulation. Circulation changes have a minor role at the time when these signals emerge. On the contrary, in the deeper ocean, circulation changes are much more sensitive to surface forcings and play an important role in setting the time scales of ocean warming, through redistributive warming or cooling.

Abstract The assessment of current and future risks for natural and human systems associated with climate change largely relies on numerical simulations performed with state‐of‐the‐art climate models. Various steps are involved in the development of such models, from development of individual components of the climate system up to free parameter calibration of the fully coupled model. Here, we describe the final tuning phase for the IPSL‐CM6A‐LR climate model. This phase alone lasted more than 3 years and relied on several pillars: (i) the tuning against present‐day conditions given a small adjustment of the ocean surface albedo to compensate for the current oceanic heat uptake, (ii) the release of successive versions after adjustments of the individual components, implying a systematic and recurrent adjustment of the atmospheric energetics, and (iii) the use of a few metrics based on large scale variables such as near‐global mean temperature, summer Arctic sea‐ice extent, as targets for the tuning. Successes, lessons and prospects of this tuning strategy are discussed.

The World Ocean is rapidly changing, with global and regional modification of temperature and salinity, resulting in widespread and irreversible impacts. While the most pronounced observed temperature and salinity changes are located in the upper ocean, changes in water masses at depth have been identified and will probably strengthen in the future. Here, using 11 climate models, we define when anthropogenic temperature and salinity changes are expected to emerge from natural variability in the ocean interior along density surfaces. The models predict that in 2020, 20–55% of the Atlantic, Pacific and Indian basins have an emergent anthropogenic signal; reaching 40–65% in 2050 and 55–80% in 2080. The well-ventilated Southern Ocean water masses emerge very rapidly, as early as the 1980–1990s, while the Northern Hemisphere water masses emerge in the 2010–2030s. Our results highlight the importance of maintaining and augmenting an ocean observing system capable of detecting and monitoring persistent anthropogenic changes. Climate models predict that by 2020, 20–55% of the three key ocean basins express an anthropogenic fingerprint of change. The well-ventilated Southern Ocean water masses are particularly sensitive, emerging as early as the 1980–1990s, consistent with observations of change over the past 30 years.