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Research reported during the past decade has shown that global warming is roughly proportional to the total amount of carbon dioxide released into the atmosphere. This makes it possible to estimate the remaining carbon budget: the total amount of anthropogenic carbon dioxide that can still be emitted into the atmosphere while holding the global average temperature increase to the limit set by the Paris Agreement. However, a wide range of estimates for the remaining carbon budget has been reported, reducing the effectiveness of the remaining carbon budget as a means of setting emission reduction targets that are consistent with the Paris Agreement. Here we present a framework that enables us to track estimates of the remaining carbon budget and to understand how these estimates can improve over time as scientific knowledge advances. We propose that application of this framework may help to reconcile differences between estimates of the remaining carbon budget and may provide a basis for reducing uncertainty in the range of future estimates. A method of tracking changes in estimates of the remaining carbon budget over time should help to reconcile differences between these estimates and clarify their usefulness for setting emission reduction targets.

Climate change is expected to modify ecological responses in the ocean, with the potential for important effects on the ecosystem services provided to humankind. Here we address the question of how rapidly multiple drivers of marine ecosystem change develop in the future ocean. By analysing an ensemble of models we find that, within the next 15 years, the climate change-driven trends in multiple ecosystem drivers emerge from the background of natural variability in 55% of the ocean and propagate rapidly to encompass 86% of the ocean by 2050 under a ‘business-as-usual’ scenario. However, we also demonstrate that the exposure of marine ecosystems to climate change-induced stress can be drastically reduced via climate mitigation measures; with mitigation, the proportion of ocean susceptible to multiple drivers within the next 15 years is reduced to 34%. Mitigation slows the pace at which multiple drivers emerge, allowing an additional 20 years for adaptation in marine ecological and socio-economic systems alike. Climate change is expected to alter ocean ecology, and to potentially impact the ecosystem services provided to humankind. Here, the authors address how rapidly multiple factors that affect marine ecosystems are likely to develop in the future ocean and the remedial effects climate mitigation might have.

Mesoscale eddies dominate the ocean kinetic energy reservoir. However, how and where this energy flows out from the mesoscale remains uncertain. Here, a simplified mesoscale energy budget is used where sources due to baroclinic instability are balanced by all the dissipative processes approximated as a linear damping rate. In this simple model, the eddy kinetic energy (EKE) dissipation is computed from a climatological mean field of density and satellite altimeter data, and is proportional to an eddy efficiency parameter α. Assuming an eddy efficiency of α = 0.1, we find a global EKE dissipation rate of 0.66 ± 0.19 TW. The results show an intense dissipation near western boundary currents and in the Antarctic Circumpolar Current, where both large levels of energy and baroclinic conversion occur. The resulting geographical distribution of the dissipation rate brings new insights for closing the ocean kinetic energy budget, as well as constraining future mesoscale parameterizations and associated mixing processes. Plain Language Summary The ocean is home to abundant and large swirls from tens to hundreds of kilometers, called “mesoscale eddies.” These eddies contain more momentum than most ocean currents and can thus impact the climate evolution. There are now good reasons to believe the effect of mesoscale eddies is directly related to their strength, and so to their kinetic energy. However, how the energy is removed from these eddies is still unclear mostly due to instrumental and theoretical limitations. In this work, a simplification of the eddy energetic behavior is used to indirectly estimate the dissipation from observations of temperature, salinity and surface currents. Our results confirm intensified dissipation near strong ocean currents and hence constitute a new attempt for the global reconstruction of the eddy kinetic energy dissipation in the world ocean. The work presented here is consistent and complementary to other studies and can help us to understand the ocean energy cycle. Key Points Global mesoscale eddy kinetic energy dissipation rate estimated to 0.66 ± 0.19 TW from observation‐based and statistically analyzed data sets More than 25% of the total dissipation occurs in the western boundary currents and 38% is found in the Antarctic Circumpolar Current Estimation of the eddy dissipation timescale from observations to inform future parameterization developments

Temperature drives global ocean patterns of biodiversity, shaping thermal niches through thresholds of thermal tolerance. Global warming is predicted to change thermal range bounds, yet research has primarily focused on temperature at the sea surface, while knowledge of changes through the depths of the water column is lacking. Here, using daily observations from ocean sites and model simulations, we track shifts in ocean temperatures, focusing on the emergence of thermal ranges whose future lower bounds exceed current upper bounds. These emerge below 50 m depth as early as ~2040 with high anthropogenic emissions, yet are delayed several decades for reduced emission scenarios. By 2100, concomitant changes in both lower and upper boundaries can expose pelagic ecosystems to thermal environments never experienced before. These results suggest the redistribution of marine species might differ across depth, highlighting a much more complex picture of the impact of climate change on marine ecosystems.The authors use daily data to understand current thermal conditions across ocean depths and project changes under various future scenarios. They show varying responses in thermal range shifts on the basis of depth, highlighting complexities in predicting marine life habitat under global change.

Measurements show large decadal variability in the rate of CO₂ accumulation in the atmosphere that is not driven by CO₂ emissions. The decade of the 1990s experienced enhanced carbon accumulation in the atmosphere relative to emissions, while in the 2000s, the atmospheric growth rate slowed, even though emissions grew rapidly. These variations are driven by natural sources and sinks of CO₂ due to the ocean and the terrestrial biosphere. In this study, we compare three independent methods for estimating oceanic CO₂ uptake and find that the ocean carbon sink could be responsible for up to 40% of the observed decadal variability in atmospheric CO₂ accumulation. Data-based estimates of the ocean carbon sink from pCO₂ mapping methods and decadal ocean inverse models generally agree on the magnitude and sign of decadal variability in the ocean CO₂ sink at both global and regional scales. Simulations with ocean biogeochemical models confirm that climate variability drove the observed decadal trends in ocean CO₂ uptake, but also demonstrate that the sensitivity of ocean CO₂ uptake to climate variability may be too weak in models. Furthermore, all estimates point toward coherent decadal variability in the oceanic and terrestrial CO₂ sinks, and this variability is not well-matched by current global vegetation models. Reconciling these differences will help to constrain the sensitivity of oceanic and terrestrial CO₂ uptake to climate variability and lead to improved climate projections and decadal climate predictions.

The United Nations’ Paris Agreement includes the aim of pursuing efforts to limit global warming to only 1.5 °C above pre-industrial levels. However, it is not clear what the resulting climate would look like across the globe and over time. Here we show that trajectories towards a ‘1.5 °C warmer world’ may result in vastly different outcomes at regional scales, owing to variations in the pace and location of climate change and their interactions with society’s mitigation, adaptation and vulnerabilities to climate change. Pursuing policies that are considered to be consistent with the 1.5 °C aim will not completely remove the risk of global temperatures being much higher or of some regional extremes reaching dangerous levels for ecosystems and societies over the coming decades. The results of efforts to limit global mean warming to below 1.5 °C may include many possible future world climates.

This study introduces CNRM‐ESM2‐1, the Earth system (ES) model of second generation developed by CNRM‐CERFACS for the sixth phase of the Coupled Model Intercomparison Project (CMIP6). CNRM‐ESM2‐1 offers a higher model complexity than the Atmosphere‐Ocean General Circulation Model CNRM‐CM6‐1 by adding interactive ES components such as carbon cycle, aerosols, and atmospheric chemistry. As both models share the same code, physical parameterizations, and grid resolution, they offer a fully traceable framework to investigate how far the represented ES processes impact the model performance over present‐day, response to external forcing and future climate projections. Using a large variety of CMIP6 experiments, we show that represented ES processes impact more prominently the model response to external forcing than the model performance over present‐day. Both models display comparable performance at replicating modern observations although the mean climate of CNRM‐ESM2‐1 is slightly warmer than that of CNRM‐CM6‐1. This difference arises from land cover‐aerosol interactions where the use of different soil vegetation distributions between both models impacts the rate of dust emissions. This interaction results in a smaller aerosol burden in CNRM‐ESM2‐1 than in CNRM‐CM6‐1, leading to a different surface radiative budget and climate. Greater differences are found when comparing the model response to external forcing and future climate projections. Represented ES processes damp future warming by up to 10% in CNRM‐ESM2‐1 with respect to CNRM‐CM6‐1. The representation of land vegetation and the CO2‐water‐stomatal feedback between both models explain about 60% of this difference. The remainder is driven by other ES feedbacks such as the natural aerosol feedback. Key Points This study introduces CNRM‐ESM2‐1 and describes its set‐up for CMIP6 Represented Earth system processes further impact the model response to external forcing than the model performance over present‐day Represented Earth system processes damp future warming by up to 10%

The ocean is a source of atmospheric carbon monoxide (CO), a key component for the oxidizing capacity of the atmosphere. It constitutes a minor source at the global scale, but could play an important role far from continental anthropized emission zones. To date, this natural source is estimated with large uncertainties, especially because the processes driving the oceanic CO are related to the biological productivity and can thus have a large spatial and temporal variability. Here we use the NEMO-PISCES (Nucleus for European Modelling of the Ocean, Pelagic Interaction Scheme for Carbon and Ecosystem Studies) ocean general circulation and biogeochemistry model to dynamically assess the oceanic CO budget and its emission to the atmosphere at the global scale. The main biochemical sources and sinks of oceanic CO are explicitly represented in the model. The sensitivity to different parameterizations is assessed. In combination to the model, we present here the first compilation of literature reported in situ oceanic CO data, collected around the world during the last 50 years. The main processes driving the CO concentration are photoproduction and bacterial consumption and are estimated to be 19.1 and 30.0 Tg C yr−1 respectively with our best-guess modeling setup. There are, however, very large uncertainties on their respective magnitude. Despite the scarcity of the in situ CO measurements in terms of spatiotemporal coverage, the proposed best simulation is able to represent most of the data (∼300 points) within a factor of 2. Overall, the global emissions of CO to the atmosphere are 4.0 Tg C yr−1, in the range of recent estimates, but are very different from those published by Erickson in (1989), which were the only gridded global emission available to date. These oceanic CO emission maps are relevant for use by atmospheric chemical models, especially to study the oxidizing capacity of the atmosphere above the remote ocean.

Anthropogenic climate change is projected to lead to ocean warming, acidification, deoxygenation, reductions in near-surface nutrients, and changes to primary production, all of which are expected to affect marine ecosystems. Here we assess projections of these drivers of environmental change over the twenty-first century from Earth system models (ESMs) participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6) that were forced under the CMIP6 Shared Socioeconomic Pathways (SSPs). Projections are compared to those from the previous generation (CMIP5) forced under the Representative Concentration Pathways (RCPs). A total of 10 CMIP5 and 13 CMIP6 models are used in the two multi-model ensembles. Under the high-emission scenario SSP5-8.5, the multi-model global mean change (2080–2099 mean values relative to 1870–1899) ± the inter-model SD in sea surface temperature, surface pH, subsurface (100–600 m) oxygen concentration, euphotic (0–100 m) nitrate concentration, and depth-integrated primary production is +3.47±0.78 ∘C, -0.44±0.005, -13.27±5.28, -1.06±0.45 mmol m−3 and -2.99±9.11 %, respectively. Under the low-emission, high-mitigation scenario SSP1-2.6, the corresponding global changes are +1.42±0.32 ∘C, -0.16±0.002, -6.36±2.92, -0.52±0.23 mmol m−3, and -0.56±4.12 %. Projected exposure of the marine ecosystem to these drivers of ocean change depends largely on the extent of future emissions, consistent with previous studies. The ESMs in CMIP6 generally project greater warming, acidification, deoxygenation, and nitrate reductions but lesser primary production declines than those from CMIP5 under comparable radiative forcing. The increased projected ocean warming results from a general increase in the climate sensitivity of CMIP6 models relative to those of CMIP5. This enhanced warming increases upper-ocean stratification in CMIP6 projections, which contributes to greater reductions in upper-ocean nitrate and subsurface oxygen ventilation. The greater surface acidification in CMIP6 is primarily a consequence of the SSPs having higher associated atmospheric CO2 concentrations than their RCP analogues for the same radiative forcing. We find no consistent reduction in inter-model uncertainties, and even an increase in net primary production inter-model uncertainties in CMIP6, as compared to CMIP5.

Emergent constraints on tropical marine primary production increase confidence in a long-term decrease in primary productivity in response to rising sea surface temperatures. The most extreme projected declines in productivity are, however, unlikely. Marine primary production is a fundamental component of the Earth system, providing the main source of food and energy to the marine food web, and influencing the concentration of atmospheric CO 2 (refs  1 , 2 ). Earth system model (ESM) projections of global marine primary production are highly uncertain with models projecting both increases 3 , 4 and declines of up to 20% by 2100 5 , 6 . This uncertainty is predominantly driven by the sensitivity of tropical ocean primary production to climate change, with the latest ESMs suggesting twenty-first-century tropical declines of between 1 and 30% (refs  5 , 6 ). Here we identify an emergent relationship 7 , 8 , 9 , 10 , 11 between the long-term sensitivity of tropical ocean primary production to rising equatorial zone sea surface temperature (SST) and the interannual sensitivity of primary production to El Niño/Southern Oscillation (ENSO)-driven SST anomalies. Satellite-based observations of the ENSO sensitivity of tropical primary production are then used to constrain projections of the long-term climate impact on primary production. We estimate that tropical primary production will decline by 3 ± 1% per kelvin increase in equatorial zone SST. Under a business-as-usual emissions scenario this results in an 11 ± 6% decline in tropical marine primary production and a 6 ± 3% decline in global marine primary production by 2100.