Edited by: Hayley Jane Fowler, Newcastle University, United Kingdom
Reviewed by: Jean-Francois Lamarque, McKinsey & Company, United States; H Damon Matthews, Concordia University, Canada
*Correspondence: Sofia Palazzo Corner,
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
How do we halt global warming? Reaching net zero carbon dioxide (CO2) emissions is understood to be a key milestone on the path to a safer planet. But how confident are we that when we stop carbon emissions, we also stop global warming? The Zero Emissions Commitment (ZEC) quantifies how much warming or cooling we can expect following a complete cessation of anthropogenic CO2 emissions. To date, the best estimate by the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report is zero change, though with substantial uncertainty. In this article, we present an overview of the changes expected in major Earth system processes after net zero and their potential impact on global surface temperature, providing an outlook toward building a more confident assessment of ZEC in the decades to come. We propose a structure to guide research into ZEC and associated changes in the climate, separating the impacts expected over decades, centuries, and millennia. As we look ahead at the century billed to mark the end of net anthropogenic CO2 emissions, we ask: what is the prospect of a stable climate in a post-net zero world?
香京julia种子在线播放
Substantial uncertainty remains in both the sign and magnitude of the Zero Emissions Commitment (ZEC): the expected additional change in global surface temperature once we achieve net zero CO2 emissions.
Uncertainty in ZEC has implications for the remaining carbon budget to stay below the temperature limits of the Paris Agreement: a positive ZEC reduces the remaining budget; a negative ZEC opens the door for more ambitious targets or more time to reach net zero.
The prospect of additional warming after net zero is both plausible and significant, with a chance that ZEC could exceed 15% of total global warming.
While a ZEC of 0 means no further change to global surface temperatures, other aspects of the Earth system, such as sea levels, will continue to change in a net zero world due to warming realized previously. These changes should be factored into the assessment of safe warming limits and adaptation plans.
Current climate models do not adequately represent the full scope of complex and interdependent Earth system processes that determine ZEC. We present a structure for quantifying uncertainty in ZEC and propose a roadmap for future research into quantifying ZEC and reducing its uncertainties.
As we advance into the 21st century, a drumbeat of climate-related disasters is sounding an ever-louder alarm. Be they floods, heatwaves, or hurricanes, their impacts are projected to increase in regularity and intensity as global warming continues (
The Zero Emissions Commitment (ZEC) describes the net change to global surface warming after a complete cessation of anthropogenic emissions, in many cases with a focus on CO2. In combination with the transient climate response to cumulative emissions (TCRE), it defines the total amount of global surface warming for a given amount of CO2. In turn, this total expected warming determines the carbon budget: the amount of carbon dioxide that can be released before reaching a certain temperature threshold. A positive ZEC indicates additional surface warming after net zero, a negative ZEC a drop in surface temperature, and a zero ZEC no further averaged surface temperature change (
Stylised schematic of how atmospheric carbon dioxide (CO2) concentrations, ocean heat uptake, and global surface temperature can evolve under net zero CO2 emissions. Process timescales are illustrative.
For global surface temperatures to become and remain stable, the behavior of large-scale planetary sub-systems must cancel each other out and equilibrate at similar timescales (
In this article, we first present key definitions and the current high-level scientific understanding of the ZEC concept. We delve into the processes that determine the ZEC outcome and their uncertainties to better understand what to expect in a world that manages to bring its carbon emissions to net zero. We describe our current best understanding as well as key unknowns and reflect on the implications of these insights for climate policy today.
Net zero CO2 emissions are defined by the IPCC as the “condition in which anthropogenic CO2 emissions are balanced by anthropogenic CO2 removals over a specified period” (
The Zero CO2 Emissions Commitment is the estimated amount of
The broader Zero Emissions Commitment concept can also be applied to non-CO2 gases and wider CO2 inclusive groups, with studies considering ZEC for all anthropogenic forcing agents combined (
ZEC is a function of the geophysical properties of the interconnected Earth system (
An overview of some of the processes that determine the Zero Emissions Commitment (ZEC), including ocean heat uptake, the ocean biogeochemical cycle, the land carbon cycle, and physical climate feedbacks.
The Zero Emissions Commitment (ZEC) can be thought of as a surface energy balance relation in a simplified “zero-dimensional” model. If, after carbon dioxide (CO2) emissions cease and Earth’s surface and lower atmosphere absorb more energy than they emit back to space or to the deep ocean, ZEC will be positive. On the other hand, if Earth’s surface and lower atmosphere emit more energy than they absorb, ZEC will be negative. Under this simplified relationship, temperature stabilization is reached when energy out equals energy in. Abbreviation: GHG, greenhouse gas.
Even a zero ZEC scenario will not stop regional temperature change (
Studies of how much warming could be expected under particular conditions originated in the 1980s with early analyses of response timescales by Hansen et al. and Wigley and Schlesinger (
The ZEC Model Intercomparison Project (ZECMIP) (
ZECMIP (
Simple models have also been used to explore what values of ZEC are consistent with observations. The results suggest that industrial-era temperatures alone do not strongly constrain ZEC, but paleoclimate records and independent evidence about equilibrium climate sensitivity could lead to potential further constraints (
Two significant caveats apply to the current modeling evidence relating to ZEC. First, ZECMIP features only a subset of available ESMs, and the 100-year zero emission extensions for ESMs cannot inform us about the potential for carbon-climate, deep ocean, and ice sheet dynamics, which occur on longer timescales. Multi-century simulations with individual ESMs have revealed markedly different behaviors, with an increase in warming after a century-long decrease despite zero CO2 emissions (
To assess the potential for significant warming or cooling post net zero emissions, we consider the evolution of key Earth subsystem processes that both drive and respond to changing carbon concentrations and heat gradients under net zero CO2 emissions. The shifting dominance of processes from decadal to millennial timescales is discussed, building a picture of interconnected Earth system elements and the changes to global mean temperature these can bring. Some processes require or result in temperature changes, while others are set to evolve even in a world with stable global temperature.
The ocean is the largest reservoir of accumulated energy as a result of anthropogenic climate forcing, having stored roughly 91% of Earth’s additional retained heat since 1971 compared with approximately 1% stored in the atmosphere (
Two parameters are particularly important to understand the role of the ocean in global surface temperature: the
While most of the energy in recent decades has been absorbed by the upper ocean, the link between the upper and deep ocean is important to understand the ocean temperature of an ocean thermal steady state, and the time needed to reach it. One simple model that accounts for a surface energy reservoir (effectively the upper ocean) and a deeper reservoir (taking surface energy down to greater depths) is the “two-layer energy balance model” (
On diurnal to seasonal timescales, the evolving upper 200 m stratification and turbulent mixed layer govern the uptake of energy, where deeper mixed layers have a higher heat capacity and thus the sea surface temperature warms less per unit of energy taken up (
The specific patterns of surface heat uptake and surface warming affect the marine boundary layer and thus the atmosphere, leading to complex cloud feedbacks that affect radiative and atmospheric warming and cooling of the oceans (
Biases where the tropical ocean mixed layer depth is excessive tend to increase the short timescale heat capacity of the ocean, while excessive mixed layer depths in the North Atlantic deep convection region tend to warm the NADW, and excessive mixed layer depths in the Southern Ocean deep convection tend to cool the NADW and warm the upper tropical oceans. The CMIP6 ensemble spread in observable indicators of these water formation processes is much larger than that which is supported by observational uncertainty. Thus, the efficacy and climate sensitivity of the CMIP6 ensemble, and two-layer models trained to match it, can likely be improved substantially through better ocean process representation (Hall and Fox-Kemper, 2022)
The timescales of the deep and bottom oceanic overturning are on the order of centuries to millennia. Neither the NADW nor AABW have yet been fully equilibrated to the surface climate changes of the last century, let alone modern changes. That means that the deep ocean will continue to warm, acidify, and deoxygenize over the coming centuries and millennia even if net zero CO2 emissions are reached (
The processes governing the ocean carbon response to the cessation of CO2 emissions result from the same processes and mechanisms that drive the sign and magnitude of the ocean carbon sink during rising emissions (
Current knowledge, mostly based on multi-model assessment, suggests that the role of the ocean carbon cycle varies substantially across timescales. On annual to multi-decadal timelines after net zero CO2, the magnitude of ocean carbon uptake is projected to be similar to that absorbed by the terrestrial biosphere (
Beyond the ocean carbon cycle, many other ocean processes may influence the overall ocean carbon contribution to ZEC, including mesoscale eddies, melting from ice sheets, sea ice cover, and CH4 release—all of which add to the uncertainty in defining the outcome of ZEC.
The land carbon contribution to ZEC is dominated by three types of processes: those that govern the carbon response to CO2 concentrations in terrestrial ecosystems; those that govern the response to climate change; and anthropogenic land use and land use change. Each of these processes may also interact with one another.
In a world with excess CO2 emissions, the magnitude of the land carbon sink increases via CO2 fertilization of leaf photosynthesis, which is one of the most important drivers of current land carbon uptake (
The land carbon sink capacity is further modulated by climate feedbacks (
To date, ZEC has been quantified in idealized scenarios that do not include historical or potential future land use dynamics. ESM results predicting continued carbon uptake by terrestrial ecosystems post zero emissions should therefore be qualified; this carbon sink may rely in part on substantial contributions from regions where historical land use, or future changes, may preclude such uptake owing to deforestation (which reduces sink capacity) and continued carbon emissions from agricultural lands. However, comparing idealized ZECMIP runs with simulations of the SSP5-3.4-overshoot scenario that include land use (
Physical climate feedbacks involve the responses of clouds, surface albedo, water vapor, and temperature via their effects on the global energy balance. A challenge in predicting the long-term evolution of global warming is that these feedbacks vary over time. Indeed, under sustained CO2 forcing, most coupled atmosphere-ocean climate models predict more amplifying feedbacks as time progresses (
On all timescales relevant to ZEC, much of the feedback time-dependence comes from low clouds via their impact on solar reflection and thus the global energy balance (
In addition to the SST pattern effect, additional drivers of time-dependence in low cloud feedback include state-dependent cloud responses, for example, in mixed-phase clouds where the feedback depends on the initial ratio of liquid versus ice phase (
On millennial timescales, the melting of ice sheets will fundamentally alter the global radiative budget, both via local changes in surface albedo and via remote climate impacts mediated by changes in ocean overturning and global sea level. Ocean circulation changes would likely interact with the SST pattern effect, allowing clouds to modulate the global climate response to ice sheet melting. The coupling of ice sheets with global climate allows for a potentially large impact on ZEC, but we currently have no confidence in the sign or magnitude of this effect.
In this section, we outline the processes in each Earth subsystem that drive uncertainty in ZEC. This uncertainty can be organized into five categories:
processes known or suspected to impact ZEC, but currently missing in models
processes with high uncertainty, whose uncertainty and interactions are not sufficiently represented in models
common structural biases in current modeling frameworks that tend to bias ZEC estimates to the upper or lower ranges
the possibility of abrupt change or irreversible tipping of system dynamics, and
the potential of state and pathway dependence for all these subsystem processes.
These uncertainties warrant caution in two aspects. First, temperature stabilization could take longer than anticipated and be both pathway- and state-dependent. Second, a world that reaches net zero emissions will still be locked into long-term changes for which we must prepare.
Many of the uncertainties in ZEC are related to uncertainties in the transient climate response to cumulative carbon emissions (TCRE) as they are driven by similar processes whose response is correlated over time. For example, MacDougall et al. (
Heat and carbon are absorbed by the ocean via the mixed layer. They are then transported and distributed vertically at a rate determined by layer depths, turbulence, convection, overturning, and detrainment. These ocean physical processes therefore control both excess heat and anthropogenic carbon and their relative uptake and transport to the deep ocean, which is a major determinant of ZEC. However, uncertainty remains in the quantification and modeling of these processes (
Future research to improve the sign and magnitude of the Zero Emissions Commitment (ZEC) must strengthen couplings between the relevant Earth system components (both physical and biogeochemical) to determine their responses to rising carbon dioxide and climate change. This table poses research questions and suggests modeling experiments to address the uncertainty of the processes listed in
| Topic | Corresponding label in |
Process | Research question | Experiment/analysis to quantify effect on ZEC | Actionability (1: start immediately; 2: start within known timeframe; 3: timeframe to start unknown) | Guidance for prioritization |
|---|---|---|---|---|---|---|
|
|
A | Land response to CO2 | What are the long-term ecosystem responses to elevated CO2 and the direct responses to declining CO2 concentrations following net zero? | Increased evaluation of land models against free-air CO2 enrichment (FACE) experiments. Dependent on site experiments, but plenty exist and new ones are coming online [see, e.g., AmazonFACE ( |
1: Can begin now. Requires more synthesis of data and use to evaluate land models, but this exists and is ongoing. A systematic land model protocol and evaluation could be made, e.g., as part of GCP budget or TRENDY (see: |
Potential for > ±10% impact on ZEC. Can be actioned now and included in systematic land evaluation. Processes A–C are all related and could be a single ongoing activity to evaluate land model response to CO2, nutrients and climate. |
| B | Nutrient limitation on carbon-concentration and carbon-climate interactions | What is the role of nutrient limitation in the ecosystem response to elevated CO2? | Climate model ZEC experiments with carbon-only, carbon-nitrogen and carbon-nitrogen-phosphorous, and also land model experiments compared to manipulation (nutrient addition) experiments [e.g., Amazon fertilisation experiment (AFEX)]. This could be piloted with one model. | 1: Can begin now, with some experiments already underway using UVic ESCM. Systematic nutrient evaluation could be carried out with a common protocol as per ( |
Potential for ±10% impact on ZEC. Can be actioned now and included in systematic land evaluation. | |
| C | Decomposition response to climate | What are the long-term timescales of soil decomposition responses to warming? | Increased evaluation of land models against soil warming experiments, e.g., ( |
1: Can begin now and be included in systematic land model evaluation, as above. | Potential for ±10% impact on ZEC. Can be actioned now and included in systematic land evaluation. | |
| D | Permafrost response to climate | How can thawing permafrost be better/more widely included in Earth system models, and what are the timescales of permafrost responses, including processes such as abrupt thaw? | Offline land model runs can be done to quantify timescale and magnitude of carbon release, then coupled into ESMs in the longer run. See ( |
1: Offline runs can be done now with some land models; 2: Some ESMs may have permafrost for CMIP7; 3: Need for more ESMs in the long term to include permafrost models. | Potential for +1% impact on decadal ZEC, and > +10% impact on millenial ZEC. Offline runs can be actioned now, and plans can be made to assess permafrost impact for models available in CMIP7. Processes D–I are all related and concern processes that are not yet widely included in models. | |
| E | Boreal ecosystem response to climate | What are the timescales of forest range shifts and disturbance shifts with warming? | Offline land model runs can be done to quantify timescale and magnitude of vegetation dynamics. In the longer run these can be coupled into ESMs and evaluated with observed evolution, particularly after disturbance/extremes. Requires more ESMs to include vegetation dynamics. | 1: Offline runs can be done now with some land models; 2: Some ESMs already have vegetation dynamics and will do for CMIP7; 3: Need for more ESMs in long term to include vegetation dynamics. | Impact on ZEC quantified only speculatively due to lack of evidence. Assessed potential of -1% impact on decadal ZEC, and -10% impact on millenial ZEC. Some models may be available for CMIP7, but there is a need for more ESMs to prepare now to include vegetation dynamics for future CMIPs. | |
| F | Tropical ecosystem response to climate | What are the long-term implications of warming on tropical forests? | Offline land model runs can be done to quantify timescale and magnitude of vegetation dynamics. In the longer run these can be coupled into ESMs. These can be used to identify the role of disturbance and extremes in observed and modeled transitions. Requires more ESMs to include vegetation dynamics. | 1. Offline runs can be done now with some land models; 2: Some ESMs already have vegetation dynamics and will do for CMIP7; 3: Need for more ESMs in long term to include vegetation dynamics. | Potential for ±10% impact on ZEC. Can be actioned now in offline land models. There is a need for more ESMs to prepare now to include vegetation dynamics for future CMIPs. | |
| G | Fire disturbance to ecosystems | What is the impact of wildfire on the land carbon flux? | Offline land model runs can be done to quantify timescale and magnitude of fire, then in the longer run coupled into ESMs. Requires more ESMs to include vegetation dynamics. | 1. Offline runs can be done now with some land models; 2: Some ESMs already have fire or vegetation dynamics, but few have both that interact. Some may do for CMIP7; 3: Need for more ESMs in the long-term to include vegetation and fire dynamics. | Potential for ±1% impact on ZEC. Can be actioned now in offline land models. There is a need for more ESMs to prepare now to include vegetation and fire dynamics for future CMIPs. | |
| H | Wetland methane | How will natural methane emissions evolve after cessation of emissions? | Offline land model runs can be done to quantify timescale and magnitude of wetland methane, then in the longer run coupled into ESMs. Requires more ESMs to include wetlands and interactive methane cycle. | 1: Offline runs can be done now with some land models; 2: For some ESMs, wetland methane diagnosis is already possible but no interactive methane runs are likely in CMIP7; 3: Need for more methane coupling in ESMs in the long term. | Potential for -1% impact on ZEC. Can be actioned now in offline land models. There is a need for more ESMs to prepare now to include methane coupling. | |
| I | Potential release of nitrous oxide from land | How will nitrous oxide emissions evolve after cessation of emissions? | Offline land model runs may already be able to quantify timescale and magnitude of N2O. Inclusion in ESMs needed in the longer term. Dependent on ESMs planning N2O cycle (both land and ocean). | 1: Offline runs can be done now with some land models; 2: ESMs might be developed to be able to do this, but no interactive N2O runs in CMIP7; 3: Need for more N2O coupling in ESMs in the long term. | Impact on ZEC quantified only speculatively due to lack of evidence. Assessed potential of ±1% impact on ZEC. Some offline runs can be actioned now, but there is a need for ESMs to prepare interactive N2O runs for the next CMIP. | |
| J | Legacy effects of land use change | How does ZEC in idealized no-land-use scenarios differ from a net zero characterized by both widespread land-based carbon sequestration and prior land use legacies? | Emissions-driven ESM scenarios, including historical land use and a net zero characterized by both gross fossil fuel emissions and land- and ocean-based sequestration. Requires wider diversity of emissions-driven climate mitigation scenarios in CMIP7. | 2: Can be undertaken in CMIP7 timeframe. | High confidence in a ±10% potential impact on decadal ZEC. Can be actioned for CMIP7. | |
|
|
K | Potential release of non-CO2 greenhouse gases from the ocean (e.g., methane and nitrous oxide) | How can marine biogeochemical processes responsible for non-CO2 GHG emissions be included in ocean models? | Both offline simulation/reconstruction and emission-driven runs as ZECMIP would be useful to quantify their impact. Requires: a larger array of observational data, including mesocosms experiments; greater computational resources to properly resolve open and coastal oceans; hybrid resolution numerical schemes to decrease modeling costs; machine learning to accelerate modeling and/or reduce costs necessary to improve performance; reduced complexity models. | 2 or 3: Timeframe depends on the trace GHG being investigated. | Potential for ±10% impact on ZEC. If the level of non-CO2 GHG is subject to change because of Earth system feedbacks, it is important to bound it. |
| L | Ocean-ice sheet interactions—change in ocean stratification affecting carbon uptake | How will global and regional ocean stratification evolve after reaching net zero emissions, in particularly in polar regions with ice sheet meltwater release; how will this influence carbon uptake? | Improve the representation of ocean-ice coupling by simple parameterization or ocean-ice sheet coupling, and run ESM in warming stabilization scenarios. Investigate impact of different warming targets and pathways, which will (1) determine the level of ocean-ice inertia after net zero emissions and (2) provide different level of stratification to assess their impact once net zero is reached. | 1: All tools are available for such assessment. Simple ocean-ice parameterization exists; 2: coupling ocean-ice sheet requires more development and process understanding but will probably become efficiently actionable in the next decade. | Potential for ±10% impact on ZEC. Some aspects are actionable now, with ocean-ice sheet coupling possible in the next decade. | |
| M | Mesoscale eddies and filaments affecting ocean carbon uptake | What are the impacts of unresolved and poorly represented mesoscale and submesoscale processes in the current generation of Earth system models, and how can these affect air-sea flux of carbon as well as carbon transfer between the sea surface and the deep ocean? | See processes O, P and Q below. | See processes O, P and Q below. | Potential for ±1% impact on ZEC with only speculative confidence due to lack of evidence. | |
| O, P, Q | Ocean surface boundary layer (OSBL) mixing, OSBL submesoscales, subduction by mesoscales | What are the key processes driving mixing of the OSBL missing in the current generation of Earth system models, and how can those affect heat and carbon flux across the sea surface? What is the impact of unresolved and poorly represented submesoscale processes in the current generation of Earth system models, and how can those affect upper ocean stratification, and heat and carbon transfer between the sea surface and the deep ocean? | Recently, the community established an ensemble of submesoscale-permitting models ( |
1: Already underway. | Potential for ±10% impact on centennial ZEC from process O. Some work already underway. | |
| R | Ocean stratification | How will global and regional ocean stratification evolve after reaching net zero emissions, particularly in the polar region with ice sheet meltwater release; how will this influence heat uptake? | See process L. | See process L. | Impact on ZEC only speculative over centennial-millenial timescales due to lack of evidence. Some aspects actionable now, with ocean-ice sheet coupling possible in the next decade. | |
| S | Changing Atlantic meridional overturning circulation (AMOC) | What are the key processes driving the AMOC mean strength and depth, and controlling its future change? How can we better represent them in Earth Sytem Models to quantify the impact of AMOC decline on ZEC? | RAPID and OSNAP and other similar monitoring systems are already in place [e.g., ( |
1: Already underway. | Potential for +10% impact on centennial ZEC. Some work already underway. | |
| T | Changing Southern Ocean overturning circulation (SMOC) | What are the key processes driving the SMOC mean strength and depth, and controlling its future change? How can we better represent them in Earth Sytem Models to quantify the impact of SMOC decline on ZEC? | One key limitation of the study of the Southern Ocean overturning is the poor process understanding of dense water formation and overflows, which hampers our confidence in the current generation of Earth system models to address this specific question. Representation of bottom water formation, export, and consumption urgently needs to improve observation of key overflow processes and develop modeling strategies to improve the representation of such processes (e.g., increase horizontal or vertical resolution, change coordinate system, develop specific parameterization). Impact of changing SMOC on ZEC can be assessed by modeling sensitivity experiments such as virtually freezing ocean circulation in a climate model, or by freshwater hosing experiments in a warming stabilization scenario. Requires national operators to target surveys in ways that are best aligned to models. International cooperation is key here, and may be a feature of the buildup to and legacy of the planned 5th International Polar Year (2032-33). | 1 or 2: Some modeling aspects are readily available; the observation part will require years to advance further. | Potential for +10% impact on ZEC over centuries. Requires international cooperation, with opportunities for this in the upcoming International Polar Year. | |
| misc. | misc. | How can Earth system components (rivers, ice shelves, aerosols) be coupled more completely in Earth System Models? | Factorial experiments including or not including the coupling across components. Depends on the level of readiness of ESMs. | 2: Several modeling groups are progressing in this direction currently. | Quantitifying ZEC implies conducting holistic simulations, where couplings are the glue of the represented components of the model. As such, novel couplings might affect the sign or the magnitude of ZEC in both directions, depending on how the coupling will affect the radiative budget of the Earth. | |
| misc. | Ocean-ice sheet interactions—change in sea level | How will global sea level change after net zero is achieved? | We note that the Southern Ocean and Antarctic ice sheet remain poorly surveyed and observed to the level needed for reliable prognostic evaluations of change. Guided by satellite observations of present change, and evaluations of past change, basic data gathering on surveys designed for models may allow us to better constrain predictions. Critical to data gathering, and modeling are ice shelf cavities and also the boundary between water and the grounded ice sheet. Both aspects are data and knowledge sparse yet appear critical to the interaction between the ocean and ice sheets from a sea level prespective. Requires national operators to target surveys in ways that are best aligned to models. International cooperation is key here, and may be a feature of the build-up to, and legacy of, the planned 5th International Polar Year (2032-33). | 1: Assembling existing data (Southern Ocean Observing System) Ocean bathymetry (GEBCO) and Antarctic subglacial topography (upcoming Bedmap3). 2: Using this information, with modeling in mind, to target high-precision survey and observations. 3: Using drones, AUVs and UAVs to collect data in a systemetic continental-wide manner. Modeling can begin immediately and would be improved in time with such data gathering. Work in the polar regions is notoriously time-limited, due to the field season being around 3-4 months per year. The 5th International Polar Year, in 2032-33, may offer an acceleration of plans and projects. The Scientific Committee on Antarctic Research could act as a convenor of nations and programs related to such work. | A long-term effect of current and expected global warming, with significant implications for policy and planning. | |
|
|
U | Sea surface temperature pattern effect affecting low cloud and lapse-rate feedbacks | What physical drivers control the time-evolution of SST patterns in response to a climate forcing? | Develop observational constraints on forced SST patterns and track particularly tropical Pacific gradients. Requires analysis of SST patterns and climate feedbacks in global high-resolution coupled climate model experiments. Depends on availability of decadal-scale 3D timeseries of ocean and atmosphere variables for climate analysis. | 1: Activity is ongoing. | Potential for +10% impact on decadal ZEC. Some work already underway. |
| V | State-dependence of the cloud response to warming | How does cloud feedback depend on the climate state? | Analyze cloud feedbacks in model experiments for different climate states e.g., paleo, historical, future. Depends on availability of paleo, historical and future climate model experiments (e.g., from CMIP). | 1: Activity is ongoing. | Potential for +10% impact on decadal ZEC. Some work already underway. | |
| W | High cloud aggregation | How do clouds and convection respond to climate change? | Assess the drivers of convective aggregation in convection-permitting simulations and in observations. Depends on availability of convection-permitting climate model experiments and availability of decadal-scale satellite observations of clouds. | 1: Activity is ongoing, primarily on a regional scale. | Potential for ±10% impact on decadal ZEC. Some work already underway. | |
| X | Nonlinear cloud response to warming | How do nonlinear climate responses control cloud feedback? | See process Z. | See process Z. | Potential for ±10% impact ZEC. Some work already underway. | |
| Y | Snow and sea ice changes affecting albedo feedback | How do snow- and ice-albedo feedbacks depend on the spatial pattern of climate change? | Assess snow- and ice-albedo feedbacks in high-resolution ocean-ice model simulations of climate change. Depends on availability of high-resolution ocean-ice model simulations and correct representation of ocean–shelf heat exchange. | 2: Ocean-ice sheet coupling requires more development and process understanding but will probably become efficiently actionable in the coming decade. | Potential for +10% impact on decadal ZEC. Some work already underway, with further development of ocean-ice sheet coupling needed. | |
| Z | Ice sheet changes affecting albedo and ocean circulation | How do ice sheets respond to climate change on centennial to millenial timescales? | Analyze climate feedbacks in coupled ice sheet–ocean–atmosphere models on millennial timescales. Depends on availability of coupled ice sheet models with millenial-scale climate change simulations. | 2: Partly underway with a new generation of active ice sheet climate models ( |
Potential for > ±10% impact on millenial ZEC. Some work already underway, with further development of ice sheet models needed. |
Abbreviations:
AMOC, Atlantic meridional overturning circulation; AUV, autonomous underwater vehicle
CMIP, Coupled Model Intercomparison Project; CO2, carbon dioxide
ESM, Earth system model; GCP, global carbon budget
N2O, nitrous oxide; OSBL, ocean surface boundary layer
OSNA, Overturning in the Subpolar North Atlantic Program; SMOC, Southern Ocean overturning circulation
SST, sea surface temperature; UAV, unmanned aerial vehicles
UVic ESCM, University of Victoria Earth System Climate Model; ZECMIP, Zero Emissions Commitment Model Intercomparison Project
In general, atmospheric anomalies in temperature and carbon concentration are well mixed and thus largely uniform across the air-sea surface, meaning the ocean uptake for heat and carbon behave similarly. However, in some regions and processes, they differ; for example, polar amplification results in increased regional warming affecting only heat uptake (and not carbon), while seawater carbonate chemistry and biological processes affect only carbon. These may lead to subtle long-term differences in ZEC. Separately, upper ocean processes involved in air-sea fluxes, such as mixing and submesoscale, change the rates and sensitivity of large-scale processes such as overturning.
The North Atlantic and Southern Ocean overturning cells are critical for the transport of carbon and heat at depth, and hence small changes in the overturning strength affect ocean storage of heat and carbon. Overturning also impacts the rate of change in ocean surface temperature, and hence physical climate feedback via the pattern effect and associated cloud response (
Changes in glacial meltwater from polar ice sheets have important consequences for upper ocean stratification, and hence the strength of the global ocean overturning (
Most climate models do not resolve the small-scale (10–100 m) turbulence that mixes the boundary layer of the ocean and the submesoscale (1–10 km) and mesoscale (10–100 km) turbulent eddies that influence the boundary layer, deep convection, and transport of water out of the boundary layers and convection zones into the global overturning (
While the circulation determines the volume of anthropogenic CO2 transported into the deep ocean, the carbonate chemistry, which is well understood, determines the concentration of anthropogenic CO2 in these water masses (
The role of biological processes in long-term carbon storage, especially the transport and transformation of living and dead particulate matter in the mesopelagic zone (
It is well known that potential CH4 emission sources in the coastal and open ocean (e.g., clathrates in marine sediments) are sensitive to climate change (
The response of the land biogeochemical cycle to net zero is tightly linked to the drivers and constraints of the current natural land carbon sink. Here, we provide an initial expert estimate of both the magnitude and direction of uncertainty in ZEC due to uncertainty in the underlying processes, and the timescales to which they apply. Some of these processes have already been included in ESMs but are uncertain, while others are still missing.
The largest driver of the land carbon sink—the productivity and allocation response to CO2—is also the largest contributor to its uncertainty. It is known that the uncertainty in the land response to CO2 exceeds that for the ocean and that the response to CO2 is more uncertain than the response to climate (
As the majority of ESMs used to quantify ZEC are biased by not including nutrient constraints and processes that affect these over long timescales, there may be consequential biases in the calculated magnitude of ZEC (
Phosphorus limitation on land is likely to further reduce the land carbon response to CO2 and climate change (
There is no evidence that the existing estimates would be biased, so this uncertainty (a) is large, (b) symmetrically impacts ZEC, and (c) operates on short to mid-term timescales. Confidence in this assessment is medium due to the relatively large body of literature and some experimental evidence but with a substantial spread in modeling results.
As there is only a small change in global temperature following zero emissions, the expected response of land carbon to the climate post net zero will largely be the long-term continuation of its response to the climate change that had occurred during the period of rising CO2. Warmer temperatures lead to accelerated decomposition of organic matter, and this remains stubbornly uncertain across CMIP6 models even with nutrient cycling considered, as it was in CMIP5. Therefore, this uncertainty (a) is moderate, (b) symmetrically impacts ZEC, and (c) operates on short timescales. Confidence in this assessment is medium due to the relatively large body of literature and some experimental evidence but with a substantial spread in modeling results.
Only represented in a small number of the models contributing to ZECMIP, permafrost thaw is a process that contributes to uncertainty in ZEC. It is virtually certain that large areas of permafrost will thaw under a warmer climate. However, the amount of carbon released with decomposition of previously frozen organic matter, the timescale of release and its form (CO2 versus CH4), and the potential compensation by plants growing in areas impacted by permafrost thaw are all uncertain. IPCC assessments (
Converse to permafrost loss of carbon from high latitude regions, woody encroachment and boreal forest expansion represent potentially long-term committed carbon sinks, while at the same time altering the biogeophysical properties of the surface and therefore the local climate (
Tropical ecosystems respond to elevated temperatures and associated changes in rainfall by potentially reducing productivity and wood growth. Research is heavily focused on the Amazon forest, which is seen as especially vulnerable to future climate change (
Conversely, assessing tropics-wide carbon sinks (
Fire may increase CO2 (and other) emissions from land. It may also reduce equilibrium carbon storage for a given ecosystem. If global climate change is small following zero emissions, then further changes in fire disturbance may also be small, but this process is missing in most ESM models and no studies have assessed the response of fire disturbance after zero emissions. Therefore, this uncertainty (a) is moderate, (b) symmetrically impacts ZEC, and (c) operates on short to mid-term timescales. Confidence in this assessment is low due to limited literature and poor model agreement.
Wetland CH4 emissions are driven by both climate (temperature and moisture) and vegetation productivity and therefore are directly responsive to CO2. Warming generally tends to increase emissions (
N2O emissions from natural and semi-natural ecosystems are driven by climate and nitrogen cycle processes, which are directly or indirectly affected by the productivity response of plants to atmospheric CO2 changes. The sign and magnitude of the response to changes in either driving factor are location-dependent, and there is no general understanding of the large-scale responses of terrestrial N2O fluxes to either CO2 or climate change. Models suggest that rising CO2 decreases and climate change increases N2O emissions (
Past land-use changes and land management have left the land out of equilibrium with regard to carbon storage and have reduced the additional sink capacity the land may have provided. The legacy effects of this disturbance could lead to either increased carbon accumulation (e.g., forest regrowth after abandonment) or carbon loss (e.g., following peatland drainage or drying) post zero emissions and hence could affect ZEC. As the land continues to adjust to past disturbances, the impact of these will decrease over time.
Continued land-use changes and changes in management are not directly related to ZEC but represent one of the dimensions in which a gross zero emissions world can differ from a net zero world. Given the large potential variations in net zero configurations (
Inter-model differences in climate feedbacks are the dominant driver of the inter-model uncertainty in surface warming projections in CMIP6 projections (following a scenario of CO2 concentrations increasing by 1% per year), as quantified by the proportionality between surface warming and cumulative carbon emissions (
Here we present an expert assessment by the article’s co-authors, supported by available published evidence from the literature where possible, to quantify the plausible impact of these Earth system processes on the magnitude and sign of ZEC over decades, centuries, and millennia (
Expert assessment of the hypothesized impact of Earth system uncertainties on the magnitude of the Zero Emissions Commitment (ZEC) for futures initially aiming to limit global warming to 2°C.
A key take-home message is the large proportion of low—and often only speculative—confidence assigned to the impact of many processes on ZEC. It is clear that uncertainty remains high in both ZEC’s sign and magnitude over short- and long-term timeframes, owing to uncertainties in the impact of its constituent parts. Though it is known that some long-term changes to the Earth system, such as sea level rise, will continue beyond net zero, it is now clear that we must also consider the possibility of significant continued temperature rise even after anthropogenic emissions cease. This uncertainty should be factored into the risk assessment of global temperature limits, carbon budgets, and emissions reduction plans for countries and businesses.
One of the key properties of ZEC is that it emerges from the coupling of the climate and the carbon cycle. As such, improving ZEC in terms of sign and magnitude requires improving the couplings between Earth system components, including both physical and biogeochemical ocean components. Current models have evolved toward a more comprehensive treatment of biological boundary conditions (e.g., atmospheric deposition, riverine inputs, sediments, ice sheets, and geothermal sources) but the latter is currently largely represented using climatological data rather than dynamic connections (
The emergence of high-resolution ocean models that include marine biogeochemical processes may be supported in a number of ways: (1) the availability of even greater computational resources; (2) the use of hybrid-resolution numerical schemes to decrease the cost of biogeochemical models (
Land carbon fluxes dominate the total uncertainty in assessments of changes in the natural carbon cycle on decadal timescales, with ocean processes dominating on longer timescales. Priorities for reducing uncertainty in ZEC are tightly related to priorities for modeling the land carbon cycle in general and its role in determining carbon cycle feedbacks, the transient response to cumulative CO2 emissions (TCRE), and carbon budgets. Notably, the response of ecosystems to elevated CO2 and the role of nutrient limitation is considered the highest priority. Model data comparisons against site-level manipulation experiments [e.g., free-air CO2 enrichment (FACE) experiments] or nutrient supply experiments [e.g., the Amazon Fertilisation Experiment (AFEX)] offer valuable ways forward. Forthcoming FACE experiments in a tropical forest in Brazil will prove extremely valuable. However, experiments must also capture the long-term response of ecosystems; it is well established that elevated CO2 causes an initial increase in productivity, but less is known about the long-term impacts on ecosystem carbon storage. Similarly, ecosystem responses to environmental changes can be improved in models with valuable observational experiments, such as the Caxiuana throughfall exclusion site (
Representing missing processes is also increasingly important, including physical and carbon permafrost processes and vegetation dynamics. Some, but not all, models have improved in their representation of permafrost physics from CMIP5 to CMIP6 (
The role of time-varying physical feedbacks in determining the future climate response is under active investigation for increasing carbon emissions. There are key uncertainties in the representation of clouds over different parts of the atmosphere and in surface albedo changes linked to the presence of snow and sea ice. We are beginning to understand the physical mechanisms controlling the forced pattern of sea surface warming and its time evolution (
Simple climate models (SCMs) provide tools for exploring how process interaction may lead to results that are not observed in the current ESM ensemble, which constitutes an “ensemble of opportunity” since the ESMs that have examined ZEC are a serendipitous selection of models from researchers interested in participating in the ZECMIP exercise. SCMs contain modules that represent many of the processes controlling ZEC, albeit in simplified ways. By tuning these modules to capture behaviors at different ends of the spectrum of scientific uncertainty, the link between ZEC and relevant processes can be examined in a simplified manner. This approach allows the exploration of results that would be expected if the behavior of multiple processes were at the extreme end of their potential range, recognizing that this simplified approach may miss couplings such as that between ocean carbon and heat uptake. Given the multiple feedbacks and non-linear processes involved, the high-end result could be much larger than a simple linear superposition would imply. The key to performing such investigations is a strong understanding of how simple climate model modules and behavior relate to the processes controlling ZEC. While the broad mappings are understood, research is needed to understand them in detail. With this detailed understanding, individual (partial) and combined (feedback) effects on ZEC can be isolated from variations in physical processes.
The identified uncertainties in ZEC come with implications for climate policy, both in the near and the long term. Over the coming decades, the potential variation in ZEC on timescales of decades to centuries is most relevant. To understand the implications for climate policy, ZEC insights need to be translated from model studies to the real world. ZEC is studied with idealized model experiments, often assuming a sudden stop in global emissions (
Because ZEC already partially materializes before total global CO2 emissions reach zero (
Over the long term, the uncertainty in ZEC comes with further implications for policy. In case of a negative ZEC over centuries to millennia, future societies will have fewer challenges to reverse some of the global warming. This would be good news. However, the assessed uncertainties on these timescales indicate a positive rather than a negative ZEC (
To counteract potential continued gradual warming over centuries to millennia, our global society will most likely have to prepare for continued active management of the global carbon cycle by drawing CO2 out of the atmosphere, while taking issues related to land use and biodiversity into account. The long-term consequences of the CO2 already emitted, and that which will be emitted until global net zero is reached, imply that net zero CO2 remains an important milestone but that it might not be enough. The uncertainties in ZEC point toward further long-term warming risks and the imperative to develop sustainable, safe, and reliable options for long-term CO2 removal to keep Earth’s climate stable. Importantly, the assessment of ZEC uncertainties also emphasizes that to limit risks it is essential to keep the initial disturbance of the Earth system as low as possible. This can be achieved first and foremost by reducing global CO2 emissions to net zero swiftly in this decade and over the coming decades, and by continuing to pursue limiting global warming to as close to 1.5°C as possible.
The Supplementary Material for this article can be found online at:
Expert assessment supporting
SPC: Investigation, Project administration, Visualization, Writing – original draft, Writing – review & editing. MS: Conceptualization, Investigation, Writing – original draft, Writing – review & editing. PC: Investigation, Writing – original draft, Writing – review & editing. BF-K: Investigation, Writing – original draft, Writing – review & editing. TF: Investigation, Writing – original draft, Writing – review & editing. AG-S: Investigation, Writing – original draft, Writing – review & editing. JH: Investigation, Writing – original draft, Writing – review & editing. GH: Investigation, Writing – original draft, Writing – review & editing. CJ: Investigation, Writing – original draft, Writing – review & editing. RK: Investigation, Writing – original draft, Writing – review & editing. CK: Investigation, Writing – original draft, Writing – review & editing. AM: Investigation, Writing – original draft, Writing – review & editing. MM: Investigation, Writing – original draft, Writing – review & editing. ZN: Investigation, Writing – original draft, Writing – review & editing. JS: Investigation, Writing – original draft, Writing – review & editing. BS: Investigation, Writing – original draft, Writing – review & editing. RS: Investigation, Writing – original draft, Writing – review & editing. MT: Investigation, Writing – original draft, Writing – review & editing. RW: Investigation, Writing – original draft, Writing – review & editing. SZ: Investigation, Writing – original draft, Writing – review & editing. JR: Conceptualization, Investigation, Visualization, Writing – original draft, Writing – review & editing.
The original contributions presented in the study are included in the article/
The author(s) declare financial support was received for the research, authorship, and/or publication of this article. The study presented in this manuscript benefitted from the direct funding of ESM2025 (grant number: 101003536), through the author SPC. The authors JR, CJ, RS, BS and SZ acknowledge support from ESM2025. The funder was not involved in the study design, collection, analysis, interpretation of data, the writing of this article, or the decision to submit it for publication. The authors also declare the following indirect funders which were not involved in the study design, collection, analysis, interpretation of data, the writing of this article, or the decision to submit it for publication: SPC was funded by the NERC 'Science and Solutions for a Changing Planet' doctoral training partnership, and the Grantham Institute for Climate Change and the Environment. JR acknowledges support from the European Union’s Horizon 2020 Research and Innovation Programme’s CONSTRAIN (grant number: 820829), ESM2025 (grant number: 101003536), and PROVIDE (grant number: 101003687) projects. AG-S received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 865403). The article reflects the authors’ view and the European Commission is not responsible for any use that may be made of the information it contains. CJ was supported by the Joint UK BEIS/Defra Met Office Hadley Centre Climate Programme (GA01101) and the European Union's Horizon 2020 research and innovation programme under Grant Agreement No 101003536 (ESM2025 - Earth System Models for the Future). CK acknowledges support by the Director, Office of Science, Office of Biological and Environmental Research of the US Department of Energy under contract DE-AC02-05CH11231 through the Regional and Global Model Analysis Program (RUBISCO SFA). TF was funded by the European Union’s Horizon 2020 research and innovation program under grant agreement no. 821003 (project 4C, Climate-Carbon Interactions in the Current Century) and no. 820989 (project COMFORT, Our common future ocean in the Earth system-quantifying coupled cycles of carbon, oxygen and nutrients for determining and achieving safe operating spaces with respect to tipping points) as well as by the Swiss National Science Foundation under grant PP00P2_198897. JS received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement N°821001. BS acknowledges 4C (Grant agreement ID: 821003), PROVIDE (grant agreement No. 101003687) and ESM2025 (Grant agreement ID: 101003536). AM was funded by the Natural Science and Engineering Research Council of Canada Discovery Grant program. RW acknowledges the U.K. Natural Environment Research Council (NE/T007788/1, NE/T010657/1).
The authors declare that the research was conducted in the absence of financial relationships that could be construed as a potential conflict of interest.
The reviewer HDM declared a past co-authorship with the authors JR, CK, SZ, RK, CJ, AM, MM to the handling editor.
The authors MS, TF, JR, CJ declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision.
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
Hall G and Fox-Kemper B. Regional mixed layer depth as a climate diagnostic and emergent constraint. (submitted to)