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The use of digital tracking of individuals throughout the coronavirus 2019 (COVID-19) pandemic renewed societal debates on the efficacy and ethics of digital surveillance to mitigate collective crises. While digital emissions tracking is being used to support climate-mitigation strategies, to date there has been limited exploration of the opportunities and challenges of deploying it at the individual level. Here, we assess temporal and regional differences in levels of support for the use of digital surveillance in times of crisis, such as climate change. Results from a global survey indicate moderate support for the use of digital tracking, including for personal carbon footprints. Response varied regionally, with the lowest support in North America and Europe. This study raises key questions-if digital surveillance tools could be part of a socially acceptable response to the climate crisis, is it worth exploring? Or is this an unacceptable risk for society?

Nature-based climate solutions (NBCSs) refer to actions that seek to protect, restore and better manage natural landscapes to reduce greenhouse gas (GHG) emission or remove CO2 from the atmosphere. NBCSs are integral part of many countries’ roadmaps to reach net zero GHG emissions by mid century in compliance with the Paris Agreement climate goals. Implementation of NBCSs not only affects cycling of CO2 and other GHGs in the Earth system, but impacts the energy balance at the Earth’s surface through biophysical effects including changes in reflectivity (albedo), surface roughness and the water cycle, with effects on surface temperature. Furthermore, storage of the sequestered CO2 in above-ground biomass is often vulnerable to natural and anthropogenic disturbances, with the risk of re-release after a few decades. Yet, frameworks that seek to balance residual GHG emissions with nature-based CO2 removal often only consider the CO2 sequestration potential, and do not take the full climate impacts of NBCSs and the vulnerability of carbon stocks into account. By implementing large-scale reforestation as an example of a NBCS in an Earth system model we show that offsetting fossil-fuel CO2 emissions with nature-based CO2 removals to achieve net zero CO2 emissions could result in additional warming compared to the case where the CO2 emissions are avoided, if biophysical effects are not considered and nature-based CO2 storage is temporary. We provide recommendations for taking into account the full climate impacts of NBCSs in net zero accounting frameworks and lay out directions for future research.

Climate change necessitates an immediate and sustained global effort to reduce greenhouse gas emissions while adapting to the increased climate risks caused by historical emissions. This broader climate transition will involve mass global interventions including renewable energy deployment, coastal protection and retreat, and enhanced space cooling, which will result in CO2 emissions from energy and materials use. Yet, the magnitude of these emissions remains largely unconstrained, leaving open the potential for under-accounting of emissions and conflicts or synergies between mitigation and adaptation goals. Here, we use a suite of models to estimate the CO2 emissions embedded in the broader climate transition. For a pathway limiting warming to 2°C, we estimate that selected adaptations will emit ~1.5GtCO2 through 2100. Emissions from energy used to deploy renewable capacity are much larger at ~95GtCO2, equivalent to over two years of current global emissions and ~8% of the remaining carbon budget for 2°C. These embedded transition emissions are reduced by 80% to 20GtCO2 under a rapid decarbonization scenario limiting warming to 1.5°C. However, they roughly double to 185GtCO2 under a low-ambition transition consistent with current policies (2.7°C warming by 2100), mainly because a slower transition relies more on fossil fuels. Under this status-quo, the emissions embedded in the transition total nearly half the remaining carbon budget for 1.5°C. Our results provide the first holistic assessment of the carbon emissions embedded in the transition itself, and suggest that these emissions can be largely minimized through rapid energy decarbonization, an underappreciated benefit of enhanced climate ambition.

Nature-based climate solutions have the potential to contribute to climate mitigation, but the vulnerability of land carbon to disturbances means that any efforts to slow or reverse land carbon losses will likely result in only temporary storage. Tonne-year accounting, which integrates the amount of carbon over the time that it remains stored, has been proposed as a method to measure the value of temporary relative to permanent carbon storage. Here we show that using tonne-year accounting to establish an equivalency of temporary with permanent storage is not grounded in climate science. However, a reimagined approach to tonne-year accounting could be effectively used as a metric to track the contribution of temporary carbon storage towards climate mitigation goals. We show that in a scenario where the total number of tonne-years accumulates at a constant or increasing rate, this results in a sustained or increasing climate benefit. However, as the rate of accumulation of tonne-years slows, this temperature benefit is eroded, and is lost completely when tonne-years stop increasing. This implies that if the world is able to sustain an increasing number of tonne-years alongside achieving net-zero fossil fuel CO2 emissions, then the resulting carbon storage (even if only temporary) would contribute to lowering peak temperature. Tonne-years of temporary carbon storage therefore have considerable climate value, not if treated interchangeably with permanent storage or fossil fuel emissions, but rather if quantified as an independent contribution to lowering the global temperature peak.

Fire is an integral part of the Earth system, interacting in complex ways with humans, vegetation and climate. Global fire activity is an important element of the carbon cycle, and understanding its role in the context of climate change is crucial. In order to represent the transient fire-climate-vegetation interactions and to integrate these in the long term climate projections of climate models, coupling these three components is necessary. Global fire models have been coupled to climate-vegetation models with complex atmosphere modules but these models are computationally intensive. In this research, we use the University of Victoria Earth System Climate Model (UVic ESCM), an ESCM of intermediate complexity to which we couple a process based global fire model, in order to develop a computationally efficient means of studying long term fire-climate-vegetation interactions. The fire model used simulates burned area based primarily on relative humidity, soil moisture and biomass density. The UViC ESCM’s simulated relative humidity is improved by parameterizing it according to the simulated precipitation, and observational variability is added to the simulated climatology to improve the variability of simulated burned area. The best parameterization achieves a moderate spatial agreement of simulated burned area with observational data. Tropical rainforests in South America and Africa, however, display very high burned fractions, due to the poorly simulated relative humidity input; indeed, when we used observed relative humidity to simulate fire activity, the pattern of burned area in the tropics improved substantially. This research demonstrates the importance of variability and regional patterns of climatology for global wildfire activity and the corresponding limitations of ESCMs that simplify atmospheric circulation. This suggests that using pattern scaling of climate variables as an input to fire models could provide such ESCMs of intermediate complexity with the ability to integrate global fire activity.

With increasing pressure for climate action, commitments to setting scientifically supported emissions targets have become more common among firms. The target-setting methods currently endorsed by the Science Based Targets Initiative (SBTi) use emission pathways that are aligned with 1.5°C and well-below 2°C long-term temperature goals to inform near-term corporate targets. However, most of these scenarios lead to a temperature overshoot, followed by a return to the temperature goal achieved via net-negative emissions in the second half of this century. When used to inform near-term (e.g., 2030) corporate targets, the result is a set of targets that are aligned with an overshoot of a temperature target, with no explicit long-term commitment to using negative emissions technologies to reverse this. To decrease the risk of this misalignment with the long-term temperature goal, we propose an alternative approach that derives corporate targets directly from the remaining global cumulative carbon budget. We illustrate this approach using global Scope 1 emissions disclosed by public firms in 2019 to estimate corporate carbon budgets and construct idealized emissions-reduction pathways that are consistent with the remaining global carbon budget for 1.5°C and well-below 2°C. While firms, or their sectors, may choose varying mitigation pathways aligned with either global temperature limit, consistency with remaining carbon budgets requires that any delayed mitigation action in the near term is followed by more rapid emissions reductions in subsequent years. This study emphasizes the need for a more precautionary and robust approach to corporate target setting.

Negative CO2 emissions are a key mitigation measure in emission scenarios consistent with temperature limits adopted by the Paris Agreement. It is commonly assumed that the climate–carbon cycle response to a negative CO2 emission is equal in magnitude and opposite in sign to the response to an equivalent positive CO2 emission. Here we test the hypothesis that this response is symmetric by forcing an Earth system model with positive and negative CO2 emission pulses of varying magnitude and applied from different climate states. Results indicate that a CO2 emission into the atmosphere is more effective at raising atmospheric CO2 than an equivalent CO2 removal is at lowering it, with the asymmetry increasing with the magnitude of the emission/removal. The findings of this study imply that offsetting positive CO2 emissions with negative emissions of the same magnitude could result in a different climate outcome than avoiding the CO2 emissions. It is commonly assumed that the climate response to CO2 removals is equal in magnitude and opposite in sign to the response to CO2 emissions. The response, however, is asymmetric, meaning that offsetting CO2 emissions with equal removals could lead to a different climate than avoiding the emissions.

Human activities have caused global temperatures to increase by 1.25°C, and the current emissions trajectory suggests that we will exceed 1.5°C in less than 10 years. Though the growth rate of global carbon dioxide emissions has slowed and many countries have strengthened their emissions targets, current midcentury net zero goals are insufficient to limit global warming to 1.5°C above preindustrial temperatures. The primary barriers to the achievement of a 1.5°C-compatible pathway are not geophysical but rather reflect inertia in our political and technological systems. Both political and corporate leadership are needed to overcome this inertia, supported by increased societal recognition of the need for system-level and individual lifestyle changes. The available evidence does not yet indicate that the world has seriously committed to achieving the 1.5°C goal.

Current greenhouse gas accounting standards allow companies to use renewable energy certificates (RECs) to report reductions in emissions from purchased electricity (scope 2) as progress towards meeting their science-based targets. However, previous analyses suggest that corporate REC purchases are unlikely to lead to additional renewable energy production. Here we show that the widespread use of RECs by companies with science-based targets has led to an inflated estimate of the effectiveness of mitigation efforts. When removing the emission reductions claimed through RECs, companies’ combined 2015–2019 scope 2 emission trajectories are no longer aligned with the 1.5 °C goal, and only barely with the well below 2 °C goal of the Paris Agreement. If this trend continues, 42% of committed scope 2 emission reductions will not result in real-world mitigation. Our findings suggest a need to revise accounting guidelines to require companies to report only real emission reductions as progress towards meeting their science-based targets.

Climate change is likely to negatively affect the habitats of non-human primate species. Recent research has identified a near-linear relationship between cumulative CO2 emissions, and the resulting regional and seasonal temperature increase. Here, we use this relationship to assess the potential impact that cumulative CO2 emissions could have on the ranges available to primate species. We used data from the International Union for Conservation of Nature on ranges for 426 species and subspecies of non-human primates, combined with spatial climate data from the Coupled Model Intercomparison Project Phase 5 that represent regional and seasonal temperature changes per unit CO2 emissions. Using these data, we estimated the portion of the area of each species’ range where annual average temperatures exceed the Pre-industrial Seasonal Maximum Temperatures (PSMT), for cumulative CO2 emissions from 600 to 2000 billion tonnes of carbon. For the level of emissions corresponding to a 2 °C global temperature increase scenario, 26.1% of all ranges had temperatures in excess of their PSMTs, and for 8% of species, the entire current range was above their PSMT. This suggests the potential for considerable loss of or compromised habitat for non-human primates on a global scale, as a result of the emergence of climate conditions that are outside of the scope of historical experience for many species. Our results point to key priorities for conservation efforts, as well as the need for future research on strategies to increase the resilience of vulnerable local non-human primate populations.

One key contribution to the wide range of 1.5 °C carbon budgets among recent studies is the non-CO2 climate forcing scenario uncertainty. Based on a partitioning of historical non-CO2 forcing, we show that currently there is a net negative non-CO2 forcing from fossil fuel combustion (FFC), and a net positive non-CO2 climate forcing from land-use change (LUC) and agricultural activities. We perform a set of future simulations in which we prescribed a 1.5 °C temperature stabilisation trajectory, and diagnosed the resulting 1.5 °C carbon budgets. Using the historical partitioning, we then prescribed adjusted non-CO2 forcing scenarios consistent with our model’s simulated decrease in FFC CO2 emissions. We compared the diagnosed carbon budgets from these adjusted scenarios to those resulting from the default RCP scenario’s non-CO2 forcing, and to a scenario in which proportionality between future CO2 and non-CO2 forcing is assumed. We find a wide range of carbon budget estimates across scenarios, with the largest budget emerging from the scenario with assumed proportionality of CO2 and non-CO2 forcing. Furthermore, our adjusted-RCP scenarios produce carbon budgets that are smaller than the corresponding default RCP scenarios. Our results suggest that ambitious mitigation scenarios will likely be characterised by an increasing contribution of non-CO2 forcing, and that an assumption of continued proportionality between CO2 and non-CO2 forcing would lead to an overestimate of the remaining carbon budget. Maintaining such proportionality under ambitious fossil fuel mitigation would require mitigation of non-CO2 emissions at a rate that is substantially faster than found in the standard RCP scenarios.

There is growing recognition that meeting the climate objectives of the Paris Agreement will require the world to achieve net-zero carbon dioxide emissions around or before mid-century. Nature-based climate solutions (NbCS), which aim to preserve and enhance carbon storage in terrestrial or aquatic ecosystems, are increasingly being evoked as a potential contributor to net-zero emissions targets. However, there is a risk that any carbon that we succeed in storing in land-based systems could be subsequently lost back to the atmosphere as a result of either climate-related or human-caused disturbances such as wildfire or deforestation. Here we quantify the climate effect of NbCS in a scenario where land-based carbon storage is enhanced over the next several decades, and this stored carbon is then returned to the atmosphere during the second half of this century. We show that temporary carbon sequestration has the potential to decrease the peak temperature increase, but only if implemented alongside an ambitious mitigation scenario where fossil fuel CO2 emissions were decreased to net-zero during the time that NbCS-sequestered carbon remained stored. We also demonstrate the importance of non-CO2 climate effects of NbCS implementation; decreases in surface albedo that result from temporary reforestation, for example, have the potential to counter almost half of the climate effect of carbon sequestration. Our results suggest that there is some climate benefit associated with NbCS, even if the carbon storage is temporary, but only if implemented as a complement (and not an alternative) to ambitious fossil fuel CO2 emissions reductions.

The climate scenarios that form the basis for current climate risk assessments have no assigned probabilities, and this impedes the analysis of future climate risks. This paper proposes an approach to estimate the probability of carbon dioxide (CO2) concentration scenarios used in key climate change modeling experiments. It computes the CO2 emissions compatible with the concentrations prescribed by Coupled Model Intercomparison Project Phase 5 (CMIP5) and CMIP6 experiments. The distribution of these compatible cumulative emissions is interpreted as the likelihood of future emissions given a concentration pathway. Using Bayesian analysis, the probability of each pathway can be estimated from a probabilistic sample of future emissions. The approach is demonstrated with five probabilistic CO2 emission simulation ensembles from four Integrated Assessment Models (IAM), leading to independent estimates of the likelihood of the CO2 concentration of Representative Concentration Pathways (RCP) and Shared Socioeconomic Pathways (SSP). Results suggest that SSP5-8.5 is unlikely for the second half of the 21st century, but offer no clear consensus on which of the remaining scenarios is most likely. Estimates of likelihoods of CO2 concentrations associated with RCP and SSP scenarios are affected by sampling errors, differences in emission sources simulated by the IAMs, and a lack of a common experimental framework for IAM simulations. These shortcomings, along with a small IAM ensemble size, limit the applicability of the results presented here. Novel joint IAM and the Earth System Model experiments are needed to deliver actionable probabilistic climate risk assessments.

There is growing recognition that meeting the climate objectives of the Paris Agreement will require the world to achieve net-zero carbon dioxide emissions around or before mid-century1–4. Natural climate solutions (NCS), which aim to preserve and enhance carbon storage in terrestrial or aquatic ecosystems5,6, are increasingly being evoked as a potential contributor to net-zero emissions targets7,8. However, there is a risk that any carbon that we succeed in storing in land-based systems could be subsequently lost back to the atmosphere as a result of either climate-related or human-caused disturbances such as wildfire or deforestation9–12. Here, we show that temporary NCS-based carbon sequestration has the potential to decrease the peak temperature increase, but only if implemented alongside an ambitious mitigation scenario where fossil fuel CO2 emissions were decreased to net-zero during the time that NCS-sequestered carbon remained stored. We also demonstrate the importance of non-CO2 climate effects of NCS implementation, which have the potential to counter a substantial portion of the climate effect of carbon sequestration. Our results suggest that there is some climate benefit associated with temporary NCS, but only if implemented as a complement (and not an alternative) to ambitious fossil fuel CO2 emissions reductions.

Atmospheric methane removal (e.g.in situmethane oxidation to carbon dioxide) may be needed to offset continued methane release and limit the global warming contribution of this potent greenhouse gas. Because mitigating most anthropogenic emissions of methane is uncertain this century, and sudden methane releases from the Arctic or elsewhere cannot be excluded, technologies for methane removal or oxidation may be required. Carbon dioxide removal has an increasingly well-established research agenda and technological foundation. No similar framework exists for methane removal. We believe that a research agenda for negative methane emissions—‘removal' or atmospheric methane oxidation—is needed. We outline some considerations for such an agenda here, including a proposed Methane Removal Model Intercomparison Project (MR-MIP).This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 1)'.

Climate policies and plans can lead to disproportionate impacts and benefits across different kinds of communities, serving to reinforce, and even exacerbate existing structural inequities and injustices. This is the case in Canada where, we argue, climate policy and planning is reproducing settler-colonial relations, violating Indigenous rights, and systematically excluding Indigenous Peoples from policy making. We conducted a critical policy analysis on two climate plans in Canada: the Pan Canadian Framework on Clean Growth and Climate Change (Pan-Canadian Framework), a federal government-led, top-down plan for reducing emissions; and the Québec ZéN (zero émissions nette, or net-zero emissions) Roadmap, a province-wide, bottom-up energy transition plan developed by civil society and environmental groups in Quebec. Our analysis found that, despite aspirational references to Indigenous Peoples and their inclusion, both the Pan-Canadian Framework and the ZéN Roadmap failed to uphold the right to self-determination and to free, prior, and informed consent, conflicting with commitments to reconciliation and a “Nation-to-Nation” relationship. Recognizing these limitations, we identify six components for an Indigenous-led climate policy agenda. These not including clear calls to action that climate policy must: prioritize the land and emphasize the need to rebalance our relationships with Mother Earth; position Indigenous Nations as Nations with the inherent right to self-determination; prioritize Indigenous knowledge systems; and advance climate-solutions that are interconnected, interdependent, and multi-dimensional. While this supports the emerging literature on Indigenous-led climate solutions, we stress that these calls offer a starting point, but additional work led by Indigenous Peoples and Nations is required to breathe life into a true Indigenous-led climate policy.

The remaining carbon budget quantifies the future CO2 emissions to limit global warming below a desired level. Carbon budgets are subject to uncertainty in the Transient Climate Response to Cumulative CO2 Emissions (TCRE), as well as to non-CO2 climate influences. Here we estimate the TCRE using observational constraints, and integrate the geophysical and socioeconomic uncertainties affecting the distribution of the remaining carbon budget. We estimate a median TCRE of 0.44 °C and 5–95% range of 0.32–0.62 °C per 1000 GtCO2 emitted. Considering only geophysical uncertainties, our median estimate of the 1.5 °C remaining carbon budget is 440 GtCO2 from 2020 onwards, with a range of 230–670 GtCO2, (for a 67–33% chance of not exceeding the target). Additional socioeconomic uncertainty related to human decisions regarding future non-CO2 emissions scenarios can further shift the median 1.5 °C remaining carbon budget by ±170 GtCO2.

The remaining carbon budget represents the total amount of CO2 that can still be emitted in the future while limiting global warming to a given temperature target. Remaining carbon budget estimates range widely, however, and this uncertainty can be used to either trivialize the most ambitious mitigation targets by characterizing them as impossible, or to argue that there is ample time to allow for a gradual transition to a low-carbon economy. Neither of these extremes is consistent with our best understanding of the policy implications of remaining carbon budgets. Understanding the scientific and socio-economic uncertainties affecting the size of the remaining carbon budgets, as well as the methodological choices and assumptions that underlie their calculation, is essential before applying them as a policy tool. Here we provide recommendations on how to calculate remaining carbon budgets in a traceable and transparent way, and discuss their uncertainties and implications for both international and national climate policies. ISSN:1752-0908 ISSN:1752-0894

Climate projections are made using a hierarchy of models of different complexities and computational efficiencies. While the most complex climate models contain the most detailed representations of many physical processes within the climate system, both parameter space exploration and integrated assessment modelling require the increased computational efficiency of reduced-complexity models. This study presents a computationally efficient method for generating probabilistic projections of local warming across the globe, using a pattern-scaling approach derived from the Climate Model Intercomparison Project phase 5 (CMIP5) ensemble, that can be coupled to any efficient model ensemble simulation of global mean surface warming. While the method can project local warming for arbitrary future scenarios, using it for scenarios with peak global mean warming ≤2 ∘C is problematic due to the large uncertainties involved. First, global mean warming is projected using a 103-member ensemble of history-matched simulations with an example reduced complexity Earth system model: the Warming Acidification and Sea-level Projector (WASP). The ensemble projection of global mean warming from this WASP ensemble is then converted into local warming projections using a pattern-scaling analysis from the CMIP5 archive, considering both the mean and uncertainty of the local to global ratio of temperature change (LGRTC) spatial patterns from the CMIP5 ensemble for high-end and mitigated scenarios. The LGRTC spatial pattern is assessed for scenario dependence in the CMIP5 ensemble using RCP2.6, RCP4.5 and RCP8.5, and spatial domains are identified where the pattern scaling is useful across a variety of arbitrary scenarios. The computational efficiency of our WASP–LGRTC model approach makes it ideal for future incorporation into an integrated assessment model framework or efficient assessment of multiple scenarios. We utilise an emergent relationship between warming and future cumulative carbon emitted in our simulations to present an approximation tool making local warming projections from total future carbon emitted.

The remaining carbon budget quantifies the allowable future CO2 emissions to keep global mean warming below a desiredlevel. Carbon budget estimates are subject to uncertainty in the Transient Climate Response to Cumulative CO2 Emissions (TCRE), which measures the warming resulting from a given total amount of CO2 emitted. Moreover, other sources of uncertainty linked to non-CO2 emissions have been shown to also strongly affect estimates of the remaining carbon budget. Here we present a new framework that estimates the TCRE using geophysical constraints derived from observations, and integrates the effect of geophysical and socioeconomic pathway uncertainties on the distribution of the remaining carbon budget. We estimate a median TCRE of 0.40 °C and likely range of 0.3 to 0.5 °C (17-83%) per 1000 GtCO2 emitted. Our 1.5 °C remaining carbon budget has a median value of 710 GtCO2 from 2020 onwards, with a range of 470 to 960 GtCO2, (for a 67% to 33% chance of not exceeding the target). Uncertainty in the amount of current warming from non-CO2 forcing is the dominant geophysical contributor to the spread in both the TCRE and remaining carbon budget estimates. The remaining carbon budget distribution is also strongly affected by current and future mitigation decisions, where the range of non-CO2forcing across scenarios has the potential to increase or decrease the median 1.5 °C remaining carbon budget by 740 GtCO2.

The majority of emissions scenarios that limit warming to 2°C, and nearly all emission scenarios that do not exceed 1.5°C warming by the year 2100 require negative CO2 emissions. Negative emission technologies (NETs) in these scenarios are required to offset emissions from sectors that are difficult or costly to decarbonize and to generate global ‘net negative’ emissions, allowing to compensate for earlier emissions and to recover a carbon budget after overshoot. It is commonly assumed that the carbon cycle and climate response to a negative CO2emission is equal in magnitude and opposite in sign to the response to an equivalent positive CO2 emission, i.e. that the climate-carbon cycle response is symmetric. This assumption, however, has not been tested for a range of emissions. Here we explore the symmetry in the climate-carbon cycle response by forcing an Earth system model with positive and negative CO2emission pulses of varying magnitude and applied from different climate states. Our results suggest that an emission of CO2into the atmosphere is more effective at raising atmospheric CO2than a CO2removal is at lowering atmospheric CO2, indicating that the carbon cycleresponse is asymmetric, particularly for emissions/removals > 100 GtC. The surface air temperature response, on the other hand, is largely symmetric. Our findings suggest that the emission and subsequent removal of a given amount of CO2 would not result in the same atmospheric CO2concentration as if the emission were avoided. Furthermore, our results imply using simple models used to estimate negative emission requirements may result in underestimating the amount of negative emissions needed to attain a given CO2concentration target.

Estimates of the 1.5°C carbon budget vary widely among recent studies. One key contribution to this range is the non-CO2 climate forcing scenario uncertainty. Based on a partitioning of historical non-CO2 forcing, we show that there is currently a net negative non-CO2 forcing from fossil fuel combustion (FFC) mainly due to the co-emission of aerosols, and a net positive non-CO2 climate forcing from land-use change (LUC) and agricultural activities. We then perform a set of future simulations in which we prescribed a 1.5°C temperature stabilization trajectory, and diagnosed the resulting 1.5°C carbon budgets. Using the results of our historical partitioning, we prescribed changing non-CO2 forcing scenarios that are consistent with our model’s simulated decrease in FFC CO2 emissions. We compared the diagnosed carbon budgets from these idealized scenarios to those resulting from the default RCP scenario non-CO2 forcing, as well as from a scenario in which we assumed proportionality between future CO2 and non-CO2 forcing. We find a large range of carbon budget estimates across scenarios, with the largest budget emerging from the scenario with assumed proportionality of CO2 and non-CO2 forcing. Furthermore, our adjusted-RCP scenarios, in which the non-CO2 forcing is consistent with model-diagnosed FFC CO2 emissions, produced carbon budgets that are smaller than the corresponding default RCP scenarios. Our results suggest that ambitious mitigation scenarios will likely be characterized by an increasing contribution of non-CO2 forcing, and that an assumption of continued proportionality between CO2 and non-CO2 forcing would lead to an overestimate of the remaining carbon budget required to avoid low-temperature targets. Maintaining such proportionality under ambitious fossil fuel mitigation would require mitigation of non-CO2 emissions from agriculture and other non-FFC sources at a rate that is substantially faster than is found in the standard RCP scenarios.

The Zero Emissions Commitment (ZEC) is the change in global mean temperature expected to occur following the cessation of net CO2 emissions, and as such is a critical parameter for calculating the remaining carbon budget. The Zero Emissions Commitment Model Intercomparison Project (ZECMIP) was established to gain a better understanding of the potential magnitude and sign of ZEC, in addition to the processes that underlie this metric. Eighteen Earth system models of both full and intermediate complexity participated in ZECMIP. All models conducted an experiment where atmospheric CO2 concentration increases exponentially until 1000 PgC has been emitted. Thereafter emissions are set to zero and models are configured to allow free evolution of atmospheric CO2 concentration. The inter-model range of ZEC 50 years after emissions cease for the 1000 PgC experiment is -0.36 to 0.29 oC with a model ensemble mean of -0.06 oC, median of -0.05 oC and standard deviation of 0.19 oC. Models exhibit a wide variety of behaviours after emissions cease, with some models continuing to warm for decades to millennia and others cooling substantially. Analysis shows that both ocean carbon uptake and carbon uptake by the terrestrial biosphere are important for counteracting the warming effect from reduction in ocean heat uptake in the decades after emissions cease. Overall, the most likely value of ZEC on decadal time-scales is assessed to be close to zero, consistent with prior work. However substantial continued warming for decades or centuries following cessation of emission is a feature of a minority of the assessed models and thus cannot be ruled out.

The warming caused by past CO2 emissions is known to persist for centuries to millennia, even in the absence of additional future emissions. Other non-CO2 greenhouse gas emission have caused additional historical warming, though the persistence of this non-CO2 warming varies among gases owing to their different atmospheric lifetimes. Under deep mitigation scenarios or in an idealized scenario of zero future greenhouse gas emissions, the past warming from shorter-lived non-CO2 gases has been shown to be considerably more reversible than that caused by CO2 emissions. Here we use an intermediate-complexity global climate model coupled to an atmospheric chemistry module to quantify the warming commitment and its reversibility for individual and groups of non-CO2 greenhouse gases. We show that warming caused by gases with short atmospheric lifetimes will decrease by more than half its peak value within 30 years following zeroed emissions at present day, with more 80 percent of peak temperature reversed by the end of this century. Despite the fast response of atmospheric temperature to the elimination of non-CO2 emissions, the ocean responds much more slowly: past ocean warming does not reverse, but rather continues for several centuries after zero emissions. Further consequences are shown for the land carbon pool, which decreases as an approximately linear function of historical non-CO2 greenhouse gas induced warming. Given that CO2 and non-CO2 greenhouse gas emissions share common emission sources, we also explore a set of scenarios where sets of emissions are zeroed according to two broad source categories: (1) fossil fuel combustion, and (2) land-use and agriculture. Using these additional mode runs, we investigate the temperature change that is avoided if all CO2 and non-CO2 greenhouse gas emissions from a particular source abruptly stops while others are allowed to continue. These results indicate the possibility of land-use change and agriculture activities continuing under deep mitigation scenarios and ambitious climate targets, without leading to exceedance of global climate targets. Though we analyze unlikely scenarios, our work provides baselines from which more realistic mitigation scenarios can be assessed. The reversibility of peak temperature caused by historical non-CO2 gases is a relevant measure for policy frameworks seeking to limit global warming to ambitious targets, such as the 1.5 oC target adopted by the Paris Agreement

The University of Victoria Earth system climate model of intermediate complexity has been a useful tool in recent assessments of long-term climate changes including paleo-climate modelling. Since the last official release of the UVic ESCM 2.9, and the two official updates during the last decade, a lot of model development has taken place in multiple groups. The new version 2.10 of the University of Victoria Earth System Climate Model (UVic ESCM), to be used in the 6th phase of the coupled model intercomparison project (CMIP6), presented here combines and brings together multiple model developments and new components that have taken place since the last official release of the model. To set the foundation of its use, we here describe the UVic ESCM 2.10 and evaluate results from transient historical simulations against observational data. We find that the UVic ESCM 2.10 is capable of reproducing well changes in historical temperature and carbon fluxes, as well as the spatial distribution of many ocean tracers, including temperature, salinity, phosphate and nitrate. This is connected to a good representation of ocean physical properties. For the moment, there remain biases in ocean alkalinity and dissolved inorganic carbon, which will be addressed in the next updates to the model.

The Zero Emissions Commitment (ZEC) is the change in global mean temperature expected to occur following the cessation of net CO2 emissions, and as such is a critical parameter for calculating the remaining carbon budget. The Zero Emissions Commitment Model Intercomparison Project (ZECMIP) was established to gain a better understanding of the potential magnitude and sign of ZEC, in addition to the processes that underlie this metric. Eighteen Earth system models of both full and intermediate complexity participated in ZECMIP. All models conducted an experiment where atmospheric CO2 concentration increases exponentially until 1000 PgC has been emitted. Thereafter emissions are set to zero and models are configured to allow free evolution of atmospheric CO2 concentration. Many models conducted additional second priority simulations with different cumulative emissions totals and an alternative idealized emissions pathway with a gradual transition to zero emissions. The inter-model range of ZEC 50 years after emissions cease for the 1000 PgC experiment is − 0.36 to 0.29 ºC with a model ensemble mean of −0.06 ºC, median of −0.05 ºC and standard deviation of 0.19 ºC. Models exhibit a wide variety of behaviours after emissions cease, with some models continuing to warm for decades to millennia and others cooling substantially. Analysis shows that both ocean carbon uptake and carbon uptake by the terrestrial biosphere are important for counteracting the warming effect from reduction in ocean heat uptake in the decades after emissions cease. Overall, the most likely value of ZEC on multi-decadal timescales is close to zero, consistent with previous model experiments.

Estimates of the 1.5 °C carbon budget vary widely among recent studies, emphasizing the need to better understand and quantify key sources of uncertainty. Here we quantify the impact of carbon cycle uncertainty and non-CO2 forcing on the 1.5 °C carbon budget in the context of a prescribed 1.5 °C temperature stabilization scenario. We use Bayes theorem to weight members of a perturbed parameter ensemble with varying land and ocean carbon uptake, to derive an estimate for the fossil fuel (FF) carbon budget of 469 PgC since 1850, with a 95% likelihood range of (411,528) PgC. CO2 emissions from land-use change (LUC) add about 230 PgC. Our best estimate of the total (FF + LUC) carbon budget for 1.5 °C is therefore 699 PgC, which corresponds to about 11 years of current emissions. Non-CO2 greenhouse gas and aerosol emissions represent equivalent cumulative CO2 emissions of about 510 PgC and −180 PgC for 1.5 °C, respectively. The increased LUC, high non-CO2 emissions and decreased aerosols in our scenario, cause the long-term FF carbon budget to decrease following temperature stabilization. In this scenario, negative emissions would be required to compensate not only for the increasing non-CO2 climate forcing, but also for the declining natural carbon sinks.

Carbon budgets, which define the total allowable CO2 emissions associated with a given global climate target, are a useful way of framing the climate mitigation challenge. In this paper, we review the geophysical basis for the idea of a carbon budget, showing how this concept emerges from a linear climate response to cumulative CO2 emissions. We then discuss the difference between a “CO2-only carbon budget” associated with a given level of CO2-induced warming and an “effective carbon budget” associated with a given level of warming caused by all human emissions. We present estimates for the CO2-only and effective carbon budgets for 1.5 and 2 °C, based on both model simulations and updated observational data. Finally, we discuss the key contributors to uncertainty in carbon budget estimates and suggest some implications of this uncertainty for decision-making. Based on the analysis presented here, we argue that while the CO2-only carbon budget is a robust upper bound on allowable emissions for a given climate target, the size of the effective carbon budget is dependent on the how quickly we are able to mitigate non-CO2 greenhouse gas and aerosol emissions. This suggests that climate mitigation efforts could benefit from being responsive to a changing effective carbon budget over time, as well as to potential new information that could narrow uncertainty associated with the climate response to CO2 emissions.