Peak shaving with solar radiation modification would shorten global temperature overshoot

Limiting global temperature rise below 2°C requires significant reduction in greenhouse gas emissions and likely large-scale carbon dioxide removal (CDR). This study assesses the CO2 sequestration and efficacy of two CDR approaches, Bioenergy with Carbon Capture and Storage (BECCS) and Ocean Alkalinity Enhancement (OAE), applied individually and in combination. Using the Norwegian Earth System Model (NorESM2-LM), simulations were designed to ramp up deployment of BECCS and OAE, to an additional area of 5.2 million km² by 2100 for bioenergy feedstock for BECCS, and a CaO deployment rate of approximately 2.7 Gt/year for OAE within the exclusive economic zones of Europe, the United States and China. The combined land-ocean CDR simulation revealed a largely additive carbon removal effect. Over 2030-2100, OAE sequestered 7 ppm of CO 22 with an accumulated 82.3 Gt CaO, achieving a CDR effectiveness of 0.08 ppm (~ 0.17 PgC) per Gt CaO, while BECCS reduced 16 ppm of CO2, with CDR effectiveness of 3.1 ppm per million km² of bioenergy crops. Together, the carbon removal achieved by BECCS and OAE corresponds to anthropogenic CO₂ emissions of 5.4 Gt CO₂/year by 2100, slightly more than 60% of current global transport sector emissions. Notably, the efficiency of BECCS and OAE alone was unaffected by their concurrent deployment. Nevertheless, simulations revealed distinct non- linear interactions, such as declines in land and soil carbon sinks in the combined scenario. Furthermore, all simulations show negligible effects on the global annual mean temperature. These results highlight near-additive CDR responses even under net-negative emissions, but feedback on land and ocean carbon sinks must be considered when designing CDR portfolios. This study provides new insights into CDR portfolio design and Earth system feedback under an overshoot scenario, highlighting both their potential and the need for continued emissions cuts and supportive policies.

Ocean negative carbon emissions: A new UN Decade program

Carbon removals from nature restoration are no substitute for steep emission reductions

Cutting through the noise on negative emissions

Global warming levels are politically relevant targets, and therefore, in public discussion and in climate science, these global warming levels are often taken as a reference for climate states. While the focus on global warming levels is a useful simplification in many cases, it becomes misleading when looking at temperature overshoot (or stabilization) scenarios. In temperature overshoot scenarios, greenhouse gas concentrations are eventually reduced leading to a decrease in global mean temperatures. In such scenarios, lagged effects, feedback mechanisms, and tipping points can result in considerably different climate states after the overshoot as compared to before at the same global warming level.Here we assess to what extent changes in regional climate signals are reversed in the period after peak warming when global mean temperature decreases. We analyze a multi-model ensemble of CMIP6 simulations of two overshoot scenarios, SSP5-34-OS and SSP119. In many regions, climate signals are decoupled from global mean temperatures in the decades after peak warming, leading to differences in regional climate signals between before and after the overshoot at the same global warming level.More dedicated climate simulations of overshoot scenarios would be required to better evaluate how long the influence of the overshoot would affect regional climate signals and to better understand the mechanisms behind these changes. The presented overview of regional climate signals in overshoot scenarios until 2100 already suggests that considerable implications of temperature overshoots for climate impacts are to be expected and that these implications need to be considered for adaptation planning and policy making.

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

ABSTRACTDespite the ambitious temperature goal of the 2015 Paris Agreement, the pace of reducing global CO2 emissions remains sluggish. This creates conditions in which the idea of temperature ‘overshoot and peak-shaving’ is emerging as a possible strategy to meet the Paris goal. An overshoot and peak-shaving scenario rests upon the ‘temporary’ use of speculative solar radiation management (SRM) technologies combined with large-scale carbon dioxide removal (CDR). Whilst some view optimistically the strategic interdependence between SRM and CDR, we argue that this strategy comes with a risk of escalating ‘climate debt’. We explain our position using the logic of debt and the analogy of subprime mortgage lending. In overshoot and peak-shaving scenarios, the role of CDR and SRM is to compensate for delayed mitigation, placing the world in a double debt: ‘emissions debt’ and ‘temperature debt’. Analogously, this can be understood as a combination of ‘subprime mortgage’ (i.e. large-scale CDR) and ‘home-equity-line-of-credit’ (i.e. temporary SRM). With this analogy, we draw some important lessons from the 2007–2009 US subprime mortgage crisis. The analogy signals that the efficacy of temporary SRM cannot be evaluated in isolation of the feasibility of large-scale CDR and that the failure of the overshoot promise will lead to prolonged peak-shaving, masking an ever-rising climate debt. Overshoot and peak-shaving scenarios should not be presented as a secured feasible investment, but rather as a high-risk speculation betting on insecure promises. Obscuring the riskiness of such scenarios is a precipitous step towards escalating a climate debt crisis.Key policy insightsThe slow progress of mitigation increases the attraction of an ‘overshoot and peak-shaving’ scenario which combines temporary SRM with large-scale CDRFollowing the logic of debt, the role of CDR and SRM in this scenario is to compensate for delayed mitigation, creating a double debt of CO2 emissions and global temperatureUsing the analogy of subprime lending, this strategy can be seen as offering a combination of subprime mortgage and open-ended ‘line-of-credit’Because the ‘success’ of peak-shaving by temporary SRM hinges critically on the overshoot promise of large-scale CDR, SRM and CDR should not be discussed separately

Limits to Paris compatibility of CO2 capture and utilization

Moving toward Net-Zero Emissions Requires New Alliances for Carbon Dioxide Removal

Does China's pathway to carbon neutrality require the integration of land-based biological negative emission solutions with geochemical and chemical alternatives?

With ongoing greenhouse gas emissions it becomes increasingly unlikely that the global mean temperature (GMT) can be stabilized at 1.5°C without considerable negative emissions. As a result, most emission scenarios that would allow to reach 1.5°C GMT at the end of the century are overshoot scenarios: In these scenarios GMT warms until net-zero emissions are reached and slowly starts to cool afterwards. Here we want to have a closer look at the local climate responses after peak warming to get a first idea of potential consequences of overshoots.The analysis is mainly based on the overshoot scenarios SSP119 and the SSP534-over from the “Coupled Model Intercomparison Project (Phase 6)”. We identify regions in which precipitation or temperature has an asymmetric response to GMT changes around peak warming. In some regions, and especially for temperature related variables, the asymmetries could result from lagged responses in the climate system. However, there are also a number of dynamic mechanisms that could influence local climate signals after peak warming and there are only few regions where analyzed earth system models (ESM) agree on the sign of change.In many regions, the projected trends in precipitation or temperature after peak warming are in the range of trends that can be found in control runs without anthropogenic forcings. Here, single model initial-condition large ensembles (SMILEs) are necessary to estimate the forced response in overshoot scenarios. For a comprehensive understanding of the mechanisms explaining these non-linear responses to GMT changes around peak warming, more large ensemble simulations of idealized overshoot scenarios for different ESMs would be required.

With the global annual mean temperature in 2024 exceeding 1.5°C above preindustrial levels, there is an urgent need to investigate pathways for returning the Earth system to lower temperature levels. In addition to stringent emission reduction, we need portfolios of Carbon Dioxide Removal (CDR) techniques to achieve the net-zero emission target. Therefore, it is crucial to evaluate various land and ocean-based CDRs for their effectiveness, environmental risks, and additional benefits.This study evaluates the CO₂ sequestration potential and efficacy of two prominent CDR methods—Bioenergy with Carbon Capture and Storage (BECCS) and Ocean Alkalinity Enhancement (OAE)—applied both individually and in combination. Using the Norwegian Earth System Model (NorESM2-LM), simulations were designed with ramped-up CDR deployment, targeting 5.2 million km² of bioenergy feedstock for BECCS and a CaO deployment rate of 2.7 Gt/year for OAE by 2100 across the exclusive economic zones of Europe, the United States, and China. The results reveal a nearly additive carbon removal effect of BECCS and OAE.   Over the period 2030-2100, OAEsequestered a total of 7 ppm of CO2 with an accumulated 82.3 Gt CaO, achieving a CDR effectiveness of 0.08 ppm per Gt of CaO, while BECCS removes 23 ppm of CO2, with CDR effectiveness of 3.1 ppm per million km² of bioenergy crops.  The combined BECCS-OAE simulation offsets anthropogenic CO₂ emissions of 5.4 Gt/year by 2100—equivalent to over 60% of current global transport sector emissions. However, the combined CDR scenario shows negligible effects on the global annual mean temperature, with no clear response detectable against the high internal variability. This underscores the limitations of current CDR approaches in addressing climate warming over the 21st century and emphasizes the need for substantial emissions reductions, supportive policies and diversified CDR strategies to facilitate a return to lower global temperatures.

ABSTRACTIn principle, many climate policymakers have accepted that large-scale carbon dioxide removal (CDR) is necessary to meet the Paris Agreement’s mitigation targets, but they have avoided proposing by whom CDR might be delivered. Given its role in international climate policy, the European Union (EU) might be expected to lead the way. But among EU climate policymakers so far there is little talk on CDR, let alone action. Here we assess how best to ‘target’ CDR to motivate EU policymakers exploring which CDR target strategy may work best to start dealing with CDR on a meaningful scale. A comprehensive CDR approach would focus on delivering the CDR volumes required from the EU by 2100, approximately at least 50 Gigatonnes (Gt) CO2, according to global model simulations aiming to keep warming below 2°C. A limited CDR approach would focus on an intermediate target to deliver the CDR needed to reach ‘net zero emissions’ (i.e. the gross negative emissions needed to offset residual positive emissions that are too expensive or even impossible to mitigate). We argue that a comprehensive CDR approach may be too intimidating for EU policymakers. A limited CDR approach that only addresses the necessary steps to reach the (intermediate) target of ‘net zero emissions’ is arguably more achievable, since it is a better match to the existing policy paradigm and would allow for a pragmatic phase-in of CDR while avoiding outright resistance by environmental NGOs and the broader public.Key policy insightsMaking CDR an integral part of EU climate policy has the potential to significantly reshape the policy landscape.Burden sharing considerations would probably play a major role, with comprehensive CDR prolonging the disparity and tensions between progressives and laggards.Introducing limited CDR in the context of ‘net zero’ pathways would retain a visible primary focus on decarbonization but acknowledge the need for a significant enhancement of removals via ‘natural’ and/or ‘engineered’ sinks.A decarbonization approach that intends to lead to a low level of ‘residual emissions’ (to be tackled by a pragmatic phase-in of CDR) should be the priority of EU climate policy.

Abstract. A growing body of evidence suggests that to achieve the temperature goals of the Paris Agreement, carbon dioxide removal (CDR) will likely be required in addition to massive carbon dioxide (CO2) emissions reductions. Nature-based CDR, which includes a range of strategies to enhance carbon storage in natural and managed land reservoirs, such as agricultural lands, could play an important role in efforts to limit climate warming to well below 2 °C above preindustrial levels. However, there remains a substantial knowledge gap on how the climate will respond to CDR when the removed carbon remains in the active carbon cycle. This study uses an intermediate-complexity climate model to perform simulations of agricultural CDR via soil carbon sequestration at rates reflecting realistic costs under three future emissions scenarios. We found that plausible levels of agricultural CDR reduced CO2 concentration by 5–19 ppm and global surface air temperature by 0.02–0.10 °C by the end of the century. This temperature decrease was non-linear with respect to cumulative removals, as the removed carbon remained part of the active carbon cycle, lessening the climate benefit than if it was removed permanently. In low-emissions scenarios, a given amount of CDR was found to be more effective at reducing surface air temperature and less effective at reducing atmospheric CO2, compared to high-emissions scenarios. This was due to a proportionally larger impact of CDR on radiative balance at lower atmospheric CO2 and reduced weakening of the carbon sinks at lower atmospheric CO2. CDR was substantially more effective when implemented at a higher rate, as CDR results in a proportionally larger difference in a climate with lower cumulative air fraction of CO2. Land and soil carbon responses were driven by the scenario-dependent balances between the impacts of CDR on primary productivity from CO2 fertilization and the impacts on soil respiration from increased soil carbon availability and global temperatures.

Adaptation and Carbon Removal