Ambitious efforts on residual emissions can reduce CO2 removal and lower peak temperatures in a net-zero future

Carbon dioxide removal (CDR) is expected to play a critical role in reaching net zero CO2 and especially net zero greenhouse gase (GHG) emissions. However, the extent to which the role of CDR in counterbalancing residual emissions can be reduced has not yet been fully quantified. Here, we use a state-of-the-art integrated assessment model to develop a ‘Maximum Sectoral Effort’ scenario which features global emissions policies alongside ambitious effort across sectors to reduce their gross GHG emissions and thereby the CDR required for offsets. We find that these efforts can reduce CDR by over 50% globally, increase both the relative and absolute role of the land sink in storing carbon, and more evenly distribute CDR contributions and associated side-effects across regions compared to CO2 pricing alone. Furthermore, the lower cumulative CO2 and nonCO2 emissions leads to earlier and lower peak temperatures. Emphasizing reductions in gross, in addition to net emissions while disallowing the substitution of less durable CDR for offsets can therefore reduce both physical and transition risks associated with high CDR deployment and temperature overshoot.


Introduction
Achieving the well-below 2 • C or the 1.5 • C goals of the Paris Agreement will be infeasible without largescale CO 2 removal (CDR) via an enhanced land sink (e.g.reforestation) and/or dedicated technologies (e.g.direct air capture) (IPCC 2022).Net-negative emissions, achieved with CDR, are required to reverse the warming effects of atmospheric CO 2 stocks after temperature overshoot in nearly all scenarios meeting these temperature outcomes (Huppmann et al. 2018, Fuhrman et al 2019).CDR is also necessary to achieve net-zero emissions by offsetting difficultto-abate emissions, including non-CO 2 greenhouse gases (GHGs) (Fankhauser et al 2021, Iyer et al 2021).The sustainability, feasibility, and political economy concerns of CDR have been explored in the literature, including with the integrated assessment models (IAM) that themselves project tens of GtCO 2 -yr −1 scale deployments (Wise et al 2009, Muratori et al 2016).In light of these concerns, several studies have evaluated the role of reduced energy and material demand, electrification, agricultural improvements, and reduced non-CO 2 emissions in limiting the need for CDR (Smith et al 2016, Grubler et al 2018, van Vuuren et al 2018, Gambhir et al 2022).Others have used alternative scenario formulations that prioritize near-term mitigation and thereby reduce the need for late-century CDR to reverse temperature overshoot (Rogelj et al 2019, Stler et al 2021).The amount of CDR required to reach a given climate goal is likely to be driven primarily by the degree to which (gross) CO 2 can be reduced as a means of reaching net-zero emissions, as well as the timing of net-zero CO 2 emissions (Prütz et al 2023).One study begins to address this by modifying an IAM to allow CDR demand to be considered separately from gross emissions reductions (Morrow et al 2023).However, as IAMs have expanded the portfolio of CDR options that they represent, the expanded CDR capacity has often resulted in scenarios with increased levels of gross emissions and/or delays in net emissions reductions (Stler et al 2018, Realmonte et al 2019, Fuhrman et al 2020, 2021).
The presumed availability of large-scale CDR has borne out in real-world policymaking: as nations have announced pledges to cut emissions to net-zero, their long-term strategies for doing so often allow for substantial levels of residual emissions that will require CDR to offset-averaging 18% of present-day emissions for Annex 1 (developed) countries (Buck et al 2023, Rogelj 2023).International organizations and trade groups, including for the most expensive-toabate sectors have announced plans to bring their industries in-line with deep mitigation efforts (Aspen Institute 2021, International Chamber of Shipping 2021, International Air Transport Association 2022, International Civil Aviation Organization 2022).Many of these envisage some level of CDR to compensate for expensive-to-abate emissions or those outside the company or industry's direct control (Joppa et al 2021, Trencher et al 2023).Many national and subnational decarbonization policies are also sector-specific and are in lieu of or in addition to CO 2 emissions pricing represented in most IAM scenario formulations (US Congress 2007, White House 2021, California Air Resources Board 2022, Maryland General Assembly 2022, 2022).Efforts to reduce non-CO 2 emissions in particular may come with disproportionate climate benefits, but these emissions will be extremely challenging to fully eliminate (Ou et al 2021).There are numerous studies and modeling exercises on deep emissions reductions pathways for individual sectors to reduce their GHG emissions (Kyle and Kim 2011, Davis et al 2018, Roe et al 2019, IEA 2020, Cao et al 2021, Levesque et al 2021, Luderer et al 2022).However, no IAM study to date has examined how comprehensive adoption of both behavioral changes and emerging technologies aimed at avoiding gross GHG emissions might contribute to a reduced role of CDR in deep mitigation, while evaluating a full portfolio of CDR representation.
To fill this gap, we used the Global Change Analysis Model (GCAM) to evaluate the extent to which maximal effort to reduce gross emissions from electricity, fuel supply, transportation, buildings, industry, and agriculture could minimize the requirement for CDR in reaching net-zero and eventually net-negative emissions (Calvin et al 2019).GCAM is a technology-rich IAM with detailed treatment of climate and global energy, land, and water systems (Bond-Lamberty et al 2022).GCAM was recently enhanced with the capability to model a diverse portfolio of CDR approaches including direct ocean capture with carbon storage (DOCCS), enhanced weathering, and biochar, in addition to its existing treatment of bioenergy with carbon capture and storage (BECCS), direct air capture with carbon storage (DACCS), and forest restoration (Fuhrman et al 2023).That study evaluated how reduced CDR demand could interact with an expanded CDR portfolio by applying a 'Sectoral Strengthening' scenario first developed for a 1.5 • C roadmap report (ClimateWorks Foundation 2020) and later used in Gambhir et al (2022).This scenario includes demand reductions through behavioral changes, higher energy and material efficiency, rapid electrification of transport, reductions in non-CO 2 GHGs, and restrictions on both bioenergy and geologic carbon storage.Here, we build upon that scenario to develop a 'Maximum Sectoral Effort' scenario that further limits residual emissions, to attempt to quantify a lower limit on offsetting CDR in Pariscompliant deep mitigation (table 1).This iterative scenario design process attempts to represent the full range of residual emissions reduction and carbon removal technologies that are being considered across sectors.

Methods
We used GCAM 6.0 and imposed a stylized constraint (Global Carbon Cap) on net CO 2 emissions that limits warming to below 1.5 • C in 2100.This constraint begins in 2025: the first of GCAM's 5 year timesteps after present-day.This scenario assumes population and GDP from SSP1, but otherwise uses standard GCAM assumptions for technology cost, performance, and demand for material goods and energy services (Bond-Lamberty et al 2022, JGCRI 2022).Model agents are generally free to choose a combination of emissions avoidance and CDR to meet the global CO 2 emissions constraint, except for a constraint on financial transfers for CDR equal to 1% of GDP from GCAM's standard release.Non-CO 2 emissions are not explicitly constrained but are abated using marginal abatement cost curves for each technology to estimate the amount of abatement that would occur at the realized carbon price each year (Ou et al 2021).Non-CO 2 radiative forcing effects are tracked by GCAM's embedded climate model Hector.Land-use CO 2 emissions are priced at an increasing, exogenously defined fraction (0.5 by 2100) of the CO 2 price required to meet the fossil fuel and industrial (FFI) CO 2 constraint.In the Global Carbon Cap scenario, carbon uptake via land-use changes (LUCs) may compensate for less ambitious reductions in FFI emissions.
In an effort to limit CDR for offsets, we identified the largest sources of residual GHG emissions from the 'sectoral strengthening' scenario in

Geologic carbon storage
• Global constraint based on IEA projections for operational, planned, and under construction plants until 2030, increasing gradually increasing to 10 GtCO2-yr −1 by 2100 after 2030.This also acts as a constraint on direct air capture and BECCS, which we do not constrain separately to reduce computational effort and avoid solution triviality.
our recent work (supplementary figure 1) (Fuhrman et al 2023).Given the already-high ambition of the demand reduction measures assumed for most sectors in this scenario, we focused the bulk of our efforts to further reduce CDR on representing elevated adoption of low-carbon technologies.These include zerocarbon shipping and aviation fuels, hydrogen combustion turbines for peaking power, point source carbon capture, electrification, accelerated compliance with the Kigali amendment to phase out hydrofluorocarbons, and more rapid retirement of unabated fossil electricity generation.A full description of the model capability developments and assumptions for this 'maximum sectoral effort' scenario is provided in the supplementary information.Under Maximum Sectoral Effort, land-use carbon removal achieved mostly via reforestation must be in addition to FFI mitigation under the constraint we imposed.Carbon storage via changes in land-use patterns is therefore incentivized by carbon pricing but may not substitute for emissions reductions and more durable CDR to meet the constraint placed on FFI CO 2 emissions.This effectively increases the stringency of the constraint, resulting in lower combined FFI and LUC emissions, but was done explicitly to address the reversibility concerns of forest planting for emissions offsets (West et al 2023).

Global and regional emissions and removals
Under a CO 2 emissions constraint (Global Carbon Cap) consistent with meeting the 1.5 • C temperature goal, CO 2 emissions reach net-zero by 2055, with about 12 GtCO 2 -yr −1 of CDR from dedicated technologies and net LUC offsetting an equivalent amount of gross-positive CO 2 emissions.Net GHG emissions are approximately 15 GtCO 2 e on a 100 year GWP basis in the year of net-zero CO 2 , and GHG emissions reach net-zero at the end of the century.In contrast, Maximum Sectoral Effort on gross emissions can enable net-zero CO 2 to be reached a decade earlier, in 2045 and net-zero GHG by 2060 (figure 1).The earlier net-zero CO 2 result is driven by more efficient agriculture practices and lower use of biomass that reduce pressure on the land system, along with policy design that requires reforestation efforts to be in addition to fossil and industrial CO 2 mitigation.This results in lower cumulative CO 2 emissions (Supplementary figures 4 and 5).Gross CO 2 emissions in the year of net-zero CO 2 are reduced by a factor of 2, to 6 GtCO 2 -yr −1 , and CDR from BECCS, DACCS, DOCCS, and enhanced weathering is reduced by a factor of 5-2 GtCO 2 -yr −1 , with approximately 4 GtCO 2 -yr −1 of net land-use removals offsetting the remaining fossil and industrial CO 2 emissions.Gross CO 2 emissions are nearly eliminated by the end of the century under Maximum Sectoral Effort.CDR deployment is sharply reduced in regions with large prospective geologic carbon storage capacities such as the United States and China, while remaining roughly constant or slightly increasing in Brazil and Western Africa, which have among the largest potentials for carbon storage via forest restoration.Nonetheless, the U.S. and China continue to play large, albeit reduced roles in CDR deployment, with the relatively low cost of reforestation allowing other regions to maintain or even slightly increase their contribution to lower overall CDR.Nearly all regions have net-negative land-use emissions at netzero CO 2 under Maximum Sectoral Effort (supplementary figure 9).

Sectoral transitions to net-zero CO 2 and GHG emissions
Reaching net-zero CO 2 under the Global Carbon Cap requires steep declines in GHG emissions from electricity (−82%), buildings (−63%), industry (−60%), energy supply (−45%), and transportation (−41%) relative to their 2015 levels (figure 2).Emissions associated with land-use, land-use change, and forestry (LULUCF) switch sign direction from +4 GtCO 2 eyr −1 in 2015 to −1 GtCO 2 e-yr −1 , whereas nonCO 2 emissions from agriculture increase from 6 GtCO 2 eyr −1 to 8 GtCO 2 e-yr −1 driven by increased population and GDP.Maximum Sectoral Effort towards net-zero CO 2 emissions could contribute a combined 10 GtCO 2 e-yr −1 additional gross emissions reduction, increase LULUCF removals by 2 GtCO 2 eyr −1 , and reduce CDR by 8 GtCO 2 -yr −1 for a net 4 GtCO 2 e-yr −1 GHG reduction.This additional abatement is driven primarily by the near-full elimination of CO 2 emissions from electricity generation and transportation, as well as by dietary shifts that reduce nonCO 2 emissions from agriculture.Further ambition to reach net-zero GHG is reached with an additional 2 GtCO 2 e-yr −1 abatement of mostly CO 2 from the industrial sector, additional 1 GtCO 2 e-yr −1 of negative GHG flux from land-use, and 2 GtCO 2 -yr −1 additional CDR.

Detailed drivers of residual GHG emissions at net-zero
While emissions from most sources are drastically reduced under a mid-century net-zero CO 2 emissions goal, several sectors increase their emissions relative to their 2015 levels in the Global Carbon Cap, driving additional CDR.
Under a Global Carbon Cap, unabated CO 2 emissions from fossil electricity generation are nearly fully eliminated.But uncaptured CO 2 from electricity generation with CCS technology installed and capture rates ranging from 80%-90% represents a new source (approximately 1 GtCO 2 e-yr −1 ) of gross GHG emissions, as do gas combustion turbines with no carbon capture installed that provide grid balancing under high deployments of intermittent renewables (1GtCO 2 e-yr −1 ) (figure 3).Further shifts away from fossil electricity (regardless of CO 2 capture) in favor of renewable generation lowers CO 2 emissions from CCS electricity by over 90% (900 MtCO2-yr-1).Improved grid management and the availability of hydrogen combustion turbines for peaking power reduces emissions associated with their gas-fired counterparts by 60% (600MtCO 2 -yr −1 ).
Under a Global Carbon Cap, nonCO 2 emissions (mostly CH 4 ) from meat, dairy, pork, and poultry also increase from their 2015 levels (supplementary figure 2).Dietary shifts in the Maximum Sectoral Effort scenario reduce demand for animal agriculture (and indirectly, the crops required to raise livestock), reducing emissions from these sources by over 4 GtCO 2 e-yr −1 beyond a CO 2 emissions cap alone.Full compliance with the Kigali Amendment in the Maximum Sectoral Effort reduces F-Gas emissions by an additional 0.5 GtCO 2 -yr −1 .Nonetheless, all netzero CO 2 and GHG scenarios have multiple GtCO 2 e of residual GHG emissions, even with ambitious demand reduction.While nonCO 2 emissions can be substantially decreased but not eliminated, residual CO 2 emissions are more amenable to ambitious but plausible efforts to drastically reduce emissions.
Full adoption of electric or hydrogen fuel cell vehicles in freight and passenger road transportation, and zero carbon fuels for shipping and aviation could eliminate approximately 3 GtCO 2 -yr −1 and 1.5 GtCO2-yr −1 , respectively of gross CO 2 emissions.Phase-out of fossil fuels in pulp and paper, food processing, and textiles (grouped as other industry in figure 3) could contribute another 0.5 GtCO 2 -yr −1 of residual emissions reductions at net-zero CO 2 .Defossilization of most sectors by mid-century has the added benefit of reducing CH 4 emissions associated with oil, gas, and coal extraction by nearly 60%.
The building heating and iron and steel sectors both have slightly higher emissions in the year of net-zero CO 2 under Maximum Sectoral Effort, due to the decade earlier achievement of this milestone.By 2055, when CO 2 emissions reach net-zero in the Global Carbon Cap scenario, emissions from both sectors are lower.To reach net-zero GHG by midcentury, the only sectors found to have increased emissions relative to net-zero CO 2 under a Global Carbon Cap are district heat and hydrogen production, both due to increased demand by other sectors as higher-emissions technologies are phased out.

Temperature and radiative forcing effects
Reductions in nonCO 2 emissions and increased landuse carbon storage have substantial impacts on radiative forcing and temperature trajectories (figure 4).The accelerated adoption of zero-emissions freight transportation and electricity generation are the largest contributors to reduced near-term forcing from black carbon and tropospheric ozone (O 3 ) of about 0.3 W m −2 by 2040, although this is offset somewhat by reductions in cooling effect from organic carbon emissions.In the longer-term, shifts towards plant-based diets and lower fossil fuel extraction lowers end-of-century radiative forcing from CH 4 by 0.25 W m −2 .The effective increase in stringency of the CO 2 emissions constraint by requiring land-use CO 2 mitigation to be in addition to fossil and industrial abatement results in an additional 0.25 W m −2 reduction in end-of-century forcing.Reductions in aerosol emissions combine with the GHG reductions for a net reduction of 0.37 W m −2 in 2050 and 0.75 W m −2 by 2100.Differences in radiative forcing contributions from Montreal Gases are negligible between the two scenarios because their phase-out has already been largely completed under the Montreal Protocol.A comparison of our temperature results to those in the AR6 1.5 • C scenario database illustrates that these radiative forcing reductions have substantial effects on both peak and endof-century temperature changes from pre-industrial levels.Peak warming is reduced by 0.1 • C and occurs 10 years sooner under Maximum Sectoral effort.Both scenarios temporarily overshoot the 1.5 • C warming goal, but Maximum Sectoral Effort changes the IPCC categorization from high to low overshoot.Maximum Sectoral Effort also reduces end-of-century warming by 0.4 • C, which is on the lower bound of 1.5 • C scenarios in the AR6.

Comparison of emissions to the 1.5 • C ensemble
While our net GHG emissions trajectories are well within the envelope of 1.5 • C scenarios, a comparison of gross emissions and CDR reveals important differences between our scenarios, and with those in the AR6 (figure 5).Under a Global Carbon Cap, both gross CDR and gross-positive emissions are near the upper end of the trajectories in the AR6, especially in late century.This is due in part to the recently expanded representation of CDR within GCAM allowing for increased residual emissions.Maximum Sectoral Effort can reduce gross GHG emissions towards the lower end of AR6 scenarios, while CDR is reduced to the middle of the AR6 distribution.The nearfull elimination of CO 2 emissions from both transportation and buildings by mid-century is consistent with the highest level of ambition for these sectors of any AR6 scenario.Near-term declines in electricity CO 2 emissions are again consistent with the scenarios exhibiting the most rapid decarbonization of this sector.However, rapid scale-up of BECCS and corresponding large net-negative emissions from the electricity sector in the global carbon cap scenario are in contrast with those of the Maximum Sectoral Effort scenario, where CO 2 emissions decline to zero but do not reach net-negative levels until late-century.Industrial CO 2 emissions decline more slowly in the near-term under Maximum Sectoral Effort due to lesser emphasis on BECCS, but eventually become net-negative with more limited BECCS deployment by the end of the century.Buildings also have slightly slower near-term emissions declines, due to additional abatement by other sectors with Maximum Sectoral Effort.AFOLU CO 2 emissions are also well within the range of AR6 scenarios.Under a Global Carbon Cap nonCO 2 GHG emissions generally follow the upper end of the AR6 range while

Discussion
Deep mitigation scenarios developed by the IAM community often project large amounts of CDR to offset gross emissions and achieve the net CO 2 and GHG reductions required to meet international climate goals.This is because IAM modeling structures almost universally treat emissions avoidance and removals on an equal basis at any given time, but also discount costs across time which may favor future removals when combined with high discount rates (Emmerling et al 2019).IAMs often use carbon pricing, which may stimulate different technologies compared to non-pricing policies (Sognnaes et al 2021).The assumed costs of technologies also play a role; still-nascent CDR technologies may prove to be much more costly (or emissions avoidance much less costly) than parametrized in the models.It is also difficult to price some policy changes, particularly behavioral or demand-side policies, or consider cobenefits (e.g.local air pollution, energy security, etc.)All these factors may lead to unrealistically large CDR projections which, when these projections are relied upon by decisionmakers, increasingly locks the world into large temperature overshoot and resultant costly physical climate risks (Drouet et al 2021).The continued relevance of deep decarbonization IAM scenarios will therefore depend on models more explicitly representing the complete range of proposed mitigation policy, technology, and behavioral levers.At the very least, models must recalibrate to the so-far sluggish deployment of CDR relative to projections.
To begin to address these issues, we developed a scenario consistent with very high levels of technological and behavioral ambition across sectors to reduce gross-along with net GHG emissions.We find that these efforts could enable net-zero CO 2 to be achieved while reducing the offsetting CDR requirement by a factor of 2. Additional land-use carbon removals enabled by more sustainable diets and lessened reliance on bioenergy, alongside policies that disallow land-based carbon storage to substitute for avoiding or permanently removing fossil and industrial emissions could also enable net-zero CO 2 to be achieved a decade earlier and allow for deeper levels of netnegative CO 2 emissions.This has the added benefit of hedging against reversibility risks of ecosystembased (e.g.forest preservation and restoration) offsets which remove CO 2 from the atmosphere, but not from the active carbon cycle (West et al 2023).Nonetheless, forest restoration and preservation in tropical regions such as Brazil and Western Africa remain critical, relatively low-cost components of overall climate mitigation efforts, as evidenced by the higher relative CDR contributions of these regions under maximum sectoral effort, even as their absolute CDR levels are maintained or even reduced.
Regions with large, estimated biomass and geologic CO 2 storage availability (e.g. the U.S. and China) sharply reduce both their CDR and gross emissions under a scenario that emphasizes reduced reliance on these resources for mitigation.This could become critical considering the uncertainty range of sustainable geologic CO 2 injection rates in many parts of the world including China. 42 2023).These efforts may also help build consensus on mitigation action amongst other countries, given the large contribution of these countries to cumulative emissions.
Our detailed analysis highlights a limited number of sectors that could determine the amount of CDR required to reach net-zero emissions, as well as the extent to which any given amount of CDR can contribute to long-term reductions in warming from cumulative emissions.Targeted technology adoption and behavioral changes for agriculture, industry and transportation could reduce gross emissions from these sources by over 7 GtCO 2 e-yr −1 at the year of net-zero CO 2 .We also identify several components of prospective low-carbon electricity grids (fossil CCS and load-following gas power plants) that could themselves become large sources of residual emissions.Efforts to accelerate the scale-up of renewable generation and consequently lower the use of fossil fuels in electricity generation could reduce up to 1 GtCO 2 -yr −1 of uncaptured emissions.Improved technologies including hydrogen turbines to balance intermittent wind and solar, coupled with grid management practices that lower reliance on combustion turbines for variable power altogether could provide an additional 0.5 GtCO 2 -yr −1 reduction in gross emissions.High spatiotemporal resolution modeling studies are needed to help realize the vast technological and management transformations required to ensure the reliability and resilience of a near-zerocarbon electricity system (Bennett et al 2019).
In addition to lowering reliance on CDR, the sharply decreased nonCO 2 emissions and enhanced land carbon sink under Maximum Sectoral Effort reduces both peak and long-term warming; both of which would correspondingly reduce climate impacts.We emphasize, however, that even with enhanced ambition on residual emissions, there will still be a need for multiple GtCO 2 -yr −1 of removals to achieve net-zero emissions.Additionally, the levels of near-term mitigation in this study diverge substantially from recent trends; higher levels of (net) CDR will likely prove necessary to make up for this divergence if the 1.5 • C goal is to still be met.
Because of the enormous uncertainty regarding the feasibility of both CDR and the levels of lowcarbon technology and behavioral changes under Maximum Sectoral Effort, it will be critical to begin investing in and scaling up both CDR and emissions avoidance together, and not delay action on one with the expectation that the other will itself enable the drastic emissions reductions required to come anywhere close to achieving the Paris goals.Clearly, CDR deployment now lags far behind what is projected in many deep mitigation scenarios, but so too does action on emissions avoidance, with many key indicators (e.g.ruminant meat consumption, air travel, and planned new unabated fossil electricity generation) moving in directions inconsistent with netzero CO 2 (let alone GHG) emissions (Ritchie et al 2017, Tong et al 2019, ICAO 2021, United Nations Environment Programme 2022).Additional CDR may prove necessary to reduce reliance on unproven technological pathways and unrealistic reversals in behavioral trends; both will have non-zero and potentially very high costs which future work can help quantify.However, prolonged high levels of warming will also bring enormous costs.Because there is no discontinuity in climate damages between 1.49 and 1.51 • C of warming, our results can help inform which sectors (and GHGs) can be most effectively targeted to reduce harm under economy-wide decarbonization.This is the ultimate objective of netzero emissions policies, regardless of when they are achieved.

Figure 1 .
Figure 1.GHG emissions and CDR under a Global Carbon Cap and Maximum Sectoral Effort 1.5 C scenarios.(a) The dashed lines represent net GHG emissions (i.e.including non-CO2 emissions) whereas the solid lines represent net CO emissions.100-year GWP equivalents are used for non-CO2 GHGs.gross CDR (including net land-use removals) by GCAM region in the year of net-zero CO2.(b) Total CDR by aggregated region at the year of net-zero CO2 (c).

Figure 2 .
Figure 2. Transition pathways from 2015 emissions to net-zero CO2 in a global carbon cap scenario (a), from net-zero CO2 in a global carbon cap scenario to net-zero CO2 with Maximum Sectoral Effort (b), and from net-zero CO2 with Maximum Sectoral Effort to net-zero GHG under Maximum Sectoral Effort (c).100-year GWPs from the IPCC's Sixth Assessment report are used to convert nonCO2 GHGs. 63Note the differences in y axis scales; the left-most bar on the lower and middle panels are equal to the right-most bar on the panel above.Grey shading in the middle indicates changes in GHG emissions between the absolute values in the left-most and right-most bars.

Figure 3 .
Figure 3. GHG emissions by source in the year of net-zero CO2 or GHG emissions for the Global Carbon cap and Maximum Sectoral Effort scenarios.100 year GWPs are used to convert non-CO2 emissions to CO2-equivalent basis.Dark grey bars indicate net decreases in GHG emissions relative to the top panel, while lighter grey negative bars indicate net increases in emissions.Gross emissions increases are reported as negative values.

Figure 4 .
Figure 4. (a) Changes in radiative forcing contribution by gas between the global carbon cap and max sectoral effort scenarios.OC = organic carbon, BC = black carbon, Montreal gases = chlorofluorocarbon (CFC) and hydrochlorofluorocarbon gases covered under the Montreal protocol.F-Gas = fluorinated gases exclusive of Montreal gases covered under the Kigali Amendment to the Montreal Protocol.(b) Global mean air temperature trajectories for the global carbon cap and max sectoral effort scenarios, compared to 1.5 • C scenarios in the AR6 database (bottom).Temperatures in the AR6 scenarios are calculated using the FAIR model; for comparability the Hector results apply an offset of +0.0338 • C to correct for different historical baselines (1850 for hector, 1850-1900 mean for FAIR).

Figure 5 .
Figure5.Gross GHG emissions (brown), CDR (teal), and net GHG (black) emissions of scenarios in this study (thick lines), compared to 1.5 • C scenarios from the AR6 database (thin lines) (a).CO2 emissions by sector for our scenarios, compared to 1.5 • C scenarios from the AR6 scenario database (b).GHG emissions for our scenarios compared to 1.5 • C scenarios from the AR6 scenario database (c).100-year GWP equivalents from the IPCC Sixth Assessment Report are used to convert non-CO2 GHG emissions to their CO2-equivalent basis.

Table 1 .
Model representation of Maximum Sectoral Effort to abate residual emissions.No new cement, fertilizer, or steel plant construction without CCS or hydrogen feedstock (fertilizer) after 2030 • Phase-out of fossil fuels for industries not requiring combustion-temperature process heat Biomass• Global constraint on first and second-generation bioenergy crops of 45 EJ by 2050, increasing to 70 EJ by 2100.
However, both countries are shown have among the largest capacities for still-substantial deployments of most types of CDR under Maximum Sectoral Effort.Cooperative research, development, and deployment efforts for CDR (e.g. the recent agreement between the two nations aiming to advance at least 5 large-scale carbon capture utilization and storage projects each by 2030) (U.S.Department of State 2023) could help reduce warming and concomitantly reduce climate impacts if successful (Galán-Martín et al 2021, Service