Jay Fuhrman

Meeting climate pledges will require gigaton-scale carbon dioxide removal (CDR). Enhanced rock weathering and biochar provide substantial opportunities beyond conventional land-based CDR, particularly in emerging economies.

Concerns about mitigation deterrence have prompted calls for pathways that avoid multigigatonne reliance on carbon dioxide removal (CDR), yet such pathways can also discourage near-term investment in CDR, leaving the technologies technically and economically underprepared if large-scale deployment becomes necessary due to unfulfilled emission reduction pledges. Here we model a new pathway in which CDR and decarbonization “Co-Scale” aggressively in parallel without one undermining the other, with the intention that we can readily course-correct if effort in one domain does not materialize. We treat this scenario as a stylized upper bound of what is climatically achievable under ideal enabling conditions, rather than what is immediately feasible under current technical, economic, and environmental constraints. We compare this pathway with two conventional designs, i.e., CDR-Led and Decarb-Led. In CDR-Led, large-scale CDR can substitute for deep decarbonization, while in Decarb-Led, rapid emissions reductions are prioritized and CDR plays a limited complementary role. We find that compared to these two conventional scenarios, Co-Scale reaches net zero CO2 seven years earlier, accumulates 4 times more net negative CO2 by 2100, cuts the 1.5 °C overshoot duration by roughly half, and limits end-of-century warming to 1 °C rather than 1.37–1.39 °C. Relative to the recent focus on Decarb-Led pathways, Co-Scale’s main constraints is the feasibility of geological carbon storage, while food, water, and cost pressures are comparatively less restrictive.

Economy-wide efforts to achieve net-zero emissions offer climate and air quality-related public health benefits from reducing fossil fuel combustion. However, carbon dioxide removal (CDR) may be necessary to meet emissions targets cost-effectively, and relying on CDR would forego some air-quality benefits. Here, we systematically quantify the regional air quality and public health implications of six CDR portfolios for the U.S. using a coupled modeling approach and compare those to a no U.S. climate action scenario. While both high- and low-CDR deployment avoid about $2.5-5.8 trillion USD2020 (or 0.4-0.8% of cumulative GDP (CGDP)) in climate damages, the high-CDR pathway costs $11-13 trillion USD2020 (or 1.8-1.9% of CGDP) by 2050, whereas the low-CDR pathway costs $16-20 trillion USD2020 (or 2.6-2.9% of CGDP) due to deeper near-term fossil fuel reductions. Public health benefits reach $2.8-6.5 trillion USD2020 (or 0.5-0.9% of CGDP) under high-CDR and are $3.5-8 trillion USD2020 (0.6-1.2% of CGDP) under low-CDR, reflecting greater reductions in particulate matter and ozone exposure and preventing approximately 12,600 additional premature deaths by mid-century. However, heavy reliance on CDR technologies could generate $5-6 trillion USD2020 (∼0.8-0.9% of CGDP) in CDR revenues by 2050, exemplifying the trade-offs between public health, economy, and climate.

Previous studies on the use of hydrogen (H ₂ ) in the future energy system typically find a limited role for the energy carrier; however, these studies have not accounted for geologic H ₂ (gH ₂ ), which by some estimates may be abundantly available at low cost. Consequently, hydrogen’s role in the energy system could shift from a minor contributor, deploying mostly in difficult-to-decarbonize sectors, to a much more prominent element of the global energy system. In this paper we examine the role of gH ₂ for global energy and industrial systems with different scenarios of future radiative forcing. We find that if favorable conditions exist, where gH ₂ is abundant and inexpensive ($1/kg), it could lead to expanded hydrogen deployment in a wide variety of energy applications, with power generation the most notable. However, deployment of gH ₂ is also found to be sensitive to its production cost, with its use declining rapidly as its production cost approaches the cost of manufactured hydrogen.

CO₂ capture and storage (CCS) in geological reservoirs is expected to play a large role in low-emissions scenarios from multi-sector human Earth system models. Yet these scenarios have often projected near-term CCS deployments that far exceed what is currently planned, let alone operational. They have also failed to consider regional differences in capacity to deploy large-scale CO₂ capture, transport, and subsurface injection. Here, we update a leading integrated energy-economy-land model by recalibrating maximum deployments to publicly announced CCS projects through to 2030. We also quantify a range of regionally explicit future scaling and maximum injection rates for the overall CCS value chain and evaluate their implications for emissions trajectories, energy mix, use of rate-limited storage capacity, and mitigation costs. Under limited CCS growth rates, deployment at mid-century and 2100 could be reduced by a factor of 7 relative to a scenario that does not consider injectivity or growth rate limits. However, sustained efforts to rapidly scale CCS could reduce transition costs by nearly $11 trillion (20%) globally, with cost reductions most heavily concentrated in regions such as China and India. Delayed mitigation combined with slower-than-expected CCS deployment could result in large and prolonged temperature overshoot. Conversely, there are lower peak and long-term temperatures with aggressive emissions cuts in anticipation of slow CCS scaling that subsequently far exceeds expectations.

Scenarios to stabilize global climate and meet international climate agreements require rapid reductions in human carbon dioxide (CO ₂ ) emissions, often augmented by substantial carbon dioxide removal (CDR) from the atmosphere. While some ocean-based removal techniques show potential promise as part of a broader CDR and decarbonization portfolio, no marine approach is ready yet for deployment at scale because of gaps in both scientific and engineering knowledge. Marine CDR spans a wide range of biotic and abiotic methods, with both common and technique-specific limitations. Further targeted research is needed on CDR efficacy, permanence, and additionality as well as on robust validation methods—measurement, monitoring, reporting, and verification—that are essential to demonstrate the safe removal and long-term storage of CO ₂ . Engineering studies are needed on constraints including scalability, costs, resource inputs, energy demands, and technical readiness. Research on possible co-benefits, ocean acidification effects, environmental and social impacts, and governance is also required.

This study explores a new approach in modeling explicit targets for decarbonization of the electricity and transport sectors combined with separate targets for carbon dioxide removal (CDR) based on the current plans and strategies put forward or expected by countries. Additionally, we examine an equitable "fair share" scenario that aligns sectoral decarbonization timelines and CDR liabilities with the respective capabilities of countries. In this "capability" burden-sharing principle, developed countries with the financial means to support their climate change mitigation efforts undertake faster energy transition while developing countries with lower incomes are allowed an extended timeline for decarbonization and are exempted from excessive CDR obligations. Here, we modify a technology-rich multi-sector model in a manner where explicit sectoral emission reduction and CDR targets can be modeled. Our analysis reveals that adopting this sector-specific strategy shows a potential reduction in residual emissions by up to 35%, and a 35-45% decrease in carbon removal requirement. Furthermore, sector-specific decarbonization and carbon removal targets not only help mitigate the adverse impacts of climate change but also promote sustainability by supporting food security and reducing the global demand for water, land, and fertilizer necessary for energy production and negative emissions.