Andres F. Clarens

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.

Deterministic energy transition planning risks uninformed decisions. Yet, the challenge of high-dimensional uncertainty–encompassing various technological, economic, social, and climatic factors–often leads to a deterministic treatment or simplification of uncertainties in planning. Here, we propose a computationally efficient framework that leverages surrogate-based sensitivity analysis to identify the key uncertainty sources driving the cost of different energy transition scenarios. We applied the proposed approach to Puerto Rico as a hurricane-prone power system that lacks efficient management. We find that changes in the frequency of hurricanes and organizational inefficiency are the two primary sources of uncertainty determining the system’s total expected cost. When examining operational costs, different transition scenarios demonstrate unique key uncertainty sources. For example, the price of biofuel would mainly drive the operational cost when transitioning to a fully renewable power system. These findings can help planners by allowing them to focus on a narrower set of uncertainties in planning.

To achieve net zero carbon emissions by mid-century, the United States may need to rely on carbon dioxide removal (CDR) to offset emissions from difficult-to-decarbonize sectors and/or shortfalls in near-term mitigation efforts. CDR can be delivered using many approaches with different requirements for land, water, geologic carbon storage capacity, energy, and other resources. The availability of these resources varies by region in the U.S. suggesting that CDR deployment will be uneven across the country. Using the global change analysis model for the United States (GCAM-USA), we modeled six classes of CDR and explored their potential using four scenarios: a scenario where all the CDR pathways are available (Full Portfolio), a scenario with restricted carbon capture and storage (Low CCS), a scenario where the availability of bio-based CDR options is limited (Low Bio), and a scenario with constraints on enhanced rock weathering (ERW) capabilities (Low ERW). We find that by employing a diverse set of CDR approaches, the U.S. could remove between 1 and 1.9 GtCO₂/yr by midcentury. In the Full Portfolio scenario, direct air carbon capture and storage (DACCS) predominates, delivering approximately 50% of CO₂ removal, with bioenergy with carbon capture and storage contributing 25%, and ERW delivering 11.5%. Texas and the agricultural Midwest lead in CDR deployment due to their abundant agricultural land and geological storage availability. In the Low CCS scenario, reliance on DACCS decreases, easing pressure on energy systems but increasing pressure on the land. In all cases CDR deployment was found to drive important impacts on energy, land, or materials supply chains (to supply ERW, for example) and these effects were generally more pronounced when fewer CDR technologies were available.

The chemical industry is a major and growing source of CO₂ emissions. Here, we extend the principal U.S.-based integrated assessment model, GCAM, to include a representation of steam cracking, the dominant process in the organic chemical industry today, and a suite of emerging decarbonization strategies, including catalytic cracking, lower-carbon process heat, and feedstock switching. We find that emerging catalytic production technologies only have a small impact on midcentury emissions mitigation. In contrast, process heat generation could achieve strong mitigation, reducing associated CO₂ emissions by ∼76% by 2050. Process heat generation is diversified to include carbon capture and storage (CCS), hydrogen, and electrification. A sensitivity analysis reveals that our results for future net CO₂ emissions are most sensitive to the amount of CCS deployed globally. The system as defined cannot reach net-zero emissions if the share of incineration increases as projected without coupling incineration with CCS. Less organic chemicals are produced in a net-zero CO₂ future than those in a no-policy scenario. Mitigation of feedstock emissions relies heavily on biogenic carbon used as an alternative feedstock and waste treatment of plastics. The only scenario that delivers net-negative CO₂ emissions from the organic chemical sector (by 2070) combines greater use of biogenic feedstocks with a continued reliance on landfilling of waste plastic, versus recycling or incineration, which has trade-offs.

: Net-zero greenhouse gas emission targets are central to current international efforts to stabilize global climate, and many of these plans rely on carbon dioxide removal (CDR) to meet mid-century goals. CDR can be performed via nature-based approaches, such as afforestation, or engineered approaches, such as direct air capture. Both will have large impacts in the regions where they are sited. We used the Global Change Analysis Model for the United States to analyze how regional resources will influence and be influenced by CDR deployment in service of United States national net-zero targets. Our modeling suggests that CDR will be deployed extensively, but unevenly, across the country. A number of US states have the resources, such as geologic carbon storage capacity and agricultural land, needed to become net exporters of negative emissions. But this will require reallocation of resources, such as natural gas and electricity, and dramatically increase water and fertilizer use in many places. Modeling these kinds of regional or sub-national impacts associated with CDR, as intrinsically uncertain as it is at this time, is critical for understanding its true potential in meeting decarbonization commitments.