Limiting global warming to 1.5 °C above pre-industrial levels requires removing up to ~1000 Gtonne CO₂ from the atmosphere by 2100. However, the benefits of deploying Negative Emissions Technologies (NETs) at a large scale against their potential damaging effects on humans and the planet remain unclear. Previous studies on Direct Air Carbon Capture and Storage (DACCS) and Bioenergy with Carbon Capture and Storage (BECCS) primarily focused on their costs and CO₂ removal potentials, often overlooking side-effects and co-benefits beyond global warming. While some studies quantified environmental impacts, their interpretation from an absolute sustainability viewpoint is difficult. A comprehensive analysis encompassing both human and planetary health implications of DACCS and BECCS is lacking. This study addresses this knowledge gap by quantifying the human health impacts (in Disability-Adjusted Life Years or DALYs) and planetary footprint (on seven key Earth-system processes) of DACCS and BECCS. The research is crucial to understand the co-benefits of CDR and minimize potential collateral damage from climate change mitigation, particularly given frequent environmental trade-offs in the energy sector, which is deeply intertwined with NETs.

Prior research on DACCS and BECCS predominantly concentrated on cost-benefit analyses and CO2 removal potential, neglecting the broader environmental and human health consequences. Studies that did assess environmental impacts often lacked a clear framework for evaluating absolute sustainability. The current research builds upon this limited body of work by integrating human health and planetary boundaries assessments into a comprehensive evaluation of NETs' lifecycle impacts.

The study employed Life Cycle Assessment (LCA) methodology to quantify the impacts of DACCS and BECCS on human health and the Earth system. The functional unit was the net removal of 5.9 Gtonne/year CO₂, aligning with the average CDR rate in the SSP2-1.9 climate change mitigation scenario (excluding CDR in agriculture, forestry, and other land-use sectors). Sixteen scenarios were modeled, varying in NET type, energy/biomass sources, and CO₂ storage configurations (ten DACCS, four BECCS, two hybrid BEDACCS). These were compared to a baseline without NETs. For DACCS, High-Temperature Liquid Sorbent (HTLS) and Low-Temperature Solid Sorbent (LTSS) technologies were evaluated with various energy sources (geothermal, onshore wind, solar PV, nuclear, natural gas with CCS, or the global electricity mix). BECCS scenarios used biomass combustion (Miscanthus or poplar) to generate electricity, replacing the global electricity mix. CO₂ storage options included geological sequestration, in situ mineral carbonation (freshwater or seawater), and ex situ mineral carbonation. Human health impacts were quantified using DALYs, considering both prevented and additional health risks (e.g., from pollutant emissions, water consumption) using the ReCiPe 2016 method. Climate-sensitive health impacts were assessed using spatially differentiated damage factors from the WHO and Tang et al. Planetary impacts were assessed across seven Earth-system processes (climate change, ocean acidification, terrestrial biosphere integrity, biogeochemical flows, freshwater use, stratospheric ozone depletion, and land-system change) relative to the Planetary Boundaries framework and associated Safe Operating Space (SOS). The study used SimaPro 9.2 software and data from the Ecoinvent 3.5 database. The attributional modeling approach was used, and the system boundary expansion method accounted for the multi-functionality of BECCS. Health externalities were monetized using a conversion factor from Weidema.

The study found that removing 5.9 Gtonne/year CO₂ through NETs could prevent a substantial health burden, equivalent to that of Parkinson's disease, with Africa and Asia benefitting most. The health co-benefits of CDR outweighed adverse effects in most scenarios, leading to net health gains. BECCS scenarios generally showed larger health benefits than DACCS scenarios, but this varied depending on the biomass source (Miscanthus generally outperformed poplar). Within DACCS scenarios, HTLS-DACCS powered by wind and nuclear energy performed best. Ex situ mineralization was the most damaging CO₂ storage option in terms of human health, while in situ mineralization with seawater minimized impacts. Fine particulate matter formation was the main driver of regional health effects in most scenarios, while freshwater usage for irrigation dominated in poplar-based BECCS scenarios. The health externalities of the assessed NETs ranged from 35 to 148 US$/tonne CO₂, comparable to their levelized costs. Regarding planetary boundaries, DACCS, especially with renewable energy, demonstrated lower environmental impacts than BECCS. While both NETs could avert climate change and ocean acidification impacts, BECCS exerted considerable pressure on the terrestrial biosphere, nitrogen biogeochemical flows, and freshwater use. DACCS showed environmental superiority due to its lower planetary impact and ability to avoid adverse biosphere effects. A regionalized analysis of climate-sensitive health impacts revealed substantial disparities, with Sub-Saharan Africa benefiting disproportionately due to averted risks of undernutrition and malaria. In relative terms, Sub-Saharan Africa received the most benefits per million inhabitants, while North America, Europe, and Russia benefited the least.

The findings highlight the significant health and planetary co-benefits of employing NETs, especially DACCS, for climate change mitigation. The substantial economic benefits revealed by monetizing the avoided health impacts could incentivize the adoption of these technologies. While prioritizing emissions reduction remains crucial, the results suggest that DACCS, with its limited side effects, could be a valuable tool for addressing historical emissions and hard-to-abate sectors. The uneven distribution of health co-benefits underscores the need for regional assessments and international cooperation to optimize NET deployment and minimize collateral damage. The study's limitations, such as the lack of regionalized analysis for non-climate health impacts and the exclusion of infrastructure impacts, should be considered when interpreting the results. Future research could focus on regional-scale analyses incorporating infrastructure effects and a broader range of SDGs, furthering the understanding of sustainable NET deployment.

This study provides quantitative evidence of the significant co-benefits of negative emissions technologies, particularly DACCS, for both human and planetary health. The substantial health and economic benefits, coupled with DACCS's comparatively lower environmental impact, position it as a promising tool in climate change mitigation. However, the uneven distribution of benefits across regions highlights the need for targeted strategies and international collaboration to ensure equitable and sustainable outcomes. Future research should focus on refining regional assessments, incorporating a wider range of indicators, and exploring strategies for optimizing the deployment of a portfolio of NETs.

The study's analysis omits impacts related to the infrastructure of NETs due to data limitations, although previous research suggests these contributions might be minor. The lack of spatially differentiated characterization factors for some stressors limits the detailed regional analysis of global health impacts (stratospheric ozone depletion, ionizing radiation). The modeling approach uses a generic, non-geographically differentiated dataset, while the actual performance of NETs is location-dependent. The models also exclude impacts related to the freshwater and marine biosphere and do not address potential unforeseen issues of NET deployment at a large scale.