Non-staff authors: Jay Fuhrman, Haewon McJeon, Pralit Patel, Scott C. Doney, Andres F. Clarens
During the 2015 UNFCCC Conference of the Parties in Paris, world leaders agreed to limit global temperature increase relative to pre-industrial levels to well below 2°C and pursue efforts to meet a 1.5°C target by 2100. These targets require rapid declines in greenhouse gas emissions, reaching net zero by mid-century. Recent progress on mitigation has been highly inconsistent with this goal. With emissions still rising, integrated assessment modelling (IAM) scenarios of the global economy and climate system have increasingly relied on the presumed ability to deploy net-negative emissions activities to meet these ambitious climate targets. There are a number of ways by which to remove already emitted CO2 from the atmosphere. Yet the vast majority of IAM scenarios include just two land-based negative emissions technologies (NETs): bioenergy with carbon capture and storage (BECCS) and afforestation. The degree to which these NETs would compete for productive agricultural and natural land, as well as their impact on water resources if deployed at climatically relevant (that is, GtCO2/yr) scales has raised concerns about the viability of these approaches.
In light of the foreseeable tradeoffs inherent to land-based negative emissions approaches, recent work has focused on developing direct air capture (DAC) technology. DAC is an engineered separation process that uses aqueous or amine sorbents to remove CO2 from ambient air, compress it and inject it into geologic reservoirs. The physical footprint of these units would be much smaller than BECCS or afforestation, and it would not require any particular land type, only proximity to a geologic reservoir for storage. However, CO2 exists in low concentrations in ambient air, so DAC is likely to be energy intensive to deploy. This is intuitively the case for DAC processes that require combustion heat, for which fossil fuels are currently the most economical source. However, processes that are capable of using renewable energy or waste heat would still entail large-scale construction of infrastructure (for example, solar photovoltaic) for the purpose of disposing of CO2 emitted previously. Due to these very high assumed costs, DAC has not been included in many integrated modelling scenarios to date. However, multiple companies now have commercial-scale prototypes, claiming much lower costs than previously estimated, and several recent IAM studies have incorporated DAC into their mitigation and negative emissions portfolios. In these deep decarbonization scenarios, the availability of DAC can reduce mitigation costs, avoid immediate stranding of fossil fuel assets and benefit energy-exporting countries by preserving the value of their fossil fuel reserves under stringent climate policies. Meeting a 1.5°C temperature target may now only be possible if large-scale DAC is available. Relying on the future availability of DAC and then failing to achieve the rapid scale-ups to global-scale deployment could risk overshooting this target by up to 0.8°C.
Increased near-term mitigation effort is required to avoid the steepest tradeoffs associated with future rapid decarbonization, and to avoid ‘lock-in’ to large-scale deployment of NETs to meet the Paris targets. But the emergence of DAC as a possible climate mitigation strategy makes it important to gain understanding of its side effects if deployed at GtCO2/yr scales, weighed against its potential to reduce some of the undesirable impacts of BECCS and afforestation (for example, land and water demand) and to offset emissions from expensive-to-mitigate sectors (for example, liquid fuels for transportation). The unprecedented financial transfers (for example, emissions offsets and direct public subsidies) that would be required to reach net-negative emissions globally make it even more critical to understand these potential side effects in advance, and minimize the extent to which the deployment of any NET generates unintended consequences of its own. Previous work on the potential benefits and side effects of DAC has emphasized its ability to reduce energy system transition burdens (for example, CO2 prices), while itself requiring large amounts of energy. It has been shown that DAC would substantially reduce water use for negative emissions compared with total evapotranspiration from bioenergy crop and forest cultivation, plus additional water demand for bioelectricity generation. However, it is also important to understand how different NETs could affect water quality (for example, through thermal and chemical pollution) associated with withdrawals from surface and groundwater, as well as consumption (that is, evaporative losses) that contribute to water scarcity. Proper contextualization of each of these relative to other current and projected anthropogenic perturbations to water resources is also imperative to best inform policymakers and other stakeholders considering multiple environmental objectives (for example, water conservation and climate mitigation). The land-use impacts of DAC are considered negligible compared with BECCS and afforestation, but detailed quantitative assessment of the implications for global agriculture systems (for example, food prices) is largely missing from the IAM literature on DAC and other NETs. In particular, spatially disaggregated results for where different NETs might be deployed under different policies and assessments of the associated impacts on food, water and energy systems are needed to better inform equity considerations of international policymaking.
Here we use the Global Change Assessment Model (GCAM), a technology-rich IAM with detailed treatment of the energy, water and land sectors, to evaluate the impacts and tradeoffs of a portfolio of three distinct types of NET (afforestation, BECCS and DAC) in meeting two representative emissions pathways from the IPCC Special Report on Global Warming of 1.5°C. We investigated whether DAC could help ameliorate costly food–water–energy tradeoffs when deployed alongside BECCS, afforestation and other technology options for avoiding CO2 emissions altogether (for example, renewables and point-source CCS). In light of recent, more optimistic estimates for the cost of DAC technology, we investigate when this technology could begin to play a role in the mitigation portfolio under aggressive near-term decarbonization policy that seeks to limit the overdraft of a small and rapidly dwindling 1.5°C global emissions budget. Additionally, the side effects associated with increased negative emissions requirements resulting from delayed mitigation ambition for meeting the same end-of-century temperature goal are quantified. Finally, we provide greater resolution as to where DAC and other negative emissions activities and associated side effects could take place spatially, at the scale of geopolitical regions. Throughout our analysis, we compare land, water and energy use for each of these NETs with other current-day and projected anthropogenic perturbations to these resources.