In these concluding remarks, we offer a reprise of the motivation for creating this primer: A new sense of urgency is required. 

We as a global community must act now to navigate away from potentially catastrophic climate change. Global warming poses significant risks to natural and human systems, and it disproportionately threatens the food security, health, safety, and even the very future of climate-vulnerable populations. Already, communities around the world are being displaced due to flooding, landslides, wildfires, and other threats related to climate change. Increased warming will bring even greater risks. The only way to stop warming is to reach net-zero global emissions. The Earth’s temperature has already increased by 1° C, so there is no time to waste.

Getting to net-zero global emissions requires steep reductions in greenhouse gas emissions, but many critical sectors are unlikely to get to zero emissions on their own, even with a massive global effort to cut climate pollution. Carbon dioxide removal (CDR) will be needed to offset emissions from these sectors, which include agriculture, transportation, and certain industrial processes. The scale of such emissions requires a massive investment in CDR, likely on the order of gigatonnes of CO2 removed per year by mid-century. Even larger amounts would have to be removed to draw down atmospheric concentrations from their peak after we reach net-zero global emissions. 

This primer aims to explain what we must do to meet this need, and it offers tools for analyzing and conceptualizing proposed CDR strategies. High-quality CDR will require an extremely large amount of land, energy, and resources and thoughtful, long-term stewardship practices; claims to the contrary are not grappling with the actual size and time scale of the problem. For this reason, one of this primer’s principles is that decision-makers should consider all options for CDR approaches and rely on clear scientific metrics and principles, rather than on fleeting assessments of cost or current ease of deployment. This principle also applies to public policymaking: The widest range of options should be open for consideration, from government subsidies to public ownership to participatory decision-making.

A portfolio of CDR solutions is required to address climate change. CDR approaches present opportunities and challenges that vary over a range of spatial and temporal scales. For example, direct air capture that is coupled with geological storage is most feasible in locations whose geology ensures permanent storage; waste alkalinity sources (e.g., steel slag, mine tailings) for carbon mineralization are not uniformly distributed; and opportunities for CDR through forests and soil have complex interactions with land management practices that limit the long-term durability of carbon storage. Full consideration of different approaches’ costs and benefits, as well as deep engagement with communities and stakeholders in broader climate policy discussions, will be required to plan successful  CDR deployment portfolios. 

Rigorous and transparent data and metrics are critical for evaluating the quality, potential, and risks of CDR approaches. Quantitative assessment methods such as life cycle analysis (LCA) and techno-economic assessment (TEA) are essential, but much more work remains to adapt these tools to the broad range of CDR solutions and the granular decisions required for specific deployments. As CDR scales up, robust analysis and verification will be required to ensure safe and responsible deployment, and to hold accountable those who might claim credit or seek to financially benefit from it. Verification will be especially challenging in the land sector, as many land-based CDR approaches involve changing existing practices, and assessing the impact thus relies on comparison to a counterfactual scenario that can only be estimated, not observed. Complete transparency in all aspects of analysis will be critical, both to ensure accountability and to help the field evolve; science moves fastest when practiced in the open and in collaboration. We must also recognize the limits of quantitative and scientific analysis in capturing the social, ethical, and cultural impacts of deployed technologies and approaches. 

Shifting to a new and sustainable future will require enormous effort. Just as CDR success depends on a portfolio of different approaches, progress will require an interdisciplinary constituency of CDR champions who are committed to the scale of the problem and the need for scientifically rigorous, just, and meaningful action. Scientists from a variety of fields (including chemical engineering, agronomy, ocean biochemistry, physics, chemistry, geology, and a broad range of social sciences) are important to the development and deployment of CDR approaches. Local and national governments, funders, businesses, and communities around the world will all need to participate in the transition to a cleaner economy. Scientists, nonprofit organizations, journalists, and the public at large will also need to hold all involved accountable. Educating ourselves about such issues can help ingrain this understanding into our global consciousness. Education broadly will aid, assist, and encourage present and future stakeholders to participate in CDR. A student studying environmental policy, a CEO instituting corporate transformation, a scientist exploring at the current edge of chemistry –all can make a substantial difference. 

For gigatonne-scale CDR deployment to result in equitable outcomes, it must be centered on social justice and pursued through participatory decision-making involving a wide range of stakeholders. The land, energy, and resource trade-offs involved in large-scale CDR are extremely complex, and they will challenge decision-makers to balance social, economic, environmental, and technological considerations. In addition to technical and environmental assessments, extensive social science research is needed to understand and guide public acceptance and to establish ethical values associated with CDR to help evaluate associated trade-offs. Moreover, CDR presents a significant potential for moral hazard if slowing the pace or extent of greenhouse gas emission reductions today is justified on the promise of carbon removal tomorrow. Such an argument, which bolsters extractive and fossil fuel industry interests, must be rejected so that decarbonization is prioritized as CDR is pursued in parallel. Considering an expanded set of policy options increases the likelihood that CDR is implemented ethically and not co-opted for private gain. CDR poses a daunting challenge, but also offers immense opportunity. If done right, ethical deployment of CDR could help reduce global inequality and poverty, benefitting frontline and marginalized communities by establishing responsible CDR stewardship at the local level and by playing a key role in helping end the climate crisis.

This primer offers an introduction and education for the next generation of CDR champions. Everyone must find their own way to fight the climate crisis and leverage their unique skills toward this daunting challenge. We hope this primer provides a starting point for learning, inspiration, justice, and action.