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.
If emissions of multiple greenhouse gases (carbon dioxide, methane, nitrous oxide, and hydrofluorocarbons) are causing the climate crisis, why does this primer focus only on removing CO2 from the atmosphere? The answer lies in the properties of greenhouse gases once they reach the atmosphere as well as their relative atmospheric concentration.
Under a common measure of cumulative long-term warming impacts, carbon dioxide is the most important greenhouse gas emitted by human activity (Edenhofer et al., 2014). This measure takes into account the total emission rate of the gas, as well as its atmospheric lifetime and ability to absorb incoming solar radiation (Myhre et al., 2013). Carbon dioxide is a very long-lived gas, with carbon cycle impacts that can last centuries to millennia (Archer et al., 2009). By contrast, other important greenhouse gases, commonly referred to as short-lived climate pollutants (SLCPs), have much shorter atmospheric lifetimes closer to 10 to 100 years. While the atmospheric concentration of CO2 may already seem low at around 410 parts per million (ppm), its concentration is significantly larger than the next-most-abundant greenhouse gas, methane, which is around 2 ppm (Saunois et al., 2020). The relative abundance of CO2, its long atmospheric lifetime, and its chemical reactivity make CO2 an appealing candidate for removal. Furthermore, the global carbon cycle flux of CO2 (its rate of movement between reservoirs) is substantially larger than that of any other gas, which allows for more biological, geological, and chemical CDR interventions to be explored.
To understand the relevance of CDR to climate change, it is necessary to put CDR in the context of the global carbon cycle (Keller et al., 2018). The carbon cycle concerns the amount and flux of carbon – in various chemical states – between the ocean, terrestrial biosphere (or “land”), atmosphere, and geologic formations in the Earth (Figure 1.2a; Friedlingstein et al., 2019). Large-scale CDR deployment will directly affect levels of atmospheric carbon, but also create feedback loops that alter fluxes among other carbon reservoirs. For this reason, removing 1 GtCO2 from the atmosphere will ultimately reduce atmospheric CO2 concentrations by less than 1 Gt. To understand how CDR perturbs the carbon cycle, we need to characterize its effects on fluxes between reservoirs as well as how carbon is stored in reservoirs. Moreover, even if net-zero emissions are achieved by the end of this century through the use of CDR to offset hard-to-avoid emissions, the particular emission and CDR pathways may leave long-lasting harmful imprints on parts of the global climate system, such as ocean acidity or ecosystem health (Mathesius et al., 2015).
The climate system’s slow timescale for removing CO2 from the atmosphere means that unless drawdown is enhanced, atmospheric CO2 concentration will continue to increase – even if only hard-to-avoid emissions continue and all other emissions are abated. Even if we stopped emitting today, a substantial portion of the anthropogenic CO2 would remain in the atmosphere over the coming century and beyond.
CDR strategies also carry the potential for serious harm to the environment, climate, and frontline communities. Harms can result from intentional behaviors, including fraudulent claims of CO2 sequestration to earn tax credits (Sylvan and Allen, 2020) and utilities knowingly neglecting infrastructure that causes catastrophic forest fires (Penn et al., 2019). Harms also can stem from inadvertent behaviors, ranging from accidentally releasing a toxic CO2 capture material to unknowingly misevaluating the economic or agricultural impacts of a proposed policy. While everything we choose to do as a society carries the potential for serious harms, thoughtful policies and frameworks can be put in place to try to foresee and account for both malice and error, and to prioritize equitable outcomes when people or future generations are harmed as a result of our collective action or inaction.
There is a significant range of capacity and permanence estimates across CDR approaches, which impact how that storage should be compared to other reservoirs and how to account for uncertainty (Table 1.4). The main distinction around permanence stems from the difference between sequestration reservoirs. Reservoir permanence ranges widely: Storage of CO2 in geologic formations deep underground can be near-permanent, while terrestrial carbon stocks can rapidly release carbon back into the atmosphere if management practices or external factors change. For example, forests are grown to sequester carbon in plant material over decades or hundreds of years, but a wildfire can return some portion of the carbon stored in plant tissues to the atmosphere as carbon dioxide in just days or weeks (Minx et al., 2018). Trees will grow back after a wildfire, but there is less carbon stored in the forest while they do. In contrast, when proper planning identifies reliable sequestration sites and establishes sound monitoring protocols, CO2 injected into geologic formations is unlikely to leak at significant scale over thousands of years. This difference in the sequestration timescale and the potential for rapid release highlights the importance of establishing robust frameworks to quantify, compare, and manage a portfolio of CDR approaches responsibly. The potential for intentionally malicious behavior, as well as simple mistakes, means that we will always need comprehensive planning, monitoring, and accountability frameworks.
With so many unknowns in how society will manage the economy, the climate system, and global emissions, the impact of the future trajectory of GHG levels on the climate is uncertain. Ultimately, it is preferable to prevent harms before they occur, rather than attempt to reverse them (Schneider, 2014). Over the long term, reducing greenhouse gas emissions whenever feasible will be economically and socially preferable to undertaking large-scale CDR.
An overview of the types of CDR approaches that have been developed or are being developed today. Together, they comprise a portfolio of approaches, or “building blocks,” for CDR systems.
As discussed in Chapter 2, achieving net-negative GHG emissions globally will require large-scale development of a portfolio of CDR systems. Additional considerations alongside carbon accounting will be required for each strategy, and many of these are fundamentally spatial: What land area can support a CDR system without competing with human activities (e.g., food production, settlements) and without disturbing natural habitats? Are construction materials available, and what do they cost? What are the social and environmental risks associated with each CDR system related to their location? Can the components of the CDR system be recycled or reused across deployments?
The primer's conclusion: Multiple CDR approaches are available in most regions of the world and depend on the availability of resources. For example, waste alkalinity sources from mine waste and industrial aggregate are produced in different volumes, depending on the region. Approaches with high TRL levels (as described in Chapter 2) are ready to be deployed immediately, and local regions can leverage their expertise and resources, ideally through global coordination, collaboration, and transparency.
Actual CDR deployment requires detailed local studies of the energy supply network and the sources of CO2 emissions. Some CDR approaches are energy-intensive and may therefore be limited in their efficacy. In all cases, carbon dioxide removal must be deployed in tandem with reducing emissions by switching to low-carbon energy sources or avoiding emissions by implementing point source capture. Also, biological CDR solutions have additional environmental co-benefits compared to technological approaches, but technological approaches may result in more durable storage and remove more CO2 per land area.
To assess the potential of CDR technologies and systems, we must first define what constitutes negative emissions and how we evaluate the system boundaries, cost, and net carbon balance of such systems. There are four main criteria that qualify systems as carbon dioxide removal (Tanzer and Ramírez, 2019).
A CDR system must meet these four criteria.