In discussions about the net-zero energy transition, many sectors are often labelled “hard-to-abate”—but what does that really mean? The term has no agreed definition, leading it to be applied loosely and potentially be used as an excuse to delay decarbonization efforts. We propose a science-based definition: hard-to-abate emissions are those for which direct air carbon capture and storage is the most cost- and energy-efficient route to achieve net zero.
As the world pushes toward net-zero energy systems, the term “hard-to-abate” often appears in policy discussions [1], corporate roadmaps [2], and energy transition strategies [3]. Yet, despite its frequent use, this term lacks a scientific definition. Instead, the term “hard-to-abate” appears in the scientific literature and in the media, followed by a list of examples of what sectors can be considered “hard-to-abate”: aviation, cement, steel, and shipping, among others. However, these examples vary substantially depending on the source (see Figure 1), creating confusion about what “hard-to-abate” truly means and, more importantly, how we should act on it.
This lack of clarity is more than a semantic problem. By allowing “hard-to-abate” to remain vaguely defined, we risk using it as a justification to delay critical decarbonization efforts. If emissions are deemed (too) “hard” to reduce, it can sound reasonable to postpone action while we search for a technological breakthrough. But to reach net-zero, we need to deal with all emissions.
Towards a science-based definition
To avoid unjustified mitigation delays, we need a robust, science-based definition of “hard-to-abate.” In our recent commentary, we propose a clear and actionable definition:
Emissions are “hard-to-abate” if these greenhouse gas emissions can be brought down to net-zero more cost- and energy-efficiently using direct air carbon capture and storage (DACCS) than through any other decarbonization method (see Figure 2). As these emissions are likely to be addressed close to the time of net-zero pledges, cost and energy of DACCS and of the other decarbonization methods should be projected to the time of net-zero pledges.
Why use DACCS as a benchmark?
Delaying climate action today increases our future dependence on carbon dioxide removal (CDR) to reach net-zero targets. CDR encompasses a range of methods that extract CO₂ from the atmosphere and permanently store it. CDR methods include natural approaches like afforestation and soil carbon sequestration, as well as technological solutions such as bioenergy with carbon capture and storage (BECCS) and direct air carbon capture and storage (DACCS) [4]. However, many CDR methods face significant limitations: trees can burn and release stored CO₂, soil carbon is difficult to measure, report and verify, and BECCS requires large amounts of land and water, making it hard to scale sustainably.
In contrast, DACCS offers durable CO₂ storage, scalable deployment, and reliable MRV [4, 5]. For these reasons, we use DACCS as the benchmark when defining what qualifies as “hard-to-abate.” The limitations of DACCS are its high cost and high energy demand, which make cost and energy the natural criteria for defining what hard-to-abate emissions should be.
Using DACCS as a benchmark and evaluating technologies based on energy and cost provides a concrete basis for understanding what is meant by “hard-to-abate”. We advocate explicitly stating what makes an emission “hard-to-abate”: if it is the cost, an emission can be characterized as economically hard-to-abate; if it is the energy, it can be characterized as energetically hard-to-abate. Indeed, these two criteria come with different implications: the cost criterion is related to profits for companies or adoption by consumers. Whereas the energy criterion is more relevant for policymakers who need to plan for a transition to net-zero energy systems.
To stay objective, our definition intentionally leaves out political and social aspects such as willingness to pay for decarbonization, social acceptance, or employment impacts. However, when deciding which emissions to reduce first, these political, and social factors should still be considered, as they can greatly affect both how quickly and how successfully decarbonization efforts move forward.
As we are investigating the most efficient options to reach net zero, cost and energy demand should be projected to the date of net-zero pledges. These projections imply that the status of emissions as “hard-to-abate” could change over time as new decarbonization options appear, and cost and energy projections evolve. While the projections of technologies such as DACCS are still uncertain, it is still possible to make conclusions on the status of emissions which are hard-to-abate.
Which emissions are not hard-to-abate, and what should we do about them?
Using our definition, we study two sources of emissions that are commonly referred to as “hard-to-abate”:
Cement is not hard-to-abate under this definition. Point-source carbon capture at cement plants is more cost- and energy-efficient than DACCS.
Aviation, on the other hand, is economically and energetically hard to abate. Alternatives like hydrogen planes or sustainable aviation fuels are likely to require more energy than DACCS and come with uncertain costs that are in similar ranges to compensating aviation emissions with DACCS.
This refined definition offers more than just academic precision. It is a tool for prioritizing action. Emissions that are not hard to abate, where viable, cost-effective, and energy-efficient alternatives exist, should be tackled immediately, for example, by deploying point-source CCS for the cement sector. In these sectors, postponing decarbonization or outsourcing it to the CDR industry only increases the burden on future generations, who will have to rely more on energy- and cost-intensive carbon removal technologies.
We hope that our definition will prevent the ambiguity of the term “hard-to-abate” to delay decarbonization. With a clear, science-based definition, we can identify where innovation is most urgently needed and where the tools already exist to make real progress. The path to net-zero demands action—starting with clarity.
[2] Explainer: what do we mean by hard-to-abate industries? — Chevron
[3] Decarbonising hard-to-abate sectors with renewables: Perspectives for the G7
[4] Fuss, S., Lamb, W. F., Callaghan, M. W., Hilaire, J., Creutzig, F., Amann, T., … & Minx, J. C. (2018). Negative emissions—Part 2: Costs, potentials and side effects. Environmental research letters, 13(6), 063002.
[5] Deprez, A., Leadley, P., Dooley, K., Williamson, P., Cramer, W., Gattuso, J. P., … & Creutzig, F. (2024). Sustainability limits needed for CO2 removal. Science, 383(6682), 484-486.
