It’s tempting to think about carbon dioxide removal (CDR) like a light switch — press a button and reverse climate change. But in reality, most CDR pathways involve lags between when the intervention happens and when its climate benefit actually accrues. A direct air capture plant must first “pay back” the upfront emissions associated with construction before any net removals register in the atmosphere. Rocks spread for enhanced weathering can take years, or even decades, to dissolve and draw down CO₂. And pathways that store biomass only deliver a benefit based on when the carbon in that biomass would have otherwise re-entered the atmosphere. All these lags matter if we want to deploy CDR in a way that stabilizes long-term temperatures and improves the warming trajectory along the way.
We dig into temporal lags and their implications in a new preprint, “Consistent temporal accounting supports credible CDR use.” This work was a collaboration with colleagues at Stanford, CSU Humboldt, Carbon Direct, Amazon, and Stripe, reflecting a growing recognition from researchers and market participants alike that these lags deserve more attention. In the preprint, we define four archetypes of temporal lags and use the FaIR climate model to quantify how they influence the global temperature response. The analysis underscores why temporal lags must be explicitly addressed in CDR policy and carbon markets, and offers a foundation for doing so.
As we wrap up the first phase of work on this topic, there are two things we’re going to keep thinking about.
First, there’s a clear need to identify and address inconsistencies in how temporal lags are accounted for in practice. In the preprint, we take initial steps toward showing that temporal lags are treated very differently (and sometimes ignored entirely) across pathways, policies, registries, and crediting protocols. But there’s a lot more work to do. Addressing this inconsistency is essential if we want to make fair comparisons across pathways and support credible neutralization claims with CDR. More precise language for describing these lags is a small, but important, first step, and we hope the nomenclature established in the preprint helps.
Second, we want to continue developing a better understanding of just how much these lags matter. The preprint makes a start toward quantitatively characterizing their potential impact. We show that ignoring temporal lags in an offsetting context can exacerbate near-term warming, which matters because even temporary increases in warming can worsen climate impacts and push systems closer to critical thresholds. And if similar practices were present in global-scale CDR deployment, they could meaningfully affect peak warming and decrease the probability of meeting climate targets like 2°C.
But these initial findings rely on illustrative assumptions about the form of temporal lags. A lot more science is needed to understand what lags actually look like on the scale of individual projects, and how much they matter in aggregate. There has already been some good progress on these questions, especially for open system CDR pathways. Recent examples include characterizing how air-sea gas exchange rates vary based on the location and season of ocean alkalinity enhancement deployments, and synthesizing the uncertainties associated with enhanced rock weathering rates. There are also examples of temporal lags researchers are just starting to understand, like how cation sorption or water transit times affect the timing of removals from enhanced rock weathering projects. Looking forward, we are especially interested to see more research into temporal lags for biomass-based CDR — both in the form of more public data about the conversion efficiency of different pathways and improved approaches to thinking through realistic feedstock counterfactuals.
So what does all of this mean for real-world CDR projects? We’re not fully sure. The majority of projects today are financed through the sale of carbon credits. In that context, better characterization of lags will generally mean longer delays before projects can earn credits and generate revenue. Even short delays in crediting could put projects that are unquestionably good for the atmosphere in the long-term, out of reach for conventional project finance. That’s not a reason to avoid the problem — market rules clearly need to be updated to account for temporal lags more rigorously and consistently. But assuming that market participants will make a good-faith effort to thread the needle between physical rigor and practical feasibility, we think it’s likely that some climate-beneficial projects simply won’t be able to make the market business model work. For us, that points to the need to continue exploring non-market financing alternatives. (If you’re interested in this topic, go read this article where we discuss similar dynamics in the context of a different enhanced rock weathering quantification challenge.)
We’re excited to continue digging into these questions in the coming year. In the meantime, read the preprint for details, explore our Github repository for the code and data behind the analysis, and reach out if you have reactions or ideas about where this work should go next.