Currently there are several ways of removing greenhouse gases (GHG) from the atmosphere, some of them already running at commercial scale. These GGR can differ in three key aspects: (1) how they capture CO2 from the atmosphere, e.g. through vegetation growth versus man-made sorbents, (2) how they store the captured carbon away from the atmosphere, e.g. in vegetation versus in geological storage, and (3) for how long carbon is stored, e.g. decades in the case of trees versus millennia for geological storage. To understand the amount of removal delivered by each GGR and the climatic merit of this removal it is critical that we analyse each GGR over its full life cycle, to ensure that the removal is not cancelled by higher emissions released elsewhere in the GGR supply chain. The climate merit of removal is determined by how and when the removal happens, and how stable is the storage of carbon out of the atmosphere. This page will detail the key indicators we suggest for evaluating and comparing GGRs in terms of the amount of removal they deliver and the climatic merit of that removal.
Coherent indicators to evaluate and compare the removal delivery of GGR are needed to inform the sustainable deployment and scale up of GGRs in the UK and abroad. These indicators need to be underpinned by scientifically sound methods and high quality verified data. To provide a robust and trustable evaluation, these indicators also need to be discussed and harmonised between users and scrutinised by wider audiences. To meet these requirements, CO2RE developed a first set of indicators between May and September 2021, and tested them with a wider audience in several stakeholder engagement workshops which ran between September and October 2021. Based on that feedback, CO2RE continued developing removal indicators in close collaboration with researchers from the five Demos in the GGR-D programme; see the working team at the bottom of this page. This collaborative work resulted in a set of indicators to characterise removal delivered by a GGR option; see below.
Note that these indicators are still in development, as our understanding of GGRs evolves with the demonstration happening in the GGR-D programme and externally, e.g. projects funded by the Department of Business, Energy & Industrial Strategy through the Direct Air Capture and Greenhouse Gas Removal (GGR) Innovation Programme. We will be updating them every year based on our own research, feedback from testing them with different stakeholders, and new evidence coming from the demonstrators and wider GGR community in the UK and abroad. You can also provide feedback by using the form at the bottom of this page, or by emailing us at firstname.lastname@example.org, mentioning CO2RE evaluation framework in the subject of your email.
Estimation of the removal capacity of a GGR
We define the removal capacity of a GGR as its life cycle net GHG emissions relative to the amount of carbon stored and the reference counterfactual scenarios, where:
- The net life cycle GHG emissions of a GGR is the sum of the GHG sequestered and the GHG emissions (e.g. CO2, CH4, N2O) emitted over the full supply chain of the GGR, considering the system boundaries described in the Quality of reporting indicators. “GHG emissions” include both biogenic and fossil GHG emissions emitted directly and indirectly by the deployment of the GGR. The GHG emissions should be reported separately for each type of GHG, so that climate impacts over different timescales can be explored, see more below. To quantify the overall life cycle emissions and removal, all GHG should be converted to carbon dioxide equivalents (CO2e) by using the 100-year global warming potential values reported by the IPCC latest report, AR6.
- The carbon stored includes the carbon stored in all sinks: in vegetation (above- and under-ground organic matter), in soil, geological storage, carbonate and silicate rock weathering, ocean carbon sinks.
- The net GHG emissions of the counterfactual scenario are the sum of the GHG sequestered and the GHG emissions emitted in the absence of the GGR deployment, considering the counterfactual definition described in the Quality of reporting indicators.
We consider that a GGR delivers removal if the following two conditions are met:
- The net life cycle GHG emissions of the GGR are negative; that is to say, the GGR removes more GHG than it emits over its full life cycle.
- The removal efficiency of the GGR is higher than the removal efficiency of its counterfactual scenarios (if more than one counterfactual is deemed plausible; see more in the Quality of reporting indicators). We define the removal efficiency of the GGR or its counterfactuals as their net life cycle emissions normalised by the amount of carbon stored durably in each case. By using this efficiency ratio of removal to carbon stored we want to ensure that the GGR delivers more removal than its counterfactual (that is what would have happened in the absence of the GGR), and that the amount of carbon stored is clearly specified so that it can be assessed in terms of storage quality, see below.
Evaluation of the climate merit of the removal
We consider that two critical aspects determine the climate merit of the removal: (1) when and how the removal happens (gradually over time versus immediately, by sequestration in growing vegetation vs industrial modules), and (2) for how long the removed carbon is stored out of the atmosphere i.e. storage quality (decades to centuries for tree planting, versus millennia in geological storage). These aspects determine the contribution of the GGR to staying within a global carbon budget compatible with the Paris Agreement (short to medium term contribution), and to the change in the global temperature (over very long timeframes in which climatic changes happen). We are still discussing the best way to formulate indicators for evaluating the climatic merits of GGRs. Please get in touch if you would like to engage in this co-development process.
Dr Isabela Butnar, UCL (CO2RE)
Dr Sylvia Vetter, University of Aberdeen (Enhanced rock weathering)
Prof Astley Hastings, University of Aberdeen (Afforestation)
Dr Jon Mckechnie, University of Nottingham (Biochar)
Dr Mirjam Roeder, Aston University (Peatlands)
Dr Phil Renforth, Heriot Watt University (Enhanced rock weathering)
Prof Timothy Cockerill, University of Leeds (Biochar)
Dr Jo House, University of Bristol (CO2RE)
Dr John Lynch, University of Oxford (CO2RE)
Dr Eleni Michalopoulou, University of Bristol (CO2RE)
Prof Stuart Haszeldine, University of Edinburgh (CO2RE)