GHG removals

The net life cycle GHG flux of a CDR is the sum of the carbon dioxide emissions sequestered from the atmosphere and the GHG emissions (e.g. CO₂, CH₄, N₂O) emitted over the full life cycle of the CDR (expressed in CO₂ equivalents). Genuine removal requires that a CDR project deliver net-negative GHG emissions over its life cycle, that is to say, the CDR removes more GHG than it emits over its full life cycle. The life cycle is defined as per ‘CDR system boundaries’ below.

Assessment of the net life cycle GHG flux requires building a comprehensive Life Cycle Inventory covering all carbon, GHG emissions, material and energy flows into, out of, and within the system due to anthropogenic activity. The “GHG emissions” that must be assessed include both biogenic and fossil GHG emissions emitted directly and indirectly by the deployment of the CDR. The GHG emissions should be reported separately for each type of GHG, so that climate impacts over different timescales can be explored (see more in “Durability”). 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, AR6.¹ The carbon stored includes that stored in all sinks: in vegetation biomass (above- and under-ground organic matter), in soil/sediment, geological storage, carbonate and silicate rock weathering, ocean carbon sinks and human-made long-life products and infrastructure.  

Assessing the removal provided by a CDR as a simple balance between emissions and sinks across its supply chain offers a partial view of the contribution of the CDR to removing atmospheric GHG. Besides net negative flux of GHG over the full life cycle, genuine removal requires that the CDR meets “additionality” requirements. This means that a given CDR must yield greater removal than would have otherwise occurred in its absence, i.e. a counterfactual scenario. For instance, a BECCS supply chain using forestry residues from an intensively harvested forest may deliver removals overall. However, if leaving the forest unharvested would have fixed more CO2  over the timeframe of analysis, e.g. 100 years, then the BECCS supply chain would in effect contribute to increasing GHG emissions over that time.  

Removed carbon must be durably stored out of the atmosphere and for a sufficient length of time to determine its contribution to climate mitigation.  

A commonly used minimum duration for carbon storage is 100 years. However, we suggest that the 100-year timeframe is only adequate if the CDR is used to offset land-based GHG emissions (comprising shorter-lived gases than CO2, and/or land-use related CO2 that would have the capacity to be re-captured within this time-frame through ecosystem processes). If the removals delivered by the CDR are to offset fossil GHG emissions, then the carbon store needs to be more permanent (1,000+ years), for instance in geological storage. This greater permanence reflects the time over which fossil emissions will continue impact the climate, lacking the potential re-capturability inherent to land related CO2 emissions.