Metrics for Emissions Removal Limits for Nature

MERLiN is series of modelling studies designed to assess the limits of Greenhouse Gas Removal (GGR) effectiveness for prevention of climate change impacts, climate change reversibility, and the accounting of GGR.

Global temperature rise is determined by cumulative emissions of CO2, with climate policy temperature targets corresponding to emissions budgets e.g. 2K = around 1000 Gt C emitted. GGR is suggested as a way of recapturing CO2 some time after it has been emitted such that the eventual net quantity emitted meets a temperature target budget. Current climate mitigation action appears reliant on future application of GGR in this role to meet the international climate warming targets of the UNFCCC Paris Agreement of less than 2K and ideally 1.5K warming.

However, even if the same eventual budget is attained, it is expected that climate change impacts resulting from an emissions overshoot and later recapture pathway will be different to impacts associated with pathways in which no recapture is required. This difference in impacts sets limits to the contribution of GGR to avoiding dangerous climate change impacts. MERLiN is designed to determine these limits, and develop simple metrics relating the difference in committed climate change impacts to the amount and duration of emissions budget excess.

MERLiN will also, with international collaborators, investigate climate change reversibility, and how the Earth system (in particular the carbon cycle) respond to GGR recapture of CO2. Here, different timescales in carbon cycle components create inertia whereby emission and later recapture of CO2 do not have equal (cancelling) temperature effect. This creates an accounting challenge for GGR which MERLiN will investigate and quantify.

These results are crucial to informing policy choices and actions on GGR. Using MERLiN researcher's strong policy experience, MERLiN will translate and communicate its findings on climate change impact GGR limits, climate change reversibility, and GGR accounting to inform policy-makers on the implications for climate mitigation action, policies and mechanisms.

Improved understanding of the potentials and implications of Greenhouse Gas Removal (GGR) as an approach to climate mitigation is crucial to inform climate policy developent and climate mitigation action. MERLiN is designed to assess the effectiveness and limitations of GGR to prevent climate change impacts following an overshoot of the emissions budget corresponding to climate mitigation policy targets.
MERLiN will benefit both the technical climate sceinec and climate policy communities. MERLiN advances initial studies of Earth system responses to GGR through detailed investigation of the reversibility of climate change, Earth system components and resulting climate change impacts, and how these relate to emissions budget and GGR quantities and timing. From these, MERLiN will establish limits and metrics for GGR contribution to climate mitigation, producing new understanding of Earth system behaviour, creating metrics relating GGR pathways to impacts, and assessing and defining approaches to GGR accounting. This work and its results will be of considerable benefit to climate modelling, climate change impacts and mitigation, and climate policy researchers.
MERLiN's collaboration in the Carbon Dioxide Removal Model Inter-comparison project (co-lead by MERLiN researcher Dr Scott) will strengthen this international multi-model analysis and enable greater interpretation of climate reversibility and GGR rebound experiment results.
These science advances will be published and presented at international conferences, and MERLiN researchers will undertake research collaboration visits to project partners and their respective GGR research programmes.
For the climate policy community, MERLiN will inform on the role of GGR in climate change mitigation pathway choices. International climate warming limit targets in the Paris Agreement of less than 2K and ideally 1.5K appear reliant on large scale future application of GGR including to deliver global net-GGR. Here, it is crucial to improve understanding of the ability of GGR to recapture excess emissions and any associated consequences for climate change impacts. MERLiN will assess and develop simple and communicable metrics that relate these net-GGR pathways and impacts. MERLiN will also analyse the accounting of GGR and suggest how it might be incorporated into climate policy mechanisms. These findings will be valuable to assessments of climate change for policy (e.g. IPCC), for UK and EU government officials and elected representatives. The MERLiN team has strong experience in policy communication and will prepare an accessible policy-focussed final report and host a workshop to discuss MERLiN's conclusions.