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Negative emissions technologies (NETs) and their potential role in meeting emission targets is a rapidly growing and contentious area of climate change mitigation research. The literature ranges in scope from general reviews of NETs options to research and development through applied case studies. Within this field, a gap exists in the application of this growing body of research to the unique limitations and opportunities of a specific nation. Ireland is a small developed island nation in the EU with a unique emissions profile, as 32% of the total comes from agriculture due to the high number of cattle. In this study we aim to assess the potential capacity of terrestrial NETs options for Ireland and review the nation-specific context for their deployment. Despite the proportionally high representation of biochar and carbon capture and storage in the international NETs research, in an Irish context afforestation and bioenergy crops are much more established practices and could readily be considered in possible emission pathways that use NETs. Higher capacities were found for NETs options that are currently unavailable (direct air carbon capture and storage and bioenergy with carbon capture and storage), while options available to deploy at scale (afforestation, soil carbon management and biochar) have capacities limited by saturation of soil carbon stock and have higher risk of reversibility due to impermanence. Hence, while we estimate a reasonable technical capacity for NETs in Ireland, emission reduction remains the highest priority for feasibly meeting a Parisaligned carbon quota for Ireland.

<p>Overshoot of the global 1.5ºC long term temperature goal is likely soon after 2030, so high emitting nations are liable to exceed their fair share of remaining warming to 1.5ºC well before 2030. Net zero globally and for high emitters will occur in overshoot, therefore the meaningful goal is a net negative world until 1.5ºC is reached. In addition to radical near-term reductions in fossil fuel and land CO<sub>2</sub> emissions, limiting and returning from overshoot will require substantial warming reduction (negative emissions), via some combination of methane mitigation and carbon dioxide removal (CDR), and limits on excessive agricultural N<sub>2</sub>O resulting from inefficient reactive nitrogen usage. Therefore, for developed nations and their decision-makers, rapid assessment of the warming impact from primary greenhouse gases for alternative society-wide policy pathway options relative to a fair share of remaining warming to 1.5ºC is required on a clearly defined equity basis. This research applies such a “Paris Test” through: a ‘micro climate model’ GWP* assessment of IPCC 1.5ºC scenarios undertaken to establish a remaining global CO<sub>2</sub> warming equivalent (CO<sub>2</sub>we) budget, aggregated for [CO<sub>2</sub>+N<sub>2</sub>O+CH<sub>4</sub>], to 1.5ºC from 2015; allocation of this budget on a global equal per capita and national population basis to set out 2015 remaining national ‘carbon’ quotas, as of 2015; and, a case study (Ireland) of alternative multi-gas national scenarios to compare aggregate society-wide cumulative CO<sub>2</sub>we outcomes relative to meeting the 1.5ºC national carbon quota well before 2100. Other equitable budget allocation principles are possible, but this case shows the importance of justifying the reference year choice, and other normative and quantitative assumptions, on a clearly defined “common but differentiated responsibility” basis. The study shows the benefits of such a rapid Paris Test national mitigation policy assessment methodology. Its outputs clarify the considerable difference for developed nations between <em>overshoot net zero</em>, commonly referred to as “no additional warming”, and <em>quota net zero</em>, the Paris Agreement aligned goal, which requires early and substantial CH<sub>4</sub> emissions rate reduction as well as CDR. The common use of GWP<sub>100</sub> CO<sub>2</sub>e in mitigation analyses is shown to undervalue the importance of early, deep, and sustained annual CH<sub>4</sub> emission rate reduction toward reducing inequitable long-term reliance on uncertain and costly large scale CDR. If the 1.5ºC goal is to be met, by limiting overshoot magnitude and quickly returning to a Paris-consistent net zero quota level, then urgent, substantial and sustained action by developed nations – to radically reduce their fossil fuel use and deforestation responsibility, and to limit nitrogen flows to intensive animal agriculture – will be required at policy ambition levels far greater than those considered ‘technically feasible’ in IPCC mitigation assessments. To meet society-wide, 1.5ºC fair share, national multi-gas quotas, so-called ‘hard-to-abate’ sectors, such as aviation and ruminant agriculture, likely have to be abated substantially and directly within developed nations through policy-directed regulation. This research confirms that the window of options for fair share 1.5ºC climate action in developed nations is closing very rapidly.</p>

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