The role of carbon budgets in translating the Paris Agreement into national climate policy

Two articles of the Paris Agreement are particularly relevant to countries as they seek to mitigate their impact on global warming. Article 2 sets a target for long-term temperature stabilisation, seeking to hold the increase in global average temperature to “well below 2 °C above pre-industrial levels, while pursuing efforts to limit the temperature increase to 1.5 °C”. Article 4 includes a target to “peak greenhouse gases (GHGs) as soon as possible” and thereafter to achieve “rapid reductions” in emissions so that sources of emissions are balanced by sinks (i.e. achieving net zero emissions) in the second half of the 21st century. Despite the ratification of the Paris Agreement, there is no political or scientific consensus on precisely how these targets should be translated into national decarbonisation trajectories. As a result, countries are developing and setting their own carbon reduction strategies in different ways: increasingly, countries are implementing net zero targets for mid-century. Although consistent with Article 4, given the linear relationship between warming and cumulative net emissions of long-lived GHGs, net-zero target dates only specify when temperatures are stabilised, but not at what temperature. To comply with Paris Agreement temperature goals, international and national climate action policy must have regard to cumulative reductions in GHG emissions, not just single year targets. Broadly, this requires a carbon budget approach. In this discussion paper we review some of the ways that a carbon budget approach has been used in climate action policy. One approach is based on developing a so-called Climate Science Carbon Budget derived from a Global Carbon Budget (GCB) associated with a particular temperature goal and an equitable effort-sharing approach to share the GCB. This is contrasted with other approaches including national frameworks for climate policy such as the UK’s net zero emissions by 2050 where the carbon budgets are for a shorter time period (i.e. 5 years). We call this latter approach Climate Policy Carbon Budgets, that is carbon budgets that aren’t derived from a global temperature target, but instead from national decarbonisation trajectories. This discussion paper also explores approaches to developing decarbonisation pathways for Ireland which are consistent with the Paris Agreement using a carbon budget approach. This discussion paper outlines a broad approach to generating a national long-term climate science carbon budget for Ireland and translating it into five-year sectoral carbon budgets. The process for developing such carbon budgets will require an appropriate set of robust energy system modelling tools, that are iteratively developed and examined, together with an extensive stakeholder engagement process. This discussion paper concludes with some reflections on the governance arrangements for setting, monitoring, and reviewing carbon budgets