Conceptualising the Paris Agreement long-term temperature goal
The central objective of the Paris Agreement is its long-term temperature goal to hold global average temperature increase to “well below 2°C above preindustrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels”. This is referred to by the CAT as the 1.5°C Paris Agreement goal ,This goal is linked to a requirement in the Paris Agreement1 that all countries work together to bring greenhouse gas emissions to zero within the second half of the 21st century, with the timing of when the zero emissions are reached being determined by the best available science in relation to the achievement of the long-term temperature goal. The long-term temperature goal in the Paris Agreement goes significantly further, both legally and substantively, than the earlier goal to hold warming to below 2°C (Schleussner et al. 2016).
The Copenhagen Accord from 2009 mentions the long-term temperature goal of holding the global temperature increase to “below 2 degrees Celsius” (UNFCCC 2010). One year later, Parties to the UNFCCC adopted the Cancun Agreements which “further recognised that deep cuts in global GHG emissions are required, with a view to hold the increase in global average temperature below 2°C above preindustrial levels”. This is referred to by the CAT as the Cancun 2°C goal.
The 1.5°C Paris Agreement goal is, by design, a strengthening of the Cancun 2°C goal.
The origin of the 1.5°C Paris Agreement limit derived from the concern amongst the vulnerable countries about the adverse consequences of a 2°C warming level. In 2014 the UNFCCC established a process to review whether the long-term Cancun goal of holding warming below 2°C was adequate to avoid dangerous climate change and to consider “strengthening the long-term global goal on the basis of the best available scientific knowledge, including in relation to a global average temperature rise of 1.5°C” 2. This process ended in 2015 with the final report of its scientific arm (Structured Expert Dialogue) concluding that using the globally-agreed warming limit of 2°C as a “guardrail” is not safe, and that governments should aim for 1.5°C instead. The 2°C limit was found not to be in line with the ultimate objective of the Convention to “prevent dangerous anthropogenic interference with the climate system.” This was a key input into the Paris Agreement negotiations then underway and led ultimately to the Paris Agreement’s long-term temperature goal in Article 2.1 as described above.
One of the key architectural concepts of the Cancun 2°C goal that carried over into the Paris agreement long-term temperature goal is that of "holding warming" below a certain level. The “hold below” formulation is clearly stronger than a returning to a certain level of warming by a certain time (up to 2100 (from an implicitly higher-level). In the negotiations of this warming goal, formulations such as returning to 2°C by 2100 were proposed and rejected. In examining any set of emission pathways consistent with a long-term temperature goal, a requirement to hold below a certain level of warming requires greater and faster would emission reductions than a temperature goal that requires returning to a certain degree of warming by, for example, 2100. This has concrete implications for policy – and emission pathways - and as a consequence the Climate Action Tracker has been careful to use pathways that are fully consistent with the goals.
The probability of achieving a given temperature target has also been a discussion point, but not as explicitly as concepts such as holding warming below a certain level or return warming to a certain level by a certain time. In general, it is fairly well accepted that pathways that assess specific warming goals should meet those goals with a likely or greater probability. Few would argue that a specific warming goal should be met with only a 50% chance (toss of a coin).
Over the years, there has been a strong consensus within the scientific community in interpreting the Cancun goal of holding warming below 2°C to use the “likely below” 2°C class of scenarios in the scientific literature. These energy-economic model scenarios have a 66% chance, or greater, of staying below a 2°C global mean warming above pre-industrial levels throughout the 21st century.
The Climate Action Tracker has used these pathways as benchmark for emissions reductions in line with Cancun 2°C goal, as well as a basis for assessing the adequacy of efforts of individual countries.
From its inception in 2009 the Climate Action Tracker itself has consistently referred to the 1.5°C global mean warming limit in its analyses, alongside the 2°C pathway, particularly in relation to comparing the global effect of pledges and commitments with emission pathways for both these warming limits.
As note above the Paris Agreement 1.5°C goal, by design, goes beyond the Cancun Agreements’ below 2°C limit and aims to hold warming to well below 2°C and to pursue efforts to limit temperature increase to 1.5°C.
IPCC Special Report on 1.5°C
The most comprehensive and up-to-date assessment of Paris Agreement compatible mitigation pathways is the recent IPCC Special Report on 1.5°C (SR1.5). Its Summary for Policy Makers (SPM)3 established 1.5°C compatible mitigation pathways as being pathways with no- or limited-overshoot. These pathways limit median global warming to 1.5°C throughout the 21st century without exceeding that level (“no-overshoot”), or allow warming to drop below 1.5°C by the end of the century (around 1.3°C warming by 2100) after a brief and limited overshoot of median peak warming below 1.6°C around the 2060s (“low-overshoot”). With a peak warming of (at most) 1.6oC these pathways meet several tests with reference to the Paris Agreement LTTG.
The “hold below 2oC” pathways hold warming below 2°C with a least 66% probability and have peak 21st century warming of up to 1.8°C, whereas 1.5°C compatible mitigation pathways in IPCC SR1.5 peak warming at a significantly lower level (1.5-1.6°C), and hold warming below 2°C with probability to at least 86%.
As a consequence, the 1.5°C compatible mitigation pathways can be said to hold warming “well below 2°C” and limit warming to 1.5°C. In these 1.5°C mitigation pathways, total greenhouse gas emissions peak around 2020 and decrease rapidly to global zero around 2070. These pathways are compatible with interpretations of the PA LTTG in its Article 2.1 and can be used for operationalising Article 4.1.
The IPCC SR1.5 also assesses other pathways that lead to higher warming levels, including pathways that hold warming below 2°C with 66% chance and do not return to 1.5°C. The IPCC SR1.5 provides an assessment of these pathways for purposes of comparison and consistency with 1.5°C compatible mitigation pathways. The IPCC SR1.5 is also very clear about the increases in climate risks between 1.5°C and 2°C, which relates to the clause of the Paris Agreement LTTG that recognises that holding warming well below 2°C and limiting it to 1.5°C significantly reduces the risks and impacts of climate change.
Schleussner, C.-F. et al., 2016. Science and policy characteristics of the Paris Agreement temperature goal. Nature Climate Change, 6(9), pp.827–835. Available at: http://www.nature.com/doifinder/10.1038/nclimate3096.
UNFCCC, 2010. Report of the Conference of the Parties on its fifteenth session, held in Copenhagen from 7 to 19 December 2009. In Copenhagen.