CAT rating system update

The Climate Action Tracker (CAT) rating system evaluates the adequacy of emissions levels resulting from emissions reductions targets against a 2°C benchmark. In order to evaluate the compatibility of targets with the stronger Paris Agreement long-term temperature limit, a methodological update of the CAT rating system is under way and will be online in 2017. In this section, we describe in which way moving to the stronger temperature goal affects our rating system and discuss in a conceptual manner the foreseen implications to the CAT rating. In short:

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 goal is linked to a requirement in the Paris Agreement [1] 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”.  

Because of disquiet amongst the vulnerable countries about the adverse consequences ultimately of a 2°C warming level at the same time the UNFCCC established a process to review whether the long-term temperature 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 [3]. 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.”

Over the years, there has been a strong consensus within the scientific community in interpreting the Cancun Agreements’ goal of holding warming below 2°C with the “likely below” 2°C class of scenarios. These energy-economic model pathways 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 those pathways as benchmark for emissions reductions in line with 2°C, 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. However, given the lack of scenarios with appropriate resolution for 1.5°C pathways the rating system of the Climate Action Tracker has drawn mainly from the published 2°C pathways and related equity and fairness studies.

The Paris Agreement 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. There are currently two interpretations of the implications:

In order to reflect the additional efforts required, the Climate Action Tracker needs to update its methodology, particularly its rating system.

Anticipated changes to the Climate Action Tracker analysis

First, we need to understand and define what the Paris Agreement long-term temperature goal means for future emission pathways. The range and depth of literature available for the evaluation of the 1.5°C limit, although catching up, remains not as ample as for the “likely below” 2°C class of scenarios. The scenarios published to date that go beyond the likely hold below 2°C scenarios, are the so-called 1.5°C class scenarios which hold warming below 2°C with 85% probability, or greater, and with a more a 50% or greater chance of remaining below 1.5°C by 2100 [4].These scenarios overshoot a 1.5°C global mean warming above preindustrial in the 21st century by about 0.1 to 0.2°C, before returning to 1.5°C or below in 2100 with a 50% likelihood (median warming in 2100 of 1.4°C).
The CAT will use these scenarios as a proxy for the long-term goal in the Paris Agreement, however, recognising that most vulnerable countries view scenarios that overshoot the 1.5°C limit as being inconsistent with the Paris Agreement long-term temperature goal. New scenarios that limit warming to 1.5°C or below throughout the 21st-century are now being developed and will be available in the open literature in early 2017. The Climate Action Tracker plans to update its assessment in 2017 based on this forthcoming new scientific literature.

Considering the 1.5°C class scenarios, we find that the Paris Agreement’s temperature goal requires that global GHG emissions are reduced by 70–95%2 below 2010 (65–90% below 1990) levels by 2050, and reach globally aggregated zero GHG emissions by 2060–2080. In contrast, the 2°C goal implied that global greenhouse gas emissions need to be reduced by 40-70% [5] in 2050 below 2010 (35–55% below 1990) levels and reach globally aggregated zero emissions by 2080–2100. Summarising, to be in line with 1.5°C, global emissions will need to decline faster in the first half of the century and global zero GHG emissions will need to be reached around 10-20 years earlier in comparison with a 2°C world.

While the technologies needed for reducing emissions to achieve 2°C are the same as those necessary to limit global warming to maximum 1.5°C by 2100, they need to be deployed faster and be complemented by a faster reduction in energy demand. The currently available likely 2°C and 1.5°C class scenarios require similar amounts of carbon dioxide emissions to be removed from the atmosphere through negative emissions technologies (Schleussner et al. 2016).

The Climate Action Tracker rating system needs to reflect these additional and faster global emissions reductions, meaning that the additional efforts need to be assigned to the countries according to a fair sharing approach. The required changes will have implications on the rating of individual governments’ emissions reduction targets.

Implications on the Climate Action Tracker rating

The CAT Effort Sharing assessment includes a wide range of literature on what researchers would consider options to determine a “fair” contribution to greenhouse gas reductions [6] , plus additional analyses the CAT has performed to complete the dataset. The studies and our own calculations apply an emissions allocation regime, based on a particular equity proposal or view that splits emissions across countries. Summing up the resulting emissions allowances of all countries leads to a global emissions pathway that is consistent with holding warming to below 2°C in the 21st century.

To be in line with the Paris Agreement long-term global goal and its more stringent 1.5°C limit, the starting carbon budget to be split across countries will be lower than the one used in the studies the CAT currently considers. On average the global emissions of 1.5°C scenarios are 14% lower in 2025 and 18% lower in 2050 than in 2°C scenarios.

Updating to studies considering the 1.5°C limit will have the following implications on the rating system and its results:

  • More stringent emission levels for most countries: Distributing a smaller carbon budget to the countries will result in lower emission allowances for most approaches and countries. As a first order estimate, the CAT effort sharing rating bars would be 14% lower in 2025 and 18% lower in 2050.
  • The impact is higher on the rating for 2050 than for 2020, 2025 and 2030: emissions pathways consistent with 1.5 and 2°C pathways start deviating from each other more robustly towards mid-century, the largest discrepancies between 2 and 1.5°C compatible categories will happen around then. As a result, the rating for 2020 pledges is likely not to be affected, the rating for NDCs is less likely to be affected whereas national long-term targets will be rated much more stringently. 

Of the 30 countries the CAT has assessed, only five are rated “sufficient” against a 2°C benchmark. The remaining 25 countries are not doing enough to be in line with 2°C, let alone 1.5°C. What remains clear, therefore, is that the NDCs’ ambitions globally fall far short of what is needed for the world to get onto a pathway that is in line with 2°C, let alone a pathway in line with the Paris Agreement long-term temperature limit.

In fact, the Decision that adopts the Paris Agreement (Decision 1/CP.21) already specifies that much greater emissions reduction efforts will be required to close the emissions gap between the estimated aggregate greenhouse gas (GHG) emissions levels resulting from (I)NDCs and emissions consistent with the long-term temperature goal of the Paris Agreement. Combined with other “ambition” elements, including the facilitative dialogue in 2018, a five-yearly “global stocktake”, and strong transparency and accountability elements, the Agreement is designed to pave the way for the mitigation efforts and ambition of GHG targets to be progressively improved. The first cycle of global stocktake and review of progress is scheduled for 2018 through the facilitative dialogue and is linked with a global political moment by 2020 when countries will need to be prepared to raise ambition in their new or updated NDCs. 


Höhne, N., den Elzen, M. & Escalante, D., 2014. Regional GHG reduction targets based on effort sharing: a comparison of studies. Climate Policy, 14(1), pp.122–147.

IPCC, 2014. Summary for Policymakers. In Stocker & V. B. and P. M. M. (eds T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, eds. Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, United Kingdom and New York, NY, USA: Cambridge University Press.

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:

UNEP, 2014. The Emissions Gap Report 2014: A UNEP synthesis Report,

UNFCCC, 2010. Report of the Conference of the Parties on its fifteenth session, held in Copenhagen from 7 to 19 December 2009. In Copenhagen.


[1] Article 4.1



[4] The 1.5°C scenarios underlying the emission numbers here have a more than 50% chance of returning to below 1.5°C by 2100 and simultaneously have a probability of about 85% to hold warming below 2°C during the 21st century (own calculation)

[5] These numbers are drawn directly from the IPCC AR5 Working Group III Summary for Policymakers (2014). The other numbers in this section draw from all scenarios assessed by the IPCC Fifth Assessment Report and the 2014 UNEP Emissions Gap Report (2014)and follow the methodologies of the 2014 UNEP Emissions Gap Report, to enable a direct comparison of these other numbers with the information provided in the 2014 UNEP Emissions Gap Report for 2°C.

[6] The “effort-sharing” studies in the CAT’s database include over 40 studies used by the IPCC (chapter 6 of WG III and Höhne et al. (2014)) plus additional analyses the CAT has performed to complete the dataset. They cover very different viewpoints of what could be fair, including considerations of equity, such as historical responsibility, capability, and equality.