Paris agreement and climate change

Global greenhouse gas emissions as implied by INDCs compared with no-policy baseline, current-policy and 2°C scenarios.

PLEDGES made for the Paris agreement on climate change last winter would lead to global temperature rise of 2.6 to 3.1°C by the end of the century, according to a new analysis published in the journal Nature. In fact, the entire carbon budget for limiting warming to below 2°C would be emitted by 2030, according to the study led by Joeri Rogelj of the International Institute for Applied Systems Analysis (IIASA).

The 2°C target aimed to limit future climate change to an average temperature increase of below 2°C above pre-industrial levels, as research suggested that this could help avoid some of the most dangerous consequences of climate change. The target was agreed upon by 190 countries at the Cancun climate meeting in 2010. In Paris last December, countries strengthened this target by requiring temperatures to be limited to “well below” 2°C and, furthermore, agreed that they should strive to limit temperature rise even further to 1.5°C, as some studies suggest that even 2°C warming would lead to unacceptable impact, particularly in vulnerable countries such as island nations and least-developed countries.

The new study provides an in-depth analysis of the pledges countries submitted at the Paris climate meeting in December, the Intended Nationally Determined Contributions (INDCs) as they are called. In order to assess what would happen after the pledge period ends in 2030, the researchers assumed that emission reduction efforts would be continued at the same level of effort after 2030. Based on these projections, and using a variety of different models, they estimated that median global temperatures would reach 2.6 to 3.1°C by 2100. The researchers also examined what additional measures would be necessary after 2030 to limit future temperature rise to 2°C or 1.5°C in 2100.

Niklas Höhne, a researcher at the New Climate Institute in Germany and Wageningen University who worked on the study, says: “To go the rest of the way, we would need to assume much more stringent action after 2030, which leads to emissions reductions of about 3 to 4 per cent a year globally. But in practice, switching to such stringent reduction right after 2030 would be challenging, and require time.”

The study also provides a careful analysis of the uncertainties surrounding future emissions and temperature targets. For one thing, the emissions reductions from the INDCs remain uncertain, since the INDCs themselves are not consistently framed, and some of the pledges include conditional statements, making implementation of emissions reductions contingent on receiving funding from others. Comparing the possible emission levels that the INDCs could imply, the researchers found a range of uncertainties of six billion tonnes of CO equivalent, or roughly the entire emissions of the United States in the year 2012.

The other major uncertainty lies in how much temperatures will rise in response to various emission levels. For this reason, temperature targets are often interpreted in terms of probabilities, with the aim to have a 66 per cent likelihood of keeping temperature to below 2°C above pre-industrial levels. The study also found that the same INDCs would only avoid 2.9-3.4°C of warming with a 66 per cent chance and 3.5-4.2°C of warming with 90 per cent chance until 2100.

Harald Winkler, a researcher at the Energy Research Centre in South Africa, also worked on the article. He says: “While some uncertainties, like the temperature response uncertainty, are virtually irreducible over the coming years, uncertainties about what the INDCs add up to in terms of emissions are not. Immediate future work should therefore focus on a better understanding of what the INDCs mean and how they link to other socio-economic objectives, including the U.N. Sustainable Development Goals.”