Understanding the origin of Paris Agreement emission uncertainties

• Published in: Nature communications 2017-06, Vol.8 (1), p.15748-15748, Article 15748
• Main Authors: Rogelj, Joeri, Fricko, Oliver, Meinshausen, Malte, Krey, Volker, Zilliacus, Johanna J. J., Riahi, Keywan
• Format: Article
• Language: English
• Subjects: 704/106/694/682; 704/844/2175; 704/844/2787; Alternative energy sources; Climate change; Emissions; Estimates; Humanities and Social Sciences; multidisciplinary; Paris Agreement; Renewable resources; Science; Science (multidisciplinary)
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/ncomms15748

The UN Paris Agreement puts in place a legally binding mechanism to increase mitigation action over time. Countries put forward pledges called nationally determined contributions (NDC) whose impact is assessed in global stocktaking exercises. Subsequently, actions can then be strengthened in light of the Paris climate objective: limiting global mean temperature increase to well below 2 °C and pursuing efforts to limit it further to 1.5 °C. However, pledged actions are currently described ambiguously and this complicates the global stocktaking exercise. Here, we systematically explore possible interpretations of NDC assumptions, and show that this results in estimated emissions for 2030 ranging from 47 to 63 GtCO 2 e yr −1 . We show that this uncertainty has critical implications for the feasibility and cost to limit warming well below 2 °C and further to 1.5 °C. Countries are currently working towards clarifying the modalities of future NDCs. We identify salient avenues to reduce the overall uncertainty by about 10 percentage points through simple, technical clarifications regarding energy accounting rules. Remaining uncertainties depend to a large extent on politically valid choices about how NDCs are expressed, and therefore raise the importance of a thorough and robust process that keeps track of where emissions are heading over time. The pledges put forward by each country to meet the goals of the Paris Agreement are ambiguous. Rogelj et al . quantify the uncertainty arising from the interpretation of these pledges and find that by 2030 global emissions can vary by −10% to +20% around their median estimate of 52 GtCO 2 e yr −1 .