Climate–carbon cycle uncertainties and the Paris Agreement

• Published in: Nature climate change 2018-07, Vol.8 (7), p.609-613
• Main Authors: Holden, P. B., Edwards, N. R., Ridgwell, A., Wilkinson, R. D., Fraedrich, K., Lunkeit, F., Pollitt, H., Mercure, J.-F., Salas, P., Lam, A., Knobloch, F., Chewpreecha, U., Viñuales, J. E.
• Format: Article
• Language: English
• Subjects: 704/106/694/1108; 704/106/694/682; Acidification; Agreements; Carbon; Carbon cycle; Climate; Climate Change; Climate Change/Climate Change Impacts; Climate models; Climate policy; Computer simulation; Decarbonization; Decision making; Earth and Environmental Science; Emissions; Environment; Environmental Law/Policy/Ecojustice; Environmental policy; Feasibility studies; Genetic transformation; Global temperatures; Letter; Mitigation; Ocean acidification; Ocean warming; Paris Agreement; Policies; Precipitation; Productivity; Regional analysis; Regional development; Regions; Temporal variability; Temporal variations; Three dimensional models; Uncertainty
• ISSN: 1758-678X; 1758-6798
• DOI: 10.1038/s41558-018-0197-7

The Paris Agreement 1 aims to address the gap between existing climate policies and policies consistent with “holding the increase in global average temperature to well below 2 C”. The feasibility of meeting the target has been questioned both in terms of the possible requirement for negative emissions 2 and ongoing debate on the sensitivity of the climate–carbon-cycle system 3 . Using a sequence of ensembles of a fully dynamic three-dimensional climate–carbon-cycle model, forced by emissions from an integrated assessment model of regional-level climate policy, economy, and technological transformation, we show that a reasonable interpretation of the Paris Agreement is still technically achievable. Specifically, limiting peak (decadal) warming to less than 1.7 °C, or end-of-century warming to less than 1.54 °C, occurs in 50% of our simulations in a policy scenario without net negative emissions or excessive stringency in any policy domain. We evaluate two mitigation scenarios, with 200 gigatonnes of carbon and 307 gigatonnes of carbon post-2017 emissions respectively, quantifying the spatio-temporal variability of warming, precipitation, ocean acidification and marine productivity. Under rapid decarbonization decadal variability dominates the mean response in critical regions, with significant implications for decision-making, demanding impact methodologies that address non-linear spatio-temporal responses. Ignoring carbon-cycle feedback uncertainties (which can explain 47% of peak warming uncertainty) becomes unreasonable under strong mitigation conditions. A reasonable interpretation of the Paris Agreement may well still be technically achievable without the need for net negative emissions or excessively stringent policies according to climate–carbon-cycle modelling.