Climate–carbon cycle uncertainties and the Paris Agreement

Holden, P. B.; Edwards, N. R.; Ridgwell, A.; Wilkinson, R. D.; Fraedrich, K.; Lunkeit, F.; Pollitt, H. E.; Mercure, J. -F.; Salas, P.; Lam, A.; Knobloch, F.; Chewpreecha, U. and Viñuales, J. E. (2018). Climate–carbon cycle uncertainties and the Paris Agreement. Nature Climate Change, 8 pp. 609–613.

DOI: https://doi.org/10.1038/s41558-018-0197-7

Abstract

The Paris Agreement 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 and ongoing debate on the sensitivity of the climate–carbon-cycle system. 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 emis- sions 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.

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