Cumulative carbon and 1.5°C: Future mitigation pathways consistent with the goals of the Paris Agreement

The Paris Agreement has opened debate on whether limiting warming to 1.5°C is compatible with warming to date and current emission pledges to 2030. An important facet of this debate is uncertainty in the estimate of future cumulative carbon emissions consistent with this warming goal. A simple rescaling of the IPCC-AR5 carbon budgets for 2.0°C indicates that the remaining carbon budget for 1.5°C may be used up within merely a decade. How realistic is this estimate? This talk will look at alternative ways of estimating the remaining 1.5°C carbon budget, taking account of the present state of the climate system. I will discuss potential future mitigation pathways consistent with limiting warming to 1.5°C in 2100 under climate response uncertainty, and show how mitigation pathways aiming to limit warming to maximum values, such as 1.5°C or 2.0°C, could be constructed in the form of 'adaptive' mitigation policies in which mitigation efforts are continually updated in response to the emerging climate change signal. Such a policy regime would be consistent with the spirit of the 'pledge and review' mechanism of the Paris Agreement.