Does the perspective of Global Mean Temperature decreasing after peak warming in an overshoot pathway matter for adaptation?

Limited global gas emissions reductions would lead to exceeding 1.5˚C and even 2˚C of global warming compared to pre-industrial levels. Yet many emission scenarios included in the last IPCC report feature a decrease in Global Mean Temperature (GMT) after peak warming, even suggesting that levels compatible with the Long-Term Temperature Goal of the Paris Agreement could be reached again within the 21^st century. In contrast with the prominence of overshoot pathways in the literature, their implications for adaptation planning have not been discussed. Many factors are taken into account when it comes to decision-making about adaptation, however it could be argued that the perspective of seeing in the future Global Mean Temperature decrease after peak warming constitutes a disincentive to deploy adaptation measures dimensioned against impacts to be expected at peak warming.

 

In this study, we take the viewpoint of a decisionmaker who is trying to determine the extent to which they should adapt their assets to optimize costs. We assume that this decision is made in 2040, when global warming has just reached 1.5˚C and that the decisionmaker has full knowledge of the future evolution of GMT and resulting potential impacts from 1-in-100-year tropical cyclones on their assets. We consider three idealized pathways: two overshoot pathways in which GMT peaks in 2060 (at 1.65 or 1.8˚C) before coming back to 1.5 in 2080, and a third one in which it stabilizes at 1.65˚C as of 2060. We then compare the sum of the one-off costs of implementing adaptation measures and of the expected damages from tropical cyclones on the assets for two options: the decisionmaker decides to adapt against the level of damages expected at 1.5˚C, or at Peak Warming (1.65 or 1.8˚C). We also assess the sensitivity of the results to the evolving perceived value of the assets via a discount or growth (inflation) rate.

 

We find that adapting to impacts at peak warming is more cost-efficient for adaptation measures characterized by high efficiency (effectiveness divided by costs), while adapting to impacts at 1.5˚C is more cost-efficient for measures with low efficiency. In contrast, which option is more cost-efficient does not depend on the GMT pathway, although the differences in total costs between the two options become stronger in pathways that reach higher levels of global warming. Higher discount rates constitute incentives to adapt to lower levels of global warming, whereas this is the opposite for higher growth rates. Overall, these results suggest that the perspective of decreasing GMT in the future plays a limited role in adaptation decisionmaking, and that it should not be perceived as an incentive not to deploy adaptation measures.