Just published in Environmental Research Letters, a collaboration with Jesse Abrams (Exeter) and John Schellnhuber (International Institute for Advanced Study) in which we explore some of the consequences of overshooting 1.5°C of warming. Our argument is that climate damage will accumulate and so further stress societies in the future, along with increasing the chance of irreversible tipping of climate elements. Open Access paper available via the journal, and some thoughts about this over on Technosphere Earth. Abstract below.
Abstract
The carbon budget framework has served as the dominant organising principle for climate science and policy, linking cumulative emissions to peak warming and providing a tractable scalar constraint for international negotiations. With 1.5 °C exceedance now imminent and virtually all mitigation scenarios involving substantial temperature overshoot before net-negative emissions could return temperatures to target, the budget framework’s utility for evaluating and minimising risk generated by warming risks becoming diminished. We propose a complementary organising principle: the overshoot integral, defined as the time-integrated temperature exceedance above a policy-relevant threshold, with units of °C·years. Analogous to degree-heating weeks in coral bleaching prediction and positive degree-days in glaciology, the overshoot integral captures both the magnitude and duration of exceedance in a single scalar quantity. Applying this metric to future climate pathways, we show that pathways with similar peak temperatures can differ by more than two orders of magnitude in their cumulative degree-year burden. Applied to CMIP7 ScenarioMIP/FaIR (Finite Amplitude Impulse Response simple climate model) trajectories, calculating overshoot integrals reveal substantial differences in cumulative thermal exposure across pathways. For Earth-system impacts whose response dynamics integrate forcing over time, spanning marine habitat, permafrost carbon and mountain glaciers, we show that cumulative thermal exposure captured by the overshoot integral contains impact-relevant information not captured by peak warming alone. A simple risk formulation combining peak and cumulative exposure improves out-of-sample prediction across all three systems. That the overshoot integral is best understood not as a reversible account that can be debited and credited, but as a cumulative dose should inform climate policy and science in the increasingly warmer future.