Overshoot pathways: reversible biophysical change, irreversible socioeconomic impacts

Alaa Al Khourdajie, Marina Andrijevic, Edward Byers , Minal Pathak, Anaa Pirani, Carl-Friedrich Schleussner, Rupert Stuart-Smith

Exceeding global warming of 1.5°C in the near term is now unavoidable. An overshoot pathway, in which exceedance is followed by decline to or below the threshold through net-negative emissions, is now the only remaining route back to 1.5°C. Permanent exceedance forecloses the recovery from climate impacts that an overshoot pathway may permit, yet even an overshoot pathway leaves lasting legacies. To characterise the reversibility of temporary overshoot impacts, we propose a three-layer analytical framework distinguishing hazards (with four dimensions: magnitude, duration, rate of exceedance, and rate of decline), biophysical responses (reversible vs. persistent change), and socioeconomic outcomes (reversible vs. irreversible impact). We introduce the socioeconomic commitment threshold, endogenous to the overshoot trajectory: how much overshoot intensity a system can experience before the losses it accumulates persist after temperatures decline, explained by three mechanisms: non-substitutability, threshold-crossing, and lock-in. A typology linking biophysical persistence with socioeconomic irreversibility illustrates how reversible biophysical change can produce irreversible socioeconomic loss, while persistent biophysical change need not produce irreversible outcomes where adaptive capacity is sufficient. Whether temporary exceedance produces permanent harm is determined primarily by socioeconomic rather than biophysical factors, except where permanent biophysical change exceeds adaptation limits. For systems and communities with low adaptive capacity, overshoot concentrates irreversible legacies even where temperatures subsequently decline, placing equity and justice at the centre of overshoot assessment.