Overshoot pathways of 1.5°C: reversible biophysical change, irreversible socioeconomic impacts

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

Exceedance of 1.5°C in the near term is now unavoidable. Among pathways consistent with the remaining carbon budget, an overshoot pathway, in which exceedance is followed by decline to or below 1.5°C through net-negative emissions, is the best case of what remains achievable. Permanent exceedance produces strictly worse outcomes, yet even an overshoot pathway leaves lasting legacies. We propose a three-layer analytical framework distinguishing the hazard (the four overshoot dimensions: magnitude, duration, rate of exceedance, and rate of decline), the biophysical response (reversible vs. persistent change), and the socioeconomic outcome (reversible vs. irreversible impact). We introduce the socioeconomic commitment threshold, an adaptation limit that is endogenous to the overshoot trajectory, beyond which cumulative overshoot effects commit socioeconomic losses that persist after temperatures decline, governed by three mechanisms: non-substitutability, threshold-crossing, and lock-in. A typology linking biophysical persistence with socioeconomic irreversibility illustrates that 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 factors, not biophysical ones. For systems and communities with low adaptive capacity, overshoot concentrates irreversible legacies even where temperatures subsequently decline, placing justice at the center of overshoot assessment.