New research led by Concordia University suggests that even if global temperatures return to a climate target after temporarily exceeding it, the damage caused by this overshoot may be irreversible. The study, published in the journal Communications Earth & Environment, highlights that going above warming targets like 1.5°C or 2°C—before coming back down—can lead to long-lasting changes in the oceans, permafrost, and other parts of the climate system. These changes may not reverse even if temperatures return to the original target, which has important implications for climate policy and planning. To explore this, the researchers used the University of Victoria Earth System Climate Model to compare 42 pairs of climate scenarios. In each pair, one scenario temporarily exceeded its warming pathway before returning to it, while the other stayed below the threshold. This approach allowed the team to examine the effects of temporary overshoot. To measure the extent of overshoot, they used a unit called "degree-years," which combines how much temperatures exceed a target with how long they remain above it. This metric helps quantify the cumulative impact of exceeding climate goals over time. The study found that degree-years were a strong predictor of lasting changes in several climate variables. For example, permafrost—frozen ground that stores large amounts of carbon—showed almost no recovery of lost carbon even when temperatures returned to baseline levels. Similarly, changes to sea-level rise, ocean warming, and oxygen levels in the ocean also largely persisted after the temperature overshoot. This means that the peak level of warming alone does not tell the whole story. The duration of time spent above a target can also determine how much lasting change occurs in the climate system. For policymakers, this research suggests that climate goals should not be viewed simply as temperature levels that can be reached after limits are temporarily breached. The pathway to those temperatures—whether it involves overshooting or staying within limits—matters significantly. Understanding the accumulated effects of overshoot could help governments better assess climate risks and plan for necessary adaptation measures. This study underscores the importance of considering not just the final temperature outcome, but also the journey to get there.