Lead author Mitchell Dickau, a postdoctoral fellow at Concordia, together with Damon Matthews, professor of Geography, Planning and Environment at Concordia, and Kirsten Zickfeld, professor of Geography at Simon Fraser University, set out to quantify what happens when warming pathways exceed their targets before returning to them. Rather than treating an overshoot as a temporary excursion with no memory, the team treated the climate system as an integrated whole whose components respond on very different timescales. The ocean, the cryosphere and the carbon cycle each carry their own inertia, and that inertia is precisely what allows the damage of an overshoot to outlast the overshoot itself.

To do this, the researchers used the University of Victoria Earth System Climate Model, a well-established intermediate-complexity model capable of simulating interactions among the atmosphere, ocean, sea ice, land surface and carbon cycle over centuries. The team constructed 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 throughout. By comparing the two members of each pair at the moment their temperatures had reconverged, the researchers could isolate the effects of the overshoot itself, controlling for the eventual temperature level that both pathways shared.

A central innovation of the study lies in how overshoot was measured. Instead of relying only on the peak temperature reached, the researchers quantified overshoot in degree-years, a metric that combines how much temperatures exceed a target with how long they remain above it. A pathway that exceeds 1.5°C by 0.2 degrees for twenty years accumulates four degree-years of overshoot, for example, while a brief spike of the same magnitude lasting only a few years accumulates far less. This framing captures the cumulative exposure of the climate system to elevated temperatures, much as accumulated dose matters in toxicology rather than a single peak concentration.

The results were striking in their consistency. Degree-years emerged as a strong predictor of lasting changes in several key climate variables. The more cumulative exposure the system experienced above a target, the larger the residual differences that remained once temperatures had fallen back. In other words, the climate system keeps a ledger, and the balance of that ledger is written in degree-years rather than in peak degrees alone. This gives policymakers and scientists a practical, quantitative handle on a problem that has often been discussed in vague terms of irreversibility.

Not all parts of the Earth system proved equally vulnerable, however. Some variables recovered substantially once temperatures returned to baseline, reflecting the relatively fast response of atmospheric and near-surface processes. Others showed almost no recovery at all. Permafrost stood out as the most unforgiving example: the carbon lost from thawing soils showed essentially no return when temperatures came back down. Once frozen ground thaws and its organic carbon is released to the atmosphere through microbial decomposition, there is no mechanism within a plausible cooling timescale that refreezes that carbon and restores the original store. The loss is, for practical human purposes, one-way.

The oceans told a similar story of persistence. Sea-level rise, ocean heat content and ocean oxygen levels all largely retained the changes imposed during the overshoot period. The physics here is well understood. The ocean absorbs enormous quantities of heat, and because of its vast volume and slow circulation, that heat is held for centuries. Thermal expansion of seawater, which contributes to sea-level rise, cannot be quickly undone; even if the surface cools, the deep ocean continues to adjust over many generations. Ocean oxygen levels, meanwhile, respond to warming through reduced solubility and altered circulation patterns, and these too recover far more slowly than atmospheric temperature itself.

These findings strike directly at the architecture of international climate policy. The Paris Agreement’s temperature goals have often been interpreted, implicitly, as thresholds that could be crossed and later reclaimed through net-negative emissions, a strategy sometimes described as overshoot-and-return. The new research shows that this interpretation understates the risks. If the peak level of warming alone does not tell the whole story, then the length of time spent above a target becomes an independent dimension of climate damage. Two pathways that both peak at, say, 1.8°C could leave very different worlds behind depending on how long they lingered above 1.5°C on the way up and on the way down.

For governments, the practical implication is that climate goals should not be viewed simply as temperature levels that can eventually be reached again after limits are breached. The pathway to those temperatures matters, and the accumulated effects of overshoot should be factored into how climate risks are assessed and how adaptation measures are planned. A country planning coastal defenses, for instance, cannot assume that sea levels projected for a stabilized temperature will apply if that temperature was reached through a long overshoot; the ocean’s memory of the excursion will already be baked into the shoreline. Similarly, carbon accounting that treats permafrost losses as reversible would systematically understate the true emissions cost of an overshoot pathway.

The study, based on computational simulation and modeling rather than direct observation, carries the usual caveats of model-based research, and the University of Victoria model, like all Earth system models, represents complex processes with parameterizations that carry uncertainty. Yet the direction of the findings aligns with a growing body of literature on the asymmetric, path-dependent behavior of the climate system, and the use of 42 scenario pairs gives the conclusions a robustness that single comparisons would lack. As nations weigh the feasibility of temporary overshoot against the harder task of never exceeding their targets at all, the message from this research is unambiguous: degree-years accumulate, and the Earth system does not forget them. The safest overshoot, the study implies, remains the one that never happens.

Subject of Research: Lasting climate system impacts of temporary temperature overshoot beyond warming targets

Article Title: RESEARCH: Climate overshoot will leave lasting impacts even after global temperatures fall

Article References: RESEARCH: Climate overshoot will leave lasting impacts even after global temperatures fall. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: climate overshoot, degree-years, permafrost carbon, sea-level rise, ocean warming, ocean oxygen, 1.5°C target, Paris Agreement, Earth system modeling, irreversibility, climate policy, Communications Earth & Environment