Concordia researchers find time above a climate target matters after temperatures fall

Concordia University reported on September 24 that a temporary rise above a planned warming pathway could leave lasting changes in permafrost and the ocean even after global temperature falls back. Its researchers found that the amount of extra warming accumulated over time helped predict which modeled changes remained. The result gives climate planners a reason to consider how long an overshoot lasts, alongside how high temperatures climb.

The finding comes from a study by Mitchell Dickau and H. Damon Matthews of Concordia University and Kirsten Zickfeld of Simon Fraser University. Their paper was published in June; Concordia’s September report is the current development. The results describe simulations, not observed recovery from a future climate overshoot.

Comparing an overshoot with a baseline

The researchers used the University of Victoria Earth System Climate Model to compare 42 pairs of scenarios. In each pair, one pathway temporarily exceeded a cumulative carbon budget before returning to it by 2100 through net-negative carbon dioxide emissions. The other remained within that budget throughout the century. The team retained pairs in which the overshoot pathway’s global mean temperature returned to its baseline counterpart before 2100; those return years ranged from 2086 to 2100.

The study calls the accumulated difference in global surface air temperature between each pair ‘degree-years’ of overshoot. That measure combines how much warmer the overshoot pathway becomes with how long the extra warmth lasts. Across the retained pairs, it ranged from approximately one to nine degree-years, while peak warming in the overshoot scenarios was roughly 1.5°C to 2°C.

What remained when temperatures matched

At the return point, the study found median reversibility near zero for modeled permafrost carbon pools, total soil carbon in permafrost regions and maximum Atlantic meridional overturning circulation strength. Under the study’s measure, nearly all of the overshoot-related difference in those variables remained when global temperatures matched.

Modeled ocean temperature, ocean oxygen and sea-level rise from thermal expansion had median reversibility of about 20% to 25%. In other words, most of their overshoot-related change also remained at that point. For the most persistent permafrost and ocean measures, degree-years closely tracked the remaining change across the selected scenarios, with reported R-squared values around or above 0.9. That relationship describes results within this model; it is not a measurement of what has happened in the real world.

Peak temperature alone did not account for those persistent differences across the selected scenarios, particularly for ocean temperature, oxygen and thermal-expansion sea-level rise. The results therefore distinguish a brief peak from a longer period of extra warming, even when both pathways eventually reach the same global temperature.

Recovery was different across measures

The model did not show the same response everywhere. Several atmospheric and sea-surface carbon measures had median reversibility above 100%, meaning their values had passed those of the paired baselines by the return year. Global mean temperature was fully reversible at that point by the study’s definition, but regional temperatures varied; high latitudes tended to remain warmer than their paired baselines.

What the model cannot establish

The University of Victoria model is an intermediate-complexity climate model. Its atmosphere is not detailed enough to assess precipitation or atmospheric circulation changes well. Its sea-level calculation includes thermal expansion but not the contribution from melting ice sheets. The study also does not quantify effects on particular communities or establish whether a real future emissions pathway could bring temperatures back down as the simulations assume.

An independently produced 2024 Nature study offers wider context. It found that climate conditions and risks after an overshoot differ from those in a world that avoids one. It also found that falling temperatures could reduce long-term risks compared with simply stabilizing at peak warming, while warning that achieving such a decline is uncertain and could require carbon dioxide removal on a scale constrained by technical, economic and sustainability factors. Those findings provide context for the new report; they do not independently verify its degree-years relationship.