Returning global temperatures to a climate target does not necessarily undo the damage caused by exceeding it, new Concordia-led research finds.
The study shows that temporarily overshooting warming targets such as 1.5°C or 2°C can leave lasting changes in the oceans, permafrost and other parts of the climate system, even if temperatures eventually fall back to below these targets.
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.
The researchers measured overshoot using "degree-years" - a measure that combines how much temperatures exceed a target with how long they remain above it. They found that degree-years were a strong predictor of lasting changes in several climate variables.
Some variables were found to be much more susceptible to warming than others. Permafrost, for instance, showed almost no recovery of lost carbon when temperatures returned to baseline levels. Changes to sea-level rise, ocean warming and ocean oxygen levels also largely persisted after overshoot.
The findings suggest that the peak level of warming alone does not tell the whole story. The length of time spent above a target can also determine how much lasting change occurs.
For policymakers, this means climate goals should not be viewed simply as temperature levels that can eventually be reached after limits are breached. The pathway to those temperatures matters. Accounting for the accumulated effects of overshoot could help governments better assess climate risks and plan adaptation measures.
The study was published in Nature Communications Earth and Environment. It was written by Concordia postdoctoral fellow Mitchell Dickau; Damon Matthews, professor, Geography, Planning and Environment; and Kirsten Zickfeld, professor, Geography, Simon Fraser University.
Read the paper: "Irreversible climate changes driven by degree-years of temperature overshoot"
