Temperature Overshoot Would Have Lasting Impacts on Hydrology and Water Resources

Abstract Models of climate change impacts could be missing significant risks to hydrologic and water infrastructure systems through a shared feature: the idea that temperatures rise monotonically. By contrast, temperature overshoot pathways describe non‐monotonic warming trajectories, in which global temperatures first exceed a given target before declining to that target. Risks from overshoot pathways are qualitatively different from risks associated with monotonic warming trajectories, and are likely underestimated in current research and policy. Models suggest overshoot may be almost unavoidable if the more stringent Paris Agreement target limiting warming to 1.5°C over preindustrial levels is to be met by 2100. While overshoot has been relatively widely described in the climate literature, the impacts of overshoot on individual system characteristics have not. We suggest that failure to consider disparities between monotonic and overshoot warming impacts on hydrology and water resources presents particular risks due to divergent adaptation needs. Processes with decadal hysteresis are especially vulnerable. These include glacial contributions to streamflow; hydrologic consequences of vegetation change; altered groundwater; higher water use for fossil fuel combustion and carbon dioxide removal; and water infrastructure and policy that depends on climate conditions. We argue that risks of overshoot cannot be fully captured in current integrated assessment models and that overshoot needs to be specifically evaluated to adequately characterize risk in the water system. We consider how current modeling tools could be adapted to evaluate overshoot consequences, but also recognize that decisions must be made even without perfect knowledge.