Future trajectory of arctic system evolution

Abstract

The Arctic climate system is undergoing changes in its multiple components, including its hydrologic cycle, as documented in the preceding chapters. The future trajectory of the Arctic climate system becomes a major issue for adapting to anticipated impacts ranging from local-scale impacts on water security (hydropower, infrastructure, and human health), to global-scale impacts such as greenhouse gas releases and sea level rise. Here we highlight projections of changes in key Arctic variables relevant to the Arctic freshwater cycle: precipitation, evapotranspiration, snow, river discharge, surface- and ground-water, and permafrost. We highlight key uncertainties arising from the future emission scenarios and across-model differences. Precipitation and evapotranspiration are both projected to increase, consistent with atmospheric warming and the Clausius-Clapeyron equation. The percentage increases of precipitation projected for the Arctic are among the largest in the world. Evapotranspiration is also projected to increase, although not enough to offset the increase in precipitation, so the freshwater runoff as river discharge is also projected to increase. The projected change signal of the river discharge increase is much greater than the model uncertainty. Projected changes in soil moisture are highly uncertain because of interactions with vegetation, topography, and permafrost, together with uncertainties in local net precipitation minus evapotranspiration. Snow cover duration and maximum accumulation are projected to decrease across most regions of the Arctic, with increased annual maximum accumulations only in high latitudes where increased snowfall dominates the decrease of snow season length. Near-surface permafrost is projected to thaw over large portions of the Arctic, with timing dependent on the rate of warming (emission scenario) and the effects of changes in snow cover. Projected changes become more sensitive to emission trajectories after about the middle of the twenty-first century with projected changes in many variables accelerating under a business as usual scenario in contrast to much slower rates of change where efforts are made to limit GHG emissions.