The Arctic, often referred to as a polar desert, is actually a region where the cycling of freshwater has produced—and could produce even greater—changes to biogeophysical and socioeconomic systems important to northern residents, as well as potentially causing extra-Arctic climatic effects. To evaluate this, a scientific assessment (the Arctic Freshwater Synthesis, AFSΣ) was conducted of the Arctic Freshwater System (AFS) at the request of three organizations (WCRP-CliC, AMAP, IASC). Co-authored by 41 scientists from 10 countries, the AFSΣ forms a Special Issue of JGR-Biogeosciences.
Freshwater has been the focus of numerous Arctic studies, which have highlighted its critical role in ecology, human settlement, hydrology, and climate. It was the effect of freshwater on thermohaline circulation that, more than a half-century ago, first focused attention on the AFS’s role in global climate. Subsequent studies, which assessed the budgets and storage of freshwater in the Arctic Ocean and related export to southern convective regions in the salinity-stratified sub-Arctic Atlantic, concluded that even small variations in freshwater content can affect such convection and produce effects analogous to historical halocline catastrophes. Given such recognition of the profound importance of the Arctic freshwater/hydrologic system, and how it could be altered by changes in climate that affect freshwater production, storage, and transport, scientific assessments of the AFS continued to evolve. One important issue affecting the foci of these programs, however, has been the lack of a consistent definition of the geographical domain (Arctic Freshwater Domain, AFD) of the AFS and, hence, the atmospheric, marine, and terrestrial areas that contribute freshwater to it.
As reviewed in the AFSΣ, the classic Arctic freshwater-marine components include the four main oceanic gateways (Bering Strait, Davis Strait, Fram Strait, and Barents Sea Opening) for water flowing to and from the Arctic Ocean, as well as the convective gyres located within the adjacent sub-Arctic Atlantic and Pacific oceans. Given that it is not simply low-salinity water transported from the Arctic Ocean through the gateways but also from latitudes south of the sub-Arctic gyres that controls the degree of gyral convection, in order to obtain a comprehensive understanding of the AFS, an analysis of an AFD broader than simply the Arctic Ocean is necessary. From a terrestrial perspective, a marine-based definition of the AFD is critical because it determines the extent of land contributing freshwater, either from river runoff or from glacier melt—especially from the Greenland Ice Sheet, given its magnitude of runoff and proximity to the North Atlantic convective gyres. From an atmospheric perspective, defining the geography of the AFD also aids in defining the sources and pathways of storm tracks that carry freshwater not only to the Arctic proper, but also to major marine and terrestrial “catchments” that transport the precipitated freshwater to critical regions of the AFD. Notably, as reviewed in the AFSΣ, the alignment, location, and structure of such storm tracks can also be influenced by oceanic freshwater fluxes.
Overall, it has been recognized in repeated freshwater-budget analyses that terrestrial runoff produces the largest input of freshwater to the AFD. Large variations in the size of such input have been reported, but importantly, this was found not due to variability in hydro-climatic conditions that determine river discharge, but more simply to the use of different geographical definitions of the terrestrial contributing area (TCA). Four TCA-definitions have been used historically by different investigators. Notably, there is a strong linear relationship between total flow and contributing area, indicating comparable runoff yields among the spatially conservative to geographically broader TCA-definitions. For the conduct of the AFSΣ, the AAR (All Arctic Regions) definition of the TCA was employed. Importantly, a large majority of the AAR land area (~77%) and precipitation (~80%) that feeds freshwater to the Arctic Ocean via direct river runoff or marine fluxes through the oceanic gateways, are found at latitudes south of the Arctic Circle.
The AFSΣ reviews how hydrologic conditions and controls are changing in the AAR, particularly its various cryosphere (snow-ice-permafrost) and vegetation regimes, which also affect a range of high-latitude ecological processes. Of particular note is the development of a new “near-shore hydro-ecological regime” formed by an intensified hydrologic cycle as the Arctic Ocean becomes increasingly ice-free. Given that the Arctic coast is also experiencing extensive permafrost thaw, the resulting geochemically enriched surface and groundwater flows from the land to the ocean are identified as having special potential for altering primary production and the emergence of new, nearshore marine ecosystems. Similarly, hydro-ecological impacts are outlined for the vast extent of freshwater lake systems affected, like that for sea ice, by decreases in ice coverage. Reductions in the thickness and duration of lake and river ice are also noted to be threatening the viability of northern ice roads, thereby restricting transportation to communities and industries.
The AFSΣ further outlines the operational/management challenges and opportunities for Arctic developments under a changing AFS, the hydro-electric industry being one example, which could expand as water availability increases at higher latitudes. However, one related policy issue that could also develop is the consideration of using some northern freshwater to supply drier southerly latitudes. As stressed by the AFSΣ, being able to more accurately quantify such future freshwater disparities will require the application of an integrated hierarchy of freshwater numerical modeling approaches. In addition, improved modelling of the full spectrum of Arctic Ocean-freshwater pathways would help further define the full southern extent of the AFD, which in the AFSΣ was argued to extend beyond the AAR and even include flow from mid-latitude regions that feed the St. Lawrence River system. Determining the final marine trajectories and potential diluting effect on the North Atlantic system of other major freshwater sources found well outside the Arctic will lead to an even broader global delineation of the AFD, and will require study of other AFS regions.
—Terry D. Prowse, Ph.D., P.Geo., Proposer, “Arctic Freshwater Synthesis,” JGR: Biogeosciences; email: [email protected]