Influence of the physical terrestrial Arctic in the eco-climate system

This synthesis paper provides a summary of the major components of the physical terrestrial Arctic and the influences of their changes upon the larger eco-climate system. Foci here are snow cover, permafrost, and land hydrology. During the last century, snow cover duration has shortened in a large portion of the circum-Arctic, mainly because of its early northward retreat in spring due to warming. Winter precipitation has generally increased, resulting in an increase in maximum snow depth over large areas. This is consistent with the increase in river discharge over large Russian watersheds. Soil temperature has also increased, and the active layer has deepened in most of the permafrost regions, whereas thinning of the seasonally frozen layer has been observed in areas not underlain by permafrost. These active components are mutually interrelated, conditioned by ambient micro- to landscape-level topography and local surface and subsurface conditions, and they are closely related with vegetation and ecology, as evidenced by evolution in the late Quaternary. Further, we provide examples and arguments for discussions on the pathways through which changes in the Arctic terrestrial system can affect or propagate to remote areas beyond the Arctic, reaching to the extratropics in the larger climate system. These considerations include dynamical and thermodynamical responses and feedbacks,'modification of hemisphere-scale atmospheric circulation associated with troposphere-stratosphere couplings, and moisture intrusion at a continental scale.

Relevance of hydro-climatic change projection and monitoring for assessment of water cycle changes in the Arctic

Rapid changes to the Arctic hydrological cycle challenge both our process understanding and our ability to find appropriate adaptation strategies. We have investigated the relevance and accuracy development of climate change projections for assessment of water cycle changes in major Arctic drainage basins. Results show relatively good agreement of climate model projections with observed temperature changes, but high model inaccuracy relative to available observation data for precipitation changes. Direct observations further show systematically larger (smaller) runoff than precipitation increases (decreases). This result is partly attributable to uncertainties and systematic bias in precipitation observations, but still indicates that some of the observed increase in Arctic river runoff is due to water storage changes, for example melting permafrost and/or groundwater storage changes, within the drainage basins. Such causes of runoff change affect sea level, in addition to ocean salinity, and inland water resources, ecosystems, and infrastructure. Process-based hydrological modeling and observations, which can resolve changes in evapotranspiration, and groundwater and permafrost storage at and below river basin scales, are needed in order to accurately interpret and translate climate-driven precipitation changes to changes in freshwater cycling and runoff. In contrast to this need, our results show that the density of Arctic runoff monitoring has become increasingly biased and less relevant by decreasing most and being lowest in river basins with the largest expected climatic changes.