Vital Signs

Snow cover is a defining characteristic of the Arctic land surface for up to 9 months each year, evolving from complete snow cover in the winter to a near total loss of snow cover by the summer. Highly reflective snow cover acts to cool the climate system, effectively insulates the underlying soil, and stores and redistributes water in solid form through the accumulation season before spring melt. Snow on land in spring has undergone significant reductions in areal extent during the satellite era, which impacts the surface energy budget, ground thermal regime (with associated effects on geochemical cycles), and hydrological processes. The loss of spring snow cover is a clear indicator of change in the terrestrial cryosphere, much in the same way summer sea ice loss is indicative of changes in the marine cryosphere.

The Greenland ice sheet plays a crucial role globally and locally, impacting the surface energy budget and climate and weather and contributing to current and future sea level rise.

The Arctic sea ice cover is vast in areal extent covering millions of square kilometers, but is only a thin veneer a few meters thick. This ice cover plays many roles. It is a barrier limiting the exchange of heat, moisture, and momentum between the atmosphere and ocean; a home to a rich marine ecosystem, including human communities; and an indicator of climate change. Sea ice extent has been monitored using passive microwave instruments on satellite platforms since 1979. The months of September and March are of particular interest because they are the months when the Arctic sea ice typically reaches its maximum and minimum extent respectively.

Summer sea surface temperatures in the Arctic Ocean are set by absorption of solar radiation into the surface layer. In the Barents and Chukchi seas, there is an additional contribution from advection of warm water from the North Atlantic and Pacific Oceans (for a recent assessment of this in the Chukchi Sea, see Serreze et al., 2016). Solar warming of the ocean surface layer is influenced by the distribution of sea ice (with more solar warming in ice-free regions), cloud cover, water color, and upper-ocean stratification (river influxes influence the latter two).

Primary productivity is the rate at which atmospheric or aqueous carbon dioxide is converted by autotrophs (primary producers) to organic material. Primary production via photosynthesis is a key process within the ecosystem, as the producers form the base of the entire food web, both on land and in the oceans. The oceans play a significant role in global carbon budgets via photosynthesis. Approximately half of all global net annual photosynthesis occurs in the oceans, with ~10-15% of production occurring on the continental shelves alone (Müller-Karger et al. 2005). Primary productivity is strongly dependent upon light availability and the presence of nutrients, and thus is highly seasonal in the Arctic region. In particular, the melting and retreat of sea ice during spring are strong drivers of primary production in the Arctic Ocean and its adjacent shelf seas due to enhanced light availability…

Vegetation in the Arctic tundra has been responding dynamically over the course of the last several decades to environmental changes, many of which are anthropogenically-induced. These vegetation changes throughout the circumpolar Arctic are not spatially homogeneous, nor are they temporally consistent (e.g. Bhatt et al. 2013), suggesting that there are complex interactions among atmosphere, ground (soils and permafrost), vegetation, and herbivore components of the Arctic system. Changes in Arctic tundra vegetation may have a relatively small impact on the global carbon budget through photosynthetic uptake of CO₂, compared to changes in other carbon cycling processes (Abbott et al. 2016). However, tundra vegetation can have important effects on permafrost, hydrological dynamics, soil carbon fluxes, and the surface energy balance (e.g. Blok et el. 2010, Myers-Smith and Hik 2013, Parker et al. 2015). Tundra vegetation dynamics also control the diversity of herbivores (birds and mammals) in the Arctic, with species richness being positively related to vegetation productivity (Barrio et al. 2016). To improve our understanding of these complex interactions and their impacts on the Arctic and global systems, we continue to evaluate the state of the circumpolar Arctic vegetation.

Arctic air temperatures are both an indicator and a driver of regional and global changes. Although there are year-to-year and regional differences in air temperatures due to natural random variability, the magnitude and Arctic-wide character of the long-term temperature increase is a major indicator of global warming (Overland 2009). Here we report on the spatial and temporal variability of Arctic air temperatures during the period October 2014 through September 2015, the 12-month period since the end of the previous reporting period (Overland et al. 2014).

The Arctic (land areas north of 60°N) is always completely snow covered in winter, so it is the transition seasons of fall and spring that are significant when characterizing variability and change…

Estimates of the spatial extent of melting across the Greenland ice sheet in 2015, derived from spaceborne brightness temperatures recorded by the Special Sensor Microwave Imager/Sounder (SSMIS) passive microwave radiometer (e.g., Mote 2007; Tedesco 2007; Tedesco et al. 2013), show that melting occurred over more than half of the ice sheet for the first time since the exceptional melt events of July 2012…

Sea ice extent is the primary descriptor of the state of the Arctic sea ice cover. Satellite-based passive microwave instruments have been used to determine sea ice extent since 1979…