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The northern high latitudes are an area of particular importance to global climate change. As a system dependent on freezing conditions, the top of the planet contains vast amounts of carbon in biomass, soils, and permafrost that have the potential to interact with the atmosphere through the biosphere, hydrosphere, lithosphere, and cryosphere. If released en masse, this carbon would greatly exacerbate the levels of greenhouse gases in the atmosphere. Over the past 2 years, a growing body of research has provided evidence of substantial but idiosyncratic environmental changes, with some surprising aspects, across the region. This article reviews some recent findings and presents a new analysis of northern vegetation photosynthetic and productivity trends tracked from Earthobserving satellites.

[1] Over the past 100 years, high northern latitude regions have experienced more rapid warming than elsewhere on Earth. This trend is expected to continue over the next century. Arctic tundra, boreal forests, and peatlands are already undergoing major changes, reinforced by the cascading effects of thawing permafrost, increasing disturbance (particularly fire and insect pests), and altered surface hydrology. These changes influence processes at the ecosystem and landscape scales, including energy balance and vegetation productivity, which feed back to regional and global climate in addition to affecting wildlife habitat and ecosystem resources available to local communities.

Deforestation and forest degradation account for between 7% and 30% of total anthropogenic carbon emissions [Canadell et al., 2007; Denman et al., 2007]. This wide range of values results from three major uncertainties: rates of deforestation, carbon stocks (biomass and soils) in forests prior to deforestation, and changes in carbon stocks within forests (i.e., both increases from growth and decreases from degradation). Historically, rates of deforestation and reforestation, together with estimates of forest biomass, have been used to calculate the net flux of carbon between terrestrial ecosystems and the atmosphere [Woodwell et al., 1983; Detwiler and Hall, 1988; Hall and Uhlig, 1991; Fearnside, 2000; DeFries et al., 2002; Achard et al., 2004; Houghton, 20037]. This net flux is the difference between the sinks of carbon in growing and recovering forests and the sources from burning and decay associated with deforestation. Satellite imagery, particularly from Landsat, has long been used to sample deforestation rates [DeFries et al., 2002; Achard et al., 2004; Skole and Tucker, 1993; Hansen et al., 2008]. Obtaining estimates of biomass, reforestation, and forest growth and degradation, however, has proven more difficult.

The extent, density, and configuration of the built environment—such as buildings, roads, parking lots, and other materials constructed for human use—have an impact on a wide range of biogeochemical and hydrological processes. These built areas, which are impervious to water infiltration, modify hydrology through the combined influence of increased peak flows, reduced base flows, flashier stream hydrographs (decreased lag times between storm events and peak discharge), and changes in bank and streambed erosion [Nilsson et al., 2003]. Additionally, increasing impervious cover has long been known to amplify point source pollution discharges into streams, including chemical runoff from parking lots and roads [Schuelen 1994]. Two maps of the built environment, expressed in terms of impervious surface area, have been derived for areas that encompass the 168,000-square-kilometer Chesapeake Bay watershed (Figure l), a region that has been highly altered by human land use [Goetz et al., 2004; Jantz et al., 2005]. One map was developed for the region at fine (30-square-meter) spatial resolution, and the other covers the extent of the conterminous United States at one-square-kilometer resolution [Elvidge et al., 2004].A finer-resolution regional map was used to assess the quality of the national map, demonstrating the utility the latter map for a range of applications related to monitoring land transformation and assessing watershed impacts.