Land use has generally been considered a local environmental issue, but it is becoming a force of global importance. Worldwide changes to forests, farmlands, waterways, and air are being driven by the need to provide food, fiber, water, and shelter to more than six billion people. Global croplands, pastures, plantations, and urban areas have expanded in recent decades, accompanied by large increases in energy, water, and fertilizer consumption, along with considerable losses of biodiversity. Such changes in land use have enabled humans to appropriate an increasing share of the planet's resources, but they also potentially undermine the capacity of ecosystems to sustain food production, maintain freshwater and forest resources, regulate climate and air quality, and ameliorate infectious diseases. We face the challenge of managing trade-offs between immediate human needs and maintaining the capacity of the biosphere to provide goods and services in the long term.

Environmental changes in the Arctic may be an early warning system for global climate change, and recent reports from the region are alarming. Several studies have indicated substantial declines in sea ice cover and earlier ice melting, which have led to the lowest level of sea ice in more than a century. And now there is evidence that the warming on the nearby continents may also be accelerating. In his Perspective, Foley discusses results reported in the same issue by Chapin et al. that suggest that reductions in highly reflective snow cover and expanding shrub and tree cover, both caused by recent warming in the Arctic, are amplifying the temperature changes in the region. Reduced snow cover and expanded shrubs and tress both act to absorb additional solar radiation (compared to highly reflective snow fields), warming the surface and the atmosphere above. Chapin et al. provide the best empirical evidence for this climate feedback mechanism to date; these results need to be more fully incorporated into models of future climate change.