The extent to which pollution from tar sands industrial activities in northeastern Alberta, Canada affects ecosystem and human health is a matter of growing concern that is exacerbated by uncertainty. In this paper we determine whether physical and ecological changes that result from tar sands industrial activities are detectable. We analyze a diverse set of environmental data on water and sediment chemistry, contaminants in wildlife, air emissions, pollution incidents, traditional ecological observations, human health, and landscape changes from the Athabasca Tar Sands region, Canada. Increases in contaminants in water, sediment, and fishes downstream of industrial sources; significant air emissions and major pollution incidents; and the loss of 65,040 ha of boreal ecosystems are documented. Present levels of some contaminants pose an ecosystem or human health risk. The effects of these pollutants on ecosystem and public health deserve immediate and systematic study. Projected tripling of tar sands activities over the next decade may result in unacceptably large and unforeseen impacts to biodiversity, ecosystem function, and public health. The attention of the world's scientific community is urgently needed.