Measuring the Human Footprint on Ecosystem Function

Major questions for the biogeosciences concern the consequences of human transformation of landscapes for biogeochemical cycling and other ecosystem functions. While it is clear that the footprint of humans extends to almost all corners of the world, there is no single measure of the anthropogenic effect on ecosystems. Unlike the "Keeling curve" representing anthropogenic carbon dioxide emissions in a single atmospheric measurement, the spatial and temporal heterogeneity of ecosystems and the patchy nature of human activities on the landscape make such a measurement impossible. Here we present two approaches for assessing the consequences of human modification of the landscape on ecosystem processes: the effects of past and future land use change on net primary production at the global scale and analysis of satellite observations over the past 18 years to quantify the carbon dioxide emissions from tropical land use change. Key issues for quantifying human influences on ecosystem function include accounting for natural variability on a variety of time scales and incorporating spatial heterogeneity in landscape-level analysis.