Measuring the planet to fill terrestrial data gaps

[first paragraph] How a growing world population can sustain itself on Planet Earth has been the focus of much research. Norman Borlaug (1), the ‘‘father of the Green Revolution,’’ argued that one answer is to intensify agriculture to produce more food on the same area of land. But what he promoted as a policy strategy, others have called the ‘‘Borlaug hypothesis,’’ aiming to add it to sustainability science theory and test it empirically. In this issue of PNAS Rudel et al. (2) follow earlier national studies (3) by testing the hypothesis at global scale. They demonstrate the existence of the general phenomenon of ‘‘land sparing,’’ by showing that cropland area has increased more slowly than population since 1970. However, they find relatively little evidence that intensification has gone further, by shrinking cropland and generating surplus ‘‘spared land,’’ and much of this evidence is linked to changes in trade patterns. Yet their article will prove important for introducing the concept of spared land into the literature, inspiring more research, and stimulating debate about how land sparing relates to existing theory. This commentary focuses on a key challenge they identify (the measurement of land sparing) and wider monitoring issues raised by it. It refers mainly to developing countries.