A Measurable Planetary Boundary for the Biosphere

Terrestrial net primary (plant) production provides a measurable boundary for human consumption of Earth's biological resources. Forty years ago, Meadows et al. published a landmark first analysis of global limits to human activity ( 1 ). Based on a primitive computer model of the Earth system, they concluded that by the early decades of the 21st century, tangible limits to key global resources would begin to emerge. A reanalysis of the original results in 2008 found that the original global resource depletion projections were remarkably accurate ( 2 ). Since then, Rockström et al. ( 3 ) have defined a new term—planetary boundaries—to describe nine variables of high importance to habitability of Earth, including climate change, ocean acidification, land-use change, and biodiversity loss. These metrics are compelling conceptually, but many are not easily measured globally; explicitly defining a critical boundary is even more challenging. I suggest a new planetary boundary, terrestrial net primary (plant) production (NPP), that may be as compelling conceptually, integrates many of the currently defined variables, and is supported by an existing global data set for defining boundaries.