Photosynthesis is the single most important transformation on Earth. Using the energy in sunlight, all plants—from single-celled algae to towering redwoods—knit carbon dioxide and water into food and release oxygen as a byproduct. Every year, humanity uses up roughly 40 percent of the planet's photosynthesis for our own purposes—from feeding a growing population to biofuels. Given that growing human population, is there a limit to how much of the world's photosynthesis we can appropriate?

Satellite measurements now allow precise measurements of the amount of photosynthesis taking place on the planet's seven continents and assorted islands—or what scientists call "net primary productivity." Such measurements are based on the amount of ground covered by plants, the density of that growth, and observations of temperature, sunlight and available water. Using these measurements, ecological modeler Steven Running of the University of Montana concludes that plants produce nearly 54 billion metric tons of carbohydrates a year—the bulk of it the complex organic chains of cellulose and lignin.

Running has also looked back over the past 30 years and discovered that the total amount of photosynthesis is surprisingly stable. Despite local weather that ranged from droughts to floods, plants soldier on producing roughly the same amount of food year in and year out, varying by less than 2 percent annually. This may be because the inputs of photosynthesis also vary so little—sunlight strength fluctuates only mildly, as does precipitation on a global basis. This finding suggests to Running that the plants' "net primary productivity" might be usefully thought of as a planetary boundary, a threshold or safe limit for human impacts on natural systems, or so he argued in Science on September 20.

A suite of 10 such planetary boundaries had already been proposed in 2009, ranging from climate change to chemical pollution. But Running notes that this measure of photosynthesis involves at least five of those proposed boundaries—land-use change, freshwater use, biodiversity loss, nitrogen and phosphorus cycles—as well as being impacted by at least one more: climate change. And there is no question that photosynthesis on land does have a planetary limit—there is only so much land on which plants can grow.

Moreover, our population is estimated to swell to 9 billion by 2050. Will the plants be able to keep up?

Already, agriculture covers 38 percent of the globe and there's little room to grow further, although once-productive lands in Eastern Europe could be brought back and better management practices could boost output elsewhere. And as the other planetary boundaries suggest, we may be approaching or have already passed geophysical limits for fertilizer application to fields in places like the U.S. or China, as well as the potential to increase the amount of irrigated land to boost crop growth. Finally, we're already diverting more and more agricultural production away from stomachs and into fuel tanks, as exemplified by the U.S. practice of making ethanol from corn.

The vast bulk of the remaining productivity not co-opted by humans is presently inaccessible to us, whether by being part of root systems or protected national parks. That's not to say humanity won't keep trying to expand those boundaries, either by colonizing parks, breaking down formerly inaccessible cellulose to make biofuels, or extending agriculture to the seas in the form of algae farms.

But Running suggests that only roughly 5 billion more metric tons of carbohydrates can be diverted to human uses, meaning a "net primary productivity" boundary of roughly 25.6 billion metric tons. We're closing in on that fast. "The question is thus not whether humans will reach the global [photosynthesis] boundary but when we will do so," he writes. "The obvious policy question must be whether the biosphere can support the 40 percent increase in global population projected for 2050 and beyond."

Image: Courtesy of NASA