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.
Uur population is estimated to swell to 9 billion by 2050. Will the photosynthesis on this planet be able to keep up?