What is Photosynthesis?

The National Geographic Society describes photosynthesis as the way plants "use sunlight, water, and carbon dioxide to create oxygen," storing the captured energy as glucose. That sugar fuels a tree's growth from roots to crown, and the oxygen is released into the air.

Land plants share that work with the sea. NOAA estimates that roughly half of Earth's oxygen comes from ocean plankton, and that one tiny bacterium, Prochlorococcus, "produces up to 20% of the oxygen in our entire biosphere."

Plant roots absorb water and nutrients and send them up through the tree's xylem to the leaves, which pull carbon dioxide in from the air. Inside each leaf cell, chloroplasts hold chlorophyll, a pigment that absorbs blue and red light and reflects green, which gives foliage its color.

Photosynthesis then unfolds in two stages. The light-dependent reactions, in the chloroplast's thylakoid membrane, use a steady stream of sunlight to split water, release oxygen, and produce the energy carriers ATP and NADPH. In the light-independent stage, the Calvin cycle, those carriers power the conversion of carbon dioxide into carbohydrates within the surrounding fluid, called the stroma. The resulting sugars travel through the tree to build new wood, leaves, roots, and fruit.

Every ton of carbon stored in a tree's trunk, branches, and roots began as atmospheric carbon dioxide drawn in through its leaves. The World Resources Institute's Global Forest Review found that between 2001 and 2025, the world's forests removed an average of 14.4 billion metric tons of carbon dioxide equivalent a year and emitted 9.3 billion metric tons through deforestation, fire, and other disturbances, for a net sink of 5.1 billion metric tons annually.

Rising carbon dioxide has also pushed plants to grow more leaves. A Nature Climate Change study reported by NASA found that a quarter to half of Earth's vegetated land grew greener over 35 years, with carbon dioxide fertilization explaining 70 percent of the effect. "The second most important driver is nitrogen, at 9 percent. So we see what an outsized role CO2 plays in this process," Ranga Myneni,  Earth and Environment professor at Boston University, said in the NASA release.

That boost has limits. "There is simply not enough land on Earth to store all the carbon released from ongoing fossil fuel emissions in trees," Simon Lewis, a tropical ecologist at the University of Leeds, said in a university statement. "Forest management can help, but reducing fossil fuel emissions is essential," he added.