Rising CO2 Is Supercharging Grass Growth in African Savannas, Study Finds

Rising atmospheric carbon dioxide is boosting grass growth in Africa's water-limited savannas, according to a new study led by the University of Sheffield and published in the journal Nature. The findings challenge the longstanding assumption that C4 grasses — a highly efficient form of photosynthesis used by most tropical and subtropical savanna grasses — gain little benefit from rising carbon dioxide. Instead, the study shows that higher carbon dioxide levels help wild savanna grasses reduce water loss through their leaves, allowing them to keep photosynthesizing and produce more growth above ground when water is scarce.

"The grasses that dominate savannas have often been considered largely unresponsive to rising carbon dioxide because they already use a highly efficient form of photosynthesis under hot and high-light conditions," said Dr. Kimberley Simpson of the University of Sheffield's School of Biosciences, the study's lead author. "Our findings challenge that assumption. Under dry conditions, higher carbon dioxide helps these grasses conserve water, reduce drought stress and continue growing."

The international research team analyzed 70 carbon dioxide experiments alongside a 32-year record of grass production from 533 locations in South Africa's Kruger National Park, which showed grass production there increased by 28% between 1989 and 2021. The researchers examined other possible causes, including rainfall, temperature, grazing, fire, nitrogen pollution and changes in grass species, but none fully explained the increase, which was instead most consistent with the rise in atmospheric carbon dioxide and was greatest in the park's driest areas. "This increase could have important impacts on fire activity and on wildlife, as well as on the carbon cycle, in this sensitive ecosystem and others like it," said Carla Staver, professor of ecology at Princeton University and study co-lead.

Savannas cover around one-fifth of Earth's land surface and contribute around 30% of global plant productivity, so the researchers warn the extra grass growth could have wide-ranging consequences, including more fuel for savanna fires and altered wildlife distribution. "More grass does not automatically mean more carbon will be stored," Simpson said. "If it is eaten by animals or burned in fires, much of the carbon is quickly released back into the atmosphere." Climate model simulations suggest the effect could continue through the 21st century, though higher temperatures and reduced rainfall may weaken it.