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.

The findings reveal an overlooked driver of change across African savannas - which cover half the land surface on the continent - that could have a significant impact on wildlife habitats, fire risks and the global carbon cycle.

The study challenges the longstanding assumption that C₄ grasses, which dominate many tropical and subtropical savannas, gain little benefit from increasing carbon dioxide.

Published in Nature, the research shows that higher carbon dioxide levels helps wild savanna grasses reduce water loss through their leaves. This allows them to maintain photosynthesis and produce more aboveground growth when water is scarce.

Dr Kimberley Simpson, Research Fellow in the University of Sheffield’s School of Biosciences, and the study lead, said: “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.

“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 analysed 70 carbon dioxide experiments and a unique 32-year record of grass production from 533 locations in South Africa’s Kruger National Park.

The data revealed a striking trend: grass production in the park increased by 28 per cent between 1989 and 2021.

“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.

The researchers examined other possible causes, including rainfall, temperature, grazing, fire, nitrogen pollution and changes in grass species. However, none of these factors fully explained the dramatic increase.

The trend was instead most consistent with the rise in atmospheric carbon dioxide over the same period. The relative increase in grass growth was also greatest in the park’s driest areas, supporting the experimental results.

Professor Colin Osborne, from the University of Sheffield’s School of Biosciences, added: “Finding the same pattern in controlled experiments and across more than three decades of observations in a natural savanna provides strong evidence that rising carbon dioxide has been changing these ecosystems for some time.”

Savannas cover around a fifth of the Earth’s land surface and contribute to around 30 per cent of global plant productivity. While increased grass growth may sound positive, researchers warn it could have complex, wide-ranging consequences:

Fire risk: More grass can increase the fuel available for savanna fires

Wildlife: Increased vegetation can provide more forage for animals, altering wildlife distribution

Carbon storage: Extra grass does not automatically mean more long-term carbon storage

Dr Simpson added: “More grass does not automatically mean more carbon will be stored. The wider impact depends on where the extra grass ends up. If it is eaten by animals or burned in fires, much of the carbon is quickly released back into the atmosphere."

“Because savannas cover such large areas, even modest changes could have major consequences for wildlife, livestock, fire and the global carbon cycle.”

Climate model simulations suggest that carbon dioxide-driven increases in wild savanna grass production could continue through the 21st century. Although higher temperatures and reduced rainfall predicted under future climatic conditions weakened the effect, they did not eliminate it.

The findings show that the direct effects of rising carbon dioxide must be considered alongside changing temperatures and rainfall when predicting the future of savanna ecosystems.

One of the study’s most intriguing findings was that taller, more productive grass species became increasingly dominant in Kruger over time. This shift explained part—but not all—of the increase in grass biomass, raising new questions about how rising carbon dioxide may reshape savanna plant communities.

Professor Osborne added: “Our next step is to understand whether rising carbon dioxide is changing not only how much grass grows, but also which species thrive. Changes in competition between grasses could have important consequences for how savannas function in the future.”

View the study in full

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