CO2 is quietly adding fuel to Africa's savannas, and the models never counted it
A Nature study reports grass production in Kruger National Park rose 28% between 1989 and 2021, with rising CO2 the only explanation that fits. C4 grasses were assumed to be indifferent.
The Scientist · Science desk
What happened
- A study led by the University of Sheffield and published in Nature finds that rising atmospheric carbon dioxide is boosting grass growth in Africa's water-limited savannas.
- African savannas cover half the continent's land surface.
- The research challenges the longstanding assumption that C4 grasses, which dominate many tropical and subtropical savannas, gain little benefit from increasing carbon dioxide.
- The international research team analysed 70 carbon dioxide addition experiments and a 32-year record of grass production from 533 locations in South Africa's Kruger National Park.
- Grass production in Kruger National Park increased by 28% between 1989 and 2021.
Why it matters
The working assumption in savanna science has been that C4 grasses, which already concentrate CO2 inside their leaves, gain little from more of it in the air. A study led by the University of Sheffield and published in Nature reports the opposite under dry conditions, and it matters because C4-dominated savannas cover half of Africa's land surface [1][2][3].
The evidence comes in two parts. A meta-analysis of 70 CO2-addition experiments plus a 32-year record of grass production from 533 locations in South Africa's Kruger National Park [4]. In the experiments, grasses narrowed their stomata at higher CO2, cutting water loss while still increasing carbon gain [10]. In the field, that improved water use efficiency showed up as biomass: production rose 28% between 1989 and 2021 [5], which works out to roughly 0.9 percentage points a year on average [22]. The paper puts the CO2-driven component at 75.1 g per square metre of annual production, with a 95% confidence interval of 74.5 to 75.8, or 0.37 tons of carbon per hectare [8]. That interval is about 1.7% wide relative to the central estimate, which tells you it is a modelled attribution rather than a spread of field measurements [23].
The authors worked through the alternatives: rainfall, temperature, grazing, fire, nitrogen deposition and shifts in grass species. None fully accounted for the increase [6]. The residual pattern tracked atmospheric CO2, and the relative gain was largest in the park's driest areas, which is what the stomatal mechanism predicts [7]. Professor Colin Osborne of Sheffield said finding the same pattern in controlled experiments and in three decades of field observation is strong evidence the effect has been operating for some time [18].
Why the assumption held for so long is instructive. The C4 pathway evolved as a CO2-concentrating mechanism that suppresses photorespiration, and in C4 crops bred hard for yield it eliminates CO2 fertilisation when water is plentiful; wild C4 grasses photosynthesise less efficiently, leaving room for a response [11]. In situ CO2 enrichment experiments have also largely skipped tropical and subtropical ecosystems [19], and the long-term savanna work that did exist was aimed at C3 woody encroachment, not the grass layer [20].
The consequences do not point one way. Savannas contribute about 30% of terrestrial net primary production [9], so a shift in the grass layer is not a rounding error in the carbon budget. But extra grass is not stored carbon. Dr Kimberley Simpson, the study lead, said the wider impact depends on where the grass ends up, and that if it is eaten or burned much of the carbon returns to the atmosphere quickly [14]. More grass also means more fine fuel for fires [17]. The Nature abstract is explicit that the fate of the carbon is uncertain, depending on feedbacks with fire, herbivory and woody vegetation [16]. Carla Staver of Princeton, the study co-lead, said the increase could matter for fire activity, wildlife and the carbon cycle [15].
Two things to watch. Simulations with the Community Land Model suggest the fertilisation effect on C4 aboveground productivity keeps increasing under future conditions [12], with warming and drying weakening it but not cancelling it [13]. And taller, more productive grass species became increasingly dominant in Kruger over the record, explaining part but not all of the biomass gain [21]. Fire-risk and land-carbon models that treat the savanna grass layer as CO2-insensitive are now carrying a known bias, and the size of it is being reported at 0.37 tons of carbon per hectare per year [8].
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence82
- Adoption
- Not scored
- Hype gap+12
- Incentives42
- Confidence68
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
A study led by the University of Sheffield and published in Nature finds that rising atmospheric carbon dioxide is boosting grass growth in Africa's water-limited savannas.
- [2]
African savannas cover half the continent's land surface.
- [3]
The research challenges the longstanding assumption that C4 grasses, which dominate many tropical and subtropical savannas, gain little benefit from increasing carbon dioxide.
- [4]
The international research team analysed 70 carbon dioxide addition experiments and a 32-year record of grass production from 533 locations in South Africa's Kruger National Park.
- [5]
Grass production in Kruger National Park increased by 28% between 1989 and 2021.
- [6]
The researchers examined other possible causes including rainfall, temperature, grazing, fire, nitrogen pollution and changes in grass species; none of these factors fully explained the increase.
- [7]
The trend was most consistent with the rise in atmospheric carbon dioxide over the same period, and the relative increase in grass growth was greatest in the park's driest areas.
- [8]
The Nature abstract reports that annual aboveground grass production in southern Africa increased by 28% over three decades, a CO2-driven increase of 75.1 g per square metre (95% confidence interval 74.5 to 75.8 g per square metre) or 0.37 tons of carbon per hectare of annual production.
- [9]
C4-grass-dominated tropical and subtropical savannas contribute approximately 30% of terrestrial net primary production.
- [10]
In the CO2-addition experiments, grasses reduced stomatal conductance under higher levels of CO2, limiting water loss while increasing carbon gain.
- [11]
The C4 photosynthetic pathway evolved as a CO2-concentrating mechanism that suppresses photorespiration; in C4 crops selected intensively for high yields this mechanism eliminates CO2 fertilization under well-watered conditions, but photosynthesis in wild C4 grasses is less efficient and CO2 fertilization has been observed across a diversity of wild C4 grass species.
- [12]
Simulations via the Community Land Model suggest that CO2 fertilization of C4 grass aboveground productivity may continue to increase under future conditions.
- [13]
Higher temperatures and reduced rainfall predicted under future climatic conditions weakened the effect but did not eliminate it.
- [14]
Dr Kimberley Simpson, the study lead, said more grass does not automatically mean more carbon will be stored, that the wider impact depends on where the extra grass ends up, and that if it is eaten by animals or burned in fires much of the carbon is quickly released back into the atmosphere.
- [15]
Carla Staver, professor of ecology at Princeton University and study co-lead, said the increase could have important impacts on fire activity, on wildlife and on the carbon cycle in this sensitive ecosystem and others like it.
- [16]
The Nature abstract states that the fate of this carbon is uncertain, depending on feedbacks with fire, herbivory and woody vegetation, and that effects on the global carbon cycle may be profound.
- [17]
More grass can increase the fuel available for savanna fires.
- [18]
Professor Colin Osborne of the University of Sheffield's School of Biosciences said 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.
- [19]
In situ CO2 enrichment experiments have largely omitted major tropical and subtropical ecosystems.
- [20]
In tropical and subtropical savannas, long-term observations and modelling have suggested a role for CO2 fertilization as a driver of C3 woody plant encroachment, but equivalent long-term analyses of CO2 responses in C4 savanna grasses were lacking.
- [21]
Taller, more productive grass species became increasingly dominant in Kruger over time, a shift that explained part but not all of the increase in grass biomass.
- [22]
A 28% increase spread over the 1989 to 2021 period averages about 0.9 percentage points of additional grass production per year.
- [23]
The reported 95% confidence interval of 74.5 to 75.8 g per square metre is 1.3 g wide, about 1.7% of the 75.1 g central estimate.
Sources
2 independent publishers whose own reporting we read for this story.
- nature.comIncreasing CO<sub>2</sub> levels fertilize C<sub>4</sub> grass production
1 article · August 18, 2026
- phys.orgRising carbon dioxide is supercharging grass growth in African savannas, study finds
1 article · August 20, 2026
Topics and entities
Topics
- CO2 Fertilization
- Earth System Modelling
- Long-Term Ecological Monitoring
- Terrestrial Carbon Cycle
Entities
- Princeton University
- University of Sheffield
- Kimberley J. Simpson
- Community Land Model
- Kruger National Park
- Nature