Brazil's Overlooked Savanna Wetlands Store Six Times More Carbon Than Amazon Forest

Larissa Verona spends a lot of her time falling over. The field ecologist trudges through Brazilian wetlands hauling a gas analyzer that costs more than most cars, sinking into mud, stepping into hidden holes, and toppling regularly. She does not mind - as long as the instrument stays dry.

The data that instrument collects, though, tells a story that could reshape how Brazil thinks about carbon. Verona and her colleagues have completed the first deep-soil carbon survey of wetlands in the Cerrado, the vast tropical savanna that covers more than a quarter of Brazil. Their findings, published in New Phytologist, show these waterlogged grasslands are among the most carbon-dense ecosystems on Earth.

1,200 tons per hectare, buried meters deep

The team extracted soil cores reaching four meters below the surface across multiple Cerrado wetland sites - the groundwater-fed ecosystems known locally as campos umidos (wet grasslands) and veredas. Layer by layer, they measured carbon density in the lab. The average: roughly 1,200 metric tons of carbon per hectare, locked in peaty soils formed from partially decomposed plant matter.

That figure is about six times greater than the average carbon density of biomass in the Amazon rainforest. It is exceptionally high by global standards, though some peatlands - including those in the Congo basin - exceed 2,000 metric tons per hectare.

The Cerrado does not look like a place that stores vast quantities of carbon. It is a savanna - open, grassy, scattered with trees. But beneath its wetlands, conditions conspire to preserve organic matter. Water-saturated soils create an oxygen-poor environment that dramatically slows decomposition. Plant material accumulates instead of breaking down, building up over millennia into thick peat deposits.

Carbon older than agriculture itself

Radiocarbon dating by collaborators at the Max Planck Institute in Germany revealed just how long this carbon has been accumulating. The average age of the stored carbon was 11,185 years. Some deposits dated back more than 20,000 years - predating the end of the last Ice Age.

That timescale matters enormously for climate policy. A replanted forest can regrow in decades. Carbon lost from these ancient peat deposits cannot be replaced on any human timescale.

An area six times larger than thought

Veredas and campos umidos are naturally fragmented, scattered in relatively small patches across the Cerrado. Mapping them required extensive modeling. Using remote sensing data combined with machine learning, Verona's team estimated these wetlands may cover 16.7 million hectares - approximately 8% of the Cerrado and 2% of Brazil. That area is at least six times greater than previous estimates.

Based on this expanded mapping, the researchers estimate these wetlands could hold the equivalent of about 20% of the carbon stored in Amazon vegetation. But the team is careful to note this figure needs verification through additional site measurements - their deep-soil sampling, while the most thorough to date, covered a limited number of locations.

When the dry season arrives, carbon escapes

Verona measured greenhouse gas emissions across the Cerrado's wet, dry, and transitional seasons. The results were stark: about 70% of the wetlands' annual carbon emissions occurred during the hot, dry season. Because the vegetation is primarily grasses, which decompose more readily than woody plants, the stored carbon breaks down quickly once soils dry out.

As the Cerrado becomes hotter and drier under climate change, more soil carbon is expected to decompose, accelerating greenhouse gas emissions. Fires compound the problem - they can release carbon accumulated over millennia in a matter of hours, and they clear the way for invasive plants to colonize the wetlands.

The sacrifice biome

But climate is only half the threat. The Cerrado sits at the center of a political tension in Brazil. The country wants to protect the Amazon. It also wants to maintain agricultural production. The result is what Verona calls a sacrifice biome - the Cerrado absorbs the agricultural expansion that might otherwise press into the rainforest.

Agribusinesses are converting Cerrado land for commodity crops, draining wetlands, or diverting their water for irrigation. Senior scientist Amy Zanne at Cary Institute of Ecosystem Studies, a co-author on the study, points out an irony in this tradeoff: the Cerrado contains the headwaters of approximately two-thirds of Brazil's major waterways, including rivers that feed the Amazon itself. Sacrificing the Cerrado for the Amazon may ultimately put the Amazon at risk too.

Brazilian law already protects groundwater-fed wetlands. But enforcement is uneven. In some regions, as much as 50% of these wetlands have already been degraded. Current carbon-protection strategies focus almost entirely on forests, overlooking ecosystems like these entirely.

What remains uncertain

Several important caveats apply. The carbon density measurements come from a limited number of field sites. Extrapolating to 16.7 million hectares introduces substantial uncertainty - the actual total carbon stock could be considerably higher or lower than the team's estimates. The mapping itself, while a major improvement over previous efforts, relies on modeling and remote sensing rather than direct ground-truthing across the full area. The team is continuing to refine both their mapping and their carbon estimates.

The study also does not address how quickly these wetlands might lose their carbon under various climate and land-use scenarios. The seasonal emission measurements provide a baseline, but projecting future losses requires longer-term monitoring across more sites.

Still, the core finding is robust: these wetlands store enormous quantities of ancient carbon that current policies largely ignore. Whether the exact figure is 1,200 or 900 or 1,500 metric tons per hectare, the implication is the same. Brazil's carbon accounting has a blind spot the size of a small country.