Cerrado’s current drought impossible without human-caused climate change: Study

Under severe drought for decades, Brazil’s Cerrado is experiencing an ongoing water crisis. Its aquifers are losing water faster than they can replenish, rivers are running thin and those living and farming in the drainage area of the once-mighty São Francisco River are beginning to doubt whether they can still rely on the dwindling supply for water and hydroelectric power.

A new study has provided one answer to the long-debated cause of the drought. After analyzing 700 years’ worth of climate data collected in a cave in Minas Gerais, a group of researchers has found that the current drought, the most severe event of its kind in at least the past seven centuries, would have been impossible without human-caused atmospheric warming.

The study, published in Nature Communications, is part of a bigger research effort to understand climate variability and change in central-eastern South America.

Home to 5% of the planet’s plant and animal species, the Cerrado is the world’s most biodiverse savanna and an essential source of water, with nine in 10 Brazilians using electricity originating from water sourced by the biome.

Researchers collected data from Onça Cave, an open-mouth cavern in Peruaçu Caves National Park, in northern Minas Gerais state.

Minas Gerais is just south of the Matopiba region, which includes the states of Maranhão, Tocantins, Piauí and Bahia, and is Brazil’s most recent and important agricultural frontier. “In this region, notably the western Bahia, which borders the northern part of Minas, deforestation and native vegetation conversion are severely impacting ecosystems, especially river flows, as well as the livelihood and culture of local communities,” said Mariana Bombo Perozzi Gameiro, senior adviser at Mighty Earth “It’s important to remember that 12 of Brazil’s major river basins and three aquifers — the Guarani, Bambuí, and Urucuia — all rely on the Cerrado as a source for much of their water.”

Originally, the research team planned to use the data to reconstruct past climate events like strong rains or severe droughts related to volcanic activity. But consulting local communities refocused the scientists’ approach.

“Everyone was asking us why the river was dry. They were speculating about agriculture pumping water or an earthquake that might have caused underground galleries to capture the river,” said Nicolás Misailidis Stríkis, lead author of the study and geoscientist at the University of São Paulo. “The drying trend was the elephant in the room. Once we started studying it, we realized it was widespread all across the drainage basin, not just the São Francisco River,” Stríkis said.

Usually, Stríkis and his team go deep into a cave to study speleothems — mineral deposits that include stalactites and stalagmites — that are untouched by the external climate. But in this case, they chose to look at speleothems close to Onça’s entrance.

“In the back of a cave, the annual variation in temperature and relative moisture levels don’t change. But near the cave entrance, the temperature and relative humidity change a lot, totally in accordance with the outside environment,” Stríkis told Mongabay.

By studying stalagmites’ geochemistry, researchers can measure rain, evaporative potential and temperature over time. When stalactites on the cave ceiling drip onto the cave floor, they build rings of calcite that provide an image of the hydrological balance — the amount of precipitation versus evaporation — at any one point in time. The calcite can also provide insights on temperature variations.

Data combined from two different speleothems — including data on magnesium and oxygen isotope levels — and cross-checked with meteorological information from a nearby weather station suggested a clear reduction in precipitation and higher evapotranspiration.

“We were able to reconstruct 700 years of wet and dry conditions. We did see some sharp changes, but no single variation like this. We observed a trend starting in the 1970s indicating dry conditions that had no counterpart across the entire record,” Stríkis said.

The researchers used the data to build a model able to predict climate conditions across time. By inputting a series of variables, they confirmed that carbon dioxide was the major driver of temperature across the last 700 years, and that the upward trend in carbon dioxide concentration began about 50 years ago.

In the 1970s, the temperature variation can’t be explained solely by the natural climate variability, researchers say. “We saw a decoupling of the temperature from that of Earth’s natural forces. That decade is critical because that’s when the temperature started warming because of carbon dioxide,” Stríkis said. “So what we are facing is anthropogenic severe drought that affects not just society, but the biomes.”

“The 1970s is when soy plantations started to move from southern Brazil to Brazil’s center-west,” Gameiro said. “A few decades later, the epicenter of Brazilian agriculture expansion, especially for soy production, became the Matopiba. Half of the Cerrado’s area was lost due to deforestation and replaced by crops and pasture. Research shows that this destruction leads to increased greenhouse gas emissions and water deficits, drivers of the drought trends observed by the mentioned study.”

For Stríkis and his team, global climate change, not just local water use, caused the region’s drought. “It’s the ecological combination of a reduction in precipitation and an increase in evapotranspiration, all caused by anthropogenic climate change. That’s the worst scenario we can have,” Stríkis said. “Because if it’s just water use causing it, we can use policy or technology to address that. But if it’s climate, it’s the kind of thing that’s complicated to change.”

In the Cerrado, lower river flows undermine access to hydroelectric energy production and water. “Large farms are expanding fast. Crops like soybeans rely on precipitation because fields are often too vast to irrigate. But the impact of this on society relies mostly on politics about sustainable development and water use,” Stríkis said.

If the drought continues, Stríkis thinks the Cerrado and neighboring tropical rainforest biome will retreat, while the semidesert will expand. “The species that live in those biomes are losing genetic diversity and habitat. This is the worst impact, especially after the amount of extinction that we have already caused in the world.” According to Mário Marcos do Espírito Santo, a biologist from the State University of Montes Claros who was not involved in the study, these results are “very important for addressing public policies and climate change.”

“There is a lot of climate-denying here in the region among government representatives and in the rural sectors. As far as I know, this is the only study from this state that shows this level of scientific refinement in a very direct manner. I actually showed this data at my university, and it was very impactful,” he told Mongabay.

While do Espírito Santo acknowledges that the drought is not wholly caused by local land use change, he told Mongabay that stopping deforestation is essential to addressing the consequences of anthropogenic climate change.

“The main source of greenhouse emissions in Brazil and other tropical countries is land use change — deforestation and degradation. You can’t tackle climate change challenges without stopping deforestation. Not just slowing it, but stopping it,” he said.

Citations:

Marengo, J. A., Jimenez, J. C., Espinoza, J., Cunha, A. P., & Aragão, L. E. (2022). Increased climate pressure on the agricultural frontier in the eastern Amazonia–cerrado transition zone. Scientific Reports, 12(1). doi:10.1038/s41598-021-04241-4

Stríkis, N. M., Buarque, P. F., Cruz, F. W., Bernal, J. P., Vuille, M., Tejedor, E., … Novello, V. F. (2024). Modern anthropogenic drought in central Brazil unprecedented during last 700 years. Nature Communications, 15(1). doi:10.1038/s41467-024-45469-8

Banner image: Made up of a mosaic of forest, savanna and grassland vegetation, the Cerrado is the second-largest biome in South America, occupying 24% of Brazil. Image © Marizilda Cruppe/Greenpeace.

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