El Niño exacerbates effects of drought on Amazon groundwater and increases fire risk, study finds

The risk of fires in the Amazon is greater in regions where groundwater storage is compromised, especially when El Niño exacerbates the drought. Using satellite images and data from fires, researchers have been able to demonstrate the relationship between the climatic phenomenon and the propensity for fires, creating a tool that could help with future prevention efforts.

The results of the study, based on information from 2004 to 2016, show a decrease in moisture conditions at three levels—topsoil (SFSM), tree root zone (RTZSM) and groundwater (GWS), with the latter being the most arid. It is these “reservoirs” that take the longest to recover when they are affected by successive and extreme droughts caused by El Niño, one of the climatic phenomena with the greatest impact on Earth.

In recent decades, human-caused (anthropogenic) forest fires have significantly altered the dynamics of vegetation in the Amazon region. These human activities are considered “ignitions” for fire in the rainforest, and the escalation of fires is linked to climatic conditions.

This year alone, the total number of fires recorded in the Brazilian Amazon from January to November 20th is the highest since 2010—132,211 in just under 11 months, according to data from Brazil’s National Institute of Space Research (INPE). Using a different methodology to that of INPE, the Laboratory of Environmental Satellite Applications (LASA) of the Federal University of Rio de Janeiro (UFRJ) points out that about 128,000 km² of the biome have already been burned this year, an area equivalent to the territory of England.

“We decided to investigate the impact of the meteorological and hydrological drought on fires in the Amazon, paying attention to the role of groundwater and El Niño events, as part of the SACRE project, which focuses more on urban areas, but also looks at rural areas and forests. And we’ve been able to demonstrate the relationship,” says Professor Bruno Conicelli of the Institute of Geosciences at the University of São Paulo (USP), corresponding author of the study.

The article, published in the journal Science of the Total Environment, was co-authored by researcher Ricardo Hirata, coordinator of “SACRE: Integrated Water Solutions for Resilient Cities.” One of the largest applied research projects on water resources in Brazil, SACRE focuses on groundwater and aims to reduce the vulnerability of cities and rural areas to crises related to global climate change.

Cross-referencing satellite data

To characterize the hydrological drought, the researchers used satellite data from the GRACE mission, which measures terrestrial water storage by integrating soil moisture, surface water, and groundwater.

They cross-referenced this with data on the severity of the drought in each of the locations studied. They were able to identify areas of lower humidity concentration in the northeast of the Amazon basin, as well as a decrease in humidity toward the east.

The largest areas burned coincided with regions that experienced drought during extreme El Niño events, with an increase between 2015 and 2016. At the time, the phenomenon was considered one of the three most intense on record (along with 1982/83 and 1997/98). According to the World Meteorological Organization (WMO), the 2023/2024 event ranks among the five strongest.

El Niño is characterized by an abnormal warming of the surface of the Pacific Ocean due to a decrease in the intensity of the trade winds. Atmospheric circulation patterns over the Pacific are altered, with a change in the distribution of humidity and temperatures in different parts of the planet. International reports indicate that the frequency and intensity of this event will increase in the coming decades.

“We know that fires in the Amazon have anthropogenic origins. However, when there’s a record of a more intense El Niño, as there was in 2016, which we investigated, and again in 2024, meteorological and hydrological droughts become more severe in the forest. Under these conditions, the vegetation depends heavily on groundwater to survive. The smaller trees, with shallower roots, are the first to suffer from the lack of water,” says Conicelli, who was the supervisor of the paper’s first author, Naomi Toledo. When the research began, she was a student at the IKIAM Amazon Regional University in Ecuador, where Conicelli was a professor for four years.

In August, an international group published the first State of Wildfires report, showing that fires in the western Amazon—which includes the Brazilian states of Amazonas, Acre, Roraima and Rondônia—between March 2023 and February 2024 were driven by prolonged droughts linked to El Niño. Combined with weather conditions, droughts explained 68% of these fires, followed by the influence of anthropogenic activities such as deforestation, agriculture and fragmentation of natural landscapes.

Alert system

Based on the results of the work, the group is developing a fire risk index adapted to the Amazon region, including both meteorological indicators (linked to rainfall) and hydrological indicators (water in the soil, rivers, aquifers and other reserves). The model can be applied to other ecosystems.

By demonstrating the link between meteorological and hydrological conditions and the aggravation of forest fires, the results can contribute to fire risk reduction and prevention strategies. “Studies like these are also important for raising awareness of how vulnerable the forest is to extreme weather events, which are increasingly frequent and intense,” adds the researcher.

In the future, Conicelli says, the system will be able to add data collected in the field to warn of declining groundwater levels.