Researchers identify causes of the extreme drought that affected the Pantanal

A natural meteorological phenomenon similar to the one that caused the water crisis in the State of São Paulo, between 2014 and 2016, was responsible for the extreme drought in the Pantanal recorded between 2019 and 2020 and considered the worst in the last 50 years.

The finding was made by researchers from the National Disaster Monitoring and Alert Center (Cemaden), the National Institute for Space Research (Inpe) and the São Paulo State University (Unesp) through a the project supported by FAPESP within the scope of the Research Program on Global Climate Change (PFPMCG).

The results of the study were published in the magazine Frontiers in Water.

“The recent drought in the Pantanal was caused by a phenomenon we call meteorological blockage, characterized by the emergence of an area of ​​high pressure that prevented the formation of rain throughout the Central-West region of South America. Consequently, the temperature became very high, and relative humidity very low”, José Marengo tells Agência FAPESP, Cemaden researcher and study coordinator.

“The combination of lack of rain with high temperatures and very low relative humidity led to an increased risk of fire, which spread not only to agricultural areas but also to natural areas of the biome”, explains Marengo.

The use of fire to clear land for livestock farming contributes to an increase in fires in the region, which become more difficult to combat due to the lack of rain, says the researcher. “The fire caused on the one hand by the hotter air and lack of rain in the Pantanal and on the other by the burning of livestock areas in the region resulted in the environmental disaster we saw in the biome”, says Marengo.

Observational data sources

In order to investigate the hydroclimatic causes of the drought in the Pantanal, the researchers used a wide range of observational data sources on hydrology (referring to the level of the Paraguay River) and also on rainfall, climate and atmospheric teleconnection indices (an estimate of how phenomena extreme weather conditions are connected) to identify changes in wind circulation. In addition, they used information on land use and employed indices derived from sensing sources to characterize water stress and drought in the biome.

Based on this data set, it was possible to describe the interannual variability of precipitation, the flow of the Paraguay River and variables related to drought. Data analyzes indicated that a complex combination of hydroclimatic teleconnections was behind the recent drought in the region.

“The lack of rain during the summers of 2019 and 2020 in the Pantanal is related to the reduction in the transport of hot and humid summer air from the Amazon to the region,” says Marengo.

Part of the rain in the Pantanal, which is one of the largest wetlands in the world, comes from the transport of winds that leave the North Atlantic, enter the Amazon, cross the biome and head to the Center-West of Brazil, where the Pantanal is located.

The formation of the high pressure area prevented this moisture coming from the Amazon from reaching the Pantanal and blocked the passage of cold fronts from the South to the region. In this way, hotter and drier air masses prevailed, contributing to the lack of summer rain in the Pantanal at the peak of the monsoon season – from seasonal winds associated with the alternation between the rainy season and the drought that occur in large areas of tropical and subtropical coastal regions.

This situation led to extreme conditions of prolonged drought throughout the Pantanal, severely impacting the hydrology of the biome, explains Marengo.

“This phenomenon is natural and happened in a similar way during the drought in São Paulo, between 2014 and 2016”, he compares.

Amplified impacts

According to Marengo, it is not yet possible to predict whether the Pantanal will face other severe droughts in the coming years. So that the situation does not repeat itself, the rains in the biome must begin and be abundant at the right time.

“There is no point in raining now, in March, at the end of the rainy season, and then between December of this year and February 2022, for example. In this way, the rainy season is compromised and increases the risk of fire outbreaks in the Pantanal,” he says.

The drought in the Pantanal also cannot be related to global climate change because it is a meteorological event with a natural cause, while climate change is a long-term process.

Some of the differences between the recent drought in the biome compared to those that occurred in the 1950s and 1960s, however, are that, at that time, the planet was not as hot as it is now, ponders Marengo.

“What is happening now is that these natural droughts are suffering the effects of climate instability and the impacts are worse because, at that time, the region did not have as much human occupation as it does today. Consequently, the population in the Pantanal is today more vulnerable to the impacts caused by droughts”, compares Marengo.

The article Extreme drought in the Brazilian Pantanal in 2019–2020: characterization, causes, and impacts (DOI: 10.3389/frwa.2021.639204), by Jose A. Marengo, Ana P. Cunha, Luz Adriana Cuartas, Karinne R. Deusdará Leal, Elisangela Broedel, Marcelo E. Seluchi, Camila Miranda Michelin, Cheila Flávia De Praga Baião, Eleazar Chuchón Ângulo, Elton K. Almeida, Marcos L. Kazmierczak, Nelson Pedro António Mateus, Rodrigo C. Silva and Fabiani Bender, can be read at here.