Climate change was big driver of historic Amazonian drought, study finds

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A historic drought that left swaths of the Amazon rainforest parched for much of last year was caused primarily by climate change, according to a new study, rather than naturally occurring weather patterns.

The research by 18 scientists from the World Weather Attribution group also found that the El Niño phenomenon that warms the Pacific Ocean — historically a driver of drought — “had a much smaller influence”.

“As the Amazon drought worsened in 2023, many people pointed to El Niño to explain the event,” said Ben Clarke, a climate scientist at Imperial College London.

“While El Niño did lead to even lower levels of rainfall, our study shows that climate change is the main driver of the drought through its influence on higher temperatures.”

The conclusion highlights the perilous risk to global ecology from climate change, with an estimated rise of at least 1.1C since pre-industrial times by the UN body of scientists in a 2021 report. Scientists believe it has worsened since as greenhouse gases from human activity continue to rise.

As the world’s largest rainforest, the Amazon acts as a giant carbon sink and is considered a bulwark of global climate stability.

But drought threatens the environmental integrity of the biome. As extreme heat and water shortages kill trees and spark fires in the summer, the forest begins to release its enormous stocks of carbon dioxide, in turn fuelling the cycle of global warming.

“The Amazon could make or break our fight against climate change,” said Regina Rodrigues, a climate scientist at Federal University of Santa Catarina and one of the study’s authors. “To protect the health of the Amazon . . . we need to move away from fossil fuels as quickly as possible.”

More than 60 municipalities in Amazonas state were placed in a state of emergency at the height of the drought in the closing months of last year, with the local government rushing basic supplies, including food and water, to remote Amazonian villages.

Water levels in normally huge rivers pivotal to the rainforest ecosystem, including the Rio Negro and Rio Solimões fell to their lowest points in 120 years. In some sections, they were completely dry.

At the time, the unprecedented drought was primarily attributed to the El Niño pattern, and an unusual patch of warm water in the Atlantic Ocean above the equator. The two phenomena inhibited the formation of clouds, causing rainfall to drop sharply.

But the scientists behind the WWA study found that, while El Niño and climate change both contributed to a reduction in rainfall, “the increase in high temperatures was driven almost entirely by climate change”.

To measure this effect, the authors analysed weather data and model simulations to compare the current climate — factoring in about 1.2C of global warming — with a cooler pre-industrial climate.

The scientists also examined two separate indices used to measure drought in an effort to establish which precise climatic factors were fuelling the event.

Statistical modelling was then used to distinguish the seasonal influence of El Niño from long-term climate change.

“This result is very worrying. Climate change and deforestation is already wrecking parts of the most important ecosystems in the world,” said Friederike Otto, a scientist at Imperial College London and one of the study’s authors.

At the same time, scientists at world weather agencies have said they have been “astonished” at the levels of warming of the oceans, which store about 90 per cent of the excess heat trapped in the earth’s system by greenhouse gases.

“If we continue burning oil, gas and coal, very soon we’ll reach 2C of warming and we’ll see similar Amazon droughts about once every 13 years,” Otto added.

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