Brazil’s deadly floods: the compound impact of El Niño and climate change

Catastrophic flooding in Brazil in April and May was a stark reminder of the compound effect of El Niño and climate change. The unprecedented rainfall – now twice as likely due to global warming – led to devastating flooding across the state of Rio Grande do Sul (RGDS).

Record-breaking rainfall in RGDS – equivalent to three normal months of rainfall in a two-week period – caused flooding over an area the size of the UK.

Two-week rainfall accumulations reached almost 1,000 mm in some parts of RGDS, while the state’s capital, Porto Alegre, registered 327 mm of rainfall in less than a week at the end of April. The flooding has also been long-lasting, persisting for almost all of May with flood depths reaching five metres in some locations. Economic losses are expected to exceed BRL22bn ($4bn).

Approximately 650,000 residents have been displaced so far, over one-third of RGDS’s population lost access to running water, hundreds of thousands were without power for weeks, and 173 fatalities were reported. The flooding also disrupted health services and infrastructure, and outbreaks of waterborne diseases, particularly leptospirosis, compounded the challenges faced by health authorities and emergency responders in the region.

RGDS, a key producer of rice, soy and livestock, is vital to the country's economy. Although some crops were harvested before the floods, storage facilities were likely damaged. Many livestock were lost, and remaining animals face feed shortages.

The regional airport in Porto Alegre was closed for several days, with 10 planes damaged or destroyed, and local transport networks have been disrupted by flooding and landslides. Additionally, the floods disrupted supply chains throughout the region. For example, major auto manufacturers such as Chevrolet and Volkswagen have reported production slowdowns or stoppages.

Compound impact of El Niño and climate change

Both the phase of the El Niño-Southern Oscillation (ENSO) and climate change are thought to have contributed to the severity of the recent flooding. The El Niño phenomenon, characterised by warmer-than-average sea surface temperatures in the central and eastern tropical Pacific Ocean, facilitates the transport of warm, moist air toward southern Brazil.

This increased moisture supply can increase the intensity and duration of rainfall events, which leads to an elevated risk of flooding.

Compared with a neutral ENSO phase, the recent El Niño phase is estimated to have increased the likelihood of this extreme rainfall event by two to five times and its intensity by 3 percent to 10 percent.

El Niño alone isn’t to blame for the severity of the recent flooding. Scientists have found that human-induced climate change doubled the likelihood of this event and increased the intensity by 6 percent to 9 percent, aligning with more general future projections of increases in both flash flooding and river flooding. This trend is already evident in southeastern Brazil, where annual precipitation and high-intensity events are increasing.

Deforestation, urbanisation and ageing infrastructure

Alongside climate change and El Niño, social factors also played a role in exacerbating the disaster’s impacts. Deforestation, urbanisation encroaching on flood-prone land and inadequate maintenance of flood protection infrastructure in cities such as Porto Alegre may have amplified the impact of the floods.

Forecasts and warnings of the floods were available nearly a week in advance, but the warnings may not have reached all of those at risk, and the public may not have understood the severity of the potential impacts or how to respond appropriately.

While improved risk assessments, warnings and flood defences may save lives and reduce damage, the low insurance penetration rates – around 30 percent for homes in southern Brazil – mean that many uninsured residents may not be able to afford to rebuild after the recent flooding.

Against the backdrop of increasing flood risk under climate change, some local people say they have no desire to return to homes they believe are unsafe, prompting discussions among officials about the once-inconceivable relocation of entire communities to higher ground.

Neil Gunn is head of flood and water management research at WTW Research Network. Jessica Boyd is modelling research and innovation lead at WTW Research Network.