West Africa’s deadly rainfall in 2022 made ‘80 times more likely’ by climate change

Extreme rainfall which triggered deadly flooding across West Africa this year was made “about 80 times more likely” by human-caused climate change, according to a new rapid attribution study.

Severe flooding killed more than 800 people in Nigeria, Niger, Chad and neighbouring countries between June and October this year. In Nigeria alone – where the event was called the “most devastating” in a decade – floodwater displaced 1.3 million people and damaged more than a million hectares of farmland.

The World Weather Attribution service finds that climate change made the region’s 2022 rainy season 20% wetter than it would have been without human-caused climate change. The study adds that shorter periods of intense rainfall in the Lower Niger Basin, which worsened the floods, are now about twice as likely to happen because of climate change.

Meanwhile, as countries across West Africa battled with deadly flooding earlier this year, the Sahel – a 5,900km-long band of semi-arid land that stretches across Africa just south of the Sahara – faced a drought-induced food crisis. The region’s 2021 rainy season was shorter and drier than usual. As crop yields fell and food prices rose, food insecurity in the already-vulnerable region took a hit.

A rapid attribution study by the same team finds that “chronic vulnerability” was the main driver of Central Sahel’s “food security crisis”. The authors were unable to find the signal of climate change in the region’s rainfall pattern, due to uncertainties in observational data.

Both studies take place in the context of “extreme vulnerability”, resulting in “massive human consequences”, Maarten van Aalst from the Red Cross Red Crescent Climate Centre told a press briefing.

An early and intense rainy season drove widespread flooding across West Africa this year. More than 800 people were killed over June-October and tens of millions more were displaced. In Nigeria and Niger, the floods were among the deadliest on record.

In Nigeria alone, floodwater inundated hundreds of thousands of hectares of land, displacing 1.3 million people and damaging more than a million hectares of farmland. Among the country’s dead are 76 people who were killed when a boat carrying flood victims capsized.

Chad declared a state of emergency in October, following the heaviest rainfall in 30 years. Meanwhile, in Niger, more than 30,000 homes and shelters, six medical centres, 126 classrooms and 234 grain stores were damaged or destroyed by the floodwater, according to the study.

The flooding was further exacerbated by the release of Cameroon’s Lagdo Dam – part of a river management system that was designed to be accompanied by another dam in Nigeria that has not been completed – the authors study says.

Cheikh Kane from the Red Cross Red Crescent Climate Centre – a co-author on the study – told a press briefing that many affected regions were “already impacted by a lot of instability and insecurity”. He highlighted the inequality in the affected regions and explained that the flooding compounds impacts for already-vulnerable people.

West Africa’s rainy season runs between May and October each year. However, this year, the rains arrived early and were more intense than usual.

Dr Frederieke Otto – senior lecturer in climate science at the Grantham Institute for Climate Change and the Environment at Imperial College London and co-author of the study – told the press briefing that as well as higher-than-average seasonal rainfall, West Africa also saw “extremely heavy short spikes of precipitation”.

The authors used observational data from weather stations across the region to put both the seasonal and shorter-term increases in rainfall into their historical context.

The maps below show average rainfall over June–September in the Lake Chad catchment (left), and the average daily rainfall from 26 August to 1 September 2022 – the wettest 7-day period over the Lower Niger catchment (right). Dark blue indicates intense rainfall, while light blue indicates low rainfall. The red lines outline the two regions studied.

The rainfall seen over West Africa is “not very rare in today’s climate” – which has already warmed by around 1.2C due to human-caused climate change – according to the study.

The authors find that seasonal rainfall over the Lake Chad Basin has increased “significantly” due to rising global temperatures. In today’s climate, rainfall on this scale is likely to happen once every 10 years in today’s climate, they say.

Meanwhile, the study finds that a seven-day maximum rainfall over the lower Niger Basin is now a one-in-five-year event.

Attribution is a fast-growing field of climate science that aims to identify the “fingerprint” of climate change on extreme-weather events, such as heatwaves and droughts.

To conduct attribution studies, scientists use models to compare the world as it is today to a “counterfactual” world without human-caused climate change. This study aims to distinguish the “signal” of climate change in West African rainfall.

The authors find that climate change made seasonal rainfall over Lake Chad about 80 times more likely and 20% more intense. Meanwhile, they find that climate change doubled the likelihood for the shorter seven-day period of extreme rainfall over the lower Niger catchment, and increased its intensity by 4-5%.

(The findings are yet to be published in a peer-reviewed journal. However, the methods used in the analysis have been published in previous peer-reviewed attribution studies.)

The authors also look to the future, finding that 2C warming would cause rainfall intensity to “increase further, but not as strongly”.

As the planet warms, the African continent faces worsening climate extremes, ranging from floods to droughts. While countries across West Africa battled with deadly floods, the Sahel grappled with a food crisis caused by erratic rainfall.

In 2021, the western Sahel saw a shorter and drier rainy season than usual. The map below shows rainfall over western and central Africa over June 2021, relative to June rainfall over 1990-2020. Red indicates dry conditions, while blue indicates wet conditions.

Crops in western Sahel received less than 40% of their needed water by the end of August, as a result of low rainfall. In Niger and Burkina Faso, crop production dropped by 36% and 10%, respectively, compared with the previous five-year average. The low harvest, combined with the war in Ukraine, caused food prices to rise sharply in 2022 – leading to widespread food insecurity.

Kiswendsiida Guigma is a technical advisor at Red Cross Red Crescent Climate Centre and co-author of a separate World Weather Attribution study investigating last year’s Sahel drought. Much of central Sahel’s population is reliant on the single annual rainfall season for its rain-fed agriculture, Giugma explained to the press briefing. He added that if you “miss” the season, there is “no way you can survive” without external support.

The authors investigated the role of climate change on the drought using the same methods as previous attribution studies. However, it was not possible to find a climate change signal in the rainfall pattern – in large part due to uncertainties in the observational data.

“We could not detect significant trends or climate change influence in the 2021 drought,” Audrey Brouillet from France’s Institute of Research for Development told the press briefing. She emphasised that, from this result, it is not possible to tell whether or not climate change change impacted the event.

She added that the results reaffirm how important it is to “invest in and maintain a network of weather stations and rain gauges”.

This article originally appeared in Carbon Brief.