Climate Change Contributed to Deadly West African Floods, Scientists Find

Heavy rains that led to recent deadly floods in Nigeria and neighboring countries were made about 80 times more likely by human caused climate change, scientists said Wednesday.

The floods, which killed more than 600 people in Nigeria and more than 200 in Niger and Chad, were the consequence of an extremely wet rainy season. The scientists, from a loose-knit coalition called World Weather Attribution, also said climate change had made the season, which runs from April to October, 20 percent wetter overall than it would have been in a world without warming.

The findings come as negotiators are meeting in Egypt at the U.N. climate summit, with the issue of “loss and damage” — whether industrialized countries should pay less-developed nations for the effects of climate change — high on the agenda. Nigeria and many other African countries produce relatively little carbon dioxide emissions that contribute to warming yet increasingly suffer from climate-related disasters like floods and heat waves.

“This is a real and present problem, and it’s particularly the poorest countries that are being hit very hard,” ” said one of the researchers, Maarten van Aalst, director of the Red Cross Red Crescent Climate Center.

“It’s not up to us as scientists to tell negotiators what to do,” said Dr. van Aalst, who is attending the climate talks, known as COP27. But this study and others show that climate disasters “are not something for the future, they’re happening today,” he said. “So we do need those solutions on loss and damage and we need particularly to deliver in those countries where that vulnerability is highest.”

The analysis looked at two aspects of the seasonal rains in the region this year: average rainfall for the entire season over a large drainage area, mostly in Chad, and spikes of extreme rainfall over weeklong periods in another drainage area, mostly in Nigeria.

Like similar studies by this and other groups, the researchers used observational data as well as climate models that simulate both the current world, where emissions of greenhouse gases since the 19th century have raised temperatures by about 1.1 degree Celsius, or 2 degrees Fahrenheit, and a hypothetical world where no emissions, and thus no warming, occurred. This study has yet to be peer-reviewed and published in a journal, but the techniques used have been peer-reviewed many times before.

Comparing results from the two models, the researchers were able to determine the influence of climate change on the rains. That influence appeared to be greater on the overall season, where climate change made such a high average rainfall 80 times more likely. Climate change made the heavy short-duration rains only twice as likely, the researchers found.

Floods are not uncommon in the rainy season is West Africa, but these were the worst in decades in Nigeria and some of the other countries. Nearly 1.5 million Nigerians were displaced, vast stretches of farmland were inundated, and fuel and food distribution were disrupted.

The researchers said there were other factors that contributed to the disaster, including poverty, military conflicts and land-use changes, especially the increasing settlement of flood plains by growing populations. In Nigeria, the flooding was also made worse by poor water management, specifically uncoordinated releases of water from a large dam in neighboring Cameroon.

But the influence of climate change was clear, said Friederike Otto, a climate scientist at Imperial College London. Such an extreme rainy season would have been very rare in a world without climate change, she said, but now it has about a 10 percent chance of occurring in any given year.

And with the planet continuing to warm, “that also means that going forward we will see more of these very intense, rainy seasons in the region,” Dr. Otto said.

The attribution group released a second analysis Wednesday, of the erratic and weak 2021 rainy season farther north in the Sahel, the semiarid region that borders the Sahara. The lack of rain affected harvests and worsened food shortages in Niger, Burkina Faso and elsewhere.

Subsistence agriculture in the Sahel is heavily dependent on seasonal rainfall, which can vary greatly in amount and timing from year to year. The rainy season in 2021 started later than usual, was shorter and included some dry periods, which stunted the development of millet and other cereal crops.

But the researchers were unable to determine whether climate change influenced the drier than normal conditions because of a lack of reliable weather data, a common problem in some less-developed countries.

The study “confirms the importance of investing in and maintaining a network of weather stations and rain gauges within Sahel countries,” said one of the researchers, Audrey Brouillet, of the Institute of Research for Development in France. “This is key to understanding the influence of climate change on droughts and other events in the region.”