1026. Factors Contributing to Extreme Floods in East Africa

From the end of March to early May, during a long rainy period in East Africa, Kenya, Tanzania, and Burundi experienced severe flooding due to extreme rains, with more than 700,000 people suffering from infrastructure damage, school closures, and the loss of livestock and crops.

On May 23, the World Weather Attribution (WWA), which analyzes the causal relationship between extreme events and climate change, issued a preliminary analysis. According to their findings, the recent heavy rains that affected East Africa were more likely caused by climate change than the effects of El Niño. They also warned that poor urban planning could increase vulnerability to flood damage as climate change is expected to intensify extreme rains.

Heavy rains in late March and April caused particularly devastating damage in the central highlands, including Lake Tanganyika, Lake Victoria, and Nairobi, as well as in low-lying areas in southeastern Kenya and coastal Tanzania.

In recent years, East Africa's resilience has been weakened by humanitarian crises such as internal displacement, infrastructure damage, food insecurity, and health risks due to repeated natural disasters, including the 2020–2023 prolonged droughts occasionally intermitted by heavy rains with severe floods.

Due to rapid urbanization, big cities in East Africa are particularly vulnerable to flooding, especially informal settlements with insufficient drainage systems, exacerbating flood damage. Land-use changes due to deforestation and the expansion of farmland are also increasing the risk of flooding.

Towards the end of the 20th century, climate models predicted wetting trends in East Africa due to warming, while observations indicated drying trends during the long rains in East Africa. This "East African paradox" phenomenon has been observed less in recent years, with a wetting trend, while new climate models show that the wetting trend is also waning, suggesting that climate observations and model interpretation in the region are becoming increasingly difficult.

The observations do not show a long-term trend, but a drying trend toward the end of the 20th century until 2008, and a wetting trend over the past 15 years. If the recent trend is intensified by human-caused climate change, increased rainfall indicates increased flood risk in the region.

The role of anthropogenic climate change in long rains is not statistically significant, but it shows an increasingly wetting trend. Under 1.2°C of warming, events like this one are twice as likely to occur on average, and are estimated to be 5% more intense. In the future, rainfall is expected to intensify further under 2°C of warming.

The WWA team also assessed the effects of El Niño and the Indian Ocean Dipole on the recent heavy rainfall, and found that both events have a negligible impact on the 2024 heavy rainfall in East Africa.

Based on these findings, it can be inferred that the increase in rainfall in the region over the past 15 years is likely due in part to human-induced climate change. The WWA emphasized the need to invest in strengthening resilience to floods in the face of future global warming.

Contributor: IIYAMA Miyuki, Information Program