How deadly are floods?

Floods kill thousands of people each year. The direct death toll is orders of magnitude lower than that of the biggest environmental killers, such as hot weather and dirty air, but scientists are unsure just how big the indirect health burden is. The aftermath of a flood can be even more deadly than the deluge itself, as crops die and disease spreads.

Floods also force vast numbers of people to flee. In 2024, fast-flowing waters displaced more than 19 million people around the world – a mix of precautionary evacuations and washed-away villages. The figure is about the equivalent of pushing every single person in Somalia, Florida or the Netherlands from their home.

What can we do about floods?

Disasters such as storms and floods have become less deadly in recent decades, as people have grown better at managing the risks, but the damage they do varies widely. A person in Somalia may receive no warnings at all before flash floods sweep away their loved ones; someone in Florida may be evacuated to safety but lack home insurance to rebuild their home; a person in the Netherlands may be so well protected by tidal parks and sponge cities that heavy rains may not even register as a threat.

People who live in flood plains and on coasts – where much of global population growth has taken place – are particularly vulnerable.

Is human-caused climate breakdown making floods worse?

The burning of fossil fuels has heated the planet, increasing the risk of extreme rains that lead to floods around the world, particularly in Europe, most of Asia, central and eastern North America, and parts of South America, Africa and Australia. A well-established rule of physics is that warm air can hold more moisture – about 7% for every 1C – though whether it does so or not depends on how much water is available. When heavy rain does fall, clouds can unleash far more water.

Perhaps surprisingly, a lack of water can also worsen floods, by drying out the ground. Hard, caked soil does not absorb water so it runs off and pools in lower-lying regions, allowing water levels to rise much faster than otherwise.

Flooding is also affected by human factors such as the existence of flood defences and land use.

What about coastal floods?

Climate change makes coastal floods stronger by raising sea levels. The effect is so strong that by 2100, the Intergovernmental Panel on Climate Change has found, high tides that used to hit once a century will hit most of the world’s coasts every single year. This hundredfold increase will take place even in optimistic scenarios for cutting pollution – and in many coastal cities will happen by the middle of the century.

How do floods affect the economy?

Floods destroy infrastructure such as bridges, roads and railways, as well as possessions such as homes and cars. They can also wipe out businesses and render offices, schools and hospitals unusable.

When flash floods in 2023 struck Slovenia, a small but prosperous country in central Europe, they caused an estimated €10bn in damage, or about 16% of the country’s GDP. In the US, floods are estimated to cause up to half a trillion dollars in direct asset losses each year.

How can we adapt to flooding?

Globally, the biggest progress in saving lives has come from early warning systems that alert people to danger and help them escape before it strikes. This has proved particularly powerful in middle-income countries, where most people live, though it has a long way to go in the poorest parts of the world.

Building dykes and retention basins can limit the damage from heavy rainfall. In cities, parks and other green spaces can also soak up rain before it turns into a flood. On coasts, sea walls can keep the waters out.

But scientists warn there are limits to adaptation as the planet heats up. They increasingly talk about “managed retreat” to permanently move people out of harm’s way – a route that some communities around the world have already gone down. This can mean abandoning homes, towns and, in the case of small island states being submerged by the sea, entire countries.