Dams burst and thousands were forced to evacuate as floods battered the south of Germany this month following days of relentless rain. 

It is the latest of several countries impacted by extreme floods this year. The United Arab Emirates and Oman experienced the heaviest rainfall since records began. Deluges in Kenya claimed numerous lives and triggered landslides. And in Brazil, floods damaged an area equivalent in size to the UK and displaced over half a million people. 

Whereas coastal flooding is largely driven by winds and high tides, river, groundwater and flash flooding are all linked to heavy rainfall. Rising global temperatures, caused by burning fossil fuels, is making rainfall more frequent and severe across most parts of the world.

What is the science behind extreme flooding? 

Modeling precipitation patterns is a complex process, but it has one clear underlying physics principle: hot air holds more moisture. 

Greenhouse gases released into the atmosphere act like a blanket on the earth, trapping heat and causing temperatures to rise. This leads to a more rapid evaporation of water on land and at sea, meaning that when it rains, there is more water to release. And when a huge amount of rain is dumped onto earth in a short space of time, this can lead to flooding. 

Air's capacity to hold moisture rises by 7% with every rise of 1 degree Celsius. Since the pre-industrial era, global air temperatures have increased by around 1.3 degrees Celsius.

Temperature rises also make more precipitation fall as rain instead of snow which can make high altitude regions vulnerable to flooding and landslides. A  2022 study published in the science journal Nature found that in snowy, high-elevation parts of the Northern Hemisphere, rainfall extremes increased by an average of 15% per 1 degree Celsius of warming.  

How is climate change impacting global rainfall? 

Climate change is impacting the frequency of heavy downpours during storms and sudden outbursts through its influence on complex atmospheric and weather patterns. 

Globally, at a 1.5C temperature rise, which the world is increasingly close to hitting, heavy precipitation that would have been a once in a 10-year rainfall event will occur 1.5 times every decade and be over 10% wetter,according to the UN's International Panel on Climate Change (IPCC). 

Last year, Europe was around 7% rainier than normal, with most of the continent experiencing wetter-than-average conditions. Heavy or record-breaking precipitation triggered floods in Italy, Norway, Sweden and Slovenia.

Advances in attribution science are allowing experts to pinpoint the causal relationship between climate change and extreme weather events more accurately. According to one estimate, on average, 1 in 4 record rainfall extremes in the last decade can be attributed to climate change.

While no attribution studies are yet available for the recent German floods, heavy rainfall is becoming more frequent. Last year the average rainfall was 20% higher than the average for 1991-2020. 

And the floods that devastated western parts of Germany, as well as Belgium and the Netherlands, in 2021 have been directly linked to climate change. 

According to scientists from World Weather Attribution, a UK-based academic institution, it made rainfall between 3% and 19% stronger and 1.2 to nine times more likely. 

More recently, Brazil's floods in April and May are believed to have been made twice as likely to occur and up to 9% heavier due to the burning of fossil fuels. 

How many people are impacted by floods and where do they live? 

Flooding, among the most widespread natural disasters, is often devastating. Rushing currents can sweep away loved ones, critical infrastructure, wildlife and fertile soil, leaving behind grief and crippling economic damage as the water recedes. 

Since 2000 the proportion of people exposed to floods is estimated to have increased by 24%.

Today, 1.8 billion people — just under a quarter of the global population — are directly exposed to one-in-100-year floods, a term used to describe a flood that is so severe it will likely only be equaled or exceeded on average once a century.

In Europe, Germany has the highest number of people at risk of flooding, followed by France and the Netherlands. In 2023, one-third of the continent's river network saw flows exceeding the 'high' flood thresholds and 16% exceeding the "severe" levels. December levels were the highest on record, with 'exceptionally' high flows in a quarter of the continent's rivers. 

While flooding is a global threat, certain regions are much more impacted than others. 

An estimated 89% of people exposed to high flood risk live in low- and middle-income countries. Most live in South and East Asia, with 395 million exposed people in China and 390 million in India.

According to one study, the number of people living in areas with a very high flood risk has risen 122% since 1985. This trend is believed to be driven by rapid urbanization, particularly in middle- and low-income countries, with cities often located beside waterways. 

Will floods increase in the future? 

Science tells us that the risk of extreme flooding will continue to rise if the world fails to limit global warming. 

According to the IPCC, at 2 degrees Celcius of warming above pre-industrial levels, what would have been a once-every-10-year rainfall event will occur 1.7 times per decade and be 14% wetter. And if the world warms to 4 degrees Celcius, heavy rains that used to hit once a decade could hit almost three times more often and release 30% more rain.

According to calculations by the Joint Research Center, the European Commission's science and knowledge service, in Europe alone, if no adaptation measures are taken and temperatures rise to 3 degrees Celsius, by 2100, floods could rack up €48 billion of damages per year and triple the number of Europeans exposed to flooding. 

Edited by: Sarah Steffen

Sources: 

IPCC 2021 Report, 
https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_SPM_final.pdf

Europe 2023 Extreme Weather
https://climate.copernicus.eu/widespread-floods-severe-heatwaves-esotc-2023-puts-europes-climate-focus

World Weather Attribution
https://www.worldweatherattribution.org/heavy-rainfall-which-led-to-severe-flooding-in-western-europe-made-more-likely-by-climate-change/