Extreme rainfall puts cities on alert

Almost 100 years ago, Copenhagen’s Enghaveparken opened to provide green space and fresh air for locals. Now it has a new function: to protect the Danish capital from increasingly erratic rainfall.

After being hit by cloudbursts — a huge amount of precipitation in a short time — including one in July 2011 that caused more than €1bn of damage, Copenhagen has stepped up its efforts to cope with extreme rainfall.

New public parks are being built and historic recreation areas such as Enghaveparken transformed to act as temporary rainfall reservoirs. Tunnels are under construction to try to prevent the city’s 18th-century combined sewers — removing a mix of effluent, domestic wastewater and stormwater — being overwhelmed by intense precipitation.

“We are facing changing rainfall patterns. We are seeing an increase in these heavy downpours,” says Lykke Leonardsen, who has been tasked with making Copenhagen more resilient and sustainable. While ensuring the city can withstand intense rainfall, the aim is also to achieve “a really liveable city when it is not raining”.

A similar scene is playing out across the world, with changing precipitation creating new challenges. A recent study by WaterAid, a non-profit body, found many of the world’s 100 most populated cities are increasingly exposed to both floods and droughts (see graphic below).

Over a 40-year period, the research found that about 13 per cent of cities examined had “flipped” from a drier to a more extreme wet climate, while another 7 per cent or so had changed from a wet to a more extreme dry climate. A further 17 per cent were experiencing an intensification of both droughts and floods — so-called climate whiplash.

Stefan Uhlenbrook, director of hydrology, water and cryosphere at the UN’s World Meteorological Organization, explains that one reason for the changes in precipitation is that a warmer atmosphere, on average, holds more moisture. Rising sea temperatures result in more evaporation, which in turn increases the moisture in the atmosphere. And a warmer atmosphere tends to hold the moisture for longer.

As a result, when it rains, “it rains harder”, Uhlenbrook says. “On a global scale, we have more heavy rainfall events. On the other hand, there are more prolonged or longer drought periods — or dry, non-precipitation days.”

In a landmark report in 2021, scientists warned that a warmer climate would “intensify very wet and very dry weather”, although this would vary by location because of regional atmospheric circulation. They also said shorter periods of extreme rainfall were more likely as temperatures rise.

David Hetherington, global water research lead at Arup, a UK engineering group, says that in some areas of the world, the annual water budget — the balance between precipitation, evaporation, plant transpiration and run-off — has remained the same overall while the pattern of rainfall has shifted dramatically. The extremes are bigger, more pronounced, he comments. “Then there’s going to be parts of the world where there’s [both] just more water and less water generally as well.”

In the UK, for example, says Lizzie Kendon, a science fellow at the Met Office, Britain’s meteorological service, the expectation is that summers will become drier overall, but bursts of rain will be intense.

A 2024 study by academics in China and the UK, published in the journal Science, found that precipitation variability had already increased globally because of human-caused climate change. Such wet-dry swings are particularly affecting Europe, Australia, and eastern North America.

The consequences can be devastating. In October last year, more than 220 people died in Valencia when the Spanish city’s infrastructure struggled to cope with intense flooding. Further north, Barcelona declared a drought emergency in February 2024 after years of low rainfall, forcing officials to bring in water restrictions and look at building new desalination plants. It was then hit by intense flooding soon after Valencia, yet not enough to alleviate the drought.

Emma Howard Boyd, who led a review last year into London’s readiness for the effects of climate change, says such extreme events “should focus minds on cities’ emergency preparedness”, but national and local governments also need to address the gradual consequences of changes under way. 

“Flood resilience is needed to underpin economic growth and security,” she says. “The longer we fail to get prepared, the more we lock in future vulnerabilities to flash flooding.”

Prolonged drought followed by intense rainfall is often particularly challenging to cope with. Drier ground absorbs less rainfall, which creates a greater risk of flash floods as rain pools on the surface and run-off increases. 

Moreover, many sewerage systems were designed more than 100 years ago for less extreme swings in rainfall patterns. That said, even newer drainage systems can become overwhelmed by intense flooding, as Dubai discovered last year.

Authorities need to plan to retain rainwater where it falls, says Hetherington. In the UK, that would be in upland areas, from where it would gradually sink into the ground and rivers, and replenish reservoirs and water tables. 

He points to so-called natural flood management schemes, in which planners are considering “where the water actually lands in the river basin and trying to delay it from getting to the city”. This might be achieved by the use of ponds in agricultural land or other solutions that work with the landscape, he adds, rather than more conventional approaches, such as building concrete walls along riverbanks in cities.

Kendon says that everyone, at national and individual level, must keep in mind that the past is “not a good indication of what is to come” in patterns of precipitation. “If you base your infrastructure on what’s been a record event in the past . . . that will be far from a conservative approach,” she says. “What is extreme now could well be a normal occurrence.”

Back in Copenhagen, the city is pushing on with its cloudburst management system. In Enghaveparken, a 2,000 cubic metre underground retention basin will capture rain­water from nearby roofs during normal rainfall. The park has been redesigned to flood in order to capture intense rain, with capacity to store up to 22,600 cu m, which will protect nearby houses and infrastructure.

While the initial work in Copenhagen focused on extreme rainfall, the city is now pondering the implications of potential periods of drier conditions for the famously damp capital, says Leonardsen. “We have experienced periods of drought. We are seeing the same change in pattern [as many other cities], especially in the summer where you have these dry spells and then extreme wet events,” adds Leonardsen, who also works with C40 Cities, a global network focused on the impact of climate change.

There’s going to be parts of the world where there’s [both] just more water and less water generally as well

Cities such as New York, which is rolling out its own cloudburst system, have looked to Copenhagen. Leonardsen, through her C40 role, regularly speaks to officials from urban areas across the world about changing precipitation. “The continuous stress of drought or more rain, combined with these extreme events, is probably going to be more of the new normal,” she says.

But preparing for such changes is expensive, she notes. The Copenhagen plan was initially expected to cost about €1.5bn spread out over decades — that is likely to rise, she adds, especially as the city ponders managing drought too.

Yet failure to improve resilience at national or city level will be costly too, Leonardsen argues, including “enormous” bills for recovery. “If you have a city that gets hit quite often, it will reduce house prices,” she says. “It will reduce the investments in the city and so on. It can actually trigger a lower economic development.”

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