Flooding can occur throughout the year in any region of the world. But discerning the link between a given flood and climate change is no small feat, experts say, made difficult by the limited historical record, especially for the most extreme floods, which occur infrequently.
It can be tempting to attribute all floods and other extreme events to the forces of a warming planet. But weather is not climate, although weather can be influenced by climate. For example, scientists are convinced that climate change is making unusually hot days more frequent. They’re not so sure that climate change is making tornadoes more severe.
Flooding falls somewhere on the confidence spectrum between heat waves (“yeah, sure”) and tornadoes (“we don’t know yet”), said Daniel Swain, a climatologist at the University of California, Los Angeles. “I’d say ‘yeah, probably, but . . .’
Floods, like other disasters, involve a number of competing factors that can affect their frequency and intensity in opposite ways. Climate change, which worsens extreme rainfall in many storms, is an increasingly important part of the mix.
What causes floods
Northeast Entrance Road to Yellowstone National Park after a historic flood. Credit… Samuel Wilson-Poole/Getty Images
Several main ingredients contribute to the development of floods: rainfall, snowmelt, topography and how wet the soil is. Depending on the type of flood, some factors may be more important than others.
For example, a river flood, also known as a river flood, occurs when a river, stream, or lake overflows with water, often after heavy rainfall or rapidly melting snow. Coastal flooding occurs when land areas near the coast are inundated with water, often after a strong storm that collides with high tides.
Flooding can also occur in areas where there are no nearby bodies of water. Flash floods, in particular, can develop anywhere there is intense rainfall in a short period of time.
How floods are measured
Many metrics are used to measure flooding, including stage height (the height of water in the river relative to a specific point) and discharge (how much water passes through a specific location in a specific period of time).
However, to describe the severity of a flood, experts often use the simpler term “100-year flood” to describe a flood that has a 1 percent chance of hitting in a given year, considered an extreme and rare event. The term is simply a description of probability, but not a promise. A region can experience two 100-year floods within a few years.
Have floods increased in recent decades?
Residents assess flooding outside their home in Far Rockaway, Queens, in October 2021. Credit… Dave Sanders for The New York Times
Not exactly. Climate change has undoubtedly increased heavy rainfall, but unexpectedly there has been no corresponding increase in flooding.
As for river floods, climate change is likely to worsen the frequency and intensity of extreme floods but reduce the number of moderate floods, researchers found in a 2021 study published in Nature.
As the climate warms, higher evaporation rates cause soils to dry out more quickly. For these moderate and more frequent floods, initial soil moisture conditions are important as drier soils can absorb most of the rainfall.
In larger floods, initial soil moisture matters less “because there’s so much water that the soil couldn’t absorb all of it anyway,” said Manuela Brunner, a hydrologist at the University of Freiburg in Germany and lead author of the 2021 study. Any additional water added after the point at which the soil is fully saturated will run off and contribute to the development of flooding, Dr. Brunner said.
Looking to the future
Scientists are convinced that some types of flooding will increase under a business-as-usual scenario where humans continue to warm the planet with greenhouse gas emissions at the current rate.
First, coastal flooding will continue to increase as sea levels rise. Melting glaciers and ice sheets add volume to the ocean, and the water itself expands as it warms.
Second, flash floods will continue to increase as there is more extreme rainfall. Warmer temperatures increase evaporation, putting more moisture into the atmosphere, which is then released as rain or snowfall.
The researchers also expect that as the climate warms, flash floods will become “brighter,” meaning the duration of floods will become shorter while the magnitude will increase. Flash floods can be more dangerous and destructive.
Flash floods can also increasingly follow catastrophic wildfires in a deadly cascade of climate disasters. This is because wildfires destroy forests and other vegetation, which in turn weakens the soil and makes it less permeable.
If heavy rains occur on fire-damaged land, the water “is not absorbed by the land surface as efficiently as it used to be,” said Andrew Hoel, a meteorologist at the National Oceanic and Atmospheric Administration’s Physical Sciences Laboratory.
While it may be counterintuitive to see both extremes, too much fire and too much water, in the same region, the sight will likely become more common, especially in the American West.
Emergency workers in Windsor, New South Wales, Australia, on Monday, where flooding caused by heavy rains forced 50,000 people from their homes. Credit… Bianca De Marchi/EPA, via Shutterstock
Are there floods in different areas?
In a recent paper published in Nature, researchers found that in the future, flash floods may be more common in the North, in the Northern Rockies and Northern Plains states.
This poses a risk to flood mitigation efforts because local authorities may not be aware of the future risk of flash floods, said Zhi Li, lead author of the 2022 study.
The pattern is due to faster snowmelt and snow melting earlier in the year, Dr Li said. Regions at higher latitudes may experience more rain-on-snow flooding like the one that hit Yellowstone in June.
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