How is climate change affecting flooding?

Floods can occur in any region of the world throughout the year. However, according to experts, discerning the relationship between a specific flood and climate change is no easy task, complicated by limited historical records, especially for the most extreme floods, which occur infrequently.

It can be tempting to attribute all flooding and other extreme events to the forces of warming the planet. But weather is not climate, even if weather can be influenced by climate. For example, scientists are confident that climate change is making unusually hot days more common. They’re not so sure that climate change is making tornadoes worse.

Flooding falls somewhere on the confidence spectrum between heatwaves (“yes, definitely”) and tornadoes (“we don’t know yet”), said Daniel Swain, a climate scientist at the University of California at Los Angeles. “I’d say, ‘Yeah, probably, but…’

Floods, like other disasters, are associated with a number of competing factors that can affect their frequency and intensity in opposing ways. Climate change, which exacerbates extreme precipitation in many storms, is an increasingly important part of the mix.

Several key components contribute to flood development: precipitation, snowmelt, topography, and how wet the soil is. Depending on the type of flood, some factors may be more important than others.

A river flood, also known as river flooding, for example, occurs when a river, stream, or lake overflows with water, often after heavy rainfall or rapidly melting snow. A coastal flood occurs when land areas near the coast are inundated by water, often after a severe storm that collides with high tides.

Flooding can also occur in areas with no nearby bodies of water. Flash floods in particular can develop wherever there is heavy precipitation in a short period of time.

Many metrics are used to measure flooding, including step height (the height of water in a river relative to a specific point) and flow rate (how much water flows past a specific location in a specific period of time).

However, to describe the severity of a flood, experts often use the simpler term “a 100-year flood” to describe a flood that has a 1 percent chance of occurring in a given year, which is considered an extreme and rare event. However, the term is only a description of probability, not a promise. A region can have two 100-year floods within a few years.

Not exactly. Climate change has undoubtedly led to an intensification of heavy precipitation events, but unexpectedly there has not been a corresponding increase in flooding events.

When it comes to river flooding, climate change will likely exacerbate the frequency and intensity of extreme flooding, but reduce the number of moderate flooding, researchers found in a 2021 study published in Nature.

As the climate warms, higher evaporation rates cause soils to dry out faster. For these moderate and more frequent floods, initial soil moisture conditions are important because drier soils may be able to absorb most of the precipitation.

For larger flood events, the initial soil moisture plays less of a role, “because there is so much water that the soil could not absorb everything anyway,” says Manuela Brunner, hydrologist at the University of Freiburg in Germany and lead author of the 2021 study. Any additional water Added beyond the point where the soil is fully saturated will run off and contribute to the development of flooding, said Dr. Bruner.

Scientists are confident that some types of flooding will increase in the business-as-usual scenario, where humans continue to warm the planet with greenhouse gas emissions at the current pace.

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 are more extreme precipitation events. Warmer temperatures increase evaporation and bring more moisture into the atmosphere, which is then released as rain or snowfall.

Researchers also expect that flash floods will become more “conspicuous” as the climate warms, meaning that the timing of floods will decrease while the magnitude increases. More noticeable floods can be more dangerous and destructive.

Flash floods may also increasingly follow catastrophic wildfires in a deadly cascade of climate disasters. That’s because wildfires destroy forests and other vegetation, which in turn weakens the soil and makes it less permeable.

When fire-damaged land experiences heavy rainfall, “the water isn’t being absorbed by the land surface as effectively as it used to be,” said Andrew Hoell, a meteorologist at the National Oceanic and Atmospheric Administration’s Physical Science Laboratory.

While it may be counter-intuitive to see the two extremes, too much fire and too much water, in the same region, the sight will most likely occur more frequently, particularly in the American West.

In a recent article in Nature, researchers found that flash flooding could become more frequent in the states of Northern Rockies and Northern Plains in the future.

This poses a risk to flood containment efforts, as local governments may be unaware of future flash flood risk, said Zhi Li, lead author of the 2022 study.

The pattern is being driven by faster-melting snow and snow that melts earlier in the year, Dr. Li. Regions at higher latitudes may experience more of the “rain-on-snow” flooding that swept through Yellowstone in June.

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