Floods can occur all year round, in any region of the world. But discerning the relationship between a particular flood and climate change is no small feat, experts say, complicated by limited historical data, especially for the most extreme floods, which are infrequent.
It may be tempting to attribute all flooding and other extreme events to the forces of the warming planet. But the weather is not a climate, even though the weather can be influenced by the climate. For example, scientists are convinced that unusually warm days are becoming more frequent due to climate change. They are not so sure that climate change is make tornadoes more severe.
Floods fall somewhere along the confidence spectrum between heat waves (“yes, obviously”) and tornadoes (“we don’t know yet”), said Daniel Swain, a climate scientist at the University of California, Los Angeles. “I would say, ‘yes, probably, but…'”
Floods, as with other disasters, involve a number of competing factors that can influence their frequency and intensity in opposite ways. Climate change, which exacerbates extreme rainfall in many storms, is an increasingly important part of the mix.
What Causes Floods?
Several main ingredients contribute to the development of floods: precipitation, melting snow, 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 by water, often after a major storm collides with high tide.
Flooding can also occur in areas with no nearby water. Flash floods in particular can develop anywhere where heavy rainfall occurs in a short period of time.
How floods are measured
Many metrics are used to measure flooding, including stage height (the height of the water in a river relative to a specific point) and flow rate (how much water passes through a particular location during a given period of time).
However, to describe the severity of a flood, experts will often use the simpler term “a 100-year flood” to describe a flood that has a 1 percent chance of hitting in any given year, considered an extreme and rare phenomenon. However, the term is only a description of probability, not a promise. A region can have two 100-year floods in a few years.
Have floods increased in recent decades?
Not exactly. Climate change has undoubtedly intensified heavy precipitation events, but unexpectedly there is no corresponding increase in flooding.
When it comes to river flooding, climate change is likely to exacerbate the frequency and intensity of the 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 the soil to dry out faster. For those moderate and more mundane floods, the initial soil moisture conditions are important, as drier soils are able to absorb most of the rainfall.
With larger floods, that initial soil moisture is less important “because there’s so much water the soil couldn’t absorb it all anyway,” said Manuela Brunner, a hydrologist at the University of Freiburg in Germany and the 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 flood development, said Dr. Brunner.
Looking to the future
Scientists are confident that some types of flooding will increase in the “business as usual” scenario where people 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 flooding will continue to increase as there are more extreme precipitation events. Warmer temperatures increase evaporation, releasing more moisture into the atmosphere which is then released as rain or snowfall.
Researchers also expect that, as the climate warms, flash floods will become “flasher,” meaning the timing of the floods will shorten as the magnitude increases. Bigger floods can be more dangerous and destructive.
Flash floods can 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 heavy rainfall occurs on land damaged by a fire, the water is “not absorbed by the land surface as effectively as it once did,” said Andrew Hoell, a meteorologist with the Physical Sciences Lab of the National Oceanic and Atmospheric Administration.
While it may be counterintuitive to see the two extremes, too much fire and too much water, in the same region, the sight will most likely become more common, especially in the American West.
Are different areas affected by flooding?
In a recent article published in Natureresearchers found that in the future, flash flooding will be more common in the north, in the states of Northern Rockies and Northern Plains.
This poses a risk to flood control efforts, as local governments may not be aware of the future risk of flash flooding, said Zhi Li, lead author of the 2022 study.
The pattern is driven by faster-melting snow and snow melting earlier in the year, said Dr. Li. Regions at higher latitudes may experience more “rain-on-snow” flooding like this one flowed through Yellowstone in June.