Floods can occur throughout the year in every part of the world. But pinpointing the relationship between a given flood and climate change is no small feat, experts say, made difficult by the limited historical record, especially of extreme floods that occur infrequently.
All floods and other extreme events can be attributed to warming planetary forces. But weather is not climate, and weather can be affected by climate. For example, scientists believe that climate change is making unusually hot days more common. They are not sure that climate change is making hurricanes more severe.
Daniel Swain, a climate scientist at the University of California, Los Angeles, said flooding falls somewhere on the confidence spectrum between heat waves (“yes, clearly”) and hurricanes (“we don’t know yet”). “Yes, probably, but…” I’d say.
Floods, like other disasters, involve many competing factors that can influence their frequency and severity in opposing ways. Climate change, which worsens extreme precipitation in many storms, is an increasingly important part of the mix.
What causes floods?
Several key factors contribute to flood development: 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 fluvial flood, occurs when a river, stream, or lake overflows with water, often following heavy rain or rapidly melting snow. Coastal flooding occurs when areas of land near coastal areas are inundated with water, often following severe storms that collide with high tides.
Flooding can also occur in areas where there are no water bodies nearby. Flash flooding, in particular, can develop anywhere that experiences heavy rainfall in a short period of time.
How floods are measured
Several metrics are used to measure flooding, including stage height (the height of water in the river relative to a specific point) and flow rate (how much water moves at a specific location in a specific period of time).
To describe the severity of a flood, experts use the simple term “100-year flood” to describe a flood that has a 1 percent chance of hitting in any given year, which is considered an extreme and rare event. This term is a description of possibilities, however, not a promise. A region may experience two 100-year floods in a few years.
Have floods increased in the last decade?
Not exactly. Climate change has undoubtedly intensified extreme rainfall events, but, unexpectedly, There was no corresponding increase in flood events.
When it comes to river flooding, the researchers found that climate change may increase the frequency and intensity of extreme flood events, but reduce the number of moderate floods. A 2021 study published in Nature.
As the climate warms, higher evaporation rates will dry out the soil more quickly. For moderate to more frequent floods, initial soil moisture levels are important because dry soils absorb most of the rainfall.
In large flood events, that initial soil moisture is low, “because the soil can’t absorb it anyway,” said lead author Manuela Brunner, a hydrologist at the University of Freiburg in Germany. 2021 is the author of the study. Dr. Brunner said any additional water added where the soil is completely saturated will run off and contribute to flooding.
Looking to the future
Scientists believe some form of flooding will increase under a “business as usual” scenario, where humans are warming the planet with greenhouse gas emissions at current rates.
First, coastal flooding will continue to increase as sea levels rise. Melting glaciers and ice sheets add to the ocean’s volume, and water expands as it warms.
Second, flash floods will continue to increase due to heavy rainfall events. Warmer temperatures increase evaporation, driving more moisture into the atmosphere, which is then released as rain or snow.
As the climate warms, researchers expect flash floods to “brighten up,” meaning that the timing of floods will be shorter while the volume will increase. Flashier floods can be more dangerous and destructive.
Flash floods can increasingly follow catastrophic wildfires in the deadly cascade of climate disasters. This is because wildfires destroy forests and other vegetation and weaken the soil and make it less permeable.
When heavy rain falls on fire-damaged land, the water “doesn’t soak into the land surface like it once did,” said Andrew Hall, a meteorologist at the National Oceanic and Atmospheric Administration’s Physical Sciences Laboratory.
Although it is counterintuitive to see two extremes of high fire and high water in the same region, the sighting can be very common, especially in the American West.
Are different areas affected by floods?
A A recent article in NatureThe researchers found that, in the future, flash floods in the northern Rockies and northern Plains states may be more common in the north.
This poses a risk to flood mitigation efforts because local governments may not be aware of future flash flood risk, said Ji Li, lead author of the 2022 study.
The model is driven by fast melting snow and snow melting earlier in the year, Dr Li said. Areas at higher latitudes may experience “rain-and-snow” flooding, similar to the flooding in Yellowstone in June.