Floods can occur all year round, in all regions of the world. But discerning the relationship between any given flood and climate change is no easy feat, experts say, made difficult by limited historical records, especially for the most extreme floods, which occur rarely.
It can be tempting to attribute all floods and other extreme events to the forces of global warming. But weather is not climate, although weather can be affected by climate. For example, scientists are convinced that climate change is making exceptionally hot days more frequent. They are not so sure that climate change makes tornadoes worse.
Flooding falls somewhere on the confidence spectrum between heat waves (“yes, definitely”) and tornadoes (“we don’t know yet”), said Daniel Swain, a climatologist at the University of California, Los Angeles. Angeles. “I would say ‘yes, 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 exacerbates extreme rainfall in many storms, is an increasingly important part of the mix.
What Causes Floods
Several main factors contribute to the development of floods: precipitation, snowmelt, topography and soil moisture. 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 as a result of heavy rains or rapid snowmelt. Coastal flooding occurs when areas of land near the coast are flooded with water, often as a result of a severe storm that encounters high tides.
Flooding can also occur in areas without nearby bodies of water. Flash floods, in particular, can develop wherever intense rainfall occurs over a short period of time.
How floods are measured
Many parameters are used to measure flooding, including watershed (the height of water in a river relative to a specific point) and discharge (the amount of water that passes through a specific location on a particular period of time).
To describe the severity of a flood, however, experts will often use the simpler term “a 100-year flood”, to describe a flood that has a 1% chance of occurring in any given year, considered an extreme and rare event. The term is only a description of the probability, however, not a promise. A region can have two hundred-year floods in a few years.
Have floods increased in recent decades?
Not exactly. Climate change has undoubtedly intensified heavy rainfall events, but, unexpectedly, there has been no corresponding increase in flood events.
As for riverine flooding, climate change likely exacerbates the frequency and intensity of extreme floods, but decreases 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 common floods, initial soil moisture conditions are important, as drier soils may be able to absorb most of the precipitation.
With greater flooding, that initial soil moisture matters less “because there’s so much water that the soil couldn’t absorb it all, anyway,” said Manuela Brunner, a 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 flood development, Dr Brunner said.
To look forward
Scientists are confident that some types of flooding will increase under the “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 caps 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, putting more moisture into the atmosphere which is then released as rain or snow.
The researchers also expect that as the climate warms, flash floods will become “flasher”, meaning the timing of floods will get shorter while the magnitude increases. More dazzling floods can be more dangerous and destructive.
Flash floods can also increasingly follow catastrophic wildfires in a deadly cascade of climate disasters. Indeed, forest fires destroy forests and other vegetation, which weakens the soil and makes it less permeable.
If heavy rain occurs over fire-damaged land, water “is no longer being absorbed by the land surface as efficiently as it used to be,” said Andrew Hoell, a meteorologist at the National Oceanic and Physical Sciences Laboratory. Atmospheric Administration.
While it may be counterintuitive to see both extremes, too much fire and too much water, in the same area, the sight will most likely become more common, especially in the American West.
Do different areas experience flooding?
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 governments may not be aware of the future risk of flash flooding, said Zhi Li, lead author of the 2022 study.
The model is driven by snow melting faster and snow melting earlier in the year, Dr Li said. Regions at higher latitudes could experience more “rain on snow” floods like those that swept through Yellowstone in June.