Floods can happen all year round, in all regions of the world. But discerning the relationship between any specific flood and climate change is not easy, experts say. The task is made difficult by the fact that historical records are limited, especially in the case of more extreme floods, which are infrequent.
It can be tempting to attribute all the floods and other extreme events to the forces that are warming the planet. But the weather conditions are not the weather, although they can be affected by the weather. For example, scientists are pretty sure that climate change increases the frequency of unusually hot days. They are not equally sure that climate change is making tornadoes worse.
Floods occupy a position in the middle of scientists’ certainty spectrum, between heat waves (“yes, evidently”) and tornadoes (“we don’t know yet”), said climate scientist Daniel Swain of the University of California, Los Angeles. . “I would say ‘yes, probably, but…'”
Like other disasters, floods involve a number of competing factors that can affect the frequency and intensity of floods in opposing ways. Increasingly, climate change, which has exacerbated extreme rainfall in many storms, has played an important role in this set.
The causes of floods
Several key ingredients contribute to the development of flooding: precipitation, snowmelt, topography, and the degree of 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 river flood, occurs when a river, stream, or lake overflows, often after heavy rains or a rapid melting of snow. A coastal flood occurs when land near the coast is flooded with water, often after a strong storm that collides with high tides.
Flooding can also occur in areas with no bodies of water nearby. Flash floods, in particular, can occur anywhere that experiences heavy rain for a short period of time.
How are floods measured?
Many methods are used to measure floods, including height (the height reached by water in a river, measured relative to a specific point) and flow (how much water passes through a specific point during a particular period of time).
But to describe the severity of a flood, experts often use the simpler term “a hundred-year flood” to describe a flood that has a 1% chance of occurring in any given year, considered an extreme and rare occurrence. But the term is just a description of probability, not a prediction. A region can experience two hundred-year floods in a few years.
Have floods increased in recent decades?
Not exactly. There is no doubt that climate change has intensified heavy precipitation events, but unexpectedly, there has not been a corresponding increase in flood events.
In the case of river floods, climate change is likely to be exacerbating the frequency and intensity of extreme flood events but reducing the number of moderate floods, scientists found in a 2021 study published in the journal Nature.
As the temperature rises, higher evaporation rates cause soils to dry out in less time. Initial soil moisture conditions are important for the most common moderate floods, as drier soils may be able to absorb most of the rain.
In the case of major floods, that initial soil moisture makes less of a difference, “because there’s so much water that the soil wouldn’t be able to absorb all of it anyway,” said Manuela Brunner, a hydrologist at the University of Freiburg in Germany and lead author of the 2021 study. According to her, any additional water added beyond the point at which the soil reaches full saturation will run off and contribute to the formation of a flood.
Looking to the future
In the “business as usual” scenario in which humans will continue to warm the planet with greenhouse gas emissions at the current rate, scientists are pretty sure that some types of flooding will increase.
For starters, coastal flooding will become more frequent 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 in frequency as more extreme precipitation events occur. The higher temperature increases evaporation, bringing more moisture into the atmosphere, which is released in the form of rain or snow.
Researchers also predict that as the climate warms, flash floods will become even more sudden, meaning they will happen in less time and will be of increasing magnitude. More sudden floods can be dangerous and destructive.
Flash floods are also increasingly likely to follow catastrophic fires, in a lethal cascade of weather disasters. This is because fires destroy forests and other vegetation, which in turn weakens the soil and makes it less permeable.
If heavy rains fall on fire-damaged terrain, “water is not absorbed by the Earth’s surface as well as it was in the past,” said meteorologist Andrew Hoell of the National Oceanic and Atmospheric Administration’s Physical Sciences Laboratory.
It might be counterintuitive to see the two extremes occurring in the same region — too many fires and too much water — but the phenomenon is likely to become more common, especially in the western United States.
Are different regions experiencing flooding?
In a recent article published in Nature, researchers concluded that flash floods could become more common in the future in more northern regions, in the north of the Rocky Mountains and in the northern states of the American region of the Great Plains.
According to Zhi Li, lead author of the 2022 study, this creates a risk for flood mitigation efforts, as local governments may not be aware of the future risk of flash floods.
This pattern is caused by faster melting snow and more snow melting earlier in the year, Li said. Higher latitude areas may experience more “rain on snow” floods, such as those that hit Yellowstone National Park in June.
Translation by Clara Allain
