Navin Singh Kadka
Function, BBC World Service, Environment
Extreme floods in different parts of the world indicate accelerated warming of the atmosphere, which now holds much more moisture than in the past, say scientists.
In April 2023, Iraq, Iran, Kuwait, and Jordan were hit by catastrophic flooding caused by intense thunderstorms, hail, and heavy rainfall.
Meteorologists later found that there was a record amount of moisture in the sky across the region, surpassing the level seen during a similar event in 2005.
Two months later, 500 millimeters of rain fell in Chile in just three days.
This amount of water melted the snow in some parts of the Andes mountain range and caused huge floods that destroyed roads, bridges and water supply systems.
A year earlier, parts of Australia were hit by a "rain bomb", as politicians called it, killing more than 20 people and evacuating thousands.
- What are El Niño and La Niña and how do they affect the weather?
- A record warm year is changing the oceans
- "Biblical disaster": More than 100 people died in a hurricane in America
Scientists say that all these events are the result of atmospheric rivers that are becoming more intense, longer and wider and are often destructive.
The American space agency (NASA) says that because of these phenomena, floods threaten hundreds of millions of people around the world.
These "rivers in the sky" are long and wide columns of water vapor that usually form over the tropics and move towards the poles.
They carry about 90 percent of the total water vapor that moves in the area that stretches between the northern and southern returns.
The average atmospheric river is about 2.000 kilometers long, 500 kilometers wide and almost three kilometers deep, but now they are becoming wider and longer, with some reaching a length of more than 5.000 kilometers.
And yet, unlike clouds, they are invisible to the human eye.
"They can be seen using infrared and microwaves," says Brian Kahn, an atmospheric phenomenon researcher at NASA's Jet Propulsion Laboratory.
"That's why satellite observations can be extremely useful for observing water vapor and atmospheric rivers around the world," he adds.
Large and powerful atmospheric rivers can carry an amount of water that is 15 times greater than the water in the Mississippi, the longest river in North America.
On average, they move twice as fast as the regular flow of the Amazon, the largest river in the world by volume of water.
Although atmospheric rivers have always existed, scientists say global warming is creating more water vapor that makes them intense and capable of dumping huge amounts of water onto land in a short period of time, causing catastrophic floods and landslides.
Studies have shown that since the 1960s, the amount of atmospheric water vapor in the world has increased by up to 20 percent and continues to increase due to rising temperatures.
Recent research from the Institute of Geosciences at the University of Potsdam in Germany has shown that conditions for the formation of atmospheric rivers last longer over tropical South America, North Africa, the Middle East and Southeast Asia.
And that can mean more abundant rains that cause harmful effects on the soil.
This is exactly what happened across the Middle East in April 2023, according to another study conducted by Khalifa University in the United Arab Emirates (UAE).
"Our simulations revealed the presence of atmospheric rivers that produced heavy precipitation as they moved at high speed from northeast Africa to western Iran," the research concluded.
More landslides and flash floods
In other parts of the world, the number of atmospheric rivers is increasing, says Sara Vallejo-Bernal, who worked on the study at the University of Potsdam.
"In East Asia, there has been a significant increase in the frequency of these phenomena since 1940, and since then atmospheric rivers have become more intense in Madagascar, Australia and Japan," she said.
A study from 2021 published in a journal dedicated to geophysical research (The Journal of Geophysical Research) found that up to 80 percent of heavy precipitation in eastern China, Korea, and western Japan during the early monsoon season (March and April) is associated with atmospheric rivers.
Meanwhile, meteorologists in India say the warming of the Indian Ocean is causing atmospheric rivers to form and affecting the monsoon rains that occur in the region between June and September.
"The result is short episodes during which all this moisture from the warm seas is expelled from the atmospheric rivers within a few hours to a few days.
"This has led to an increase in landslides and flash floods across the country," says Roxy Mathew Cole, a scientist at the Indian Institute of Tropical Climate.
However, not all floods and landslides are caused by atmospheric rivers.
There are other factors, including cyclones and storms.
Atmospheric rivers also reach new cities.
Scientists say one reason is changing wind patterns and jet streams (fast-moving, narrow, meandering currents of air that move from west to east on Earth) in changing weather conditions.
"Increased tortuosity of winds and jet streams means the formation of larger meanders and deviation from their typical flows.
"This can cause more meandering paths of atmospheric rivers and extend their duration and impact on different areas," says Denise Bozkurt, a meteorologist at the University of Valparaiso in Chile.
Given the risks of catastrophic flooding and landslides they can cause around the world, atmospheric rivers are categorized into five types based on their size and strength - just like hurricanes.
However, not all atmospheric rivers are harmful, especially if they are of low intensity.
Some may be useful if poured in areas of prolonged drought.
But experts say monitoring and predicting storm surges is largely limited to the U.S. West Coast, where impacts have been well-monitored for decades.
"The problem is that regional meteorological forecasts do not take into account the concept of atmospheric rivers.
"The main problem is the lack of data, especially on atmospheric rivers over complex terrain," says Valparaiso University's Bozkurt.
See also this story
BBC is in Serbian from now on and on YouTube, follow us HERE.
Follow us on Facebook, Twitter, Instagram, YouTube i Viber. If you have a topic proposal for us, contact us at bbcnasrpskom@bbc.co.uk
Bonus video:
