Days of relentless rainfall in the west of Washington state has forced authorities to order tens of thousands of residents to evacuate as catastrophic flooding hits.

As rivers reach near historic levels, the state's governer, Bob Ferguson, declared a statewide emergency. "Lives will be at stake in the coming days," he said.

Meteorologists say the extreme rains have been caused by an "atmospheric river" soaking the region along the coast of Oregon and Washington. 

Former National Oceanic and Atmospheric Administration chief scientist Ryan Maue, now a private meteorologist, said between 20 and 30 inches (51 to 76 centimeters) could fall in a three-week period, which is "quite extreme." 

What is the science behind extreme flooding?

Rising global temperatures, caused by burning fossil fuels, is making rainfall more frequent and severe across most parts of the world.

Tropical storms and intensified monsoon systems have also triggered catastrophic flooding and landslides across Sri Lanka, Indonesia, Thailand, Malaysia and Vietnam since mid-November. 

"Increasing heavy rainfall and extreme flood heights in a warming climate threaten densely populated regions across Sri Lanka and the Malacca Strait," according to a study this week by World Weather Attribution, a UK-based academic institution.

While modeling precipitation patterns is complex, a clear guiding principle is that when air gets hotter it holds more moisture.

Greenhouse gases released into the atmosphere act to insulate Earth, trapping heat and causing temperatures to rise. One consequence is more rapid evaporation of water on land and at sea, so more water is released when it rains, which can result in flooding.

Air's capacity to hold moisture rises by 7% with every rise of 1 degree Celsius (1.8 degrees Fahrenheit). Global air temperatures increased 1.55 C above pre-industrial levels in 2024, which was the hottest on record according to the World Meteorological Organization.

Temperature rises also make more precipitation fall as rain instead of snow which can make high-altitude regions vulnerable to flooding and landslides. A 2022 study published in the science journal Nature found that in snowy, high-elevation parts of the Northern Hemisphere, rainfall extremes increased by an average of 15% per 1 degree Celsius of warming.

How hotter temperatures are affecting rainfall

Climate change is altering the frequency of heavy downpours during storms and sudden outbursts through its influence on complex atmospheric and weather patterns.

Advances in attribution science are allowing experts to pinpoint the causal relationship between climate change and extreme weather events more accurately. According to one estimate, on average, one in four record rainfall extremes in the past decade can be attributed to climate change. 

Severe flash flooding in the US state of Texas in July that killed more than 100 people was also made worse by climate, say experts. 

Edward Gryspeerdt, a Research Fellow at the Department of Physics at Imperial College London in the UK, concluded that "the floods in Texas were driven by record amounts of atmospheric moisture over a region that is known for flash floods."

He warned that, as the atmosphere warms due to climate change, such extreme rainfall events are likely to become worse. 

"Climate models predict that while Texas may become slightly drier on average due to climate change, the intensity of rainfall will likely increase, suggesting that these types of thunderstorms will become more common in the future."

Will floods increase in the future?

Climate science shows that the risk of extreme flooding will increase if the world fails to limit global warming. 

If the 1.5 degrees Celsius temperature rise limit set at the Paris climate summit is breached, extreme rain deemed a once-a-decade rainfall event will occur 1.5 times every 10 years and be over 10% wetter, according to the UN's International Panel on Climate Change (IPCC). 

At 2 degrees Celsius of warming above pre-industrial levels, such events would be likely to hit 1.7 times per decade and be 14% wetter, according to the IPCC. 

And if the world warms to 4 degrees Celsius, heavy rains could hit almost three times more often and unleash 30% more rain. 

Edited by: Tamsin Walker, Sarah Steffen