On the other side of the globe last week, winding Hong Kong streets became surging rapids. Wise Hui, a 20-year old student, said downpours tied to a typhoon came on more suddenly than she had ever seen.
Then came a torrent of rain over northeastern Libya on Monday, leaving 5,300 dead and thousands still missing after perhaps the most ferocious of a spate of recent floods that have inundated communities in countries from Japan to Greece and the United States..
This summer’s record heat helps explain the floods’ intensity and persistence, scientists say, a phenomenon that climate models have long predicted would come with rising temperatures.
Yet “I’m a little shocked at how many are coming this year,” said Michael Bosilovich, a scientist at NASA’s Goddard Space Flight Center who focuses on Earth’s water cycle.
In each case, factors leading to the disasters have varied: A stagnant weather pattern allowed storms to park over Spain, Greece, Turkey and Bulgaria. In southern China, the tail of Typhoon Haikui collided with monsoons. In Libya, as much as 16 inches of rain across desert landscapes overwhelmed reservoirs and dams.
But the globe’s remarkable warmth — especially of its oceans, most of which have been running several degrees warmer than normal for months — served as a backdrop for all of the floods.
It’s too soon to know the degree to which global warming, driven by humans’ use of fossil fuels, contributed to any single deluge. But scientists said there is no question that warmer water is more prone to evaporation, and warmer air can carry more water vapor, factors that can produce more intense rains and storms.
“As long as the average temperatures keep going up, that’s just going to continue,” said Gerald Meehl, a senior scientist at the National Center for Atmospheric Research.
A spate of floods spanning the globe
At the end of what has already been a summer of extremes, floods have spanned the Northern Hemisphere with remarkable intensity in recent days.
Across Brazil, deadly floods had already hit at least eight states this year before the most recent deluged ares in the southern state of Rio Grande do Sul. Floods from nearly a foot of rain from a tropical cyclone killed at least 46 people and displaced more than 25,000 people.
Heck de Carvalho and other survivors face an uncertain recovery. “We are traumatized,” he said. “I can’t stay in a place like this. I don’t know how long we will stay at this shelter we are now, because we are surviving on donations. But I want to move away from here.”
In parts of the Japanese island of Honshu, record rainfall from remnants of Tropical Storm Yun-yeung killed at least three people and triggered some 200 landslides, according to FloodList.com.
And in Hong Kong, authorities said rain fell at rates of as much as six inches per hour — the most intense since record-keeping began in 1884. The city was at a standstill for 16 hours, while some of the worst flooding was in the north of Hong Kong, near the border with mainland China, where local farmers lost hundreds of pigs to the torrents.
Even with road closures and advanced warnings, more than 150 injuries and at least two deaths were recorded at the height of the storm, the Hong Kong Hospital Authority said.
“I never thought that the water levels could go this high,” Hui said. “The weather has become much more extreme.”
Around the Mediterranean, a stagnant weather pattern and warm seas contributed to flooding from Spain to Greece, Bulgaria and Turkey. Those floods developed on either side of a stubborn high-pressure heat dome parked over northern and central Europe, a pattern that has been the hallmark of recent summers and which has contributed to other major floods, including across Germany in 2021, said Hayley Fowler, a professor at Newcastle University.
Low-pressure systems that developed and lingered around the heat dome killed at least three people in Spain, Reuters reported, and 15 people across Greece, Bulgaria and Turkey, according to the Associated Press.
As it shifted southward from Greece, that moisture gained strength over the Mediterranean to become a cyclone-like storm that inundated Libya.
It poured as much as 16 inches of rain within six hours across terrain that usually sees half an inch of September precipitation. It flowed violently through dams and over waterfalls into Derna, a seaside city of about 100,000 people.
“They tell us that almost a quarter [of Derna] was vanished away by the hurricane,” said Tamer Ramadan of the International Federation of Red Cross and Red Crescent Societies, relaying reports from aid workers. “They tell us that the dead bodies, you can see them on the streets everywhere.”
One day later, major flooding hit Massachusetts and Rhode Island, dumping as much as 9 inches in the Leominster, Mass., area and causing “catastrophic damage,” authorities said. No casualties were reported.
A recipe for exceptional rainfall
While land across the Northern Hemisphere hit its warmest temperature for the year in July, because it takes longer for water to heat up, many bodies of water are still close to or reaching their peak temperatures. Global average sea surface temperatures, excluding polar regions, have been hovering at or around record highs for six weeks, according to National Oceanic and Atmospheric Administration satellite data charted by the University of Maine’s Climate Reanalyzer.
Average global temperatures across both land and sea are meanwhile on pace to be among Earth’s warmest on record, if not setting a record high, surpassing 2016.
That is a recipe for exceptional storms, scientists said. About 80 percent of the moisture that feeds into storms comes from oceans and other large bodies of water, Meehl said. And with each degree of warming, the air can hold about four percent more water vapor.
That is not to say conditions would not produce flooding rains absent the warming trends, said Kenneth Kunkel, a research professor at North Carolina State University. Summer storms and cyclones have always contained large amounts of moisture and still require the right conditions to develop.
But given the current climate, “they’re happening with a background that’s richer in water vapor,” he said.
In the past, warming was already proved to be fueling more severe precipitation. A study published in March found that since 2002, precipitation extremes have been closely correlated with rising temperatures. While warmer air can produce heavier downpours, it can also suck more moisture from land.
The study found that instances of extremely wet or dry conditions became about 33 percent more common during the warmest years, from 2008 through 2021.
“As the planet continues to warm, what this says is that it seems more likely we’re going to be having more of these extreme wet and dry events around the world,” said Matthew Rodell, the study’s lead author and deputy director of Earth sciences for hydrosphere, biosphere and geophysics at NASA Goddard.
The climate pattern El Niño threatens to produce more precipitation extremes as it approaches an expected peak this winter, with intensity that could rival a historic El Niño in 1997 and 1998. It is known for bringing stormy conditions to the southern United States and drought to Southeast Asia and southern Africa.
That, along with the unprecedented global warmth, could mean conditions remain conducive to heavy precipitation events into next year.
“I’ve come to expect — I don’t want to say the unexpected — but I’ve come to expect that some areas will get something highly unusual and record-breaking virtually every year,” Kunkel said.
Dan Stillman and Sarah Dadouch contributed to this report.
