The catastrophic flooding in Libya that is feared to have left as many as 10,000 people dead is just the latest in a string of intense rain events to hammer various parts of the globe over the past two weeks.
In the first 11 days of September, eight devastating flooding events have unfolded on four continents. Before Mediterranean storm Daniel sent floodwaters surging through eastern Libya, severe rain inundated parts of central Greece, northwestern Turkey, southern Brazil, central and coastal Spain, southern China, Hong Kong and the southwestern U.S.
Seeing that many unrelated extreme weather events around the world in such a short time is unusual, said Andrew Hoell, a research meteorologist at the National Oceanic and Atmospheric Administration’s Physical Sciences Laboratory.
“Sometimes we have a clustering of these events, whether it’s in a given country, given hemisphere or globally,” he said. “And it seems like right now, globally, this is prime time for a number of flooding events.”
As with many other forms of extreme weather, scientists say climate change is most likely having an impact on rainfall and flooding, but understanding precisely what that relationship is can be tricky.
In general, studies have shown that global warming is intensifying the planet’s water cycle. Warmer temperatures increase evaporation, which means a warmer atmosphere can hold more moisture. As a result, when storms can unleash more intense precipitation and thus cause severe flooding.
Researchers have observed those changes over time as the world warms. Since 1901, global precipitation has increased at an average rate of 0.04 inches per decade, according to the Environmental Protection Agency.
However, a number of factors can influence flooding events and their severity, and teasing out the fingerprints of climate change when they all interact can be challenging, Hoell said.
“From a 1,000-foot view, it’s definitely true that if you have higher temperatures, you have more water vapor, and therefore you can have more precipitation fall from the sky,” he said. “But when you look at a specific event and the specific set of physical processes relevant to that event, it then becomes difficult to attribute every single process in that causal chain.”
For one thing, the types of extreme weather that caused each of the eight catastrophic flooding events this month had different origins.
It was a Mediterranean storm named Daniel that dumped heavy rain over central Greece and Libya. Typhoon Haikui and its remnants lashed Hong Kong and southern China with record rain, waterlogging urban and rural areas, destroying roads and causing more than 100 landslides.
Torrential downpours caused flash flooding in central and coastal regions of Spain, northwest Turkey and thousands of miles away in the Brazilian state of Rio Grande do Sul.
And fast-moving thunderstorms over southern Nevada this month caused flash flooding across the region, swamping the Las Vegas Strip and stranding more than 70,000 people at the Burning Man festival in Black Rock Desert.
With certain types of extreme flooding events, such as those associated with Mediterranean cyclones like Daniel, there simply isn't enough data to observe shifts over time.
“We really don’t have a long enough sample or record to be able to detect a change, because they’re not really that common of an occurrence,” Hoell said.
In other cases, local factors, such as how wet or dry the ground is or an area’s basic topography, can have enormous influences on how floods develop — and their consequences.
Beyond loss of life and property, floods increase the risks of exposing people to waterborne pathogens, which have important implications for outbreaks of deadly disease.
Hoell said the number of devastating floods this month is distressing, but he said he's especially concerned about the situation unfolding in Libya.
“If you look at the damage and the amount of people who have lost their lives,” he said, “it just blows your mind.”