What is the reason behind heavy rains around the world?
The catastrophic floods in Libya that are feared to have left up to 10,000 people dead are the latest in a series of heavy rain events to hit different parts of the world over the past two weeks.
In the first 11 days of September, eight devastating flood events occurred on four continents. Before a Mediterranean storm sent floodwaters into eastern Libya, heavy rains inundated parts of central Greece, northwestern Turkey, southern Brazil, central and coastal Spain, southern China, Hong Kong and the southwestern United States.
Seeing so many unrelated extreme weather events around the world in such a short time is unusual, said Andrew Howell, a research meteorologist at NOAA’s Physical Sciences Laboratory.
“Sometimes we have a combination of these events, whether it’s in a particular country, in a hemisphere, or globally,” he said. “This appears to be the right time for a number of flood events globally.”
As with many other forms of extreme weather, scientists say climate change likely has an impact on rainfall and flooding, but understanding this relationship precisely can be difficult.
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, storms can bring more intense rainfall and thus cause severe flooding.
Researchers have observed these 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 flood events and their severity, and detecting the signatures of climate change when they all interact can be difficult, Hoyle said.
“From a 1,000-foot perspective, certainly if temperatures are higher, you’ll have more water vapor, so you could have more rain falling from the sky,” he said. “But when you look at a specific event and a specific set of physical processes related to that event, it becomes difficult to attribute each individual process to that causal chain.”
For one thing, the types of extreme weather that caused each of the eight catastrophic flood events this month had different origins.
It was a Mediterranean storm named Daniel that brought heavy rains to central Greece and Libya. Typhoon Haikui and its remnants hit Hong Kong and southern China with record rains and flooded urban and rural areas, destroying roads and causing more than 100 landslides.
Heavy rains caused flash floods in central and coastal regions of Spain, northwestern Turkey and thousands of miles away in the Brazilian state of Rio Grande do Sul.
Fast-moving thunderstorms over southern Nevada this month caused flash flooding across the region, flooding the Las Vegas Strip and stranding more than 70,000 people at the Burning Man festival in the Black Rock Desert.
With certain types of extreme flooding events, such as those associated with Mediterranean hurricanes like Daniel, there simply is not enough data to monitor shifts over time.
“We don’t really have a sample or a long enough record to be able to detect the change, because it’s not really a common occurrence,” Howell said.
In other cases, local factors, such as how wet or dry the ground is or the underlying topography of the area, can have huge impacts on how floods develop and their consequences.
In addition to the loss of life and property, floods increase the risk of exposing people to waterborne pathogens, which has important implications for the outbreak of deadly diseases.
Hoyle said the number of devastating floods this month is distressing, but said he was particularly concerned about the unfolding situation in Libya.
“If you look at the damage and the number of people who lost their lives, it amazes you,” he added.