Compound disasters — heavy rain followed by heat waves — triple in intensity, occur five times more often

A farmer rests after weeding a rice paddy under the scorching sun in Okpo-eup, Dalseong-gun, Daegu, on Thursday. [Yonhap]

The number of days with severe compound disasters — such as intense heat waves striking immediately after heavy downpours — has increased nearly fivefold, according to new research.

Greenpeace announced Monday that a study it commissioned from a research team led by Kim Hyung-jun, a professor at KAIST's Graduate School of Future Strategy, found these results in an analysis titled "Changes in Compound Disaster Risk from Heat Waves and Heavy Rain in South Korea."

Compound disasters occur when multiple extreme weather events — such as drought and heavy rain — driven by climate change interact with, follow or strike simultaneously alongside one another.

Typical examples include large wildfires breaking out after a drought, or high temperatures and dry conditions appearing at the same time. Cases in which one extreme weather event, such as heavy rain, is quickly followed by a heat wave also qualify as compound disasters.

South Korea has recently seen this pattern play out during summer, with severe heat waves striking shortly after concentrated downpours.

Kim's team analyzed the Compound Intensity index (CI28) for the June-to-September period over the past 30 years (1991–2020) and the most recent four years (2021–2024), finding that compound disaster intensity has nearly tripled.

The CI28 combines extreme precipitation levels and peak temperatures recorded over roughly one month (28 days) into a single index, allowing researchers to gauge how severely heat waves and heavy rain occurred together within that window.

The median CI28 value stood at 0.55 over the past 30 years but rose to 1.65 over the most recent four years.

The rise in the median — rather than the average — indicates that overall disaster severity over the past four years has worsened compared with the past, not merely that a handful of extreme outlier days pulled the figure up.

Kim's team said the findings reflect a structural deepening of compound disasters. "As the frequency of two phenomena — heavy rain and heat waves — occurring together increases, the compounded risk is growing," the team said.

The number of days with extreme compound disasters has also risen sharply.

Days on which the CI28 fell in the top 10 percent of severity increased from 12.2 days over the past 30 years to 25 days over the most recent four years — roughly double. Days in the top 5 percent rose from 5.5 to 14.2 days, a 2.6-fold increase, while days in the most severe top 1 percent jumped from 1.1 to 5.2 days, a 4.8-fold increase.

"What used to be about one day a year of extreme risk has recently appeared on five or more days," Kim said. "This means that disasters that were once very rare are likely to occur more frequently going forward."

Compound disaster intensity increased across the entire country. North Chungcheong Province, Gangwon Province and Jeju in particular saw intensity more than double compared with the past 30 years.

The report "Changes in Compound Disaster Risk from Heat Waves and Heavy Rain in South Korea" [Greenpeace]

Kang Seong-won, a Greenpeace activist, said the current disaster-response system — designed to handle one type of disaster at a time, such as heavy rain or wildfires — is not equipped to deal with compound disasters. "We need to build a proactive preparedness framework that accounts for the scale and frequency of compound disasters," Kang said.

thlee@heraldcorp.com