According to a study published in the journal Nature Climate Change on Monday, the biggest cities of America are at extremely high risk of experiencing serious flooding in the coming decades. The risk is much higher than previously thought. The factors responsible for increasing flood risk in coastal US cities such as San Francisco, Houston, San Diego, Los Angeles, Boston and New York include heavy rainfall, sea-level rise and storm surge.

A co-author of the study Steven Meyers, a scientist representing the University of South Florida, said that the three factors can together be referred to as “a triple threat”. According to Meyers, this shows that there’s a growing risk of rainfall and storm surge induced compound flooding.

The coastal cities of the country house around 40% of its entire population. These cities are already experiencing a lot of problems due to damages caused by storm-induced flooding. Such floods are particularly proving costly in electricity stations, subway lines and built-up areas.

The new study on Nature is among the first ones to have explored the combined risk of sea level rise, storm surges and heavy rainfall on the US coat.

The researchers revealed that in the past 60 years, the risks of flooding in the New York City have doubled. During this period, the city has experienced the 5-inch increase in rainfall and a storm surge of 4 feet once in every 42 years. During the 1940s, this used to happen only once in a century.

Study leader Thomas Wahl, a researcher from the University of South Florida, said that all these factors (storm surge, heavy rainfall, sea level rise, and increased flood risk) are somehow interconnected. He added that if the sea levels keep on rising, it would have a strong impact on storm surges, which in turn will induce compound flooding.

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This means there’s no need of having a huge amount of rainfall for putting coastal cities like New York underwater. According to Wahl, a storm surge is enough for doing that.

The researchers, however, haven’t yet understood the exact nature of connections between heavy rainfall, storm surge and sea level rise. They feel that it’s too early to state whether climate change is the only factor responsible for the increase in risk of flood in the coastal cities.