The largest cities in the United States are at much bigger risk of catastrophic flooding than previously thought. According to a new study published in the journal Nature Climate Change, the way scientists traditionally analyzed heavy rainfall and storm surge grossly underestimated flood risk in coastal United States. In the past, disaster experts analyzed these two factors separately to define flood zones.

Climate Change Trio Exposes Major U.S. Cities To Catastrophic Flooding

Climate change doubles the odds of New York flooding

The new study led by Thomas Wahl of the University of South Florida says the traditional methods clearly ignore the threat of “compound flooding” caused by the “triple threat” of climate change. The climate change trio — which include storm surge, sea level rise, and heavy rainfall — expose major U.S. cities like New York, Los Angeles, San Francisco, Boston and Houston to a much greater degree of flooding.

Approximately 40% of the U.S. population lives in coastal cities, where floods caused by storms are already proving costly. It is the first study that combines the risk of the climate change “triple threat” over broad stretches of the American coast. Researchers found that the risk of flooding due to the three factors of climate change has doubled in the last 60 years.

Are all three factors inter-connected?

They found that a 4-feet of storm surge combined with 5-inch of rainfall would occur in the New York City every 42 years, compared to once in 100 years in the 1940s. Thomas Wahl said all the three factors were “somehow interconnected.” A rise in sea levels would have an effect on storm surges, which in turn, have an effect on compound flooding. That means it wouldn’t take a heavy rainfall to flood the New York City. A storm surge could do that alone.

However, Wahl noted that it was too early to say whether the increased risks were entirely due to climate change. Researchers estimated the risk of future flooding based on weather records and tide gauges.