According to an old saying, a rising tide floats all boats. And in the decades ahead, rising waters will threaten all coastal cities.

As our planet gets warmer as the result of human activities, sea level is rising. So cities along the coast will see more flooding—more often, with higher water levels. But a recent study says the risk isn’t the same for everyone.

Researchers calculated the possible effects of climate change combined with natural fluctuations in sea level. That included El Niño, tropical cyclones, and other events. They used a temperature increase at the high end of current predictions, then compared their numbers to conditions in 2006.

They found that, by the end of the century, the combination could boost the risk of coastal flooding around the world by 20 to 30 percent compared to global warming alone.

The study said the risk will go up for most coastal cities, including those in the United States and Australia. New York, for example, could see 18 times more flooding events with climate change alone, but 25 times more when natural events are factored in.

The numbers are even higher in Bangkok, Manila, and other major cities in Asia. Manila, for example, could see 18 times more floods with climate change alone, but 96 times more with the combo. And those same Asian cities would have to build much more extensive defenses to protect themselves—from the combined impact of Mother Nature and human-produced climate change.