Hurricane Sandy wreaked havoc on New York City in 2012. The city was caught totally unprepared for the storm; it has become the second costliest hurricane in US history. Yet, what if such an event was to happen every 25 years?

In the study published on Monday in journal Proceedings of the National Academy of Sciences, researchers argue that the storms that previously would have struck once every 500 years are now more likely to occur once every few decades.

“This is going from something you probably won’t see in your lifetime to something you may see several times in your lifetime,” said Andra Reed, a researcher at Penn State University.

The team first evaluated the sediment at various points along the New Jersey shore in order to compare sea levels between 1970 and 2005. They then went further afield to determine the sea levels between 850 and 1800.

Records show that prior to 1800, a flood that was 7.4 feet above sea level occurred only once every 500 years. Since the 1970s however, a storm of that size has occurred every 24 years.

“We have seen more intense storms with a greater ability to produce high storm surges at The Battery in NYC during the anthropogenic era than during the pre-anthropogenic era,” wrote the researchers.

Prior to 1800, sea levels rose and fell naturally. Yet in recent decades, the sea level has risen by 0.08 inches a year.

The rise in sea levels caused by warming water temperatures and melting ice caps have caused the flood level height to rise by about four feet. As a result, the risk of New York City flooding is has increased dramatically.

“Sea level is rising because of climate change. But climate change also appears to be leading to larger and more intense tropical storms,” said study author Michael Mann, a meteorologist at Penn State University.

“I think the punch line is, we made Sandy much more likely already,” said Mann. “We’re already dealing with greatly elevated risk. We’re not just talking about the future. Climate change is already costing us dearly, but it’ll be a whole lot worse if we do nothing about it.”