The strongest of the infamous New England gales known as nor’easters have gotten even stronger since the 1940s, threatening to do more damage to the US north-east coast. This is probably due to warmer ocean temperatures.

“We know what’s causing the warming of sea surface temperature: the emission of greenhouse gases. And it’s that increase that’s driving the trend,” says Michael Mann at the University of Pennsylvania.

Mann and his colleagues assembled a dataset of nor’easters and their meandering paths over the past 85 years. They used a statistical method to identify any trends in the maximum wind speed of the storms, as well as any changes in precipitation.

“What we found is that, while we couldn’t isolate any significant trend in the average intensity of these storms, we found the strongest of these storms are getting stronger,” says Mann.

This dynamic occurs because of how the ocean temperatures that fuel the storms interact with other factors, such as wind shear, to decide their ultimate intensity, he says.

Weaker storms are more likely to be influenced by factors other than ocean temperature, which determine the maximum potential intensity of a storm. “The biggest storms, to be a bit anthropomorphic here, have the opportunity to reach their potential,” says Mann.

While hurricanes at tropical latitudes were known to behave this way, it was less clear how more complex nor’easters would respond to warmer temperatures. “Nor’easters, in contrast to hurricanes, derive their energy from a lot of other factors,” says Brian Tang at the University at Albany in New York state.

The upward trend in both intensity and precipitation is small – the change in wind speed of even the strongest storms amounts to just under 2 metres per second since 1940. But this can still affect how much damage the nor’easters do.

Storm surge, combined with sea level rise, drives flooding along the coast, and more snow and rain boost inland flooding. “I think the big danger is water,” says Tang.