Bigger, as in three storms that hit as Category 4 or higher. More intense, because when you add up the level and duration of wind energy from the 10 hurricanes, the "accumulated cyclone energy" total is unusually high.

And longer lasting, as when Harvey hit Texas. Hurricanes normally wither on land. Harvey persisted, at least when it came to rainfall. It hovered for days, dropping record rain and recharging itself by extending spiral bands into the Gulf of Mexico like some airborne octopus.

Heavy rain is another sign of climate change. A warmer ocean means more water vapor rises into the atmosphere during the storms, which makes them wetter. Harvey's rains were the most intense in Texas history.

There are other factors at work besides warming. A global weather pattern called El Niño was absent this year, and that often means a stormier Atlantic Ocean. One reason is that high-altitude "shear winds" in the Atlantic that often nip the tops off budding hurricanes were absent, as well.

Atmospheric scientist Kerry Emanuel at MIT notes that there's also a rather mysterious cycle for Atlantic hurricanes. It seems to repeat about every decade. "We don't really understand it, [but] it's there," he says, and it also influences when and how often hurricanes arise. Sure enough, it was 12 years ago, in 2005, when the Atlantic saw such a similarly huge hurricane year.

Emanuel says scientists aren't clear whether hurricanes are becoming more common; reliable records for hurricanes go back only about 50 years in the Atlantic, so it's difficult to detect a long-term trend, and data for the Pacific are even scarcer. Theoretically, he says, some aspects of warming could actually decrease the frequency of hurricanes by drying out parts of the atmosphere. But those that do arise are likely to be nastier. "Insofar as the storms are intense and produce a lot of rain," he says, "they're a window into our long-term future."

And it's a costly future, Emanuel says. "We're just going to get more and more destruction," he predicts, "because we just encourage people to live and build in dangerous places" like low-lying coastlines — a fact independent of climate change.