But what really stands out is the intensity and duration of the storms, reflected in a measure known as accumulated cyclone energy. By that index, 2017 ranks eighth since modern hurricane records began in 1851, and its position is likely to rise before the season is over. (The 1893 season, which produced the last 10-hurricane streak, ranks fourth.)
This year, Mr. Henson said, the Atlantic has experienced 18.75 major hurricane days — that is, the total number of days logged by Category 3, 4 and 5 storms. While there is significant variation from year to year, he said, this is nearly five times the full-season average of 3.9 days.
All of this leads, inevitably, to one question: Why?
The link between climate change and hurricanes is not as simple as the link between climate change and other extreme weather events, like heat waves and droughts, scientists say — in part because, compared with those other events, the sample size of hurricanes is small. But climate change is a factor.
Katharine Hayhoe, a climate scientist at Texas Tech University, told The New York Times in August that climate change might not increase the number of hurricanes, but that warmer ocean temperatures could “have a role in intensifying a storm that already exists.”
“We care about a changing climate because it exacerbates the natural risks and hazards that we already face,” Dr. Hayhoe said.
That fits with what we have seen this year: a remarkable number of the strongest storms. Warmer waters simply provide more fuel for hurricanes to intensify.
“Any given season could have waters that are warmer or cooler than average,” Mr. Henson said. “That’s simply how hurricane seasons work.” But the ordinary seasonal variations are occurring on top of the long-term warming trend. These two elements can combine to produce monstrous storms like Hurricane Irma — which was one of only five hurricanes on record to reach wind speeds of 185 miles per hour, and which sustained those winds for a record 37 hours.
Generally speaking, hurricanes can develop when the water beneath them is 26 degrees Celsius (about 79 degrees Fahrenheit) or higher. Climate change, Mr. Henson said, is increasing both the number of days that present those water temperatures and the size of the areas that have them.
The warm waters have combined with atmospheric conditions to make this season historically destructive. Wind shear has been very low, Mr. Henson said, which has allowed tropical systems to intensify “without being tilted, sheared, torn apart by stronger winds.” And steering currents have tended to drive storms over land rather than pushing them out to sea, creating an enormous human toll.