And only once before in history, in 2007, did multiple Category 5 storms make landfall in North America.

The new record verifies that something extremely unusual is happening in the Atlantic Ocean this year. It also sets the coming autumn in perspective: With a La Niña current budding in the Pacific, the final two months of the Atlantic season will likely see a higher-than-normal number of tropical cyclones. (Hurricanes normally stop forming around November 30.)

But the record also provides a glance into the messy nature of the Atlantic hurricane record. Over the past decade, our understanding of that record has changed—as has what researchers can safely say about the effects of human-caused climate change.

Accumulated cyclone energy, abbreviated ACE, measures the strength of a single hurricane over its lifetime. Because it’s a durational measure, it can also gauge the intensity of an individual month or an entire hurricane season. Meteorologists calculate ACE by taking a hurricane’s maximum wind speed every six hours, then summing the squares of those figures. They divide the final product by 10,000 to make it easier to use.

Hurricane records in some form go back more than a century. The U.S. Signal Service, a predecessor to the National Weather Service, began tracking Atlantic hurricanes in 1878. In that context, September 2017’s record can seem extraordinary: the strongest hurricane month in 139 years.

But climate scientists say that there are issues extrapolating hurricane history that far back, especially when measuring storms with ACE. Accumulated cyclone energy requires that meteorologists correctly observe the strongest winds in a cyclone every day. The National Weather Service does a much better job of that now, they say, than they did even a couple decades ago.

“The probability of us observing the true maximum of a storm now is much higher than it was 50 years, or 100 years, or even 20 years ago, honestly,” says Gabriel Vecchi, a professor of geoscience at Princeton University and a former climate modeler for the National Oceanic and Atmospheric Administration.

Why? In part, weather satellites have gotten better and more accurate. But meteorologists didn’t always have access to weather satellites. In fact, this exact problem—comparing hurricane intensity across 100-year time spans—is a microcosm of the current challenge facing climate scientists studying hurricanes. Over the past decade, it’s become clear that long-term hurricane data does not mesh together as seamlessly as some researchers had once hoped.

Before satellites, the National Weather Service (and its predecessors) pieced hurricanes together from a few types of data. The most significant of these were shipping records: observations taken by commercial or passenger vessels caught up in a storm. By slicing together multiple observations of the same storm, hurricanes emerge from the data.