The average hurricane season is officially more intense

The oceans are warming and the hurricanes are coming.

A new decade’s worth of hurricane data is in, and the revised average season is predicted to be longer and have more storms than before. Previously, the National Oceanic and Atmospheric Administration (NOAA) predicted an average of 12 named storms, six hurricanes, and three major hurricanes (those Category 3 and higher). But the new data, which covers from 1991 to 2020, adds two more named storms and one more hurricane to that list. 

This probably shouldn’t have come as a surprise, given that 2020 had a record-breaking number of storms—we ran out of names for new hurricanes—and was the fifth consecutive year of above-average hurricane seasons.  But the update to the predicted averages cements what we already knew: hurricane seasons are getting worse. 

Though you might think that increase could come simply from better technology, that’s not actually the case. 

“We’re not having more storms because we observe them better and because the technology is better. We have for the last 40 years … had satellite data.” explains Cindy Bruyère, executive director of the Capacity Center for Climate and Weather Extremes at the National Center for Atmospheric Research. “If you look at the numbers of storms in the 1950s versus the numbers of storms now, then technology becomes a really big question because we miss storms in the ‘50s and ‘60s, But we didn’t really miss storms from the beginning of the 1980s.” And since the 1980s are where the modern averages come from, that means improving technology isn’t the reason for the increasing stats.

The explanation, as with many other worrying weather patterns, is global warming. “We have a lot of confidence that climate change is causing warmer sea surface temperatures,” says Dereka Caroll-Smith, a research meteorologist for the National Institutes of Standards and Technology.  “And the fuel for hurricanes is warm sea surface temperature.” 

The new predicted averages are just that: averages. The exact number of storms in any given year is uncertain, but researchers have more certainty about the nature of the storms and their increased severity. Bruyère says that no matter the number of storms in a season, there will be a shift to having a larger proportion of major storms in the future. Major storms tend to bring more rain than weaker ones, and with every additional degree of global warming (in Celsius) there’s a predicted increase of around seven percent more rain for every storm. In 2017, Hurricane Harvey surpassed that expected rate. In some areas, Harvey rained down at least 18.8 percent more than predicted due to warming temperatures in the Gulf of Mexico. This has led more scientists to believe that human-induced warming will force us beyond a seven percent rainfall rate. Flooding and heavy rains are only going to become more of a problem. 

Climate researchers also predict that storms could stall more and move more slowly once they make landfall, effectively putting people at risk of flooding for longer. According to Bruyère, there is even an indication that the effects of climate change will mean hurricanes will be able to move further away from the equator before dying out. As a consequence, places that have historically not experienced these storms could find themselves at risk. That shift likely won’t happen for many decades, but it is coming. 

As for the 2021 season, the El Niño-Southern Oscillation (ENSO) cycle will also play a role. Warming Pacific waters are known as El Niño, whereas the cooling phase of the cycle is known as La Niña. ENSO is the cycle of fluctuation between the surface temperature of the Pacific and the air pressure of the atmosphere above. Last year the La Niña system dominated, meaning  there is more hurricane activity in the Atlantic basin. Easterly winds grow stronger during this phase and the Pacific waters cool. Together, this means more intense tropical storms in the Atlantic. Currently, Bruyère says, we are moving towards a more neutral year in the cycle. But since the oscillation pattern is not regular and can span between two to seven years, the pendulum could swing either way.

“It’s still pretty early. So it could still go either way,”  Bruyère says. “Right about this time, in 2020, we had a similar prediction as what we have right now for 2021. Then, later in the year, the [number of predicted hurricanes] were increased, because La Niña intensified.”

As meteorologists move to a new 30-year period, there will also be an update to our current understanding of “normal” weather. What was previously above normal is now the new baseline for hurricanes, since warming temperatures have raised our averages over the last few years.

“That is actually really important when we look at all weather events and specifically extreme events,” Bruyère says, “because normal is changing.”