Hurricane season is still three weeks away. But among hurricane scientists an intense debate has long been churning:
Is the Atlantic Ocean in the midst of a long period of hyperactive hurricane seasons? Or have we entered a period of sedate storm seasons?
"That is the million dollar question right now," said Jim Kossin, a climate scientist with the National Oceanic and Atmospheric Administration.
Every hurricane season brings the risk of destruction. But what's at stake in this debate is how frequently residents along the Gulf of Mexico and the Eastern Seaboard will have to live with those risks in the coming decades.
Historically, the Atlantic Ocean has alternated between strong periods, during which hurricane seasons produce many storms and stronger storms, and sleepier times, characterized by seasons with fewer, weaker storms.
Those periods are caused by a phenomenon called the Atlantic multidecadal oscillation (AMO), the cyclical rise and fall of Atlantic ocean temperatures over periods of 25 to 40 years. Since hurricanes feed off heat, warmer water promotes cyclone activity, while cooler water tempers it.
Scientists agree the Atlantic's last inactive period was about 1970-1994. In those 25 or so years, ocean temperatures were below average, and so was hurricane activity. There were very few major hurricanes, considered to be Category 3 or higher storms with wind speeds of at least 111 mph.
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Researchers at Colorado State University's Department of Atmospheric Science recorded just 10 six-hour periods within that quarter century during which a major hurricane was present in the Atlantic.
Then we entered an active period starting in 1995. Ocean temperatures spiked that year, and so did cyclone frequency and strength. Since 1995, there have been 33 six-hour periods during which a major hurricane was present.
In recent years, ocean temperatures and activity started to falter. The hurricane seasons from 2013-16 were quieter, prompting scientists to wonder if the Atlantic hurricane machine was slowing down again.
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Then came 2017.
Last year's hurricane season was one of the top 10 most active hurricane seasons in recorded history — by nearly every metric. It saw the formation of 17 named storms, 10 hurricanes and six major hurricanes.
The season's three most memorable storms, Harvey, Irma and Maria, caused a combined estimated $200 billion worth of damage to Texas, Florida and Puerto Rico. Those three powerful storms not only caused historic flooding, damage and misery, but may also end up being responsible for more than 1,000 total deaths.
The question, then, is if those 2013 to 2016 quiet seasons were off years in an otherwise active period, or if the Atlantic switched back to an inactive period in 2013, and last year's hyperactive season was the anomaly.
Without historical perspective, scientists said, it's difficult to say.
"There are a lot of questions about whether 2017 was kind of a one-off," CSU hurricane scientist Phil Klotzbach said.
"A fluke, so to speak."
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What the future holds depends on what drives the Atlantic's temperature oscillation. There are two competing theories about what may come:
The first theory, strongly supported by Klotzbach, is the Atlantic Ocean's long-term temperature variation is driven by long-term changes in ocean water salinity. The salt content of North Atlantic water tends to naturally wax and wane over decades. And salt, he said, generally makes for warmer water.
Klotzbach said the salinity in the North Atlantic is dropping, meaning the ocean could be in for a cold streak — not conducive for hurricanes.
Even if Klotzbach is right, colder Atlantic waters don't guarantee quiet storm seasons. The North Atlantic was frigid last winter before it rapidly warmed in the spring, resulting in last year's historic hyperactive season.
Of course, even quiet seasons can be dangerous: Hurricane Andrew devastated South Florida during the below-average 1992 season.
The North Atlantic is again colder than normal this spring, but Klotzbach's team nonetheless predicted a slightly above-average hurricane season this year. That's because on any given year, stronger, short-term variables like the presence of El Niño — high-altitude wind that impedes hurricane formation — more heavily influence cyclone activity.
"I would argue that we should be moving into a quieter period" in general, Klotzbach said. "If you're looking for an active hurricane season, you don't make the far North Atlantic cold in March."
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Others favor the theory that cleaner air and greenhouse gases are warming the ocean.
Kossin explained it this way: Smokestacks used to fill the atmosphere with particles, blocking sunlight from the ocean, causing it to cool off.
Then the Clean Air Act was passed in 1963. Ever since, as the rate of humans pumping pollutants into the atmosphere declined, sunlight has warmed the seas. Kossin said greenhouse gases have also trapped heat in the atmosphere, warming oceans, too.
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Ocean temperatures have trended warm ever since the 1990s. If those are the main drivers of ocean temperature fluctuation, Kossin said, that likely means an end to quieter, weaker storm seasons. Under that theory, 2017 could mark a continuation of this hyperactive period.
"There's clearly two very different things people feel contribute to the AMO," he said. "One is very natural, and one is very not natural. There's no great consensus on this."
He said it's likely that both theories — ocean salinity and air quality — contribute to hurricane activity.
In an era of cleaner air and warmer ocean temperatures, Kossin worries low-salinity periods won't be as weak as they used to — and high-salinity periods will mean even stronger hurricane seasons.
He said that raises an even scarier question: "What is the new normal?"
Contact Josh Solomon at (813) 909-4613 or firstname.lastname@example.org. Follow @ByJoshSolomon.