Monday kicked off the start of another busy hurricane season, forecast to be the fifth in a row with an above average number of storms.
Part of that forecast is based on a variability in ocean temperatures that researchers believed was tied to the ocean’s massive circulation. Over the last 150 years, as the oceans gradually warmed, seasons shifted between active and less active phases on 60 to 80-year cycles.
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“We've been in what's called the warm phase of the Atlantic Multidecadal Oscillation since 1995,” lead forecaster Gerry Bell said when NOAA announced the preseason forecast last month.
But increasingly, researchers think the pattern is no pattern at all, but the result of soot in the atmosphere blocking the sun.
“What we have found was that this doesn't appear to be an oscillation, but actually appears to be almost an accident of history,” said Amy Clement, a University of Miami atmospheric and marine scientist at the Rosenstiel School of Marine and Atmospheric Science.
In 2015, Clement led a research team that modeled the record of warming ocean temperatures and found dips in temperatures believed to be tied to the ocean’s circulation coincided with soot produced by erupting volcanoes and air pollution.
Following World War II, Clement said coal burning in the U.S. and Europe expanded, fueling widespread health and ecological damage. Between the 1940s and '60s, toxic clouds of smog led to dozens of deaths in London, Pennsylvania and New York City. That triggered widespread air pollution laws. Volcanic eruptions also produced enough soot to cool the planet.
The clearing of the skies, she said, occurred amid the increase in fossil fuel burning that pumped the atmosphere full of longer-lasting greenhouse gases.
“If you believe that there are these natural cycles that come and go in the Atlantic Ocean, you might expect that we'll go back to another period of time when hurricane activity is lower than it is today for an extended period of time,” she said. “But what our research is showing is that is not likely to happen.”
It’s often difficult to tease out the human impact on hurricanes because tropical cyclones occur so infrequently, providing a short record to study. Climate scientists believe storms will become more intense, but whether the overall number increases is less certain. ANOAA study published last month concluded that a warming planet had changed the distribution of hurricanes, with more occurring in the North Atlantic and Central Pacific bookending the U.S., and fewer in the Indian Ocean and western Pacific.
The study suggested as the planet contines to warm, there could even be fewer storms because heating both the ocean and the atmopshere creates more stability, and less chance for cooler and warmer air to collide and strike up a storm.
But that remains unclear. Some models suggest they increase.
Models that incorporate aerosols from soot also show storms have likely been less intense because of the shading provided by the aerosols, said Adam Sobel, a Columbia University physicist who oversees the university’s extreme weather and climate research.
Before air pollution laws reduced aerosols, the soot likely masked some of the warming that might have otherwise occurred from greenhouse gases. Laws are now likely to prevent that kind of soot from returning, he said.
“There's compelling evidence, although the oceanographers are still fighting over it, that the slow Atlantic hurricane period in the '70s and '80s is not as likely to come back as we might have thought,” he said at a UM climate symposium earlier this year.
That’s led some researchers to turn to more radical solutions to engineer a cooler atmosphere as emissions continue to climb.
“When you put carbon dioxide in the atmosphere, it takes decades to centuries to extract it, because it has to get either absorbed into the plant matter on land or absorbed into the ocean. And that just takes time,” Clement said. “So there are people arguing that one thing to do in the short term is to basically mimic what volcanic eruptions do.”
But the geo-engineering solutions still need research. Spreading aerosols to stop warming completely could worsen droughts or hurricanes. But if aerosols were used to reduce just half the warming, climate hazards could be significantly reduced around the planet while worsening conditions in a fraction of areas, researchers from Harvard University and University College Londonsaid in a March study.
The authors warned that the fixes should not replace cutting greenhouse gases and hazards to the small areas adversely affected still need to be studied more closely.
