Princeton University and National Oceanic and Atmospheric Administration scientists have presented evidence against a favorite argument of researchers trying to prove a link between man-made global warming and hurricane activity.
The study found the lull in major Atlantic hurricane frequency from 2005 to 2015 was brought on by “a weakening of the Atlantic Meridional Overturning Circulation (AMOC) inferred from ocean observations.”
However, researchers also found evidence “suggesting the decline of the Atlantic major hurricane frequency during 2005–2015 is not likely due to recent changes in anthropogenic sulfate aerosols.”
This is in direct conflict with a theory advanced by some of the most prominent climate scientists who claim air pollution from cars and power plants has kept hurricane activity in check, despite warming ocean temperatures.
“Instead, we find coherent multidecadal variations involving the inferred AMOC and Atlantic major hurricane frequency, along with indices of Atlantic Multidecadal Variability and inverted vertical wind shear,” reads the study, led by scientist Xiaoqin Yan.
“Our results provide evidence for an important role of the AMOC in the recent decline of Atlantic major hurricane frequency,” Yan and his colleagues wrote.
Prominent climate scientists, including Penn State’s Michael Mann and MIT’s Kerry Emanuel, have argued for years that “sulfate aerosol pollution” is the main reason why Atlantic hurricane activity was so muted in the 1970s and 1980s.
Atlantic hurricane frequency began increasing in the mid-1980s, peaking around 2005. In a 2006 study, Emanuel and Mann attributed the rise in hurricanes to clean air regulations that kept more climate cooling pollution particles out of the atmosphere.
“A debate continues about why the 1970s and 80s were relatively quiet in the Atlantic,” Emanuel told The Washington Post in September.
“Some believe that it was the consequence of a natural climate oscillation called the Atlantic Multi-decadal Oscillation (AMO), while others, including me, think is was mostly a consequence of sulfate aerosol pollution,” Emauel said.
That theory hasn’t been totally accepted by scientists, and Yan’s study adds weight to the counter argument that ocean circulation plays a greater role in hurricane activity.
“Directly observed North Atlantic sulfate aerosol optical depth has not increased (but shows a modest decline) over this period,” Yan wrote, suggesting the decline in hurricane activity was not caused by efforts to improve air quality.
Instead, Yan says the AMOC, an ocean circulation pattern, is more closely related to the increase and decrease in Atlantic major hurricane frequency. The AMOC switched from a warm phase to a cool one around 2005, taking hurricane activity down with it.
Yan expects “changes in the Atlantic major hurricane frequency in the next decade would be closely linked to future AMOC changes.”
Here is a key graphic from Yan’s study showing the relationship between the AMOC and major hurricane activity:
Figure from the paper showing incidence of Cat 3+ hurricanes in the North Atlantic pic.twitter.com/PYTnyiqyB1
— Roger Pielke Jr. (@RogerPielkeJr) November 22, 2017
Of course, this year’s Atlantic hurricane season was an active one, with three major U.S. hurricane landfalls in the span of a couple of months. Hurricane Nate became the fourth hurricane to make U.S. landfall this year, but it was only a Category 1 storm.
The Atlantic saw well above normal hurricane activity, including one of the strongest storms on record, however, the Northern Hemisphere collectively saw a normal season since other basins were relatively quiet.
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