A group of four prominent climate scientists have put forward a startling claim that the number of major hurricanes has increased two-fold or even three-fold in recent decades.
There’s just one problem — it’s probably not accurate.
“Storms of 200 km/h and more have doubled in number, and those of 250 km/h and more have tripled,” scientists Stefan Rahmstorf, Kerry Emanuel, Mike Mann and Jim Kossin wrote on the blog RealClimate.org.
Scientists based their startling claim on work done by Emanuel, a hurricane expert at the Massachusetts Institute of Technology (MIT). Rahmstorf and his colleagues admitted “some of the trend may be owing to improved observation techniques,” but argued it “provides some evidence that a global increase in the most intense tropical storms due to global warming is not just predicted by models but already happening.”
The scientists are referring to hurricanes, or tropical cyclones, of about Category 3 and higher on the Saffir-Simpson scale of storm strength. Their data shows a roughly 100-percent increase in Category 3 hurricanes and a more than 300-percent increase in Category 5 storms.
Atlantic hurricane season officially began on Friday, so the scientists’ analysis of global warming and hurricanes gained some media attention, but reporters should have been more skeptical of the claims made.
Cato Institute atmospheric scientist Ryan Maue, also an expert on hurricanes, called out the four scientists for making a claim he couldn’t replicate.
Something is very confusing with this new analysis. I cannot replicate this figure using 1980-2016 Global best track data … there is not a 100% increase in the number of global Category 3 hurricanes. Closer to 0%. (https://t.co/ORoAYEaooy)
Any help on data here? @rahmstorf pic.twitter.com/C7i09VtkD5— Ryan | weather.us (@RyanMaue) June 4, 2018
Maue is one of several experts who tracks hurricane strength and intensity over time. Maue’s data shows a decrease in hurricane-strength storms in the last four decades with a statistically insignificant increase in the number of major hurricanes.
The National Oceanic and Atmospheric Administration (NOAA) also weighed in on the question of whether global warming is making hurricanes stronger or more frequent — the evidence was lacking.
“It is premature to conclude that human activities — and particularly greenhouse gas emissions that cause global warming — have already had a detectable impact on Atlantic hurricane or global tropical cyclone activity,” reads an assessment published by NOAA’s Geophysical Fluid Dynamics Laboratory in 2017.
Indeed, the Intergovernmental Panel on Climate Change (IPCC) found in 2013 there were “no significant observed trends in global tropical cyclone frequency over the past century.”
While some studies purport to find the “fingerprints” of global warming on particular storms, scientific assessments have failed to find the climate signal in global, or even regional, tropical cyclone trends.
A 2015 research paper by @philklotzbach investigated the global number of Category 4-5+ tropical cyclones using the same global best-track data-set. https://t.co/4hJ763wWOF
“the global frequency of category 4 and 5 hurricanes has shown a small, insignificant downward trend” pic.twitter.com/a6ALf2GFH3
— Ryan | weather.us (@RyanMaue) June 4, 2018
Rahmstorf, Emanuel, Mann and Kossin did not reference NOAA’s assessment or the IPCC in their analysis of hurricanes and global warming. Instead they relied on MIT data and a satellite dataset that went back to 1979 to claim the “strongest tropical storms in most ocean basins” have gotten stronger.
The scientists said satellite data was “homogenized” using “a variety of satellite, and air- and ground-based instruments whose capabilities have improved over time.” The scientists also noted this is the “period over which three quarters of global warming has occurred.”
The strongest storms have become stronger, they argued, and the “strongest increase can be found in the North Atlantic (which is more than 99% significant) where the trend has likely been boosted by the decrease in sulfate aerosols over this period.”
Except Maue also has data from the Atlantic Ocean basin based on a measure called accumulated cyclone energy (ACE), which “wind speed (knots) squared during tropical lifecycle” of a storm. ACE is used by NOAA to compare the intensity of storms and hurricane seasons.
Final data for 2017 in the Hurricane database. ACE:
2017: 224.877
2004: 224.748
ACE is wind speed (knots) squared during tropical lifecycle of each tropical storm or hurricane. NOAA defines seasons below/above/near average based upon comparison to historical ACE median. pic.twitter.com/4afs4Md1O8— Ryan | weather.us (@RyanMaue) June 4, 2018
Not surprisingly, the Atlantic Basin shows an increase since the 1980s when there was a lull in hurricane activity from earlier decades. And like Rahmstorf and his colleagues admitted, hurricanes might have been undercounted before the satellite era.
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