Given that roughly 60% of Atlantic tropical cyclones since 1950 have occurred in August and September, and that we're coming off of three seasons with twelve, seven and ten hurricanes, respectively, this year's measly two hurricanes is puzzling.
"This year we didn't even get a category two hurricane," says Dr. Jeff Masters, director of meteorology at Weather Underground. "That's really rare. You have to go back to, I think, 1968 to see a season when we didn't even have a category two hurricane. Very unusual."
Image: National Hurricane Center
According to many meteorologists, this annual fluctuation is attributed to natural variability. Dry, sinking air and strong upper level winds are the likely culprit behind this season's abnormally low numbers, but hurricane climatologists caution the public from drawing too many conclusions from a single year.
James Elsner, Ph.D, professor of hurricane climatology at Florida State University, elaborates. "Not to say that, given we have a low season, we shouldn't look for why, but there is this large variability that we just have to accept as part of the way nature works," Elsner tells Mashable.
"In either case, you can conclude incorrectly that the sky is falling or incorrectly that nothing is happening."
"In either case, you can conclude incorrectly that the sky is falling or incorrectly that nothing is happening."
To see what's really happening with hurricanes, you have to look at the bigger picture.
"We've observed an increase in the length of the Atlantic hurricane season in recent decades," says Masters. "Things are beginning earlier and ending later, and that's been correlated to record sea surface temperatures, no surprise."
The Intergovernmental Panel on Climate Change (IPCC), the leading international body for the assessment of climate change, confirmed the increase in ocean temperatures in its Fifth Assessment Report released in June. From the summary:
Ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90% of the energy accumulated between 1971 and 2010 (high confidence). It is virtually certain that the upper ocean (0−700 m) warmed from 1971 to 2010, and it likely warmed between the 1870s and 1971.
So what could warming oceans mean for season length? According to a 2008 paper by Dr. Jim Kossin of the National Climatic Data Center, the beginning and end of hurricane season lengthened by 20 days, per degree Celsius of sea surface temperature increase — though he notes the uncertainty in these relationships is high.
The graph below expands upon this warming trend.
Image: NOAA
Warming oceans, says Masters, also work to increase the intensity and severity of storms.
Hurricanes are heat engines, explains Masters. They use energy from the ocean and convert it to wind. If the world's oceans are warmer, potentially, the maximum strength of a storm increases, too.
"In a future where the oceans are three or four degrees warmer, every now and then you're gonna get an off-the-charts hurricane like we've never seen before."
"In a future where the oceans are three or four degrees warmer, every now and then you're gonna get an off-the-charts hurricane like we've never seen before."
Aside from season length and storm strength, the number of named storms in the Atlantic has also increased over a relatively short time period.
"We certainly — over the last 50 years and especially since about the mid-'90s — have seen an increase in the frequency of tropical storms and hurricanes in the Atlantic," says Elsner.
While our technology and ability to detect storms via satellite has certainly improved, Masters believes there's been a real increase as well.
Image: John Nelson, IDV Solutions
Natural variability and recent advancements in our observational capabilities have made the link between human-induced climate change and fluctuating hurricane patterns a controversial one. Because trends in data only become discernible with time, Masters believes it will be five or 10 years before we have a firm handle on what's going on.
However, he has his own theory: "We've had way too many of these one in 100, one in 500, one in 1,000 year type events to be a coincidence, in my view. There has to be something up," he says. "We need to expect the unexpected in the future. We're just going to have to hang on for the ride and see what happens."
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