https://insideclimatenews.org/news/06092017/hurricane-irma-harvey-climate-change-warm-atlantic-ocean-questions
People who don't keep an eye on such things, the way I do, might not be aware of the big hurricanes we've been having, because in recent years most of them have been deflected from the U.S. by weather fronts, and when they get near the U.S., they veer back to the east, sometimes going all the way back across the Atlantic and ending up in Europe, like hurricane Gert did a few weeks ago.
By Sabrina Shankman
Sept. 6, 2017
A third of the way into the 2017 Atlantic hurricane season, NOAA looked at the ocean and air temperatures and issued an ominous new forecast: the region would likely experience "an above normal hurricane season" that "could be extremely active," with more named storms than previously expected—14 to 19 this season—and two to five major hurricanes.
Now, halfway through the season, Hurricane Harvey's destruction stretches along the Texas coast, Hurricane Irma is hitting islands in the Caribbean with record wind speeds as it heads toward Florida, and two more named storms—Jose and Katia—have grown to hurricane strength.
As global temperatures continue to rise, climate scientists have said this is what we should expect—more huge storms, with drastic impacts.
Though scientists are still wrestling with some of the specifics of how climate change is impacting hurricanes, a lot is known, including the fact that hurricane seasons like this one could be the new norm.
Records are tumbling in quick succession this year. Irma, among the strongest Atlantic hurricanes on record, is barreling over the islands of the Caribbean as a Category 5 storm this week, while Houston, Texas, is still draining from Harvey's five-day deluge that broke the continental U.S. rainfall total for a single event.
Major storms are falling outside their normal range (Irma is the easternmost on record), and at strange times of the year (Tropical Storm Arlene hit in April of this year—one of only two named tropical storms in April, and the northernmost on record for that time of year).
As climate change progresses, scientists aren't projecting an increase in total storms, but they are expecting a jump in the number of major storms—just like we're seeing now.
If Hurricane Harvey had happened at the end of the 20th century, that amount of rain that falling in Houston in a single storm would have been rare—a 1-in-2,000-year event, said Kerry Emanuel, an MIT professor of atmospheric sciences. But as temperatures continue to rise, those rare events are becoming increasingly less rare, he said.
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According to the 2014 National Climate Assessment, the intensity, frequency and duration of North Atlantic hurricanes have increased since the early 1980s. The frequency of the strongest storms—category 4 and 5 hurricanes—has increased too.
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Those higher [ocean] temperatures (as well as higher temperatures in the atmosphere) feed the storms, helping them strengthen. One study based on two decades of data found that hurricanes intensify significantly faster now than they used to. The researchers found that storms reach Category 3 wind speeds nine hours faster than they did in the 1980s.
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Those higher temperatures don't just result in more intense wind speeds. Warmer air also retains more water vapor, which can result in dramatic rainfall like what happened during Hurricane Harvey.
In the case of Harvey, the rain volume was exacerbated by the fact that the storm stalled over the Houston area, bringing days of relentless downpours.
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This stalling is a frequent feature of extreme events, but at this point, scientists have not found a conclusive link to climate change. There may be a connection, though, according to climate scientist Michael Mann. "More tenuous, but possibly relevant still, is the fact that very persistent, nearly 'stationary' summer weather patterns of this sort, where weather anomalies (both high pressure dry hot regions and low-pressure stormy/rainy regions) stay locked in place for many days at a time, appears to be favored by human-caused climate change," he wrote in a Facebook post late last month.
In a study published online in the journal Nature in March, Mann and coauthors wrote that amplified warming in the Arctic driven by anthropogenic [human-caused] climate change may be leading to an increase in extreme weather events that linger in one place for extended periods of time.
I wonder if the same applies to conditions like the fronts that lingered in place deflected a bunch of hurricanes from the U.S. in the past few years?
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While no "major hurricanes" made landfall in the United States between 2005 and this year, those weren't weak tropical storm years—the biggest storms just didn't hit the U.S. In 2013, Typhoon Haiyan devastated the Philippines with the highest wind speeds ever seen—until Hurricane Patricia broke that record two years later off Mexico's Pacific Coast, and several other cyclones wreaked havoc elsewhere around the world in the intervening years.
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People in some parts of the United States might also disagree with the concept of a "hurricane drought" during that period.
"Tell the people of coastal Texas that Ike was not a major hurricane," said Emanuel, the MIT scientist. "Well, Ike was technically just under the ranking of major hurricane, and it completely destroyed a huge part of coastal Texas. Now, tell the people of New York that Sandy wasn't a major hurricane."
"There were plenty of hurricanes in that stretch of 12 years," he said. "They just didn't happen to make landfall as strong storms in the United States."
tags: Extreme Weather, severe weather
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