A National Oceanic and Atmospheric Administration map for Wednesday shows sea surface temperatures of 1.8 to 3.15 degrees Fahrenheit above normal. Photo: National Oceanic And Atmospheric Administration
Global warming is making the oceans hotter, fueling the intensity and flooding potential of storms like Harvey, climate scientists said as the hurricane approached...
Driven by higher-than-average temperatures in the Gulf of Mexico, Harvey quickly intensified Thursday and is likely to reach Category 3 hurricane status before it hits the Texas Coast.
Sea surface temperatures in the Gulf on Thursday were up to 2 degrees above normal, the National Oceanic and Atmospheric Administration reported.
The average temperature for most of the Gulf was 86 degrees, said Kevin Trenberth, a climate scientist with the National Center for Atmospheric Research in Boulder, Colorado.
“That makes it almost the hottest spot on the planet” for sea surface temperatures, Trenberth said. “It’s an area that’s ripe for vigorous development to occur.”
Harvey is projected to make landfall late today or early Saturday. As it sits over the inland Gulf Coast, it could bring rainfall of 10 to 20 inches Friday through Tuesday along and east of Interstate 35,the National Weather Service said. More than 25 inches could fall near and south of Interstate 10, and the Hill Country could get 5 to 10 inches.
As the Earth’s climate warms because of human burning of fossil fuels, scientists have seen tropical cyclones become more intense and predict they will continue doing so.
Since the 1980s when high-quality satellite observations became available, scientists have seen an increase in the “intensity, frequency, and duration” of Atlantic hurricanes, along with the number of Category 4 and 5 storms, according to the 2014 National Climate Assessment.
The reason warmer oceans fuel stronger hurricanes is pretty easy to understand, University of Texas climate scientist Kerry Cook said.
“Hurricanes are fueled by the condensation of water in the atmosphere that evaporated from the surface,” she said. “If the surface temperature is warmer, it increases the evaporation rate.”
Scientists even have figured out how much more evaporation to expect.
For every nearly 2 degrees of average sea surface temperature warming, evaporation increases 7 percent, Cook said. The concept relies on a well-known physics principle called the Clausius-Clapeyron relation, she said.
Hurricanes, “which are really a collective of thunderstorms,” draw their moisture from a roughly 930-mile radius and can be affected by ocean temperatures up to 650 feet deep, Trenberth said.
Tropical cyclones exchange heat between the oceans and the atmosphere. In general, more heat in the system, the more intense the storm.
“In the process, they actually cool off the tropical oceans,” Trenberth said. “This is really one of the fundamental roles that hurricanes and typhoons play in the climate system. … They’re sort of a relief valve.”
Some of the most severe floods ever seen in Texas have been caused by hurricanes.
In 1979, Tropical Storm Claudette brought thunderstorms that dumped 42 inches of rain — almost an entire year’s worth — over 24 hours in an area south of Houston, Texas A&M University professor and State Climatologist John Nielsen-Gammon said. It was the heaviest 24-hour rainfall ever recorded in the continental U.S., he said.
San Antonio also has suffered deluges tied to hurricanes.
In July 2002, a tropical disturbance in the Gulf led to 12.78 inches, the second-highest amount recorded, falling at San Antonio International Airport, he said.
The most rain recorded at that station was 15.61 inches in a three-day period that ended Oct. 19, 1998. Moisture from hurricanes in the Atlantic and Pacific contributed to that flooding, according to the city’s Office of Emergency Management.
What scientists can’t say for sure is whether hurricanes and tropical storms are happening more frequently.
Unlike some other extreme weather events like heat waves, blizzards and rainstorms, hurricanes are relatively rare. In Texas, two named storms make landfall every three years, on average, Nielsen-Gammon said. For Category 3 storms and above, the average rate is one per decade, he said.
That means there’s not enough data to say with certainty that the frequency of these storms is going up, Cook said.
“When you try to look at observations, it takes a long time to build up enough numbers of hurricanes so that you can say with statistical significance that you have observed an (increase),” she said, adding it might take another 20 years for a pattern to emerge.
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