Shared from the 9/4/2020 Houston Chronicle eEdition

Winds of (climate) change

Laura’s ferocity could be indicative of future hurricanes as ocean and air temperatures grow warmer

Elizabeth Conley / Staff photographer

Hurricane Laura caused extensive damage near Holly Beach, La. Category 4 and 5 hurricanes are likely to become less rare.

Elizabeth Conley / Staff photographer

Tashianna Williams, 16, adjusts her shoes last Friday in the backyard of her grandfather’s home in Orange, where a tree was uprooted by Hurricane Laura’s powerful winds.

Hurricane Laura’s 150 mph winds as it barreled across Cameron, La., made it a rarity. But storms that strong likely will become more common.

Warmer ocean and air temperatures, rising sea levels and other effects of climate change are expected to fuel stronger hurricanes. The flooding from storm surge could be higher, rainfall could be heavier and winds could be stronger. The percent of storms that reach Category 4 or Category 5, with sustained winds of at least 130 mph, is expected to increase.

It’s difficult, though, to determine the extent to which human-caused climate change is contributing to this year’s especially active Atlantic hurricane season. Researchers have to consider a variety of natural factors, and studies are hampered by a lack of reliable long-term data.

“It’s making it hard to kind of tease out just the greenhouse part,” said Tom Knutson, a research meteorologist at the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory.

But for Phil Bedient, director of the Severe Storm Prediction, Education and Evacuation from Disasters Center at Rice University, it’s hard to deny that warmer temperatures in the Gulf of Mexico — prompting more water to evaporate, which fuels hurricanes — sped up the strengthening of Hurricane Laura.

“Climate change, in my mind, is certainly playing a role here,” Bedient said. “It is certainly a parameter that is helping to drive it.”

According to an article in Yale Climate Connections, Laura tied with the Last Island Hurricane of 1856 as the strongest hurricane to make landfall in Louisiana. It also tied as the fifth-strongest hurricane to make acontinental U.S. landfall based on wind speeds.

Atlantic hurricane activity goes through active and quiet periods. Activity was high in the 1940s, ’50s and ’60s, but then dropped off in the ’70s and ’80s. It picked back up in the mid-1990s and has since remained active.

Some researchers believe this is caused by naturally occurring, long-duration changes in sea surface temperatures, Knutson said. Others believe aerosol emissions might have caused the most recent quiet period; acting as a giant shade that lowered the temperature over the Atlantic Ocean and reduced hurricane activity. Storms picked up again as legislation reduced this form of emissions.

It’s unclear if one or perhaps both are correct, Knutson said. But it’s just one factor to consider.

Wind shear is the change of wind speed and direction at different heights. If winds at different parts of the atmosphere are blowing in the same direction and at the same speed, that’s conducive to hurricanes. It’s disruptive if the winds are blowing in different directions or at different speeds.

El Niño helps suppress Atlantic hurricane activity as it increases vertical wind shear, which basically cuts the top off a storm before astrong circulation can form. La Niña, however, enhances hurricane activity as it causes less vertical wind shear.

“Those changes vary from year to year,” said Robert Korty, an associate professor at Texas A&M University, “and they’re superimposed on this background of a global climate change.”

Knutson and a team of climate and hurricane researchers recently examined past and possible future changes in hurricane activity due to human-caused climate change. They published two papers in the Bulletin of the American Meteorological Society journal.

The first paper looked at past hurricane activity using two approaches: one requiring very strong evidence of climate change and the second allowing for more subtle changes and emerging trends.

There weren’t many indicators of climate change when using the stricter approach. The paper noted a gradual movement of typhoons going higher north in an area that includes the Philippines, Japan, China and the surrounding waters. But even this was given only “low to medium confidence” of being caused by human-induced climate change.

The less conservative approach, however, identified several cases of possible human influence emerging in the data. For example, the average intensity of the world’s strongest tropical cyclones had increased since the early 1980s. There was an increase in the percent of global hurricanes that reached Category 4 or Category 5, and there was an increased frequency of Hurricane Harvey-like extreme rain events in the Texas region.

The team’s second paper then looked at how tropical cyclone activity would change if the world’s temperatures rose by 2 degrees Celsius (3.6 degrees Fahrenheit). They estimated that hurricanes’ rainfall rates would be 14 percent higher, average wind speeds would increase by 5 percent, the percent of storms that reach Category 4 or Category 5 would increase by 13 percent and overall storm surge inundation levels — this includes storm surge, tidal effects and the expected rise in sea level — would also increase.

But such changes could depend on geography. A paper in Nature Communications found regional differences when looking at climate change and hurricane storm surge. The northeast U.S. would mostly be affected by higher mean sea levels, which could cause present-day 100-year floods to occur every year by the end of the 21st century.

But in the Gulf of Mexico, both higher sea levels and stronger hurricanes would affect storm surge, prompting 100-year floods to occur every one to 30 years, said lead author Reza Marsooli, an assistant professor at Stevens Institute of Technology in Hoboken, N.J.

Pedram Hassanzadeh, an assistant professor at Rice University, likewise led a paper in Nature Communications that found Texas hurricanes would more likely be fast-moving storms during the last 25 years of this century.

Based on a strengthening Atlantic subtropical high pressure to the east of Texas (which would cause stronger northward winds) and a weakening North American monsoon to the west of Texas (which would cause weaker southward winds), climate change could create strong northward winds that have storms move more like Hurricane Ike rather than the stalling, rainmaker Hurricane Harvey.

“Fast-moving hurricanes and slow-moving hurricanes, they both can be very damaging,” Hassanzadeh said. “But the type of damage could be different.”

Warmer climates could prompt storms to form earlier and later in the hurricane season, but Korty, with Texas A&M, said peak season for Texas should remain between June and September. Autumn cold fronts should deflect late-forming hurricanes to the east, making them less likely to reach Texas.

“We’ve learned that hurricanes exist really at all times, whether they’re ice ages or very hot climates,” Korty said. “But … as a planet shifts from a cooler to a warmer period the conditions that support hurricanes (improve).”

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