- Hurricane Laura is expected to make landfall near the border of Texas and Louisiana as a Category 4 storm Wednesday night.
- Between Tuesday and Wednesday, the hurricane strengthened from a Category 1 storm to a major hurricane – a process called rapid intensification.
- As the planet continues to warm, such rapid intensification events are expected to get more frequent, and storms will become stronger and wetter.
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Hurricane Laura is rapidly gaining strength as it barrels toward Louisiana and Texas.
The major hurricane is poised to strengthen even more and make landfall Wednesday night as a Category 4 storm, just days before the 15th anniversary of Hurricane Katrina.
Scientists can’t definitively say whether Hurricane Laura or other individual storms are directly caused by climate change, but warming overall makes hurricanes more frequent and devastating than they would otherwise be.
That’s because storms feed on warm water, and higher water temperatures also lead to sea-level rise, which in turn increases the risk of flooding during high tides and in the event of storm surges. Warmer air also holds more atmospheric water vapour, which enables tropical storms to strengthen and unleash more precipitation.
“Our confidence continues to grow that storms have become stronger, and it is linked to climate change, and they will continue to get stronger as the world continues to warm,” James Kossin, an atmospheric scientist at NOAA who studies how climate change affects tropical cyclones, told the Washington Post.
Here’s why storms are getting so much stronger, wetter, and more frequent in the Atlantic.
How a hurricane forms
Hurricanes are vast, low-pressure tropical cyclones with wind speeds over 74 mph.
The Atlantic hurricane season generally runs from June through November, with storm activity peaking around September 10. On average, the Atlantic sees six hurricanes during a season, with three of them developing into major hurricanes (Category 3 or above). The storms form over warm ocean water near the equator, when sea surface temperature is at least 80 degrees, according to the National Hurricane Centre.
As warm moisture rises, it releases energy, forming thunderstorms. As more thunderstorms are created, the winds spiral upward and outward, creating a vortex. Clouds then form in the upper atmosphere as the warm air condenses. As the winds churn, an area of low pressure forms over the the ocean’s surface. At this point, hurricanes need low wind shear – or a lack of prevailing wind – to form the cyclonic shape associated with a hurricane.
Cyclones start out as tropical depressions, with sustained wind speeds below 39 mph. Once wind speeds pass that threshold, the cyclone becomes a tropical storm. Then above 74 mph, the storm is considered a Category 1 hurricane on the Saffir-Simpson scale.
So far this year, the Atlantic Ocean has produced a record 13 tropical storms in just three months. Three have become hurricanes – Hanna, Isaias, and Laura.
The National Oceanic and Atmospheric Administration has predicted an “extremely active” 2020 hurricane season, with 19 to 25 named storms – the first time in NOAA’s history the number is that high. The forecast includes seven to 11 hurricanes, with three to six of them reaching Category 3 or higher (that’s considered a “major hurricane”).
“This is one of the most active seasonal forecasts that NOAA has produced in its 22-year history of hurricane outlooks,” US Secretary of Commerce Wilbur Ross said in an August press release, adding, “we encourage all Americans to do their part by getting prepared, remaining vigilant, and being ready to take action when necessary.”
Hurricanes are getting stronger
As ocean temperatures increase, we’re seeing more severe hurricanes. That’s because a storm’s wind speed is influenced by the temperature of the water below: A 1-degree Fahrenheit rise in ocean temperature can increase a storm’s wind speed by 15 to 20 miles per hour, according to Yale Climate Connections.
That also enables storms to intensify and develop into powerful hurricanes in less time.
That’s what Laura did this week in its process of rapid intensification – the term for an increase of at least 35 mph in a storm’s wind speeds within 24 hours.
“Rapid intensification events are more likely because of climate change,” Kossin said.
In a recent study, Kossin’s team found that each new decade over the last 40 years has brought an 8% increase in the chance that a storm turns into a major hurricane.
Generally, a strong storm also brings a storm surge: an abnormal rise of water above the predicted tide level. If a storm’s winds are blowing toward the shore and the tide is high, storm surges can cause water levels to rise as rapidly as a few feet per minute along a coast.
“Almost all of the damage and mortality caused by hurricanes is done by major hurricanes,” Kossin told CNN. “Increasing the likelihood of having a major hurricane will certainly increase this risk.”
Higher sea levels also lead to more destructive storm surges. Unfortunately, even if we were to cut emissions dramatically starting today, some sea-level rise is already inevitable, since the planet’s oceans absorb 93% of the extra heat that greenhouse gases trap, and water (like most things) expands when heated.
Storms are moving more slowly and dropping more rain
Over the past 70 years or so, the speed of hurricanes and tropical storms has slowed about 10% on average, according to a 2018 study. Over land in the North Atlantic and Western North Pacific specifically, storms are moving 20% to 30% more slowly, the study showed.
That gives a storm more time to pummel an area with powerful winds and rain.
Hurricane Harvey in 2017 was a prime example of this: After it made landfall, Harvey weakened to a tropical storm, but then stalled for days over the Houston area – scientist Tom Di Liberto described it as the “storm that refused to leave.”
Harvey dumped unprecedented amounts of rain, flooded Houston, killed more than 100 people, and caused $US125 billion in damages.
Climate scientist Michael Mann wrote on Facebook that Hurricane Harvey “was almost certainly more intense than it would have been in the absence of human-caused warming, which means stronger winds, more wind damage, and a larger storm surge.”
Hurricane Dorian did something similar last year when it stalled over the Bahamas, pummelling the islands with 185-mph winds and up to 30 inches of rain. Although the country’s official death toll was 74, Dr. Duane Sands, who served as the Minister of Health at the time, has said the true death toll “remains unknown,” according to the Miami Herald.
To make matters worse, a warmer atmosphere can hold more moisture, so a 10% slowdown in a storm’s pace could double the amount of rainfall and flooding that an area experiences. The peak rain rates of storms have increased by 30% over the past 60 years – that means up to 4 inches of water can fall in an hour.