A couple walks on the beach on November 8, 2020, as Tropical Storm ETA hit Miami Beach, Florida. ETA was the 28th named storm of the year, on par with 2005, the most active hurricane season on record.
Photograph by WILFREDO LEE, AP
Written by: SARAH GIBBENS
Meteorologically, 2020 is a historic year: Subtropical Storm Sita has already formed in the Atlantic Ocean, bringing the total number of named storms in the 2020 hurricane season to 29, breaking the record set in 2005, and the year is still a few weeks away. As of September, the National Hurricane Center has exhausted the names available on the hurricane list and will use greek alphabet naming. In the eyes of Americans, storm seasons like these are increasingly linked to climate change. A CBS poll released last year found that 45 percent of respondents believe climate change "largely" caused severe hurricanes. Still, when climate scientists were asked about that, many said the explanation for this year's hurricane season activity wasn't that simple. They say it's not easy to think of a single storm or even a single hurricane season as a symptom of climate change. James Kossin, a climate scientist at the National Oceanic and Atmospheric Administration, said: "Given what we know about how climate change warms the oceans, and this year's hurricane season is very active and the oceans are very warm, it is likely that climate change contributed to the active hurricane season." But how much of a difference did it play? Now we face a more intractable problem. ”
Hurricane Eta brought heavy rainfall, causing the Bambito River in Panama to flood, an aerial view of the flooded Bambito River.
摄影：LUIS ACOSTA， AFP、GETTY IMAGES
How much do we know about hurricanes and climate warming
The 2020 hurricane season is forecast to be more active than ever, and it is. Storms began to form ahead of schedule, and the number increased at a record-breaking rate. Hurricane ETA, the 28th storm of the year, brought heavy rainfall to Florida, Nicaragua and Honduras, which are still battling flooding. This year's hurricane season can even extend into winter: in 2005, the 28th tropical storm of the year, Zeta, formed at the end of December.
One of the reasons for so many storms this year is the formation of a La Niña weather pattern in the Pacific Ocean, which has led to weakening winds in the Atlantic, causing the storm to accelerate into a hurricane.
However, there are also some storms that exhibit characteristics that scientists believe are related to climate change: rapidly strengthening, slow movement, and heavy rainfall. All of this is related to heat.
Kossin said: "For this year's hurricane season, the most important thing is that the Atlantic Ocean has already been unusually warm, which undoubtedly helps to produce an overactive hurricane season. ”
Climate change is undoubtedly the cause of abnormal warming: the average temperature of the ocean surface has been steadily rising since the end of the 19th century. However, the National Oceanic and Atmospheric Administration blames this year's unusually warm in part on the natural climate cycle of the Atlantic's multi-year interdeerational oscillation (AMO), which warms or cools the North Atlantic every few decades. Scientists aren't sure which impact is greater — climate change or AMO.
Either way, the warm waters act as fuel for hurricanes and lead to a rapidly strengthening process. During this process, the maximum wind speed of the storm can increase by at least 56 km/h over a 24-hour period. Eight of the 2020 storms experienced rapid strengthening in the warm waters of the Caribbean and Gulf of Mexico.
"This process gives these storms more energy. The storm draws energy from the ocean, which greatly accelerates the rotation speed of the storm," Kossin said. "It's like someone modifying their own car engine to make the car run faster. The ocean makes storms faster. ”
Rapid strengthening means that tropical storms can escalate to hurricanes more quickly, or hurricanes can escalate to intense hurricanes more quickly. However, this does not mean that hurricanes will necessarily be faster on the oceans and land; the speed of hurricanes on the oceans and land is controlled by another force. Hurricanes are propelled forward by winds in the atmosphere. That's why Hurricane Laura swept across the Gulf Coast at 24 kilometers per hour, while Hurricane Sally, propelling along a similar path, was only 5 kilometers per hour.
In the study, Kossin found that hurricanes are likely to be moving slower and slower, with an estimated 17 percent slower pace over the past 120 years. This may also be related to climate change.
"Wind is driven by the temperature difference between the equator and the poles," he said. Kossin and other scientists believe that the Arctic is warming faster than in the tropics, so the temperature difference between the two is reduced, which in turn slows down the speed of the wind.
Hurricanes that move more slowly tend to bring more precipitation, as Hurricane Harvey hit Houston in 2017, Hurricane Dorian hit the Bahamas in 2019, and Hurricane Sulli hit Alabama this year, all of which brought heavy rainfall. The situation is compounded by the fact that hurricanes bring in a lot of precipitation as they bring in more precipitation in a warming atmosphere.
"When it comes to climate change, I think one of the most immediate impacts is increased rainfall. A warmer atmosphere contains more moisture," said Phil Klotzbach, an atmospheric scientist at Colorado State University. As humidity increases, so does the probability that storms will bring more rainfall.
The city of Galveston, Texas, experienced the deadliest natural disaster in U.S. history: the September 8, 1900 hurricane that killed 6,000-12,000 people.
摄影：KEYSTONE VIEW COMPANY，LIBRARY OF CONGRESS
LALIMA, Honduras, November 5, 2020: Many villages were flooded after tropical low-pressure ETA brought heavy rainfall to the entire region. Landslides and flooding killed 13 people in Honduras, 5 in Guatemala and Panama, and 2 each in Costa Rica and Nicaragua. However, the number of victims is likely to continue to rise sharply.
摄影：PICTURE ALLIANCE，GETTY IMAGES
Peddogki, Florida, Sept. 17, 2020: Hurricane Sally overturned the roof of an apartment managed by a real estate agent as she passed through Peddydokee, Florida, Sept. 17, 2020. She said the tenants had left before the storm came. After Sally landed, she brought strong winds and torrential rain.
Photo by JOE RAEDLE, GETTY IMAGES
Not just climate change?
Data from Atlantic tropical storms provided by the National Oceanic and Atmospheric Administration shows that the number of tropical storms has been increasing since the 1880s. But the researchers attribute this to technological improvements: Satellites can now detect short-term storms that were previously undetectable, especially in remote parts of the ocean.
However, this does not explain the fact that the increase in tropical storms observed since the 1980s. Kerry Emanuel, a climate scientist at the Massachusetts Institute of Technology, says recent trends are real, not observational bias. Still, it could have something to do with another type of air pollution (unlike the carbon emissions that cause global warming) — or rather, the way we clean it up.
Aerosol contamination from cars, power plants and factories is known to cause local cooling of the surrounding environment. Sulfate aerosols are very efficient at reflecting sunlight. In the 1970s, summers on the East Coast of the United States were often foggy weather, the result of sunlight being reflected into space.
Emanuel said the phenomenon also inhibited the formation of storms. However, a study published in 2013 linked the passage of the Clean Air Act of 1970 to the subsequent decline in aerosols, rising temperatures and an increase in tropical storms that began in the late 1980s.
Emanuel notes that one challenge in validating this theory is that storm data was very limited before the 1970s, when meteorological satellites became commonplace. Kossin calls the exact effects of aerosol pollution "one of the most important questions about hurricanes that we haven't answered yet."
There is no evidence that climate change caused by carbon emissions will lead to an increase in storms. "We didn't expect the number of storms to increase with climate change," Klotzbach said. "More likely is an increase in storm intensity." On the other hand, he said, this year's strong La Niña weather pattern could mean that the Atlantic Ocean will be very active by the end of the year.
"I don't think ETA is the last hurricane we've seen this year," he said.
Probability problems
Kossin and Emanuel suggest that we consider the impact of climate change on hurricanes from a probabilistic perspective.
Emanuel gives an example of how to say, "For Hurricane Harvey in 2017, we can say that it probably brought two to three times as much rainfall in 2017 as it did in 1970." ”
Two studies published in late 2017 found that climate change has tripled the likelihood of slow-moving rainy storms like Hurricane Harvey.
Kossin said what applies to a storm also applies to a hurricane season. You can't say that climate change caused a rapidly strengthening, slow-moving, rain-rich storm in 2020, only that it made such a year more likely: "In the context of climate change, the hurricane season we just experienced may be more common in the future." ”
(Translator: Stray Dog)