It was a record season for storms. On Monday evening, subtropical storm Theta became the 29th named storm of the 2020 hurricane season, exceeding the total count for 2005.
Theta formed after Tropical Storm Eta spent the day hitting Florida, causing heavy rains and flooding in the southern state and the Keys.
The tumultuous season has raised questions about the extent of climate change affecting hurricanes in the Atlantic. Researchers can’t say for sure whether human-caused climate change will mean longer or more active hurricane seasons in the future, but there is broad agreement on one thing: global warming is changing storms.
Scientists say, for example, that unusually warm Atlantic surface temperatures have helped increase storm activity this season. Warmer ocean temperatures are “absolutely responsible for the overactive season,” said James P. Kossin, a climatologist with the National Oceanic and Atmospheric Administration. “It is very likely that man-made climate change contributed to this unusually warm ocean.”
It’s still not clear if this is an exception or part of an uptrend, Dr Kossin said. He noted that climate change could ultimately lead to fewer storms.
Either way, he said, “climate change makes hurricanes more likely to behave in certain ways.”
Here are some of those ways.
There is a strong scientific consensus that hurricanes are getting stronger and stronger.
Hurricanes are complex, but one of the key factors that determine the final strength of a given storm is the ocean surface temperature, as warmer water provides more energy that powers storms.
“The potential intensity is increasing,” said Kerry Emanuel, professor of atmospheric sciences at the Massachusetts Institute of Technology. “We predicted it would increase 30 years ago, and the observations show it.”
Stronger winds mean blown power lines, damaged roofs and, when paired with rising sea levels, more severe coastal flooding.
“Even if the storms themselves don’t change, the storm surge is rising to high sea level,” said Dr Emanuel. He used New York as an example, where the sea level has risen by about a foot over the past century. “If Sandy’s storm surge had happened in 1912 rather than 2012,” he says, “it probably wouldn’t have flooded Lower Manhattan.”
2. More rain
Warming also increases the amount of water vapor the atmosphere can hold. In fact, each degree Celsius of warming allows the air to contain about 7% more water.
This means that we can expect future storms to bring more rainfall.
3. Slower storms
Researchers don’t yet know why storms move slower, but they are. Some say a slowdown in global atmospheric circulation, or global winds, could be in part to blame.
In a 2018 article, Dr Kossin found that hurricanes over the United States had slowed by 17% since 1947. Combined with increasing rainfall rates, storms cause a 25% increase in local precipitation. in the United States, he said.
Slower, wetter storms also make flooding worse. Dr. Kossin likened the problem to walking around your backyard while using a garden hose to spray water on the ground. If you walk fast, the water will not have a chance to start collecting. But if you walk slowly, he said, “you will have a lot of rain below you.”
4. Larger storms
Because warmer water helps fuel hurricanes, climate change is expanding the area where hurricanes can form.
There is a “migration of tropical cyclones out of the tropics to subtropics and mid-latitudes,” said Dr Kossin. This could mean more storms will make landfall in higher latitudes, like in the United States or Japan.
5. More volatility
As the climate warms, researchers also say they expect storms to intensify more quickly. Researchers still don’t know why this is happening, but the trend seems clear.
In a 2017 article based on climate and hurricane models, Dr Emanuel found that rapidly intensifying storms – those that increased wind speeds by 70 miles per hour or more in the 24 hours leading up to the landings – were rare in the period 1976 to 2005. On average, he estimated, their probability during these years was about once per century.
By the end of the 21st century, he found, these storms could form once every five or ten years.
“It’s a forecaster’s nightmare,” said Dr Emanuel. If a tropical storm or Category 1 hurricane turns into a Category 4 hurricane overnight, he said, “there is no time to evacuate people.”