Cyclones and climate change: Connecting the dots

ByAgence France-Presse, adapted by Newsela staff

Grade Level7

08/30/2017

Word Count977

A NASA satellite image of Hurricane Harvey approaching southeastern Texas on August 25, 2017. In this image, it was a Category 4 hurricane. Image from NASA.govScientists acknowledge they don't know everything about how global warming affects hurricanes like Hurricane Harvey, the one that hit southeast Texas hard.

However, what they do know is enough to keep them up at night.

The intensifying effect of sea level rise, warming oceans and hotter air is basic physics, they say. All are clear consequences of climate change.

The same goes for faster shifts in intensity. A sudden strengthening turned Harvey from a Category 2 hurricane to a Category 4 — on a scale of 5 — right before it made landfall last Friday.

"Hurricanes Are Fairly Rare," So There Is Limited Data

What's missing is a long historical record of hurricanes. Climate scientists need decades-long logs of measurements to determine the fingerprint of human influence.

Hurricane tracking didn't begin until the 1970s when satellite data became available, but even back then that wasn't enough.

"It is awfully difficult to see climate change in historical data so far because hurricanes are fairly rare," Kerry Emanuel toldAgence France-Presse. He is a professor of atmospheric science at the Massachusetts Institute of Technology in Boston.

In other words, expertsdo not disagree on the potential effect man-made climate change has on tropical storms. The storms are known around the world as cyclones, hurricanes and typhoons.

Rather, they are puzzled, for now, by a lack of information.

Cyclones Are Moving Toward The Poles

Just because the information doesn't allow for clear detection yet, doesn't mean that the changes haven't been occurring, said James Kossin. He is a scientist at the U.S. National Oceanic and Atmospheric Administration's Center for Weather and Climate in Madison, Wisconsin.

Kossin figured out that cyclones have drifted toward the north and south poles over the last three decades. The finding is hailed by other hurricane gurus as the clearest evidence so far that climate change has already had a direct effect. It used to be that cyclones would form and stay near the equator. This where the ocean water and climate are warm. Climate change has increased the temperature of oceans and this means that cyclones are now forming and moving away from the equator.

Rising Sea Levels Are Affecting Storm Surges

When it comes to cyclones and climate change, there are many points of wide agreement, said Emanuel.

One is the consequence of rising sea levels.

The deadliest part of hurricanes is storm surge, he said in an interview.

"It is physically the same phenomenon as a tsunami, except that it is excited by wind rather than a sea floor shaken by an earthquake," explained Emanuel.

If Hurricane Sandy — which caused $50 billion in damage — had happened a century earlier, it probably would not have flooded lower Manhattan because the sea level was lower, he pointed out. The sea level was about 30 centimeters (1 foot) lower then, he said.

Global warming is likely to add roughly a meter (3 feet) to the global watermark by century's end. That number is based on recently revised estimates.

"The surge from these storms will be more devastating — higher and more penetrating," said James Elsner. He is an atmospheric scientist and hurricane expert at Florida State University.

Heavy Rainfall May Lead To Destructive Flooding

The second point of agreement is that hurricanes will hold more water, raising the threat of destructive flooding.

"We calculate that one degree Celsius (1.8 degrees Fahrenheit) of warming translates into a 7 percent increase in humidity in the atmosphere," said French scientist Valerie Masson-Delmotte. She is co-chair of the U.N.’s Intergovernmental Panel on Climate Change.

The U.S. National Hurricane Center reported on Tuesday that Harvey has set a new record for most rainfall from a tropical storm in the United States. Harvey has dumped 49 inches of rain, with more rain on the way. The previous record was 48 inches.

Hurricane Mitch was the second deadliest Atlantic hurricane on record. It left some 19,000 dead in Central America, "all from fresh-water flooding," said Emanuel.

Hurricanes are known for theirwind, he said. Yet, their winds are third on the list of deadly threats. Wind follows storm surges and flooding caused by rain, he said.

Forecasting Hurricane Strength Is Hard To Do

Earlier this year, Emanuel published a study pointing to another worrying climate "signal."

Scientists have made great progress in anticipating a storm's path. They have extended their predictions from a day or two to about a week.

At the same time, they have made little headway in forecasting hurricane strength.

"The thing that keeps forecasters up at night is the prospect that a storm will rapidly gain strength just before it hits land," Emanuel said. He used Harvey as an example. It strengthened right before it hit Texas.

Harvey isn't the first to strengthen so rapidly. In 2015, Hurricane Patricia in the Pacific Ocean intensified more rapidly than any storm on record.

Global warming can heighten that acceleration in intensity, Emanuel said.

A Few Storms Did A Lot Of Damage

Scientists don't know if there will be more or fewer such storms in the future. It is oftencited as evidence that the jury is still out on whether climate change will boost cyclones.

But even if there are fewer, which seems likely, that misses the point, the experts interviewed agreed. Some storms appear to be getting stronger.

Since 1971, tropical cyclones have claimed about 470,000 lives and caused some $700 billion in damages globally, said the Centre for Research on the Epidemiology of Disasters. The Centre is in Belgium.

But most of that death and destruction came from a handful of storms. Just three, for example, have caused well over half of all storm-related deaths in the U.S. since 1900.

So even if the number of smaller storms decreases, that's not what counts.

"The idea of 'fewer but stronger' seems to be the fingerprint of climate change on tropical cyclones," Elsner said.