How climate change makes storms stronger

On Monday, a hurricane battered Nicaragua before moving across Central America, claiming at least 57 lives. The day before, on the other side of the world, one of the strongest storms to ever hit land struck the Philippines. Its house-ripping winds reached speeds of 310 kilometers per hour (195 miles per hour) — as fast as a Japanese bullet train — but only grazed the area around the capital, narrowly missing 14 million people.

The storms are yet another example of how extreme weather is becoming terrifyingly ordinary as the climate changes.

So many hurricanes formed over the Atlantic Ocean this season that the World Meteorological Organization exhausted its 21-name-strong alphabetical list of storm names for only the second time in history. Tropical cyclones — rotating winds that are also known as hurricanes and typhoons depending on where in the world they form — even contributed to the locust swarms that plagued East Africa and South Asia earlier this year. 

Separately, record-breaking floods put a third of Bangladesh under water in September and, according to financial news outlet Bloomberg, wiped out a quarter of Nigeria's rice harvest. 

"We will need to adapt," said Alejandro Di Luca, a climate scientist at the University of Quebec in Montreal, Canada. Tropical cyclones "will get worse, there is no question about it."

How does climate change affect storms?

Each year the world is struck by about 86 tropical cyclones, according to a study published in the journal PNAS in May, and this has stayed constant over the last four decades. While scientists predict the number of cyclones may fall because of changing ocean conditions, the ones that do form will get stronger.

This is because of simple physics. Warmer air holds more moisture: For every 1 degree Celsius (1.8 Fahrenheit) the atmosphere warms, the air holds about 7% more water. By burning fossil fuels and heating the planet — particularly the oceans — we've put more water into the air.

That matters for tropical cyclones, like the ones raging across Central America and the Philippines this week, because they are powered by warm, humid ocean air. Rain falls when water vapor condenses. More water means more rain. And the heat released in that process strengthens the storm even further.

What else links climate change and storms?

When Hurricane Harvey struck Texas and Louisiana in 2017, with total rainfall over the course of a week among the highest in US history, three studies found that the extra rainfall attributable to climate change was several times greater than expected. 

"Before we started doing these studies, I'd thought the change [in rainfall] would be controlled by the amount of moisture the atmosphere could hold," said Michael Wehner, author of one of the studies and a senior scientist at the US Department of Energy's Lawrence Berkeley National Laboratory. "It turns out, in these very intense storms, that there are other changes going on that cause the storm to be even rainier."

In the Atlantic Ocean, for instance, climate change may be slowing storms as wind patterns change. That sounds less threatening. But slow-moving storms can still have high wind speeds — they just take longer to move along their path. By pounding the areas they lingered over with stronger winds and heavier rains, this effect increased the destruction from Hurricane Harvey, which stalled over the Texan city of Houston, as well as Hurricane Florence the next year and Hurricane Dorian the year after.

The rainfall from the Central American storm Eta, which is also slow-moving, brought "catastrophic, life-threatening flash flooding and river flooding, along with landslides," according to a weather warning from the National Hurricane Center of the National Oceanic and Atmospheric Administration (NOAA) in the US.

How does climate change affect coastal flooding?

By 2050, coastal floods that used to hit once a century will strike many cities every year, according to the IPCC, the gold standard on climate science. This is because climate change has made sea levels rise. 

Higher seas make storms worse in two ways. First, tropical cyclones create storm surges. This means fast winds and low atmospheric pressure raise the level of water hitting the coast. This increases the frequency and intensity of flooding by raising the platform from which storm surges strike.

Then there's the rain. After a storm, rainfall further inland that has built up tries to escape into the oceans. But this can overwhelm the capacity of drainage channels in the ground and burst riverbanks. As that travels downstream to the oceans, it can worsen flooding in coastal cities. 

Does climate change make storms and floods deadlier?

Together with stronger storms, rising sea levels will push the annual damages from coastal floods tens to hundreds of times greater by the end of the century than they are today, the IPCC projects. But while tropical cyclones have become more financially damaging, tearing roofs off homes and washing away possessions, experts say they have not become more deadly. 

The storm that battered the Philippines on Sunday killed at least 20 people and forced nearly 350,000 to flee their homes before the worst of the rains hit. Such preemptive evacuations, which have become increasingly sophisticated, save lives. Early warning systems allowed India, Bangladesh, China and the Philippines to move millions of people out of the path of tropical cyclones Fani, Bulbul, Lekima and Kamuri last year.

Two-thirds of the 33.4 million people displaced within their own countries last year were fleeing storms and floods, according to a report by the Internal Displacement Monitoring Center in April, and most of these were preemptive evacuations that saved lives. Some were able to return home and rebuild, while others were still living in camps months later. 

Cyclones are growing stronger from climate change, but their destructiveness also depends on "how well you deal with [them] as a society," said climate scientist Di Luca. The difference in damage between countries like Haiti and the US, for example, is how much adaptation they can afford. "Very often it is the same storm producing thousands of deaths versus five," he said.

Correction: This article has been updated to correct the affiliations of scientists Alejandro di Luca and Michael Wehner.