The Future of Storms? Are powerful hurricanes like Harvey and Irma a sign that climate change is beginning to wreak havoc on our lives?

• Author: Anastasia, Laura
• Position: Cover story

Early on the morning of August 27, as Hurricane Harvey pummeled Texas, floodwaters began lapping at the Nguyen family's tree-lined front yard in Houston. Within hours, water rushed into their two-story brick house. The dad, Viet, plunged through the currents to grab life jackets and a raft from the garage.

"Are we going to die?" Brayden, 9, asked.

The Nguyens survived, but Harvey ruined most of their belongings and did major damage to their home. It was one of the worst natural disasters in U.S. history. As the hurricane turned city streets into raging rivers, tens of thousands had to flee their homes. Thousands of others were left stranded, some waiting desperately on rooftops for rescue by boat or helicopter. At least 70 people died.

Less than two weeks after Harvey, Hurricane Irma--the strongest Atlantic storm ever recorded--led to a mass evacuation of millions of people in Florida after it flattened several islands in the Caribbean. Irma killed more than 50 people.

What made these storms so intense? Scientists are now trying to figure that out--including what role, if any, climate change may have played.

"People always want to know is it climate change or is it not?" says Katharine Hayhoe, a climate scientist at Texas Tech University. "The answer is, it's in between."

While climate change didn't necessarily cause Harvey or Irma, many scientists say, it likely made an already bad situation worse, deepening the storms' impact.

Hurricanes form over warm oceans, drawing their destructive power from the hot moist air evaporating upward. Over the past century, the Earth and its oceans have gotten warmer, creating more opportunities for tropical storms and hurricanes.

Most scientists say the planet is getting warmer because of a rise in greenhouse gases. Some of these gases occur naturally; others are produced by human actions. For example, the burning of fossil fuels like oil and coal--mostly from cars and power plants--creates carbon dioxide, the main greenhouse gas.

Greenhouse gases let sunlight through but also trap heat in the atmosphere, acting like a greenhouse. In the past century, Earth's average temperature has risen about 1.5 degrees Fahrenheit (see graph). And for three years straight, the planet has broken its record for highest global average surface temperature, according to NASA.

Harvey didn't behave like a typical hurricane, meteorologists say, because it continued to gain momentum even as it neared shore. Harvey also stayed intact for several days on land, and its punishing 115-mile-per-hour winds and rain were concentrated in the same area. Storms usually move steadily inland or back out to sea, so their damage is spread out.

Searing Summer Temperatures

Some scientists say Harvey's last-minute power surge and its onshore duration can be tied to water temperature. The Gulf of Mexico, where Harvey formed, was warmer than usual due to searing summer air temperatures. That heat transformed Harvey from a thunderstorm with strong winds into a major hurricane in roughly 48 hours, a day or two faster than is typical.

Irma formed in the Atlantic off Africa's coast. The ocean was warmer than average and the storm grew to massive proportions--reaching 425 miles across--as it moved west. The heat helped the hurricane's winds swirl at more than 185 miles per hour for a record 37 straight hours at its peak.

Climate change also likely contributed to Harvey's record-breaking rainfall, scientists say. Warmer air holds more water, so mega storms tend to dump more rain as global temperatures rise. In Harvey's case, Houston got nearly 52 inches of rain in five days-more than the city usually gets in a year. So while scientists don't tie Harvey directly to climate change, many say it made the hurricane much more...