How climate change causes extreme weather

Mammoth hurricanes have become more destructive and more common. Does climate change play a role? Here's everything you need to know:

Are hurricanes worsening?

Last year brought a trio of devastating Category 4 and 5 hurricanes: Harvey dumped a record-breaking 27 trillion gallons of rain on Texas; Irma was the most intense storm to hit the continental U.S. since Katrina; and Maria's winds reached 175 mph, flattening much of the Caribbean and knocking out Puerto Rico's entire power grid. Those storms killed thousands and did hundreds of billions of dollars in damage. They were very unusual in how quickly they intensified, how slowly they moved after making landfall, and how much rainfall they produced. Continuing that trend, Hurricane Florence exploded from Category 1 to 4 in barely 24 hours, traveling an astonishing 350-plus miles each day before slowing to an ominous crawl as it neared the Carolinas. The intensity of storms is growing so markedly that meteorologists have considered adding a Category 6 label in anticipation of future epic storms.

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Is that because of global warming?

Scientists have debated the connection for years, but new research has provided strong evidence for that conclusion. Because Harvey crossed the Gulf of Mexico when waters were abnormally calm, scientists could collect highly specific data about ocean conditions before and after the storm. They found that energy leaving the ocean in the form of water vapor equaled the amount of energy dropped in rain — the first time that's been documented. There are two simple reasons why warmth exacerbates hurricanes. First, for every 1.8-degree Fahrenheit increase in temperature, the atmosphere can hold up to 10 percent more water. Second, a warmer ocean gives off more vapor, and the Gulf of Mexico was several degrees hotter than usual, leading scientists to estimate Harvey's rainfall was up to 38 percent greater than it would have been otherwise.

How else are storms changing?

In June, Nature published the first study to analyze hurricane speeds worldwide. It found that hurricanes move about 10 percent more slowly over land than they did 50 years ago. The study's author, James Kossin, a scientist at the U.S. National Oceanic and Atmospheric Administration (NOAA), believes slower-moving hurricanes result from changes in wind currents caused by global warming. Naturally, more time lingering over land means more damage, and Kossin says a 10 percent reduction in hurricane speed corresponds with a 10 percent increase in rainfall. Warming also affects ocean currents, which could begin producing storms farther from the equator; as a result, hurricane season, now June 1 through November, could expand from late spring to winter. Warmer waters, however, can cause an "intensity-frequency trade-off," in which there are more severe storms but fewer weak ones.

How bad could things get?

Researchers at NOAA use supercomputers to simulate the effects of climate change on hurricanes. From 2016 to 2035, they project more hurricanes in general and 11 percent more Category 3, 4, and 5 hurricanes. By the end of the century, they expect 20 percent more of the worst storms, some with winds above 190 mph. The flooding from hurricanes will also worsen as sea levels rise because of global warming. Sea levels are rising faster than at any time in the past 2,000 years, yet even with that in mind, geological scientist Andrea Dutton says, "projections give you a false sense of security." A crisis might look decades away, "but in reality, it can happen much faster."

How can we plan accordingly?

Experts champion "resilient infrastructure," with buildings, roads, hospitals, and power systems all designed to endure disasters. Taiwan is an exemplar; it experienced four typhoons in 2015 but was fully operational within about four days after each storm. A 2017 study by the nonprofit National Institute of Building Sciences found that every dollar spent on hazard prevention saves $6 in future costs. Some pre-emptive planning specifically takes global warming into account. In 2015, President Obama issued an executive order requiring the federal government to take flood risk and climate change into account before committing funds for new public infrastructure projects, such as hospitals, roads, and bridges. That could have major consequences for many coastal areas. Climate scientists, for example, have predicted the sea level along the North Carolina coast could rise by a catastrophic 39 inches by 2100.

Has North Carolina taken action?

Yes, but only by passing a law preventing state officials from using the prediction of rising sea levels to make coastal development decisions. More recently, President Trump reversed Obama's executive order, so that federally funded construction projects do not have to take climate change into account. Scott Pruitt, Trump's former Environmental Protection Agency director, called it "insensitive" to discuss climate change when a hurricane strikes. Irwin Redlener, director of Columbia University's National Center for Disaster Preparedness, warns that we continue to ignore what nature is telling us at our peril. "We keep calling these storms a wake-up call, but they keep turning into a snooze alarm," Redlener said. "When the cameras go away, we slip into complacency."

Flood coverage drowning in debt

More than 5 million Americans receive flood insurance from the Federal Emergency Management Agency through a 50-year-old program that's more than $20 billion in debt. Last year alone, it paid out $8.7 billion to policyholders, leading Congress to forgive $16 billion the program owed to the federal government. Yet despite the increasing costliness of covering floods as sea levels rise and hurricanes grow more severe, a bill to update FEMA's insurance setup remains stalled in Congress. The 21st Century Flood Reform Act passed the House last November, and would limit coverage for high-risk properties, expand opportunities for private policies, and authorize financing to mitigate flood damage. But the bill has gone nowhere in the Senate. Most standard homeowner insurance policies don't cover flood damage, and many homeowners in the Carolinas who could be affected by Hurricane Florence live outside a FEMA-designated high-risk flood zone, where flood insurance is required. In fact, earlier this year a study found that 40.8 million Americans are now exposed to serious flooding — roughly three times more than the government projected.