The 2018 hurricane season has already proven to be deadly. But the link between hurricanes and climate change is often misunderstood, so we asked our friends over at The Weather Channel to help break it down. Here’s what Hurricane Specialist Carl Parker had to say – the next time you’re talking to someone skeptical about the effects of climate change, these expert-vetted facts will come in handy.

Hurricanes and warmer waters

The world’s oceans are getting warmer, and warmer oceans increase the available energy for tropical cyclones. A whole series of conditions are necessary to produce hurricanes, but when those conditions are met, a warmer ocean raises the potential intensity of hurricanes, or the upper limit of the strength of the storms. The depth of the ocean warmth is also increasing, and that can allow hurricanes to maintain high intensities longer.

Hurricanes are getting stronger

While the nature of the record makes it challenging to make definitive statements about observed hurricane intensity, climate models have long indicated that hurricanes are expected to get stronger.

Right now, there is at least circumstantial evidence that we are already beginning to see an increase in the intensity of tropical cyclones:

• In 2018, Hurricane Michael was the third strongest U.S. hurricane at landfall on record, and the strongest to hit the Florida Panhandle.
• 2017’s Hurricane Irma had sustained winds of 185 mph for 37 straight hours, a far longer duration than any previous tropical cyclone on record, worldwide.
• 2015’s Hurricane Patricia had sustained winds of 215 mph, the highest of any tropical cyclone on record, worldwide.
• And 2013’s Typhoon Haiyan was the strongest at landfall of any such storm on record, worldwide, with sustained winds of 195 mph.

Hurricanes are getting wetter

An area where the link between tropical cyclones and climate change is clearer is that of the observed increase in extreme rainfall and flooding. As the atmosphere warms, it can hold more moisture. For every degree of warming in Celsius, we expect an increase in atmospheric moisture of about 7 percent, and rainfall amounts can be greatly amplified in extreme events.

There has also been an increase in the prevalence of unusually strong areas of high pressure called atmospheric blocks. These large, slow-moving weather systems can stop tropical cyclones in their tracks, allowing rain bands to stall, and produce devastating floods. A 2018 study (Kossin) reported a 20 percent reduction in forward speed for Atlantic storms, and a 20 percent increase in rainfall amounts.

Recent Hurricanes have been especially damaging

Both tropical cyclones Florence and Harvey have been linked to climate change. Florence flooded vast areas of the Carolinas in 2018, closing interstates, and causing nearly $40 billion in damage. Warming was estimated to have made Florence’s rains 50% more likely than they would have been otherwise (Reed 2018), because of high atmospheric moisture content.

In 2017, Harvey dumped a staggering maximum of 60 inches of rain in Southeast Texas. The 1000-year flood inundated an area the size of New Jersey. 300,000 structures were flooded, and damages totaled $125 billion dollars, making Harvey and Hurricane Katrina the two most expensive natural disasters in U.S. history. One recent study (van Oldenbergh 2017) found Harvey’s rains to have been as much as five times more likely due to warming, largely due to the exceptionally slow forward speed of the storm—related to a strong atmospheric block.

There’s still more to learn about hurricanes

While recent studies have demonstrated a link between warming and tropical cyclones, attribution and detection in this arena is a work in progress. Part of the challenge is that we don’t know precisely how strong hurricanes were in the distant past. Reliable satellite measurements only date back to about 1970. For storms that occurred prior to that time, we have only estimates as to just how strong they were, particularly while well out to sea.

But the physics of tropical cyclone intensity are well-understood, and warmer oceans are expected to increase hurricane intensity. What is also well-understood is the impact of warming on atmospheric moisture, and the forward speed of tropical cyclones, both of which have increased the odds of extreme rainfall in tropical cyclones. We expect these trends to worsen, and now is the time to act on climate.