In the space of two weeks, two record-breaking hurricanes. Hurricane Harvey was the wettest storm in history, and now Hurricane Irma is barreling towards south Florida packing the highest winds ever recorded. Out in the Atlantic, Hurricane Jose is trailing Irma, and in the Gulf of Mexico, Hurricane Katia lurks in Harvey’s shadow. Three major Atlantic hurricanes at once is unusual. Since records have been kept, this has only happened three times: in 1893, 1961 and 2010. But, never before have three hurricanes threatened to make landfall simultaneously.

This year’s violent hurricane season has revived the debate about the role of climate change in extreme weather. Warming temperatures don’t necessarily mean more hurricanes, but scientists believe that storms will grow more intense as temperatures rise. Is that what’s happening this year?

Scientists have long agreed that temperatures are rising due to increased carbon dioxide from human activity. But, it’s hard to say how this impacts the severity of any particular storm. We can’t say with certainty that Hurricanes Irma and Harvey were more intense than they would have otherwise been as a result of climate change. But, we can say that science predicts that intense storms will be more likely at temperatures rise. A recent article in Scientific American, provided a useful analogy:

“[I]f a baseball player on steroids is hitting 20 percent more home runs, we can’t attribute a particular home run to steroids. But we can say steroids made it 20 percent more likely to have occurred.”

So, let’s set aside the question of whether climate change was responsible for Hurricanes Irma and Harvey. We can’t know that for sure. Instead, let’s talk about the basic physics involved and why it might suggest that climate change is making storms stronger.

The Warmer the Wetter

Last year, global average temperatures over land and water were 0.95 C warmer than the 20th century average, making 2016 the hottest year on record. We know from basic physics that warmer air holds more moisture at a given pressure than cooler air. This can be calculated using the Clausius–Clapeyron relation, which shows that at standard pressures, a 1 C rise in temperature yields a 7% increase in the moisture-holding capacity of the air. As ocean temperatures warm, evaporation off the water’s surface increases too. The result will be more moisture-laden air that generates storms with heavier rainfall.

According to climate scientist Michael Mann of Penn State University, sea temperatures in the mid-Gulf have risen by about 0.5 C to 1 C over the past few decades. That translates to 3-5% more moisture in the air. So, it’s reasonable to suppose that this dynamic could have contributed to Hurricane Harvey’s unprecedented rainfall.

Hurricane Harvey caused so much flooding not only because it rained so hard, but because it lingered so long. Harvey was pinned in by sub-tropical regions of high pressure over the southern and northern U.S. and a jet stream that has shifted northward. These are both conditions that are predicted by climate change models. There is also some evidence that suggests warming in the arctic may make it more likely that mid-latitude weather systems will stall, but this has yet to be conclusively proven.

Prevailing winds that would normally have moved Harvey along more quickly have also been weaker in recent years. Scientists suspect that weaker steering winds might result from climate change. But, they aren’t all that sure that climate change was to blame for the relatively calm prevailing winds that allowed Harvey to wobble over Texas for so long, according to Michael Wehner, a senior staff scientist at the Department of Energy’s Lawrence Berkeley National Laboratory. “If there is a climate signal, it’s one that’s so weak we haven’t been able to detect it,” he said.

Stronger, Faster

Hurricanes usually diminish as they approach land. Access to warmer water caused Hurricane Harvey’s winds to intensify by 45 mph in the 24-hours before it made landfall. Friday night, Hurricane Irma strengthened as well, back to a category 5 storm. While this is not unheard of, it is not common. Recent research suggests that climate change will make this more likely. The reason is simple, says Kerry Emanuel, a researcher with the Massachusetts Institute of Technology. “Hurricanes are powered by the evaporation of sea water,” he told National Geographic. “Water evaporates faster from a hot surface than a cold surface.”

A study Emanuel published last year, compared 6,000 simulated storms under 20th century conditions and those predicted at the end of the 21st century if emissions of heat trapping gases continue to rise. The study found that by the end of the century, storms that increase in intensity by 60 knots 24-hours before landfall will occur not once a century as previously expected, but once every five to 10 years. This effect may or may not have contributed to Harvey and Irene’s intensification, but it suggests that more storms might follow the same pattern in the future.

Hurricane Irma registered 185 mph sustained winds over 64 hours, the most intense ever recorded. It’s well known that hurricanes gain strength from hotter waters. Rising warm air cause storms to churn more violently. As temperatures rise higher maximum sustained winds in storms would be expected. Indeed, wind speeds have increased 5% over the last couple of decades according to NASA. Irma likely got a boost from warm sea temperatures as it moved towards Florida.

Higher Sea Levels, Higher Storm Surge

Global ocean temperatures were 0.75 C higher warmer in 2016 than the 20th century average. Warm water takes up more volume and melts ice. So, it comes as no surprise that sea levels have risen 8 inches on average. Higher sea levels make it easier for hurricane storm surges to wash over land and cause flooding. This effect may be magnified as Hurricane Irma makes landfall.

In Florida, sea levels have risen as much as 52% more than in other parts of the world according to a study published this week in Geophysical Research Letters. This is because sea level rise is uneven due to changes in the pull of gravity as glaciers in Greenland and Antarctica melt, says Isabella Velicogna, an earth science professor at the University of California, Irvine, and the paper’s author.

“With the warming, we expect an intensification of the strength, the frequency of hurricanes and extreme events,” Velicogna said. “The sea levels are rising more there, so it’s going to intensify again those effects.”

A Perfect Season

Climate scientists are careful to point out that the blame for this year’s unusually violent hurricanes can’t be pinned entirely on climate change. There is a natural rhythm of hurricane seasons of variable intensity, as this superb chart from Chris Canipe of Axios illustrates. According to Tom Knutson, a NOAA meteorologist at the Geophysical Fluid Dynamics Laboratory in Princeton, N.J., the data doesn’t conclusively demonstrate that this year is a new normal of super intense storms.

Chart: Chris Canipe / Axios. Click for full image.

“We really can’t detect these changes yet in the existing data in the way we can detect in changes, for example, in the global mean temperature,” Knutson said. Although we’ve seen some unprecedented storms, he says that there isn’t a “clear existing signal in storm data” that proves this year is more than just natural variability.

But, it does seem like hurricanes are growing stronger, says Jennifer Collins, a hurricane expert at the University of South Florida School of Geosciences Tampa. “The research is showing that with climate change we might be seeing more intense storms,” she said. “So the fact that they are major, that does link up with the climate change research.”

Yet, it’s important to remember that hurricanes require just the right combination of warm water, moist air, and low wind sheer. High wind sheer, which is the difference in speed and direction between upper and lower level winds, literally shears hurricanes apart. The low wind shear this year has been a big reason that the tropics are birthing hurricanes at such a rapid rate, Collins points out. The may not have anything to do with climate change.

“We’ve also got a season where we have these conducive conditions, so the environment is just right for them,” Collins said.

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