Seas Are Rising. It Rains More. How Much Of That Is Making Hurricanes Worse?
As the planet heats up, polar ice melts, seas rise and Biblical-size rains become more frequent, hurricanes are expected to get wetter and more intense.
But less certain is how much climate change is making these fierce storms, which target Florida more than any other U.S. state, more punishing now.
That uncertainty comes from the complex set of variables that drive hurricanes, the rarity and short record for the storms and the modeling that researchers rely on to tease out changes linked to climate change, scientists say. Some studies have suggested storms are slowing down, becoming more intense and dumping more rain. A higher number of intense storms may already be striking further north as oceans warm.
But the National Oceanic and Atmospheric Administration said in a July report from the Geophysical Fluid Dynamics Laboratory that most signals don't yet fall out of the normal range.
"They’re rare events, so when you have rare events uncertainty is hard to characterize," said Frank Marks, chief of NOAA's hurricane research division and lead investigator for its forecast improvement project. "This is what we expect, we just haven’t seen much yet to say, 'Yep that’s right.'"
Solving the hurricane question could have major consquences. The storms are among the most destructive natural events on the planet: Maria killed nearly 3,000 people in Puerto Rico, according to a government-commissioned study. Harvey, the wettest hurricane on record, caused at least $125 billion in damage. Irma made landfall seven times, four as a Catagory 5 hurricane.
In South Florida, each hurricane season brings renewed dread and a close watch on seasonal forecasts. On Thursday, NOAA revised its forecast calling for five to nine hurricanes during the ongoing season that ends in November.
There's no debate that sea levels and rainfall are increasing, scientists say. And those are two factors that make hurricanes worse.
In 2014, global sea levels were about 2.6 inches higher than in the 1990s. NOAA estimates seas are continuing to rise about an eighth of an inch each year. In South Florida, water levels at Virginia Key have risen about 5 inches since the 90s. Higher seas make storm surge worse.
Rainfall is also increasing. Scientists know this because it occurs all the time, all over the place, so it's easier to measure.
"Rain events in general happen throughout the country, not just in coastal areas, so we can see very clear trends in rainfall and intense rainfall," said Brian Soden, an atmospheric scientist at the University of Miami's Rosenstiel School of Marine and Atmospheric Science.
But isolating changes in rainfall in tropical systems is more difficult.
"Rainfall is a very difficult output or element of numerical model prediction. It’s very sensitive to how the model is designed and how it works. And rain is very nonlinear in the way it works," Marks said. "We know in South Florida you can have 3 inches in Fort Lauderdale and nothing here in Miami."
And that's caused debate among how much rain in storms scientists can now blame on climate change.
In July, the University of California, Berkeley published a study that used thunderstorm modeling to see how much rain would have been generated in Hurricanes Katrina, Irma and Maria in conditions before the Industrial Revolution, when the planet began to warm. They found climate change "enhanced" extreme rainfall in the hurricanes. The research followed up on a 2017 Berkeley study that found global warming likely generated about 19 percent more rain in Harvey.
Studies have also argued that rising temperatures are already increasing the number of Category 4 and 5 storms. Earlier this year, a NOAA-led study found the number of storms that underwent rapid intensification between the 1980s and 2000s nearly tripled.
"In the last two years, we set a record for the most storms that intensified to Category 5 in a row. It was pretty unique," said UM oceanographer Brian Haus. "So that's a pretty clear indication that there's this capacity for rapidly growing hurricanes and there's a strong indication that that is occurring."
But even that is up for debate inside NOAA.
"These climate change detection results are suggestive but not definitive, and depend on the [the] model’s ability to simulate natural variability," such as longterm fluctuations in sea surface temperatures, NOAA's fluid dynamics lab team wrote of the study.
UM has invested heavily in finding out just how much climate change is driving hurricanes, and how to better predict those storms. In 2015, the school opened its $15 million SUSTAIN tank, an elevated wind and water machine with 4-inch thick acryllic walls capable of generating Category 5 hurricanes.
Haus runs the lab and has been working to better measure how water, air and temperature conspire to form monster storms. A smaller tank helped improve forecast models for Cat 3 storms. But this bigger tank is helping unravel monster Cat 5 hurricanes, he said.
Hurricanes are fueled in large part by how much heat they can suck up from the ocean. It's why hurricane season falls during the hottest part of the year between June and November. The heat is transferred in water vapor. How hurricanes move across and through water - their drag co-efficients - also matters in the calculations.
"This can apply to the forecast models and it can apply to climate change models," Haus said. "The models are complex, and they have a lot of different different things that are happening. So the more that we can physically constrain them, with realistic observations, then the better chance we have of being able to understand the overall processes and predict them."
If most everyone agrees that storms will likely be more dangerous in the future, why does it matter whether or not they're happening now?
Marks said it's because it can obscure more immediate risks.
"The Harvey disaster. The Florence disaster. What are ways we could mitigate what happened? The Houston area is a morass of building codes, where they put up chemical plants next to residential areas. South Carolina and North Carolina, the Piedmont area, have rivers that can flood and they add pig farms with manure pools," he said. "There’s all sorts of things that happen where we create the problem. And that’s now, without any climate change signal that we can detect. What’s going to happen when it really starts to show up? If those projections are right, then we're really in trouble."