Over a massive region of southeast North Carolina and northeast South Carolina, Florence produced an extraordinary rainstorm that statistically has a 1-in-100 chance of occurring each year. Over substantial areas, the deluge had a 0.1 percent chance of happening, what is known as a 1,000-year event.
These exceptional rainfall events keep happening and appear to be part of a trend toward more extreme tropical rainmakers, probably connected to climate change.
Since August 2017, three hurricanes have set rainfall records for tropical weather systems in four states.
First came Harvey, which dumped an unheard-of five feet of rain in Texas last August. No storm in recorded history had produced so much water in the United States. In all, the hurricane and its remnants generated 33 trillion gallons of water over the country, enough to engulf Houston in a tank of water 3.1 miles high.
Then came Lane in August, which bombarded the Big Island with more than 50 inches, becoming Hawaii’s rainiest tropical storm.
As a point of exclamation, Florence slammed into the Carolinas over the past week, setting tropical storm rainfall records in two states, surpassing 20 inches in South Carolina and 35 inches in North Carolina. Ryan Maue, meteorologist with weathermodels.com, tweeted that the storm dispensed 11 trillion gallons of rainfall along the way.
Florence’s rainfall in North Carolina was the most for any tropical weather system north of Florida along the East Coast on record, and fourth most for any state.
Records like these may only be the beginning as Earth continues to warm.
Recently published research has shown hurricanes are slowing down, taking in more water and growing bigger. “If you have bigger, slower, wetter storms, you’re going to set rainfall records,” said David Titley, professor of meteorology at Pennsylvania State University.
Although it’s not certain how much of the change is the result of global warming, a study published in June found that storms have slowed down by about 10 percent since 1946. “Every one of the hazards that we know tropical cyclones carry with them, all of them are just going to stick around longer,” Jim Kossin, study author and a scientist with the National Oceanic and Atmospheric Administration, told The Washington Post’s Chris Mooney. “And so that’s never a good thing.”
Basic physics dictates that as the oceans warm, evaporation speeds up, which increases the amount of water vapor in the atmosphere available to storms. Studies published after Hurricane Harvey determined that global warming boosted its rainfall by at least 15 percent and up to 38 percent. “Climate change made this event more likely and heavier,” Karin van der Wiel, co-author of one of the studies, told The Washington Post’s Joel Achenbach.
A preliminary analysis, which was not peer-reviewed, conducted at Stony Brook University projected that rainfall in the heaviest part of Florence was intensified by 50 percent “due to human interference in the climate system.”
Oceans are warming not only at the surface but at great depths, increasing the reservoir heat from which storms can draw. Climate scientist Kevin Trenberth, author of a study exploring the link between ocean heat and hurricane intensity, said the extra heat not only increases the rainfall intensity but also “enlarges the storm,” expanding its rainfall footprint. Ocean heat content set a record high in 2017.
Scientists are careful to note that the extreme rainfall events over the past year represent a small sample size among the storms that have occurred through history. “Traditional statistical methods will take decades to show change,” said Titley, who was the lead author of a study from the National Academy of Sciences exploring linkages between extreme weather events and climate change.
But Titley said that while it may take years for a signal to emerge, “it seems like there is pretty strong evidence that we are in fact seeing wetter storms. The smart way to bet is that this is happening.”
Characterizing these exceptional tropical rainfall events as part of the “new normal” may even undersell what is happening. Because the planet is expected to warm more as humans add greenhouse gases to the atmosphere, the rainfall potential of tropical weather systems may increase well beyond the present.
“What worries me is all of that extra warming occurring in the ocean making its way back into the atmosphere whether it’s through hurricanes or something else,” said J. Marshall Shepherd, a climate scientist at the University of Georgia, who also participated in the National Academies study. “This latent warming is a dangerous reservoir of heat.”