Tropical Storm Eta’s record-breaking deluge may be part of a pattern of more intense rains and higher tides that have turned late fall in South Florida into a season of super floods.
A new study from climate researchers at the University of Miami adds to the evidence that humans —and their greenhouse gas emissions — are largely to blame.
The study looked at the relationship between sea surface temperatures in the Atlantic Ocean and rainfall in South Florida. It argues that greenhouse gas emissions from humans are the likely source of the hotter sea water and the more intense rains.
“The water is warmer, and so the air above it is warmer. Warm air can hold more water. So that means there is more water available to rain when a storm comes through," said Jeremy Klavans, one of the study’s co-authors and a doctoral candidate at the Rosenstiel School of Marine and Atmospheric Sciences.
Scientists have known for decades that warmer sea surface temperatures play a role in the over 50 inches of rain the region receives a year, on average. The region gets more rain during decades when the sea temperature is warmer and is drier when the Atlantic is cooler.
But recently, the amount of rain that falls on the southern portion of the state appears to be increasing.
"There’s a slight upward trend in annual rainfall totals,” said Brian McNoldy, a senior research associate at the Rosenstiel School of Marine and Atmospheric Science who was not involved in the study.
There’s also been a steady increase in sea surface temperatures around the region. All-time record high water temperatures were recorded for January, April, July, August and October this year in South Florida, according to data from McNoldy.
“It’s very unusual for us to have this kind of rainfall this late in the fall,” said David Enfield, a retired research scientist at the University of Miami. Back in 2001, Enfield published one of the first papers that analyzed the relationship between ocean temperatures and rainfall levels in South Florida.
The University of Miami study doesn’t look at the effects of the increasing ocean temperatures on hurricanes and tropical storms, but instead teases out the relationship between human greenhouse gas emissions, warmer oceans and heavier rainfall totals, regardless if they fall as part of a tropical system or a smaller storm.
In the study, researchers used climate models to show that the intense late fall rains in South Florida only start getting harsher as a result of human greenhouse gas emissions. Without all the extra carbon that humans have put in the atmosphere, and the added heat the extra carbon entails, the researchers' models produced no extra rains.
The increase in rain doesn’t necessarily lead to the overflowing canals, flooded living rooms, and stalled-out cars at washed-out intersections we saw this weekend as a result of Eta.
But along with the increased rainfall levels have come higher tides, and that means poorer drainage.
“The local tide levels have broken records in recent days,” McNoldy said. “The water level has never reached this high so late in the year before. You have to go back to Oct. 28, 2012, to find a water level record that’s higher.”
Scientists have observed that South Florida saw a 400% increase in high-tide flooding between 2006 and 2016, and sea levels around the peninsula have risen about 5½ inches since 1993.
All the extra water in the ocean means that the 12 to 15 inches of rain that fell in parts of Broward due to Eta are having trouble draining into the sea.
Fortunately, this storm didn’t come near a full or new moon, so tides were not naturally very high when the storm surge came.
This story was produced in partnership with the Florida Climate Reporting Network, a multi-newsroom initiative founded by the Miami Herald, the South Florida Sun Sentinel, The Palm Beach Post, the Orlando Sentinel, WLRN Public Media and the Tampa Bay Times.
