But Wyneken said that for the past few years, especially since scorching summers of 2015 and 2016, she hasn't seen a significant difference in the sex ratios of the species in South Florida: it's girls, girls and more girls, in every nest.
And things are getting worse. July was the hottest month ever recorded in the world, coming in slightly higher than the previous record, which was July 2016, according to data from the World Meteorological Organization and the Copernicus Climate Change Service. There were wildfires in the Arctic, a huge ice melt event in Greenland and 90-degree weather in Alaska. European cities baked, with the temperature in Paris reaching 108.7 degrees. And this year's record temperatures did not get a boost from a strong "El Nino," which heats up surface sea water and contributes to warmer temperatures, as what happened in 2016.
"I never thought that what started as research on sea turtle gender ratios on a couple of Florida beaches would become a measuring stick for climate change, but that's what I'm seeing," Wyneken said. Before warming temperatures began to dramatically skew the sex of sea turtles toward female, the same nest would produce girls and boys, with eggs buried deeper in the sand leading to males and eggs on the warmer surface generating females.
What scientists have observed in South Florida is happening in other sea turtle nesting areas around the world.
On Australia's Raine Island, the biggest green turtle nesting ground in the Pacific, the ratio was 116 females to one male in a 2018 study led by Michael Jensen and Camryn Allen, scientists with the National Oceanic and Atmospheric Administration. The study found that older turtles that had hatched 30 or 40 years earlier were mostly female, but only by a 6 to 1 ratio. Younger turtles, however, born during the last 20 years, were more than 99% female.
Jensen and Allen "combined genetic and endocrine techniques to show that an important green turtle population has produced primarily females for two decades, suggesting that complete feminization is possible in the near future," said the research.
Another study done recently with green turtles in Guinea-Bissau, West Africa, by the University of Exeter and Portugal's Marine and Environmental Sciences Center showed similar results.
But how soon could turtle populations run out of males?
Sea turtles have been around in some form for more than 200 million years, weathering all kinds of extreme weather events and even survived the extinction of the dinosaurs some 66 million years ago.
"These sex ratio findings are very concerning and we want to raise awareness about the issue, but there is also some hope," said David Godfrey, executive director of the Sea Turtle Conservancy, a Florida research and conservation nonprofit. "Sea turtles are amazing at adapting to changes in their habitat and in the environment."