For the green sea turtles that hatched at the northern end of the Great Barrier Reef, something essential for their species' survival is missing: males. We have a very good idea why this is happening, and once again it is humanity's fault.

For many reptiles, sex is determined not by chromosomes, but by the temperature at which their eggs are incubated. For years this has sparked concerns that human-induced climate change will cause some species to only produce one sex, leading to extinction. Now it seems that day has arrived for the much beloved green sea turtles, at least in the far north of Queensland. Local extinction may not be far behind.

Sea turtles have a form of what is known as temperature-dependent sex determination (TSD). When eggs hatch from sand that averaged 29ºC (84ºF) over the incubation period, they produce a 50-50 mix of males and females. Cooler temperatures mean more males, while warmer ones mean more females.

In the past, a particularly hot or cold year might have produced more of one sex than the other, but for a species that doesn't mate until at least the age of 25, this didn't matter much as long as it evened out over several years. But global warming has created long-standing concerns that we could lose these great seafarers, along with their important role in the ocean ecosystem as almost the only thing that can eat jellyfish. In 2014, a study found that most newly hatched loggerhead turtles were female, but there were still enough males to ensure the future of the population for a long time to come.

Unfortunately, Dr Michael Jensen of the National Oceanic and Atmospheric Administration has found things are more serious for the endangered green sea turtles (Chelonia mydas) of the Great Barrier Reef. At the southern end of the reef, 65-69 percent of turtles are female, which no doubt makes for some very happy males, and could actually improve breeding prospects compared to a 50-50 ratio in a species where some males mate with many females. The mid-reef region has almost no hatcheries. Among those born north of Cooktown, however, 99.1 percent of the juvenile turtles Jensen examined were female, along with 86.8 percent of adults. Among the subadult population, the male proportion was a shocking 0.2 percent, suggesting things have gotten much worse as the world has warmed.

“The northern GBR rookeries have been producing primarily females for more than two decades, and the complete feminization of this population is possible in the near future,” Jensen and co-authors wrote in Current Biology. 

Turtles from both ends of the reef feed in the same places, so Jensen used genetic analysis to classify where a turtle was born while establishing its sex. The same feeding grounds also had some turtles from New Caledonia and other Pacific islands, whose sex ratios were quite similar to those of turtles born at the southern end of the Great Barrier Reef.

The findings have a certain irony in the light of a recent study that found some men reject environmentally friendly options when shopping because they see them as too feminine.

As bad as things are for sea turtles, they are likely to be worse for the tuataras – the last survivors of an order of lizard-like reptiles that predates the dinosaurs. When temperatures rise, more male tuataras hatch, a far more threatening outcome for species survival.

TSD species have survived periods when the world was much hotter or colder before, as they can evolve to a different temperature mid-point where populations are balanced. Green sea turtles, which currently incubate their eggs during the hottest part of the year, could also even out their sex ratio by nesting at a different time. Some reptile species even switch to using sex chromosomes when it suits them best. However, these changes don't always happen quickly, and may not be possible in the face of current rates of warming, which are much faster than anything the turtles have experienced in their multi-million-year history. Although some intermixing is thought to occur between populations, most southern males return to mate near where they were born, rather than seizing the opportunities further north.

Experiments are being run on shading beaches favored by loggerhead turtles in the south, and the idea could be extended to those preferred by northern green turtles.