More sea turtles will be born female as climate warms, study shows.

Hotter sands triggered by a warming climate could cause greater numbers of sea turtles to be born female, increasing the reptiles’ numbers in the short term, research shows.

But the study, published in the scientific journal Nature Climate Change, warns that once the sands in which sea turtle eggs incubate grow too warm, the population could become entirely female, risking the animal’s extinction.

“Sea turtles are unusual in that the gender of the offspring is not driven by sex chromosomes, as in humans,” said Professor Graeme Hays, one of the lead authors of the study.

Instead, a sea turtle’s sex is determined by the temperature of the sand in which the female turtle buries her eggs.

“The logic is that warming temperatures will lead to more female hatchlings being produced, because the females are produced at the warmer temperatures,” Hays said.

Sand temperature depends strongly on its colour, Hays said. “The darker the sand, the more heat it absorbs from sunlight. So blacker sand would be much warmer than lighter coloured sand.”

At the “pivotal temperature”, 29C, the gender ratio of turtle hatchling is approximately 50:50. The ratio is upset as the sand temperature increases, until it reaches around 31C. Sand that hot will “almost exclusively produce female hatchlings”, Hays said.

Similarly, sand at 28C will likely produce only males.

The study, carried out on a loggerhead turtle rookery in Cape Verde, in the Atlantic Ocean, modelled the likely impact of unchecked temperature increases over the next 150 years.

“Over the next 20 to 30 years, it’s not going to create problems,” Hays said. “In fact there’s going to be a benefit to the turtles, because there’s going to be more females produced, which means more females laying eggs. More females will lead to a population expansion.

“But ultimately, if you extrapolate long enough into the future … once you get 100 years or more into the future, then things start to look serious. You have so few males left that it’s likely to be a problem. There will be heaps of female but not enough males to fertilise all those eggs.”

It is possible the hardy reptile could adjust by laying its eggs in a cooler season or shifting to a colder part of the world, Hays added. Humans could adjust too, by avoiding the development of hotels and resorts on lighter beaches. "They're going to be the most important ones to protect,” he said.

Other reptiles whose sex relies on temperature, including alligators, crocodiles and many species of lizard, could be exposed to the same process.