The temperature at which sea turtle embryos incubate determines the sex of an individual, which is known as Temperature-Dependent Sex Determination (TSD).

The pivotal temperature for TSD is 29 degrees Celsius as both males and females are produced in equal proportions - above 29 degrees Celsius mainly females are produced while below 29 degrees Celsius more males are born.

"Up to a certain point, warmer incubation temperatures benefit sea turtles because they increase the natural growth rate of the population: more females are produced because of TSD, which leads to more eggs being laid on the beaches," said Jacques-Olivier Laloe from Swansea University in the UK.

However, beyond a critical temperature, the natural growth rate of the population decreases because of an increase of temperature-linked in-nest mortality, researchers said.

"Temperatures are too high and the developing embryos do not survive. This threatens the long-term survival of this sea turtle population," Laloe said.

Within the context of climate change and warming temperatures, all else being equal, sea turtle populations are expected to be more female-biased in the future.

While it is known that males can mate with more than one female during the breeding season, if there are too few males in the population this could threaten population viability, researchers said.

Sea turtle eggs only develop successfully in a relatively narrow thermal range of about 25-35 degrees Celsius, so if incubation temperatures are too low the embryo does not develop but if they are too high then development fails, they said.

This means that if incubation temperatures increase in the future as part of climate warming, then more sea turtle nests will fail.

Researchers recorded sand temperatures at a globally important loggerhead sea turtle nesting site in Cape Verde off the northwest coast of Africa over six years.

They also recorded the survival rates of over 3,000 nests to study the relationship between incubation temperature and hatchling survival.

Using local climate projections, the team then modelled how turtle numbers are likely to change throughout the century at this nesting site.

"In recent years, in places like Florida - another important sea turtle nesting site - more and more turtle nests are reported to have lower survival rates than in the past," Laloe said.

"This shows that we should really keep a close eye on incubation temperatures and the in-nest survival rates of sea turtles if we want to successfully protect them," he added.

The study was published in the journal Global Change Biology.