About 99 in 100 newly hatched turtle babies are female at one of the biggest sea turtle nesting sites in the world — and warming temperatures are to blame for the lack of male babies, a new study suggests.

That's because the sex of young sea turtles (and alligators, crocodiles and some other kinds of turtles) is determined by the temperature at which their eggs are incubated, with warmer temperatures producing females, and cooler temperatures producing males.

Because of the role of water temperature in the sex of offspring, scientists have been worried for some time that climate change, which has caused a rapid increase in the average global temperature in recent decades, could push the sex ratios of some populations of those animals to skew female.

A study published Monday in the journal Current Biology shows that's already happening, and has been for about two decades, at nesting sites at Raine Island and Moulter Cay in the northern Great Barrier Reef "such that virtually no male turtles are now being produced from these nesting beaches." That area, off the coast of Australia, has experienced very warm temperatures, leading to a range of other problems such as deadly coral bleaching.

The researchers expressed concern that in the future, the lack of males could leave many females unable to find a mate and "eventually impact the overall fertility of females in the population."

Great Barrier Reef nesting area

That's a concern because the northern Great Barrier Reef is one of the biggest sea turtle nesting areas in the world, where about 200,000 females go to lay their eggs, reported the scientists from the U.S. National Oceanic and Atmospheric Administration Fisheries Department and the Queensland Department of Environment and Heritage Protection.

A Green Sea turtle swims over a reef near the surf break known as 'Pipeline' on the North Shore of Oahu, Hawaii March 20, 2013. (Hugh Gentry/Reuters)

They made the discovery after they caught and examined 400 green sea turtles of various ages feeding off the Howick Group of Islands in the Great Barrier Reef off the coast of Queensland in Australia, studying which were male and which were female. They then used genetic analysis to trace each turtle to the beach where it originally hatched, as the turtles almost always return to lay eggs at the beach where they were born. That means their birthplace can be determined using DNA from their maternal lineage.

Among turtles from the southern Great Barrier Reef, about 65 to 69 per cent of turtles were female.

But among those from the northern Great Barrier Reef, 99.1 of juvenile, 99.8 per cent of subadult and 86.8 per cent of adult turtles were female.

Since the 1990s

When the researchers looked at sand temperatures from those beaches, they found the green sea turtle nests in those areas have been incubated above the temperature that produces a balanced sex ratio since the early 1990s. The higher proportion of females among younger turtles shows the problem has been getting worse.

While male sea turtles will mate with multiple females, the researchers say they don't know how many are needed to sustain sea turtle populations. They added that males tend to court females very close to the beaches where they're born, reducing the chance that females from the northern Great Barrier Reef will be able to find a mate from a different nest site.

Researchers release an adult green sea turtle after it was caught during the survey. The team caught and examined 400 green sea turtles of various ages feeding off the Howick Group of Islands in the Great Barrier Reef off the coast of Queensland in Australia to figure out which were male and which were female. (Michael Jensen/NOAA)

The researchers also express doubt that natural selection can save the day, as long-lived species like turtles evolve slowly.

"With temperatures predicted to increase by several degrees in only a few turtle generations," they wrote, "many sea turtle populations… will have little room to adapt to a rapidly changing climate."

Michael Jensen, the lead researcher in the study, and a scientist at NOAA's Southest Fisheries Science Center in La Jolla, Calif., declined to be interviewed by CBC News. His office said he was too overwhelmed by media requests.

'It bears noticing'

The bad news is that this could be happening to other species around the world – including other sea turtles – that rely on hatching temperature to determine the sex of their offspring.

Kathleen Martin, executive director of the Canadian Sea Turtle Network, said the new study shows "there are real repercussions to climate change… it bears noticing."

She said the problem had been "a worry for some time" among scientists.

"But I'm surprised it's happening so quickly."

Martin said that while sea turtles don't nest in Canada, Atlantic Canada is one of the most important feeding grounds in the world for leatherback sea turtles. Both it and the other sea turtle commonly found in Canadian waters, the loggerhead, are listed as endangered under Canada's Species at Risk Act.

Green sea turtles are also occasionally found in Canadian waters.