Green sea turtles may stop basking on beaches around the world within a century due to rising sea temperatures, a new study suggests. Basking on sun-warmed beaches helps the threatened turtles regulate their body temperatures and may aid their immune systems and digestion.

By analyzing six years of turtle surveys and 24 years of satellite data, researchers from Duke University, the National Oceanic and Atmospheric Administration (NOAA) Pacific Islands Fisheries Science Center and the University of Ioannina in Greece have found the turtles bask more often each year when sea surface temperatures drop.

If global warming trends continue, this behavior may cease globally by 2102, the study projects. In Hawaii, where the study was primarily focused, green turtles might stop basking much earlier, by 2039.

The scientists published their peer-reviewed findings last week in the journal Biology Letters.

"By comparing turtle basking counts with sea surface temperatures, we found that green turtles tend not to bask when local winter sea surface temperatures stay above 23 degrees Celsius," said lead researcher Kyle Van Houtan, adjunct associate professor at Duke's Nicholas School of the Environment.

To conduct the study, Van Houtan and his colleagues used six years of turtle count data collected daily by the Hawaiian nonprofit M?lama na Honu on Laniakea Beach in Oahu. The counts showed regular, seasonal fluctuations in the number of turtles basking on the beach. These fluctuations correlated with sea temperatures at Laniakea, indicating that sea turtles bask more when waters are cooler.

The scientists then compared these fluctuations in temperature and basking to growth marks in the humerus bone of several green turtles. They found that the growth lines occurred at the same time of year when turtles bask more, between February and April.

The turtles' growth lines are similar to tree rings in that they indicate periods of stress for the organism, said Van Houtan, who is also a scientist in NOAA's Turtle Research Program. In trees, growth rings can indicate winter, dry seasons, or periods of drought. In green turtles, the lines seem to reflect periods when seas are colder and body temperatures are consequently lower, prompting the turtles to haul out on beaches to warm in the sun.

More research is needed to fully understand the importance of basking and the effect climate change will have on basking behaviors of green turtle populations around the world. said Van Houtan.

Not all green turtles bask on land, he noted. Though the turtles are found in tropical and subtropical oceans around the world, beach basking has only been observed in Hawaii, the Galapagos Islands and Australia. Sea surface temperatures at these sites have been observed to be warming at three times the global average rate.

It is not yet clear whether populations that currently bask on land during cooler months will adapt to warming sea temperatures and begin to bask exclusively in the water, as do some other populations around the world.

"When looking at climate change, which is this vast geopolitical issue, you have to drill down to specific climate variables impacting specific aspects of an organism's life," said Van Houtan. "The next step for us is to look at how turtles are storing climate data in their bodies -- in their tissues, shells, and bones, and how we can tease that out."