Plastics Make Beaches Hotter During the Day and Colder at Night

Henderson Island and the Cocos Islands are remote paradises. They have beautiful white-sand beaches and swaying palm trees. But they are also covered in plastic.

Henderson Island, which sits at the edge of the infamous “Pacific garbage patch,” made headlines a few years ago, when a study found 18 million tons of plastic littered the island despite the fact that it is thousands of miles from a major landmass. Now, the same research team has found that all those plastics are affecting sand temperatures on Henderson Island and the Cocos Islands, in the Indian Ocean. The study team’s results, published online in the Journal of Hazardous Materials in May, show that plastic pollution changes sand temperatures. That could be a problem for heat-sensitive beach dwellers like crabs, mussels and snails, as well as for animals like shore birds and sea turtles that use beaches for nesting.

Plastics harm wildlife in myriad ways, many of which scientists are just beginning to grasp. When birds, fish and other larger animals eat plastics, the material can get tangled up inside their bodies and cause damage; plastics can also make animals feel falsely full, so they stop eating. Hermit crabs try to use slippery plastic cups for shells, then get stuck and die. Microplastics leach chemicals into tissues and organs once they’ve been ingested, posing potential risks like organ failure and reproductive stress at the base of food chains as chemicals bioaccumulate. (Effects on humans remain unknown.)

Even less research has focused on how plastic pollution affects environments physically. That knowledge gap prompted Jennifer Lavers, a marine ecotoxicologist at the University of Tasmania in Australia, to measure beach temperatures.

“Up until this point, so much of the literature focused on charismatic species or the type or source of plastic,” Lavers writes. “Year after year, I’d return to some of the most far-flung corners of our planet and the plastic I witnessed 12, 24 months ago would still be sitting there (except perhaps even more!). I knew it had to be having some kind of effect, but there was no data.”

To track plastic pollution and measure sand temperatures, Lavers and colleagues set up a network of ten roughly three square-foot plots across Henderson and the Cocos Islands. The team counted how many plastic pieces were present and installed temperature sensors at two depths, roughly 2- and 12-inches deep in the sand. It was a simple experimental set-up, but they ran into one fundamental problem: “They couldn’t find a blank space as a control site,” says Jack Auty, an interdisciplinary biologist at the University of Tasmania who led the data analysis of the study. Plastic was so pervasive that the field scientists could not find a single area of sand that did not have any litter. They settled for the least-polluted patches to use instead, and then they waited.

After the sensors collected temperature data for three months, Auty analyzed temperature by hour and by day. “I could see the story coming out as I explored the data,” he says. “I began to see how the [temperatures’] circadian rhythms were being massively impacted by the surface levels of plastic.”

In the shallow sands with moderate levels of plastic, daily maximum temperatures were around 2.5 degrees C warmer than low- and high-plastic sites, and their daily minimum temperatures were about 1.5 degrees cooler.

The temperatures were more stable at the deeper sensors at every study site, and the effect dissipated once the plastic layer got too thick.

“Interestingly, even the hot effect disappeared for the highest level of plastic, where you can’t even see the sand,” Auty says. “Light can’t penetrate there. At that point, the sand is buried so deep under plastic that it just can’t go through that extreme circadian cycle.”

While the researchers need to carry out further experiments to ascertain the reasons for heating and cooling, they have ideas to explain the daily temperature patterns they found. The daytime warming could be due to an insulating effect from the plastic, like a greenhouse trapping heat and moisture inside. The nighttime cooling effect, Auty says, is more “perplexing.” Plastics in the sand could serve as pathways for air and water that allow heat to dissipate from just under the surface more readily once the sun’s radiation isn’t a factor. Scientists will need to conduct another study to figure out why the temperature change happens.

A mere degree or two of temperature change can have big impacts on coastal communities, and the temperature changes might be even larger in higher-latitude regions with a bigger daily temperature range to begin with. Because tropical temperatures tend to be very stable, equatorial critters have evolved to thrive in narrow ecological niches, and budging just a little outside their ideal temperature range can be disastrous.

Cold-blooded beach dwellers, like sea turtles, are particularly at risk. “Reptiles are ectotherms—they’re cold-blooded—and they’re intrinsically tied to the thermal characteristics of their environment,” says Leo Clarke, a marine ecologist at Bangor University who was not involved in the study. “So temperature influences lots of parts of their life cycle, and reproduction in particular.”

Sea turtles bury their eggs in the sand and research has shown that nest temperature can affect the sex ratios of baby turtles, with warmer nests resulting in more young females. High temperatures can also decrease hatchling fitness and success, with stronger, larger hatchlings coming from cooler nests. Other ectotherms, like tropical iguanas and beach-nesting snakes could be affected similarly.

Some ectotherms might be less negatively impacted. The temperature stability Lavers and Auty found at depth could be good news for loggerhead sea turtles in particular. They typically nest deep enough that their eggs won’t be affected by the temperature variability.

Shore birds and seabirds that rely on beaches for nesting and burrowing could be impacted by plastics’ warming effect in multiple ways, according to conservation ecologist Stephanie Borrelle, who was not involved in the study. “If there's plastic at the nest sites and it's incorporated into nesting materials or into the burrows, and that's increasing the temperature, that could potentially have impacts on the development of an egg or a chick,” she says. “That's a big, important question for us to start looking at. We're really at the beginning of trying to understand it.”

Temperature-driven shifts could affect microscopic communities, too. Bacteria and tiny invertebrates like barnacles can be highly sensitive to temperature, so more extreme daily highs and lows could let some organisms thrive while others perish. “We talk about sea turtles because they’re charismatic, but temperature changes would also affect invertebrates and microorganisms and soils,” says Auty. “They’re really the foundation for coastal, intertidal ecosystems, and they could be incredibly vulnerable to these temperature changes,”

And a weak foundation could topple ecosystems. “If plastics in burrows affect the soil biota, that could impact mites or other disease-carrying microorganisms,” says Borrelle, who works for BirdLife International. “With climate change, that’s already an issue for seabirds.” Plastic-driven temperature changes could exacerbate the spread of diseases like avian malaria. As negative effects ripple up the food chain, with whole populations or species potentially suffering, tiny changes at the surface will have increasingly visible, outsize impacts.

With plastics posing poorly understood threats that could destabilize ecosystems, looking for other ways to protect and prop up ecosystems is critical. Borrelle, who even stumbled across plastic bottle rings while carrying out research in Antarctica, suspects it’s too late to focus on cleaning up the plastic, so efforts should focus on making species and ecosystems resilient to threats that are better understood.

“Unfortunately, it's like climate change. We've already done a lot of damage to the environment with plastics,” she says. “So at this point, I think it's more about finding ways to mitigate other threats and buffer those populations to try and avoid further declines, or the cumulative impact of multiple threats.”

Clarke’s perspective on sea turtles was strikingly similar. “The populations with the most potential to adapt and to be resilient to environmental change are healthy ones, so conservation efforts need to focus on not just temperature and climate change, but all threats turtles face, from pollution, to overfishing, to exploitation.”

The base of the problem is still global plastic consumption, and addressing that will only help these ecosystems survive. “We have filled the atmosphere, the oceans, the world with plastics, and there’s no way to reverse it, which is terrifying,” Auty says. “There really is only one solution, and that is to stop buying and producing plastic. And then hopefully things will recover.”