Ultraviolet light, present in the sun, then triggers this chemical decomposition in the concentrated pollutants. Without it, the compounds remain relatively inert, like the food in your freezer. But under UV lighting, “overall, we’re seeing faster decay rates in ice than in water,” says Halsall. These accelerated decay rates may show up more noticeably in ice at the poles, where “you can get 24 hours of sunlight at certain parts of the year,” says Grannas. “It drives a lot of chemistry.”
Microplastics, plastic fragments less than 5 millimeters long, also break down faster in ice than in water. Chemists at Central South University in China found that more than 48 days, beads of microplastic less than a thousandth of a millimeter in diameter have deteriorated in the ice to the point where they would have deteriorated in 33 years in the Yangtze River. “Microplastics take hundreds of years, if not thousands, to break down,” Chen Tian of Central South University in China told WIRED, in Chinese. “We didn’t have that much time, so we only studied the first stage of degradation. But we think the whole degradation process should be faster in ice.
Plastic litter is the most common form of marine debris – around 10 million tonnes of plastic ends up in the ocean each year, much of it breaking down into microplastics – so the ice at the poles may be churning through this waste. This could be good news, as it could help scientists find methods to break down microplastics faster, Tian and his colleagues point out in their paper. But by breaking down the microplastic into smaller and smaller pieces, the ice could also turn it into an increasingly prevalent pollutant. The smaller the plastic fragments, the deeper they penetrate into organisms. Microscopic plastic particles have been found in the brains of fishcausing brain damage.
For Halsall, whose research aims to track human activity in Antarctic ice, the degradation of pollutants makes life more difficult. He is particularly interested in perfluoroalkyl and polyfluoroalkyl substances, or PFAS. These “forever chemicals” persist in the environment and end up in non-stick pans, motor oils, and all manner of consumer products. In 2017, Halsall collaborators dug in Antarctica to extract a 10-meter-long cylinder of packed snow that had accumulated since 1958. Specimens like this reveal climate and human activity, just as tree rings do so in more temperate latitudes. The deeper the snow sample, the further back in time you go.
Many chemical companies abandoned the use of “longer chain” PFAS around the year 2000. In snow deposited that year and after, Halsall’s team found less of this pollutant and more of its compounds. replacement, “shorter chain” PFAS. “We can spot in this snow core when the industry has changed,” says Halsall. But to accurately understand what was used when, Halsall must also consider the amount of pollutants degraded, as this can help explain differences in the chemicals found at different depths.
These reactions transmitted by the ice also have impacts on the rest of us. As glaciers melt at the poles, pollutants processed by sunlight are released into the environment. “You might think, ‘We’re degrading a pollutant. It’s a good thing,” says Grannas. “In some cases it is. But we’ve found that for some pollutants, the products they transform may actually be more toxic than the original. For example, Grannas and his colleagues found that the chemical aldrin , historically used in pesticides, could be transformed more easily in the even more toxic chemical dieldrin in ice. (Farmers also used dieldrin extensively in pesticides in the 20th century, and use of both chemicals is banned in most countries.)
On a more optimistic note, Grannas says studying how ice degrades pollutants will help researchers evaluate new substances. “We are introducing new chemicals into our agricultural systems, our pharmaceuticals, and our daily use — laundry detergents, perfumes, and personal products,” says Grannas. “We want to understand from the start what will happen if we use it on a large scale and release it into the environment.” Some of these pollutants will eventually freeze in glaciers or at the poles, and tracking the evolution of chemicals in the ice gives researchers a clearer idea of their potential impact on the environment. At the Earth’s poles, the interior of an ice cube is a turbulent place.