Inscribed in any chunk of Antarctic snow, Crispin Halsall will tell you, is a story about how humans have treated the planet. Over the years, each round of precipitation at the South Pole has brought down the atmospheric detritus of the day: pollen; volcanic ash; and of particular interest to Halsall, human pollution. Antarctic pollution can originate as far away as the northern hemisphere, with volatile chemicals floating in the wind to arrive at the South Pole in a matter of days. “Those layers of snow become an environmental record of contamination, going back decades,” says Halsall, who is a chemist at Lancaster University in the UK.
The world’s icy landscapes also foretell our environmental future. As icebergs and glaciers melt, pollutants trapped inside are released back into seas, waterways, and the air. Melting ice can unleash harmful molecules that damage ecosystems, deplete the ozone layer, or mess with the weather. And due to rising global temperatures, more and more of the world’s frozen landscapes are thawing. In the Alps and the Himalayas, “we are seeing the rerelease of old contaminants that have been locked up in ice for many decades,” says Halsall. It’s vital to know what’s being emitted.
But interpreting what’s trapped in Antarctic snow is more complicated than previously thought. Researchers have discovered that the frozen water at Earth’s poles—contrary to conventional wisdom—is a hotbed of chemical reactions. What’s trapped within may transform over time.
For a long time, scientists assumed the opposite: that frozen pollutants remain inert. “Most of the time, if you freeze something or make something colder, it slows things down,” says chemist Amanda Grannas of Villanova University in the US. Molecules move slower in solid ice and snow compared to liquid water, which means they collide less, leading to fewer opportunities to participate in chemical reactions. It’s why freezing raw meat keeps it from spoiling. It’s also why the bodies of several woolly mammoths, some 30,000 years old, have emerged preserved from frozen ground as it thaws.
But in laboratory experiments, scientists have found that many pollutants—illuminated using bright light simulating the sun—break down faster in ice than in liquid water. In 2020, a team at the University of California, Davis observed that guaiacol, a molecule found in woodsmoke and consequently in bacon and whiskey, broke down into smaller compounds faster in ice than in liquid water. In 2022, they saw that the same applied to dimethoxybenzene, another molecule produced in smoke. This February, Halsall and his colleagues found that pollutants in car exhaust fumes—known as polycyclic aromatic hydrocarbons—also degraded faster in ice than in water.
Researchers attribute this flurry of chemical activity in ice to a phenomenon known as the “freeze concentration effect.” As water cools to form ice, its constituent molecules line up in hexagonal crystals. “The stuff dissolved in the water gets forced out of that ice crystal structure,” says Grannas. “To the naked eye, it looks like a frozen ice cube. But microscopically, there’s these little pockets of liquid where the other chemicals get concentrated. The reactants have been shoved into this tiny volume together, and that makes the chemistry go a lot faster.”