Thawing permafrost driven by climate change could free trapped microbes and radioactive material.

As climate change warms the Arctic, melting ice could release hazardous chemicals and radioactive material dating to the Cold War. Vanishing permafrost could also free viruses and bacteria that have slumbered beneath Arctic ice for tens of thousands of years, a new study shows.

By poring over historical records and past studies on contamination, the researchers found that in addition to fallout from nuclear explosions and pollutants such as mercury, arsenic and DDT, so-called Methuselah microorganisms — microbes that have been locked in permafrost for millennia — may awaken if climate change melts Arctic ice and the microbes defrost. That could release bacteria that are resistant to antibiotics, or introduce viruses that humans have never encountered before.

The term “permafrost” describes ground that has been continuously frozen for two years or longer and can include soil alone or dirt mixed with ice and covered by snow, according to the National Snow and Ice Data Center (NSIDC). Permafrost covers about 9 million square miles (23 million square kilometers) of the Northern Hemisphere, and it ranges in thickness from less than 3 feet (1 meter) to more than 3,000 feet (1,000 m), according to NSIDC.

Most Arctic permafrost cover has persisted for 800,000 to 1 million years, but climate change is eating away at even some of the most ancient ice reserves. Warming in the Arctic is progressing at least twice as rapidly as elsewhere in the world, and the past 15 years have warmed and melted the region to the point where the frozen landscape has been permanently transformed, according to the 2020 Arctic Report Card released by the National Oceanic and Atmospheric Administration (NOAA).

One of the known hazards of Arctic warming is the release of vast reserves of greenhouse gases. Melting permafrost releases millions of tons of carbon dioxide and methane each year, and that amount is likely to increase as Earth continues to warm, Live Science reported in 2020.

But until now, scientists did not know the extent of hazards posed by pollutants stored in permafrost — “everything from microbes and potential viruses, to nuclear waste, chemicals and mercury,” said lead study author Kimberley Miner, a science systems engineer with NASA’s Jet Propulsion Lab at the California Institute of Technology (JPL-Caltech).

“Almost no one had ever put all of these different things together,” Miner told.

What’s in permafrost?
Scientists reviewed hundreds of prior studies “to catalogue emergent microbial, viral and chemical hazards within the new Arctic, and recommend research priorities to quantify and address these risks,” the authors wrote.

Since nuclear testing began in the 1950s, radioactive materials have been dumped in the Arctic. During the Cold War, from the end of World War II until 1991, the United States and the Soviet Union conducted nuclear testing and research in the Arctic that left high levels of radioactive waste in soil and permafrost, the researchers discovered.

Detonations by the Soviet Union in the country’s Novaya Zemlya archipelago, between 1959 and 1991, released 265 megatons of nuclear energy; the Russians also scuttled more than 100 decommissioned nuclear submarines in the Barents and Kara seas, releasing radioactive plutonium and cesium that can be detected today in sea bottom sediments and ice sheets, and in plants and soil beneath glaciers, according to the study.

The U.S. Camp Century, a nuclear-powered research center in Greenland, generated radioactive waste that was abandoned beneath the ice when the site was decommissioned in 1967. That ice is now rapidly retreating, with losses of about 268 tons (243 metric tons) per year, as the Arctic warms. And when a U.S. B-52 bomber crashed near Denmark’s Thule Air Base in Greenland in 1968, its nuclear missile payload ruptured and released uranium and plutonium from four bombs into the ice sheet. Arctic radiation levels could remain harmful until 2500, the study authors reported.

Decades of mining in the Arctic across tens of thousands of square miles also left behind waste rich in toxic heavy metals such as mercury, arsenic and nickel. These pollutants have since sunk deep into Arctic soil and could threaten wildlife and human communities in Alaska, Canada, Greenland, Scandinavia and Russia, according to the study. An estimated 880,000 tons (800,000 metric tons) of mercury alone is stored in permafrost, and current warming trends could increase Arctic mercury emissions by up to 200% by 2300, the researchers found.

Arctic permafrost also traps reservoirs of hazardous chemicals that were banned in the early 2000s, such as the insecticide DDT (dichloro-diphenyl-trichloroethane) and PCBs (polychlorinated biphenyls), a group of chemicals that were widely used in coolant fluids. These and other persistent organic pollutants, or POPs, traveled to the Arctic atmospherically and over time became concentrated in permafrost. However, “few studies have traced POP transport and risk,” suggesting that “the impact of these chemicals within Arctic systems is underestimated,” according to the study.

Microbial threats could lurk in Arctic permafrost, too. Because Arctic microbes have evolved to survive subzero temperatures with minimal access to nutrients or water, many are capable of coming back to life even after thousands of years in a deep freeze. In prior studies, other researchers revived bacterial populations in permafrost dating to 30,000, 120,000 and even a million years ago, the scientists reported.

Finding the risk

But identifying pollutants in permafrost is just one part of calculating their risk to the Arctic and beyond; the other part of the equation is how quickly the permafrost is melting, Miner said.

“There’s gradual thaw, which is just year-over-year thaw that moves down slowly from the top. And then there’s abrupt thaw, where, for example, you can lose an entire side of a permafrost hill in a series of weeks. That’s the kind of difference that will need to be mapped in order to understand when and how these things can emerge,” Miner said.

Another important factor is that different pollutants pose varying levels of risk depending on pollutant quantity, duration of exposure, and how people and wildlife might come into contact with it, she added. For that reason, a next step for researchers could be assigning a risk profile to the recently identified pollutants in permafrost. But it’s harder to evaluate the risks of permafrost’s Methuselah microbes, as it’s unknown which types of bacteria and viruses could emerge from ancient frozen soil.

“We have a very small understanding of what kind of extremophiles — microbes that live in lots of different conditions for a long time — have the potential to reemerge,” Miner said. “These are microbes that have coevolved with things like giant sloths or mammoths, and we have no idea what they could do when released into our ecosystems.”

In the long run, keeping these organisms and pollutants in their permafrost tombs would be preferable to trying to contain them once they’ve escaped, Miner said.

“It’s absolutely critical to make sure that we do everything in our power to keep the permafrost — and generally the Arctic — frozen,” she said. “It would be so much easier if we didn’t have to deal with any of these, besides long-term remediation proposals.”

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