Warmer temperatures in the Arctic is causing the region’s permafrost — the frozen layer of soil below the ground — to melt and potentially reawaken viruses that, after being dormant for tens of thousands of years, could threaten animal health and people.

Although a pandemic caused by a disease from the distant past sounds like the plot of a sci-fi movie, scientists warn that the risks are underestimated, reports CNN.

“There’s a lot going on with the permafrost that is of concern, and (it) really shows why it’s super important that we keep as much of the permafrost frozen as possible,” said Kimberley Miner, a climate scientist at the NASA Jet Propulsion Laboratory at the California Institute of Technology in Pasadena, California.

Permafrost covers a fifth of the Northern Hemisphere, supporting the arctic tundra and boreal forests of Alaska, Canada, and russia for millennia. It serves as a kind of time capsule, preserving, in addition to ancient viruses, the mummified remains of a number of extinct animals that scientists have been able to dig up and study in recent years, including two cave lion cubs and a woolly rhino.

The reason permafrost is a good storage medium isn’t just because it’s cold; it’s an oxygen-free environment that light doesn’t penetrate. But today the temperature in the Arctic is rising four times faster , than on the rest of the planet, which weakens the upper layer of permafrost in the region.

To better understand the risks posed by frozen viruses, Jean-Michel Claverie, an Emeritus professor of medicine and genomics at the Aix-Marseille University School of Medicine in Marseille, France, has tested earth samples taken from Siberian permafrost to see whether any viral particles contained therein are still infectious. He’s in search of what he describes as “zombie viruses” — and he has found some.

Scientists revived the "zombie" virus that spent 48.5 thousand years in permafrost
Jean-Michel Claverie

Claverie studies a special type of virus that he first discovered in 2003. Known as giant viruses, they are much larger than normal viruses and can be seen with a normal light microscope rather than a more powerful electron microscope. This makes them a convenient model for this type of laboratory work.

His efforts to identify viruses frozen in permafrost were partly inspired by a group of russian scientists who in 2012 revived a wildflower from 30,000-year-old seed tissue found in a squirrel’s burrow. Since that time, scientists have also successfully brought ancient microscopic animals back to life.

In 2014, Claverie managed to revive a virus that he and his team isolated from permafrost, making it infectious for the first time in 30,000 years by injecting it into cultured cells. To be safe, he decided to study a virus that can only infect single-celled amoebas, not animals or humans.

In 2015, he repeated this feat, by isolating another type of virus that also affected the amoeba. In his latest study, published on February 18 in the journal Viruses, Claverie and his team isolated several strains of the ancient virus from multiple permafrost samples taken from seven different locations in Siberia and showed that each could infect cultured amoeba cells.

Scientists revived the "zombie" virus that spent 48.5 thousand years in permafrost
Computer-enhanced microphoto of Pithovirus sibericum that was isolated from a 30,000-year-old sample of permafrost in 2014.4

These latest strains represent five new families of viruses,in addition to the two he had revived earlier. The oldest is nearly 48,500 years old, according to radiocarbon dating of the soil, and comes from a soil sample taken from an underground lake 16 meters below the surface. The youngest specimens found in the stomach contents and fur of woolly mammoth remains were 27,000 years old.

The fact the viruses that infect the amoeba are still infectious after such a long time suggests a potentially larger problem, Claverie says. He fears that people view his research as a scientific curiosity and do not perceive the prospect of ancient viruses returning to life as a serious threat to public health.

“We see the traces of many, many, many other viruses,” he added. “So we know they are there. We don’t know for sure that they are still alive. But our reasoning is that if the amoeba viruses are still alive, there is no reason why the other viruses will not be still alive, and capable of infecting their own hosts.”

Traces of viruses and bacteria that can infect humans were found in the permafrost.

A lung sample from the body of a woman exhumed in 1997 from permafrost in a village on Alaska’s Seward Peninsula contained genomic material from the strain of influenza responsible for the 1918 pandemic. In 2012, scientists confirmed that the 300-year-old mummified remains of a woman buried in Siberia contain genetic features of the virus that causes smallpox.

An anthrax outbreak in Siberia, which affected dozens of people and more than 2,000 reindeer in July and August 2016, was also linked to deeper melting of permafrost during an extremely hot summer that allowed old Bacillus anthracis spores to surface from old burials or animal carcasses.

Birgitta Evengård, professor emerita at Umea University’s Department of Clinical Microbiology in Sweden, said there should be better surveillance of the risk posed by potential pathogens in thawing permafrost, but warned against panic.

“You must remember our immune defense has been developed in close contact with microbiological surroundings,” said Evengård, who is part of the CLINF Nordic Centre of Excellence, a group that investigates the effects of climate change on the prevalence of infectious diseases in humans and animals in northern regions. If there is a virus hidden in the permafrost that we have not been in contact with for thousands of years, it might be that our immune defense is not sufficient. It is correct to have respect for the situation and be proactive and not just reactive. And the way to fight fear is to have knowledge.”

Of course, scientists don’t know how long these viruses can remain infectious under current conditions, or how likely it is that the virus will encounter an appropriate host. Not all viruses are pathogens capable of causing disease, some are benign or even beneficial to their hosts. And while the Arctic is home to 3.6 million people, it is still a sparsely populated region, making the risk of human contact with ancient viruses very low.

“The risk is bound to increase in the context of global warming,” Claverie said, “in which permafrost thawing will keep accelerating, and more people will populate the Arctic in the wake of industrial ventures.”

And Claverie is not alone in warning that the region could become a breeding ground for the spread of the virus.

Last year, a group of scientists published study of soil and lake sediment samples taken from Lake Hazen, a freshwater lake in Canada located above the Arctic Circle. They sequenced the genetic material in the sediment to identify viral signatures and the genomes of potential plant and animal hosts in the area.

Using computer model analysis, they suggested that the risk of virus transmission to new hosts is higher in areas close to areas where large amounts of glacial meltwater flow into the lakeand is a scenario that is becoming more likely as the climate warms.

Identifying viruses and other hazards contained in the melting permafrost is the first step toward understanding the danger they pose to the Arctic.

The melting can be a gradual process, lasting only a few centimeters per decade, but it can also happen faster, as in the case of massive landslides that can suddenly expose deep and ancient layers of permafrost. This process also releases methane and carbon dioxide into the atmosphere, which is an underestimated driver of climate change.