Siberia frozen lake If global warming releases ancient diseases from glaciers and permafrost, humans could become exposed to viruses they have not encountered in thousands of years. (Photo: Serg Zastavkin/Shutterstock)

In 1999, Russian scientists famously dug a long-dead frozen woolly mammoth out of the Siberian permafrost. Other things lurking in the frozen earth may more alive — and more dangerous. Scientists warn that global warming could release ancient bacterial, viruses and fungi from frozen lakes, glaciers and permafrost. If this happens, humans could become exposed to viruses and diseases they have not encountered in thousands of years.

It happened just last year in a remote part of Siberia in the Arctic. As the BBC reports, an exceptionally warm summer in 2016 thawed a layer of permafrost, revealing the carcass of a reindeer infected with anthrax some 75 years ago. Anthrax is caused by a bacteria, Bacillus anthracis, which leaked into the water supply, soil and food supply. A 12-year-old boy died from the infection, as did 2,300 reindeer; dozens more people were sickened and hospitalized.

"Permafrost is a very good preserver of microbes and viruses, because it is cold, there is no oxygen, and it is dark," evolutionary biologist Jean-Michel Claverie at Aix-Marseille University in France, told the BBC. "Pathogenic viruses that can infect humans or animals might be preserved in old permafrost layers, including some that have caused global epidemics in the past."

Or as Montana State University professor John Priscu told Scientific American: "You put something on the surface of the ice and a million years later it comes back out."

What else lurks under the ice?

Melting sea ice on Antarctica Under Siberia's ice, bacteria such as anthrax and viruses like smallpox may be frozen and could be released into the environment if the ice thaws. (Photo: ssguy/Shutterstock)

Scientists around the world have been studying Arctic and Antarctic ice for years. For example, scientists found the 1918 Spanish flu virus, which killed 20 to 40 million people worldwide, intact on corpses frozen in Alaska. And researchers studying the anthrax outbreak in Siberia believe smallpox is frozen in the same area. One 2009 study of Antarctica's frozen freshwater lakes revealed DNA from nearly 10,000 species of viruses, including many that had not previously been identified by science.

Frozen viruses may have been making their way back into the environment for centuries, even without global warming. Scientists theorize that periodically melting Arctic lakes release previously frozen influenza viruses, which are picked up by migrating birds and transported toward human populations.

One virus seems to have reappeared in the 1930s, 1960s and most recently in 2006 when a Siberian lake melted. "This phenomenon may take place regularly, far beyond what we witness," Dany Shoham a biological warfare researcher at Israel's Bar-Ilan University, told Wired. Many viruses won't remain viable after freezing, but others are more adaptable. For example, influenza has properties that allow it to survive the ice and transfer between animals and humans once it is out, Shoham said.

Ice isn't the only repository for diseases. Many also are carried by insects, some of which are expanding their range due to warming climates. Humans won't be the only ones affected. Climate change will stress out some organisms, such as coral, leaving them more vulnerable to new viruses. "It's really a double whammy, not only does the host become more stressed and susceptible, but also the pathogens are growing faster," Drew Harvell of Cornell University told LiveScience. "That's the key to why a warmer world can be a sicker world."

Editor's note: This article has been updated with new information since it was originally published in November 2012.