As global warming melts the world's ice sheets, rising sea levels are not the only danger. Viruses hidden for thousands of years may thaw and escape - and we will have no resistance to them.

Last week, the latest study to track global warming revealed that Alaska's snowless season is lengthening. As the world warms and ice-sheets and glaciers begin to melt, most of us worry about how the earth will respond and what kind of impact climate change will have. Will flooding become a regular feature, or is the land going to become parched? Are hurricanes and typhoons going to spring up in places they have never visited before? Is the rising sea level going to swallow some of the world's most fertile farmland, along with millions of homes?

All of these are valid concerns, but now it turns out that the impact of global warming could be worse than we first imagined. Ice sheets are mostly frozen water, but during the freezing process they can also incorporate organisms such as fungi, bacteria and viruses. Some scientists believe that climate change could unleash ancient illnesses as ice sheets drip away and bacteria and viruses defrost. Illnesses we thought we had eradicated, like polio, could reappear, while common viruses like human influenza could have a devastating effect if melting glaciers release a bygone strain to which we have no resistance. What is more, new species unknown to science may re-emerge. And it is not just humans who are at risk: animals, plants and marine creatures could also suffer as ancient microbes thaw out.

In 1999, Scott Rogers from Bowling Green State University in Ohio and his colleagues reported finding the tomato mosaic tobamovirus (ToMV) in 17 different ice-core sections at two locations deep inside the Greenland ice pack. Gentle defrosting in the lab revealed that this common plant pathogen had survived being entombed in ice for 140,000 years. "ToMV belongs to a family of viruses with a particularly tough protein coat, which helps it to survive in these extreme environments," says Rogers.

Since then Rogers has found many other microbes in ice samples from Greenland, Antarctica, and Siberia. And this has turned out to be just the tip of the microbial iceberg. Over the last 10 years biologists have discovered bacteria, fungi, viruses, algae and yeast hibernating under as much as 4km of solid ice, in locations all over the world.

Most recently Rogers and his colleagues found the human influenza virus in one-year-old Siberian lake ice. "The influenza virus isn't quite as hardy as ToMV, but this finding showed that it is capable of surviving in ice," says Rogers. This particular strain of influenza had only hibernated for one year and doesn't present much of a threat to humans, but it shows that there is potential for a human virus to survive the freezing process for much longer. Imagine if older, more vicious strains, such as the virus responsible for the Spanish flu pandemic, which killed somewhere between 20 and 40 million people in 1918 - 1919, were to re-emerge.

Not all scientists are convinced by these viral discoveries, and some argue that they are more likely to have arrived in the ice via contamination during the drilling process. However, Rogers is confident that this is not the case. "We use a chemical called sodium hypochlorite to decontaminate the outer ice surface, which is then followed by extraction or melting of an interior section of the core," he explains.

So if these viruses have been huddled in the ice for thousands of years, how did they get there in the first place? According to Rogers one very effective way for viruses to travel the world is to hitch a ride in the guts of migrating birds. "The Siberian lake ice where we found the human influenza virus is on a bird migration route. This is the most likely way that the virus arrived," he says. Other modes of transport could include riding on aquatic mammals such as seals, clinging to grains of dust, or water transport via rivers and ocean currents.

"Human beings have been more prevalent in northern areas for a long time and so human viruses are more likely to have been frozen into Northern Hemisphere ice sheets," says Dany Shoham, one of Rogers' colleagues from Bar-Ilan University in Israel. Humans have lived close to glaciers in the European Alps, frozen fjords in Scandinavia and frosty Siberian lakes for thousands of years, making it an easy hop for viruses looking for a place to hibernate for a while. None the less, Shoham says that this doesn't mean the ice sheets of the Southern Hemisphere don't contain viruses.

Thankfully, not all viruses will remain viable after thawing out from hibernation in an ice sheet. "We routinely keep viruses at minus 80C when we want to store them in the lab, so viruses can certainly survive freezing, but they are often fragile to processes such as freeze-thaw," explains Geoffrey Smith, head of the virology department at Imperial College London. In the lab it is possible to defrost viruses gently, but outside they are subject to climatic extremes. Only viruses that contain the tough protein coat, like ToMV, are likely to be able to retain all the information they need while being repeatedly frozen and defrosted. This rules out plenty of human viruses, but still leaves a few very nasty options including smallpox, polio, hepatitis A and, of course, influenza.

Shoham believes that the influenza virus is the most likely to emerge from the freeze/thaw process in a fit enough state to re-infect humans. "It has the properties that would allow it to survive the ice and the ability to transfer between animals and humans once it is out," he says. What is more, Shoham contends that an ancient version of human influenza could be a very potent weapon. "Ancient viruses are more dangerous because the natural herd immunity is reduced over time. After just one or two generations the natural herd immunity is eliminated," he says. Water-borne viruses, such as hepatitis A and polio, are less of a threat because they rely on water currents to reach their victims.

One worrying scenario would be the creation of a super virus via the recombination of ancient and modern strains. "If only one or two genes from an ancient influenza virus were to interchange with the modern avian influenza, it could become contagious and generate a new pandemic," says Shoham.

By hiding in the deep freeze for a few thousand years, viruses could be avoiding unfavourable conditions on the earth's surface, such as hosts with a strong immunity. Rogers and his colleagues think that these icy holidays may even be a deliberate part of viral evolution. Equally, the same argument could mean that it is harder for a virus to slot back into the world once it has been defrosted. "Evolutionary change over time may mean that an emerging ancient virus finds it difficult to adopt a niche," says Shoham.

If viruses do hide away in ice-sheets periodically, then there should be evidence of pandemics occurring during the earth's warmer periods in the past.

"It may be possible to relate historical extinction events with outbreaks of specific pathogens like influenza and cholera," says Rogers. As yet no research team has managed to prove this link, but it is something that Rogers and his colleagues are keen to investigate further.

So how much of a risk do these frozen viruses really represent? Without having any definite evidence that viruses are able to complete the full freeze-thaw cycle and go on to re-infect, it is hard to say. Some scientists are not too concerned, while others think it is worth looking into.

"It is certainly conceivable that viruses can survive frozen for thousands of years, but it is not top of the list of my worries. We have enough to think about with the number of dangerous viruses at high concentration around today," says Geoffrey Smith.

Meanwhile, Dany Shoham believes that the potential consequences are too dire to be ignored, but agrees that there is little we can do to protect ourselves.

"The likelihood of infection from an ancient virus is, in general, low, but once it does take place the impact will be enormous," he says. "None the less, this freezing mechanism is so complex, vague and unpredictable that there is really nothing we can do to protect ourselves."

Perhaps the only grain of comfort is that this won't be the first time that viruses have emerged from the ice. We must have survived such an event before.