Last week a new study revealed that Alaska's snowless season is lengthening. As ice sheets and glaciers begin to melt, most of us worry at 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 could be worse than first imagined. Ice sheets are mostly frozen water, but they can incorporate organisms such as fungi, bacteria and viruses.

Some scientists believe climate change could unleash ancient illnesses as ice sheets drip away and bacteria and viruses defrost. Common viruses such as 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 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 he 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 past 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 team found the human influenza virus in one-year-old Siberian lake ice.

"The influenza virus isn't as hardy as ToMV, but this finding showed 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.

Not all scientists are convinced by these viral discoveries, and some argue that they are more likely to have arrived in the ice through contamination during the drilling process. However, Rogers is confident 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."