London: 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 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?
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 bacteria and viruses. Some scientists believe that climate change could unleash ancient illnesses as ice melts and bacteria and viruses defrost.
Illnesses we thought we had eradicated as common viruses, like human influenza, could have a devastating effect if melting ice releases 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 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 four kilometres of ice all over the world.
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 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 is to hitch a ride in the guts of migrating birds. Other modes of transport could include riding on aquatic mammals, clinging to grains of dust, or water transport via rivers and ocean currents.
"We routinely keep viruses at -80ūC 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 like smallpox, polio, hepatitis A and influenza.
It is believed that the influenza virus is the most likely to emerge from the freeze/thaw process in a fit enough state to re-infect humans. What is more, an ancient version of human influenza could be a very potent weapon. One worrying scenario would be the creation of a super virus via the recombination of ancient and modern strains.
By hiding in the deep freeze for a few thousand years, viruses could be avoiding un-favourable conditions, 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.
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 concerned, while others think it is worth looking into.
Some believe that the potential consequences are too dire to be ignored, but agree that there is little 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.
