Ottawa — Data compiled from the journals of early Arctic explorers casts doubt on the assumption that recent thinning of Arctic ice is the result of human-induced climate change.
A Norwegian study using the explorers' ancient log books suggests that dramatic shrinkage of sea ice, widely cited as evidence for global warming in recent years, has occurred before.
That doesn't necessarily prove that recent disappearance of sea ice is natural, but raises the possibility that it could be, researchers say.
Adventurers of the 1700s, who took meticulous notes on their voyages, encountered ice conditions similar to those seen today, researcher Chad Dick said in an interview from Norway.
"If you go back to the early 1700s you find that sea ice extent was about the same then as it is now," said Mr. Dick of the Arctic Climate Systems Study, an international research program.
In Canada there has been alarm at reduced ice cover in Hudson Bay which is causing problems for polar bears.
There's also been debate about disappearing ice in the Northwest Passage, which could result in challenges to Canadian sovereignty over the passage.
Those phenomena have been cited as evidence that humans are causing the global climate to warm.
But similar shrinkage has occurred before, according to the Norwegian researchers who have drawn up Arctic ice charts covering 500 years.
The charts show sea ice has declined by about 33 per cent over the past 135 years, but much of that thinning occurred in the early part of that period, before the industrial revolution unleashed greenhouse pollution on a large scale.
In the more distant past, ice conditions were similar to those seen today.
That raises the possibility — but does not prove — that recent ice shrinkage could be part of a natural cycle, rather than the result of human-caused greenhouse emissions, Mr. Dick said.
"The evidence at the moment is fairly inconclusive."
"The fact is there are natural cycles in sea ice extent and we're not outside the range of those natural cycles at the moment."
He said natural climate cycles like ice ages are driven by the way the earth orbits and wobbles in its orbit and resulting changes in the amount of solar radiation reaching Earth.
If the current reduction in ice cover is part of a natural cycle, ice cover should soon start to grow again, said Mr. Dick.
"We've definitely lost a lot of sea ice over the past 20 years or so.
"If this is a natural cycle, then things should start returning to a more average condition, so we should see sea ice thickening up and extending further south.
"If we don't see that, if we see it continuing to thin and disappear, then in 10 years time we're pretty well going to be beyond the range of natural cycles we've seen up to now."
He emphasized that the study doesn't refute the theory of global warming, but points to the inadequacy of current climate models.
"Just to say, it was the same in the 1700s and therefore it's natural, doesn't follow. It's not necessarily wrong but it doesn't follow.
"What we need to understand is what these natural cycles are about and why they occur and if we could do that we could tell where in the natural cycle we were meant to be."
The World Wildlife Fund is publishing the sea-ice charts on CD-ROM for researchers around the world to use.
"I would say that in about 10 years time we'll know whether this is a human effect or not," said Mr. Dick.
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