Satellite observations of Arctic sea ice in recent years have shown a tendency that was not entirely unexpected. Since the 1970s or thereabouts, the area covered by the ice pack has been decreasing at a rate of about 3 per cent per decade.
Moreover, submarines using sonar techniques have been able to monitor the thickness of the ice, and it has become clear that the ice is also getting thinner.
Taken as a whole, the data indicate a significant decrease in the volume of Arctic ice in the last 30 to 50 years.
Scientists involved in climate modelling were cock-a-hoop at this discovery, and said "We told you so!". Most of their models predict that the effects of global warming will be seen first in polar regions, and that they could be quite dramatic.
Indeed some of the models suggest there will be very little Arctic polar ice in summer late in the 21st century, and these results were seen as confirmation that global warming had begun to take effect.
Another team of scientists decided to look further back. There was little to go on, except that Russian aircraft had collected ice data during the 1930s to support Northern Sea Route navigation. There were also sporadic surface observations here and there from various expeditions to the polar regions.
Nonetheless, this tenuous information was sufficient to build a rough picture of the ebb and flow of the Arctic ice throughout the 20th century - and the conclusion was surprising.
It was found that during the 1920s and 1930s there was a marked decrease in polar ice compared to previous decades. The volume increased again until the 1970s, and now there was a recently observed reduction in Arctic ice. All this suggests the amount of ice to be found over polar regions may vary with time quite independently of global warming.
Nobody knows why this should be; it could be due to large-scale atmospheric oscillations of some kind, or perhaps to subtle variations in the solar output. But it makes the task of trying to detect the effects of global warming even more difficult than before.
Was the missing ice destined to disappear from the Arctic regions anyway? Is the recently observed decrease a result of global warming, or is it just part of a natural fluctuation of unknown duration which may reverse itself in some years' time?
Or are there two processes in operation, working against, or perhaps in tandem with, each other? As yet, we can only answer that we do not know.