The eight lowest measurements of Arctic summer sea-ice extent occurred in the last eight years, scientists confirmed today.
The findings were presented by Professor Julienne Stroeve from the National Snow & Ice Data Centre (NSIDC) at a Royal Society conference on Arctic sea ice reduction.
On the 17th September satellites recorded the Arctic summer minimum extent at 5.01 million square kilometers (sq km). Stroeve confirmed that this year’s summer sea-ice extent is the sixth lowest on record, in a series of satellite measurments stretching back over thirty years.
Mid to late September marks the end of the Arctic summer, and the point when Arctic ice is at its smallest extent, before it freezes up again as temperatures fall in the autumn.
Measurements of sea ice taken over the past decades suggest the rate of sea-ice loss is accelerating.
Between 1979 and 1996 Arctic sea ice declined at around 36,000 sq km a year, on average. Since 1997, the rate of loss has accelerated to dramatically, to 130,000 sq km per year.
The two trends are statistically different from each other, which means there is less than five per cent chance the change has happened by chance. “We can argue that in the last several years there is an accelerated rate of decline,” Stroeve says.
She also says there’s a clear link to rising temperatures. While sea ice conditions vary a lot from one year to the next, the summers with the warmest summer temperatures have seen the lowest sea ice extents:
“If you look at the last two minimum [Arctic sea-ice] low years – 2007 and 2012 – especially 2007 it was very warm… 2007 is the warmest summer we’ve had.”
Scientists have also found that the amount of older, thicker ice is diminishing. Stroeve’s research finds a reduction in the amount of ice at least five years old.
Usually around 90 per cent of old ice persists through the summer melt season and into the winter. In recent years this has dropped to around 70 percent, she says.
This is important because old ice will be replaced by new, thinner ice as the sea refreezes in winter. Thin ice is more susceptible to being broken up by storms and will melt more easily the following year, say scientists. Only around 30 per cent of first-year ice survives the average summer.
During the 1980s or 1990s, in an average year, around 54 to 58 per cent of ice in the Arctic would be first-year ice. Last year it was 77 per cent.
Another important factor in monitoring changes in the Arctic is the length of the melt season. Ice melt is starting earlier and lasting longer, says Stroeve.
In particular, scientists have found a delay in when the sea-ice starts to freeze again after the summer. For example, in some areas of the Beaufort Sea north of Canada and Alaska, the sea isn’t freezing again until almost two months later than usual.
Stroeve points to warmer sea surface temperatures as the likely cause. While this might not come as a surprise, this could suggest a link between the sea surface temperature at an earlier point in the year and the length of the meltwater season.
This could allow scientists to predict when the melt season is going to end and when the ice would start to freeze again.
A later session at the conference examined the current state of Antarctic sea ice, that we’ll be covering tomorrow.