The state of the world’s sea ice is one of the most telling indicators of a warming planet. The news from the poles is not good. [20 September 2011 | Peter Boyer]
While Federal Parliament debates the merits of carbon pricing to deal with climate change, out there in the real world the climate really is changing. Nowhere is this more evident than in the cryosphere.
You may not have come across that word before. It’s worth remembering because it’s an increasing part of conversations about our climate future. The cryosphere is the frozen part of Earth’s surface, including ice on land — snow, permafrost, ice sheets and glaciers — and frozen sea.
Sea ice, the ice that floats on the sea’s surface around the North and South Poles, is currently in the news because it’s around now, at the end of the northern summer, that Arctic sea ice is at its annual minimum extent. And this 2011 summer is shaping up to be a record-breaker.
The growth and decay of sea ice is one of the world’s great seasonal events. The Arctic Ocean’s winter ice cover of 14 to 15 million square kilometres declines to around 7 million at the end of summer. In the Antarctic the shift is even more dramatic, from as much as 20 million square kilometres in winter to 4 million or less at the end of summer.
It wasn’t always like that. While evidence from the pre-satellite era is less reliable, a study of Arctic sea ice for an oil pipeline calculated that winter ice cover in the1950s was close to 16 million square kilometres, and the summer minimum didn’t get below 9.5 million.
In 2007 the minimum Arctic sea ice cover dropped to 5.5 million square kilometres, dramatically below all previous years’ records. It startled the world’s cryosphere scientists, many of whom confidently predicted that within decades the Arctic Ocean would experience ice-free summers.
They felt able to say so because of albedo — the well-understood phenomenon of light-coloured surfaces reflecting heat and dark-coloured ones absorbing it. Dark-coloured ocean waters absorb far more heat than white ice, so an increasingly ice-free Arctic Ocean will add to background warming to increase the rate of ice melt.
The record 2007 minimum resulted only in part from rising surface temperatures. Other causes were a coincidence of weather events, including cloudless skies and strong winds that compacted the sea ice edge to reduce the overall area.
Which brings us to this year’s northern summer, just finished. There’s usually a lag before the cooling autumn air begins to have an impact on sea ice, but by about this time each year it’s beginning to re-form ahead of winter. Most indicators suggest that 2011 will at least match 2007 as the year of lowest Arctic ice cover.
Sea ice specialists at the University of Bremen in Germany calculated 2007 cover at 4.267 million square kilometres (well below the widely-accepted estimate, 5.5 million, by the US National Snow and Ice Data Center). This year they say the ice cover reached a historic low of 4.24 million square kilometres on September 8, allowing simultaneous use of both the North West (Canada) and the North East (Siberia) Arctic routes by commercial ships.
But area is only one sea-ice measure. A more telling one is volume, and a 2009 study found that over about three decades Arctic sea ice thickness had decreased by nearly half, from 3.64 metres in 1980 to only 1.89 metres in 2008.
Antarctica is at first sight a very different story. The average area of sea ice throughout the year is seems to have changed little over the years, and summer minimum coverage is currently higher, though not by much, than it was 30 years ago.
The differences can be explained by the radical contrast between a marine North Pole and a continental South Pole and the well-understood climate processes resulting from this, according to Dr Rob Massom and Dr Jan Lieser, of the Antarctic Climate and Ecosystems research centre at the University of Tasmania.
As they explain, the small overall increase in Antarctic sea ice area reflects a strengthening in atmospheric circulation around Antarctica caused by decreased stratospheric ozone (the ozone hole) and — you guessed it — rising greenhouse gas levels. But research is also indicating that, like the Arctic, southern sea ice as a whole is steadily getting thinner.
Then there’s regional variation. Parts of Antarctica are warming rapidly, especially the Antarctic Peninsula region south of South America. Massom, Lieser and others are tracking a big multi-year decline in sea ice along the Peninsula’s west coast and the nearby West Antarctic coast, caused by persistent northerly winds.
While decreasing sea ice will have no impact on sea levels (they are affected only by loss of land-based ice, which I’ll address in coming weeks), it has big implications not just as an indicator of surface temperature trends but also for life in polar waters.
Loss of polar sea ice affects not just large land animals — polar bears, seals and penguins — that rely on it as a platform from which to hunt for food, but also much smaller organisms at the base of the marine food chain that live in or under the ice. The large-scale loss of sea ice means a rapid decline in these myriad species, and that has implications for all marine life.
These are momentous times for the cryosphere. The changing state of our frozen polar seas is a significant guide to long-term climate shifts. If the politicians debating climate policy want a reality check (God knows they need it), they’d do well to take a good look at sea ice.
• 24 September: For the Planet (Hobart). This Moving Planet event, led by the global climate movement 350.org, begins at 10am with a bike (or skateboard/scooter) ride or walk from Cornelian Bay to to Mawson Pavilion on the Hobart waterfront, where there will be a rally from 10:45am- 12:00 noon. There will be speakers, stalls, the latest cycling technology and a ‘best decorated bike’ competition, and participants will form a human sign to support strong action on carbon emissions.