Why isn’t the Arctic sea ice free already?
23 September, 2014
Diminishing Arctic sea ice is perhaps the most iconic impact of climate change. There's a good chance that before too long we'll lose it altogether in summer unless we cut emissions, scientists say.
But according to polar scientist Dr Dirk Notz, the question isn't why is Arctic sea ice melting so fast, it's how come we have any left at all?
Notz told a Royal Society conference in London this morning that with the additional heat earth is absorbing Arctic sea ice "should be long gone by now".
So why isn't it?
Arctic sea ice cover is declining by about four per cent per decade. The seasonal low at the height of summer is shrinking particularly quickly, at more like 11.5 per cent per decade.
The National Snow and Ice Data Centre announced this year's Arctic sea ice minimum on the 17th September. At 5.02 million square kilometers, it was the 6th lowest on record. Orange line is 1981 to 2010 average extent for that day. Source: NSIDCThe National Snow and Ice Data Centre announced this year's Arctic sea ice minimum on the 17th September. At 5.02 million square kilometers, it was the 6th lowest on record. Orange line is 1981 to 2010 average extent for that day. Source: NSIDC
Dirk Notz, a polar scientist at the Max Planck Institute for Meteorology in Germany, told today's conference that if you consider how much heat the Arctic is absorbing, it should be more than enough to melt the sea ice completely.
Globally, we've already seen 0.85 degrees of warming since 1850. But temperatures are rising twice as fast in the Arctic than in the rest of the world, with the region warming about two degrees Celsius over the same period.
Accelerating ice loss
In the time since satellites started monitoring the region 30 years ago, we've seen at least enough heat absorbed in the Arctic to make all the sea ice in the Arctic a whole metre thinner, Notz calculates.
And as soon as sea ice starts to thin by that much, a number of processes kick in to accelerate the pace of ice loss even more, he says.
Thinner sea ice is weak and easier for passing storms to break up, for example. And as sea ice retreats, sunlight that would have been reflected is absorbed by open water instead. The less sunlight is reflected, the more heat the ocean absorbs, and the faster sea ice melts.
Scientist know both these processes are happening. Arctic sea ice is thinning and the region is reflecting less heat back into space. In theory, these processes working together should accelerate ice loss until there's none left to melt, Notz says:
"With [these factors] and nothing else, Arctic sea ice melt should be much faster. There shouldn't be any sea ice in the Arctic"
But that hasn't happened, which means something else must be going on.
On top of the overall decline in sea ice over the last few decades, the amount we see from one year to the next varies a lot. You can see this in the spikes and troughs in the graph below of the decline in sea ice extent over the past 30 years.
Scientists are seeing something unexpected in the way Arctic sea ice behaves year to year.
After a year with a big drop in sea ice - such as 2007 and 2012 - the amount of sea ice seems to be swiftly replaced the following year. As Notz puts it:
"Sea ice in one year is closely connected to sea ice the year after … Strong loss in one year is followed by a recovery".
This suggests there's something going on to stabilise the sea ice system each time it drops particularly low, or rises particularly high.
Notz says the likely explanation is that when sea ice disappears, it's much easier for the surface of the ocean to kick heat out to the atmosphere, cooling the water down again to conditions that favour sea ice to form or grow thicker again. And thin ice grows much faster than thicker ice, Notz says.
A sea ice free Arctic
Such large natural variability makes it difficult to predict Arctic sea ice behaviour from one year to the next, says Notz. Natural variability is the main reason scientists see a mismatch between what climate models predict will happen and what happens in reality, especially over the relatively short period of time over which scientists have reliable satellite data.
But while Notz's research suggests Arctic sea ice may be more resilient than expected from one year to the next, the longer-term picture is clear.
If emissions stay high, almost all climate models predict the Arctic will become sea ice-free in summer by mid-century. But pinpointing a precise year is far more difficult, Notz says:
"Because it's a chaotic system, there will always be large uncertainty as to when Arctic sea ice will be gone in summer."
The prospect of sea ice-free summers has wide-ranging consequences. It would affect people and ecosystems, unlock new shipping routes and expose more fossil fuel reserves.
All the available information from climate models puts the likelihood that we'll be seeing sea ice free Arctic summers by 2040 at around 30 per cent, Notz estimates.
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