A closer look at how Arctic sea ice is changing
Scientists know from decades of measurements that climate change is causing Arctic sea ice to shrink. But why do some years lead to particularly low levels of ice while others stick to the long term downward trend more closely? A new study sheds some light on what could be driving dramatic year-to-year differences.
Rapid decline
Satellite data show Arctic sea ice is declining by about four per cent per decade. A recent estimate suggests between 70 and 95 per cent of Arctic sea ice loss in the past three decades is due to human-induced warming.
This long term declining trend is particularly noticeable in September, when sea ice reaches its seasonal low. In September 2012, Arctic sea ice reached its lowest extent since satellite records began in 1979.
But as a new study published today in Nature Climate Change shows, the summer minimum doesn’t get gradually lower each year. If it did, we might expect 2011 to be the second lowest on record – in fact it was 2007.
Image - Arctic _sea _ice _extent (note)
New record low Arctic sea ice extent, reached in September 2012, compared to the average summer minimum extent for the last 30 years in yellow. Source: NASA
Fluctuations
The new study found that on top of the long term declining trend, sea ice extent in September varies by several million square kilometres from year to year – sometimes up to 25 per cent of the long term average for the month.
Image - Arctic _sea _ice _decline (note)
Arctic sea-ice extent for September 1979 to 2010, with the trend marked by the black line. The new study defines low ice years as those below the bottom red dashed line and high ice years as those above the top red dashed line. Source: Kapsch et al., (2013)
So the long term decline is clear. But why do summer ice levels fluctuate from year to year? According to the research, in years when the summer ice cover is particularly low it’s due to a sequence of naturally-occurring events starting in Spring.
The study explains how a change in atmospheric circulation brings more humid air than usual into the Arctic region, causing more clouds to form. The extra clouds prevent heat escaping from the atmosphere back into space.
This effectively strengthens the greenhouse effect in the Arctic – driving up temperatures and enhancing ice melt. Lead researcher Marie-Luise Kapsch from Stockholm University tells us:
“Over the summer more solar radiation is then absorbed by the ice and the ice-free ocean water which further increases the melt … [This is] known as ice-albedo feedback”
The researchers calculated that changes in cloud cover can account for the observed differences in ice loss from year to year. But they say changes in wind forcing and ocean currents could also be having an effect, and it’s likely to be a combination of all three. The researchers say the process simply reverses in years with anomalously high ice cover.
Kapsch tells us that although climate change might impact the atmospheric circulations driving the changes in cloud cover, if and how is does so is not within the scope of the study.
Ice-free summers
Pinpointing when the Arctic could be sea ice free in summer is difficult, but most recent projections suggest it could be within the next few decades. As well as amplifying warming, there are many other consequences of sea ice loss. It might even affect atmospheric circulations like the jet stream, which influence weather systems in the UK.
It may be that we don’t need an ice free Arctic in summer to start feeling the effects. But getting a better handle on how natural variability affects the rate of ice loss on top of the long term decline will certainly provide a more accurate picture of how ice cover is changing.