Melting Ice - Part 4 - Sea Ice

Gosh, there is just so much ice to talk about, but now for one with a bit of a difference. In addition to Greenland, the Arctic Circle is also home to the Arctic sea ice - a huge blanket of ice that forms over the North Pole. And this is serious ice - up to 5 metres at it's thickest points.

 

 Average Arctic sea ice extent for March and September over the period 1979-2000 (NSIDC). The grey circle in the middle is where there is no satellite coverage. I'm gonna hazard a guess and suggest that there might be ice there too.

 

Similar to ice everywhere, sea ice varies naturally throughout the year. The Arctic sea ice sheet undergoes ablation in the summer and then accumulation in the winter culminating in a peak ice extent in March. Then, unlike Antarctica, because the Arctic Sea is mostly enclosed by land, it is more difficult for the ice to move southwards and melt. Instead it's a bit like dodgems - the ice floats around bumping into other bits of ice and gradually getting thicker. In some parts it become so thick that it doesn't even melt during the summer (NSIDC). Exciting times - a permanent ice rink at the top of the world.

 

 

But summer Arctic sea ice extent and thickness has been in decline since the 1970s, the average rate of decline of sea ice extent since 1979 has  been -4.1% a decade (Xia et al 2014). The average age of the ice left at the pole is also decreasing (NASA).  An ice free North Pole during the summer is becoming an increasing possibility. There is some debate as to the cause of sea ice decline - it could be part of the natural variability  and changes in atmospheric circulation or anthropogenic or a combination (Vihma, 2014).

Decline of Arctic sea ice extent since 1979. 2007 and 2012 were particularly bad years - remember that the thickness of the ice is important too, the ice is thinning at the same time as the extent is shrinking (NSIDC).

Overland et al (2011) used a selection of coupled ocean-atmosphere models to estimate that there would be ice free summers at the pole by 2050. But is very difficult to model the rate that the Arctic is declining, some models produce earlier dates, while others are more conservative. In general, models tend to underestimate sea ice extent decline, and we have observed much faster rates of decline than expected (IPCC). We've even started to see the direct implications of melting sea ice, with shipping routes through the Arctic Circle becoming possible for longer during the summer (TheGuardian). But how is this decline in thousands and thousands of square kilometres of ice affecting sea level?

 

 

Well, in a change to seemingly all other forms of ice, not really!

 

 

Because sea is is already in the water, even if it melts it will have little effect on sea level (NSIDC). It will have a very small effect because the ice in the sea ice is fresher and therefore slightly less dense than the ocean water surrounding it, but it is essentially negligible. I have been unable to find an actual estimate for the amount that sea ice melting will contribute to sea level - do let me know if you find one.

 

 

But even if melting all the sea ice won't have much of a direct effect on sea level, it is important in other ways:

• Firstly, albedo, ice has a much higher albedo than ocean water. Melting the sea ice will decrease the planetary albedo, allowing the Earth to absorb more radiation, so more warming will occur. Pistone et al (2014) used satellites to measure the radiation budget of the Arctic, their measurements suggest a decrease in Arctic planetary albedo from 0.52 to 0.48 from 1979 to 2011. So ultimately sea ice melting will help to promote sea level rise through  thermal expansion, and melting of other ice sources.
• Secondly, because the sea ice is fresher than the oceans, when it melts it will, in effect, be diluting the oceans. A freshening of water, particularly in the north where much of the deep ocean water is formed, could have serious implications for the ocean circulation. But as I mentioned in the Greenland post, I'm saving the effects of ocean circulation on sea level for another time.
• Thirdly, loss of the sea ice could result in an increase of heat transfer between from the oceans to the atmosphere, at the moment the ice sort of acts as insulation. Without there will be more warming of the atmospshere and so more ice melt and thermal expansion (Vihma, 2014).

So, even if we start to see ice free summers at the North Pole in the near future, they will not have much of a direct impact on the global mean sea level. But, as Tesco says 'Every little Helps' so I think although the direct impact of sea ice melting on sea level may sound insignificant, the indirect effects of the melting sea ice are really really important. It's definitely not one to be ignored.

To finish with, here is a little video that NASA made showing the variability of Arctic sea ice this year: