Thinking about a fun bit of mutual dissing with the notorious Lubos on his site, it occurred to the old man that it isn’t necessarily obvious why we should be interested in Arctic sea ice levels; after all, they do change all the time, and its all a long way away: why should it matter?
First is the idea that the state of Arctic sea-ice is generally considered to be an early indicator of climate change. Since very few people would even pretend that the climate hasn’t been warming over recent decades, this is sort of a moot point; we know the climate is warming, so it doesn’t signify. Less frequently discussed is the notion that the level and rate of decline might be strongly correlated to the level and rate of global warming. If this is the case, we can use the trends in the Arctic to look ahead and, in concert with other measurements of temperature and climate changes, project forward to an expected rate of increase of global temperatures. But this is fraught with complications.
Second, is the matter of the ice-albedo feedback mechanism. The presence of ice over the ocean keeps the Arctic surface temperatures down; a loss of ice implies maybe 20-40C of local atmospheric warming. The reflectivity of the ice, especially when compared to the surrounding land or ocean, even compared to snow or areas of melt water, is very high, which means that less heat from solar radiation is ‘pumped’ into the global system. There is a lot of concern that the relationship between the albedo feedback and the Polar regions is such that a relatively small change in albedo values will produce a relatively large change in other conditions, which in turn will further effect the albedo, etc. etc…
Then there’s the matter of the relationship between prevailing conditions in the Arctic and sub-Arctic regions and the surrounding land masses and oceans. There appears to be a relatively strong connection between the state of the Arctic and the state of the North Atlantic Oscillation. This in turn has an effect on weather (and, longer term, climate) conditions right across Europe into Eurasia, and, possibly, the West Pacific. It has implications for the direction and strength of the jetstream, and the important positioning of the northern boundaries of the Hadley cells., which in turn influence how much atmospheric heat is lifted from the tropic to the temperate regions.
There are, as always, lots of other influences to take into consideration, but the role of the Arctic in Northern hemisphere conditions, and vice-versa, is, whilst not completely understood, at the least known to be significant.
Finally, and more uncertainly, there’s the matter of freshwater. This isn’t ‘fresh’ in the sense that river or lake water is; it is ocean water which is less salty (saline) than average sea water. For a while now, there’s been conjecture that large changes in the amount of freshwater leaving the Arctic Ocean via the Denmark/Fram Strait, and the Labrador Sea, will have some measurable impact on the circulation of the ocean in the North Atlantic, an important area for ocean circulation, as it is one of the key areas where water is ‘circulated’ downward into the deep ocean, and heat is thereby ‘lost’. An interrupted ocean circulation might lead to a more rapid heating of the ocean surface generally, and another feedback starts to kick in. For those in the UK and Northern Europe, this has the added concern of changes to weather and climate caused by a slowing of the circulation patterns which contribute to the relatively mild conditions we currently experience.
It is important to point out here that the reasons for the changes in Arctic sea ice levels are not entirely understood. There appears to be a link to changes in atmospheric temperatures globally, but this can, by a recent study, account for no more than 40% of the observed decline. Current thinking seems to be that 60% of the decline is related to a warming (and freshening) of the waters entering the Arctic, and the water beneath the ice in general, mainly via the Bering Straits, but also via the Denmark Strait. Influx of freshwater from the Siberian and Canadian/Alaskan river systems, connected with regional warming trends over these land masses, probably also has some effect, though this is probably relatively small by comparison.
It is also important to note that sea-ice levels have always changed, sometimes rapidly, and that not all of the factors effecting its relative long-term stability are necessarily known, so the situation might improve, especially if there is a noticeable drop in solar radiation over the coming years; this might reasonably be expected to encourage some recovery of levels. Against this, we need to look at the paper I linked to yesterday, which posits a strong link between ozone levels and local temperature trends; if this is correct, then this signal may well override the solar variability completely.
Hopefully, all of this will help you understand why we look at the sea ice.