Arctic Ocean: fast forwarding the last 1,450 years

To date, the Arctic sea ice extent is more than 2 million sq km smaller than in the late 20th century. It's been observed that the late-summer ice cover, mainly composed of thick multi-year ice, has been shrinking at about 8.6% per decade, which is unprecedented in the last 1,450 years. 

Just to refresh your memory, loss of sea ice in the Arctic Ocean will have disastrous consequences on climate, ocean circulation (which will be expounded in this post), ecology (remember the polar bears) and economics of the Arctic (shipping lines, eskimos etc etc). Sea ice cover responses to temperature changes and is influenced by the atmosphere and the ocean. This study by Kinnard et al. confirms (as expected) that the recent decrease in summer Arctic sea ice is 'consistent with anthropogenically forced warming' (Kinnard et at., 2011) through charting Arctic sea ice changes in the last 1,450 years. 

Methodology

They took raw data from areas around the Arctic basin (circum-Arctic) (Fig. 1). In total, there were 69 proxies used.They were obtained from ice cores, tree-ring chronologies, lake sediments. Two historical series of sea ice observations were used to get an anthropogenic view . 

Ice core proxies were mainly stable isotope ratio of oxygen. It's an air temperature indicator amongst other uses. Data of sea ice openness and sea surface windiness was also obtained through measurements of sea salt ions concentrations amongst others. (Read the paper for more descriptions about the proxies and their uses)

Results

Coverage of the central Arctic is poor because they were unable to get any samples from there. Sea ice variability is the greatest in the marginal ice zone as expected. They found that the proxy-based reconstructed history of the late summer Arctic sea ice extent from AD 561 to 1995 showed that pronounced decline in summer Arctic sea ice cover that began in the late 20th century is 'unprecedented in both magnitude and duration when compared with the range of variability (of sea ice extent) of the previous 1,450 years'. The low ice extent observed since the mid-1990s is well below the range of natural variability when we study the reconstruction. 

The reconstructed history of the summer ice in the Arctic sea was also paralled in the Barents sea: low values at AD 1600 and an increase afterwards and a minimum in the late 20th century. (Figure 3)

While the Barents Sea and Chukchi Sea and Fram Strait show similarity in changes in ice extent with the Arctic Circle, North Iceland diverges slightly with a significantly lesser extent of sea ice before AD 1200 and an overall increase afterwards (Fig. 3e). The reason for sea ice along its coast during winter and spring is partly because of sea drifting out of the Arctic rather than it being formed there. 

Surprisingly, there seems to be a poor correlation between NAO index and the Arctic sea ice cover in the reconstructed history. Only in the 20th century, did the positive NAO index correspond with the pronounced warming in the 20th century. This result only confirms that while the NAO plays an important part influencing ice extent in the Arctic, it is a combination of it with other factors that ultimately causes the changes in ice extent. However, the reconstructed NAO indices are still severely limited in showing us the changes of meridional atmospheric circulation which could confirm the theory of the lesser extent of sea ice during the Little Ice Age compared to the Medieval Warm Optimum was caused solely due to enhanced southerly advection of warm air into the Arctic.

Figure 3h suggests that there was an increase in invasion of warm and saline water from the North Atlantic into the Arctic Ocean causing a decrease in sea ice cover in the Arctic Ocean.  

Conclusion

So basically, what this article is saying is that the Arctic sea ice cover during the pre-industrial period was probably influenced by the intrusion of warm Atlantic water into the Arctic Ocean. The reason for this periodic intrusion is probably just part of the ocean-atmosphere-sea ice dynamics. From figure 3, the clear decrease of Arctic ice extent in recent decades can be linked to increasing warming of the Arctic that hasn't been observed in the last 1,450 years. For now, the most possible reason for that is because the warming it's experiencing recently is anthropogenically forced. 

This study is quite well-rounded in terms of the data that they obtained to reconstruct the Arctic's history. They were able to study cores taken from areas around the Arctic basin that had significant co-relation to the Arctic itself so parallels could be made between them. 

However, we still need to know what's going on in central Arctic. We need to know if the center of the Arctic (which this study was unable to obtain data from) is unstable and vulnerable to melting because that would mean that the multi-year ice may be gone and the Arctic Ocean might be ice free in the summer sooner than expected. 

Reference:

Kinnard, C. et al (2011) 'Reconstructed changes in Arctic sea ice over the past 1,450 years', Nature, 479.