by Judith Curry
How should we interpret the record low minimum sea ice extent?
Summary of Observations
Here is the basic story from the observations:
On Aug 26, the Arctic Ocean seasonal minimum sea extent surpassed the previous satellite-era record minimum set in 2007 (4.17 M sq km). The current sea ice extent is below 4 M sq km, and apparently a minimum has not yet been reached, although a minimum around 3.5 M sq km seems likely.
The Arctic sea ice extent undergoes a pronounced annual cycle, with maximum extent of about 14 M sq km in March. In the 1980’s and 199o’s, sea ice minimum extent (in September) averaged about 6.5-7.5 M sq km.
The 2007 minimum ice extent was also associated with a new record in ice volume (which includes ice thickness). Minimum ice volume records were also set in 2010, 2011, and now 2012, with the current ice volume tracking slightly below the 2010/2011 values.
For background on this issues, see my previous posts:
- Pondering the Arctic Ocean. Part I: Climate dynamics
- Likely causes of the recent changes in Arctic sea ice
There are three main theories for the recent decline in sea ice extent and volume:
I. Greenhouse induced ‘spiral of death': Mark Serreze and Peter Wadhams are the primary proponents of a rapid spiral of death. The main idea is that melting triggers more melting through the ice albedo feedback mechanism. The evidence for this theory seems based on ‘trendology’ of observed sea ice extent and volume. Spiral of death proponents project faster melting than climate model projections. More moderate versions of this theory rely primarily on climate model projections of sea ice and acknowledge a significant amount of noise from interannual variability.
II. Natural variability only: Joe Bastardi is one of the chief proponents of this theory, see his recent article. This is basically an argument whereby the PDO and AMO set up ocean circulation patterns that are melting the sea ice.
III. Climate shifts hypothesis: JC is a proponent of this (see my previous post) new evolving theory. This hypothesis is based on the network synchronization ideas of Tsonis et al. and bifurcation analysis (e.g. Livina and Lenton). In the particular instance of the past 5 years of low summer sea ice, it seems that the shift in sea ice characteristics was induced by a combination of local factors and hemispheric indices.
Livina and Lenton have a new paper that lays out some of these ideas: A recent bifurcation in Arctic sea ice cover. From the abstract:
Here we show that a new low ice cover state has appeared from 2007 onwards, which is distinct from the normal state of seasonal sea ice variation, suggesting a bifurcation has occurred from one attractor to two. There was no robust early warning signal of critical slowing down prior to this bifurcation, consistent with it representing the appearance of a new ice cover state rather than the loss of stability of the existing state. The new low ice cover state has been sampled predominantly in summer-autumn and seasonal forcing combined with internal climate variability are likely responsible for triggering recent transitions between the two ice cover states. However, all early warning indicators show destabilization of the summer-autumn sea-ice since 2007. This suggests the new low ice cover state may be a transient feature and further abrupt changes in summer-autumn Arctic sea-ice cover could lie ahead; either reversion to the normal state or a yet larger ice loss.
Mike Wallace combines the regime shift idea with the spiral of death theory:
I view the question of whether the minimum sea ice extent sets a new record this year as secondary. The important news is that in five summers the sea ice extent over the Arctic Ocean has not recovered significantly from its precipitous decline in 2007. This is one of the clearest examples of what appears to be a regime shift in the recent historical record. I think we still need to be open to the possibility that natural variability has played a role in the recent warming of the Arctic and that the summer ice could come back, but with each year that goes by without a return to the pre-2007 summertime Arctic climatology it seems a bit more likely that the remarkable change that we have witnessed will prove to be irreversible on a human time scale.
- Thickness and compactness of sea ice at the beginning of the melt season: ice that starts out thinner is more easily melted away. Further, first year ice has different optical and thermodynamic characteristics than multi-year ice.
- Transport of ice through the Fram Strait (between Greenland and Europe), which depends on a combination of atmospheric and ocean circulation patterns
- Weather patterns that act to either break up or consolidate the ice
- Radiative forcing (which is dominated by the cloud patterns)
- Melting from below by warm ocean currents.
- Melting from above by warm atmospheric temperatures.
- Geographic distribution of the sea ice, which depends on a combination of all of the above