In my previous post I used a graph showing the cumulative sum of interannual differences of September average NSIDC sea ice Extent. In the following copy of that graph blue (deltas) is the interannual difference, red is the running sum of those numbers.
The sum of the interannual differences isn't really a physical number in the sense that in no way do the differences between years accumulate like that. This is unlike volume where such differences do accumulate to produce the overall volume loss, indeed the plot of volume can be plotted on top of the running sum of interannual extent differences and the two track fairly closely.
The point of the running sum of inter-annual differences is merely to show that up to 1997 for each loss, on average there was a compensating gain in extent, so although there was a downward trend it was slight, with the series staying close to the zero axis. It was after 1997 that the losses began to dominate over gains such that the running sum sharply veered off the zero extent line and started to decline. Something happened around this time that lead to a change in behaviour from near balance (with only a slight decline in extent) to a marked decline trend post 1997.
Using this observation as a basis for splitting
The slope of the loss trend after 1997 is four times greater than before. Using the lower left equation to project extent forward, extent reaches zero in 2038, and drops below 1M km^2 (virtually sea ice free) in 2030. Omitting 2013 as an outlier and using only the years 1997 to 2012 gives a zero extent in 2033, and virtually sea ice free (less than 1M km^2) in 2026.
I've plotted the relationship between NSIDC Extent and PIOMAS volume for September. A power function has been used to relate the two because of the available simple functions this one converges to zero volume at the same time as zero extent.
The clustering found underlines that there is a clear relationship between extent and volume in the month of September, the month of minimum volume.Using a linear fit produces a similar R2 (0.8778), but is unphysical, giving 3.23M km^2 extent at zero volume. What this suggests to me is that as volume declines below about 2.5k km^3 we should see increasingly aggressive open water formation with attendant ice albedo feedback (low September extent and volumes are a result of low extent and volumes earlier in the summer). However it provides no information about the speed of progression along the fit curve, or even if the fit will continue to hold. That noted, due to considerations of thickness and resultant efficiency of open water formation, it is clear that the volume loss is driving the declines in extent that are seen in the NSIDC September extent, as argued in my previous post. The thinner ice is at the start of the melt season, the more open water will be formed by the end of the melt season, and as more open water forms so ice/ocean albedo feedback becomes stronger (white ice reflects more sunlight than darker ocean, as as the ice retreats the ocean gains more energy which can increase melt).
The reason I wanted to show that, and turn thoughts towards open water formation efficiency and the ice/ocean albedo feedback is that there is research using an earlier version of PIOMAS that shows what caused the volume loss in that version. The process shown is so fundamental, it also applies to the volume drop we have been seeing in PIOMAS, and as I will argue, this is probably the process behind the virtually simultaneous increase in loss rate shown in the NSIDC data set after 1997.
Lindsay & Zhang 2004 examine whether a tipping point was passed in the 1990s, but in studying that also account for what has caused the volume decline in the 'feedback' period, i.e. the strongest period of volume decline in the PIOMAS model data.
They identify three distinct phases as shown in the above graph, although the study runs the model from 1948 to 2003. The phases are as follows:
- Preconditioning. During this period, 1948 to approximately 1989, warming winter air temperatures reduced the thickness of undeformed ice while the thickness of ridged ice increased. Undeformed ice is that which has thickened thermodynamically, as heat is lost through the ice to the atmosphere new ice grows on the underside of ice floes. Critical to this ice formation is the heat flux through the ice which is set by the temperature difference between the ice/ocean boundary at the underside of the ice (freezing point) and the temperature of the atmosphere at the ice surface, changes in atmospheric temperature dictate thermodynamic freezing. Ridged (or deformed) ice thickens as mechanical compression of ice causes the ice to buckle under compression and form ridges. So as the air warmed over this early period it reduced the thickening of ice over winter by reducing heat flux through the ice and reducing growth of new ice.
- Trigger. (Approximately 1990 to 1995) During the 1990s the Arctic Oscillation and Pacific Decadal Oscillation lead to a shift in the strength and centre of action of the Beaufort Gyre, from 1990 this lead to larger amounts of open water in summer. Due to export of ice through the Fram Strait (as noted by other authors, e.g. Rigor et al) there is a large drop in deformed ice thickness as multi-year ice North of Greenland and the Canadian Arctic Archipelago (CAA) was reduced by large volumes exporting out of the Fram Strait. This period can be seen in the first graphic of this post as a succession of large interannual variations in NSIDC sea ice extent.
- Feedback. (Approximately 1996 onwards) Following the thinning of the pack in the Preconditioning and Trigger phases, the stage was set for ice albedo feedback to take effect. Ice was thin enough such that in the marginal oceans summer thinning was able to open up more open water and lower concentrations of ice, allowing heat gain within the open water (and the pack behind the ice edge - not stated in the paper). Lindsay & Zhang find that from 1988 to 2003 the loss of thickness due to ocean heat flux is about 2m, this is countered by infra-red emission of heat from freeze season open water and thin ice which accounts for a gain of about 2m thickness of ice. However the change in absorbed solar flux is equivalent to about 3m of ice loss, and as the model doesn't simulate yearly changes in insolation this must be due to albedo reducing as the sea ice recedes.
That the model shows ice albedo feedback is driving most of the volume decline should not be a surprise, nor should it be a surprise if the model results can be transferred to the real world, for some time it has been known as a basic factor in Arctic sea ice, e.g. Curry et al. Furthermore as found by other researchers ice/ocean albedo feedback is causing ocean warming, e.g. Perovitch et al, and ice albedo is falling, as discussed previously. Referring back to the first graphic and previous post, I think that the change seen in NSIDC Extent is evidence that the process found by Lindsay & Zhang in their model has been happening in reality. This is evidently a case of self-acceleration. That the incidence of loss years has increased in NSIDC Extent, such that the cumulative sum went negative at almost precisely the same time as the model showed ice albedo feedback driven self acceleration kicked in, seems too much of a coincidence to me.
Furthermore PIOMAS is validated against available observational data, Schweiger et al, Laxon et al (i.e. figure 3), and is able to reproduce large scale patterns of multi-year ice, see here. In my opinion, taking into account that corroboration and long term detailed data, PIOMAS is the best available proxy for sea ice volume. The above noted behaviour of NSIDC Extent and the coincident timing with the start of the 'feedback' period from Lindsay & Zhang 2004 adds to that opinion.
One counter intuitive issue is that the volume loss has mainly been coming from the thicker ice within the Central Arctic. The following graphic is for September PIOMAS volume based on gridded data.
Bitz & Roe show that because first year ice can grow back in a season, but multi-year ice takes years to grow back, in a declining ice pack the first year ice will appear to be largely stable, while the thickest oldest ice effectively has a long memory of losses. However ice/ocean albedo feedback hadn't penetrated into the middle of the pack in the early years of the Feedback period. A particular mechanism relevant here will have been the failure of the Beaufort Gyre Flywheel. The following graphic is amended from Cryosphere Today.
In past years ice moving in the Beaufort Gyre survived the summer, so was able to age (4 years typically) and return to the central Arctic via the transpolar drift (from Siberia to the CAA/Northern Greenland). With the lower summer extents in recent years, ice exported out from the Central Arctic region into Beaufort and Chuckchi has tended to melt out in summer, especially in a year like 2012.
Lindsay and Zhang find that in the recent feedback period internal thermodynamic process (ice albedo feedback) dominate the loss of volume, not external forcing (i.e. anthropogenic global warming, AGW). However the atmospheric data they use to drive the model is from NCEP/NCAR, this includes the signal of AGW because it is reanalysis data derived from observational data. So does Lindsay & Zhang really discount a role for AGW in Arctic sea ice loss?
Notz & Marotzke found no evidence of self acceleration in sea ice loss. However they looked at sea ice extent and concluded that any year of increased sea ice could have broken the trend in loss, therefore self-acceleration was not at work. They did not look at volume, where the decline has been staggering and it is clear why even a single year like 2013 is insufficient to break the trend of losses, the Arctic would need a succession of cold years to increase volume and start to rebuild the ice, bucking the trend and starting a recovery. But that is not going to happen.
Notz & Marotzke plotted scatter plots of sea ice extent and various factors that could be reasonably assumed to be affecting sea ice.
The only factor that shows the sort of grouping one might expect of a causal factor is CO2 concentration, the others including the AMO, don't cut it. What is going on is volume loss caused by self acceleration that is triggered, and continues to be enabled, by anthropogenic global warming (AGW).
The alternative view would require that the sort of loss of sea ice we are seeing happens from time to time. I might suggest that the 1930s warming didn't lead to the sort of precipitous crash in sea ice seen at present, that could countered by citing the following cooling that occurred.
But taking the long view, that what happening now is occurring merely as a coincidence with AGW, and that AGW has no causal role, such a stance becomes hard to maintain given sea ice throughout much of the history of civilisation (fig 3a of Kinnard et al).
Going back further there is no evidence of such low sea ice levels as at present since the Holocene Climatic Optimum, 6 to 8 thousand years ago. In the following graphic low sea ice concentration from various lines of evidence is shown in dark turquoise. The black line trace in the lower panel is a proxy for temperature from a Greenland ice sheet ice core. The blue trace in the lower panel is July insolation at 65degN (strength of sunlight).
So the last time that sea ice was in a worse state than at present was under summer sunlight that was about 10% stronger than at present.
Then there are the model results. Climate models only show sea ice decline with anthropogenic factors included in the model runs. In the following from Wang & Overland 2012 (fig2) coloured plots are for the sea ice simulations of seven different models with natural and anthropogenic forcings, the grey shaded plots are for simulations with natural forcings only.
Despite the different behaviour of the models and the differences between model designs the common behaviour is clear: Without anthropogenic forcing the ice does not decline.
So while Lindsay & Zhang state that internal thermodynamic processes dominate the volume loss from the mid 1990s to 2003, not external forcings. To take that as discounting external forcing would be wrong. The wider evidence supports the view that the self acceleration is happening against an enabling background of external anthropogenic forcing.
The Arctic sea ice volume loss is driven by anthropogenic forcing, but with the background of that forcing the sea ice is now in a state of self accelerated decline due to the ice albedo feedback. With ten more years of data and published science since Lindsay & Zhang was published in 2004, the position has become more clear, within a few decades at most we will almost certainly see the Arctic ocean in a state it hasn't been in for about 7000 years, virtually sea ice free at the end of the summer melt.
Bitz & Roe, A Mechanism for the high rate of thinning in the Arctic Ocean.
Curry et al, 1999, Sea ice albedo climate feedback mechanism.
Jakobsson et al, 2010, New insights on Arctic Quaternary climate variability from palaeo-records
and numerical modelling.
Kinnard et al, 2011, Reconstructed changes in Arctic sea ice over the past 1,450 years
Laxon et al, CryoSat-2 estimates of Arctic sea ice thickness and volume.
Lindsay & Zhang, 2004, The Thinning of Arctic Sea Ice, 1988–2003: Have We Passed a Tipping Point? Abstract - PDF should be available for free via that link.
Notz & Marotzke, 2012, Observations reveal external driver for Arctic sea-ice retreat.
Perovitch et al, 2007, Increasing solar heating of the Arctic Ocean and adjacent seas, 1979–2005: Attribution and role in the ice-albedo feedback.
Rigor et al, 2002, On the Response of Sea Ice to the Arctic Oscillation.
Schweiger et al, 2011, Uncertainty in Modeled Arctic Sea Ice Volume.
Wang & Overland, 2012, A sea ice free summer Arctic within 30 years? - CMIP5 Update.