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Thursday, September 19, 2013

David Spratt: Is climate change already dangerous? Part II. Arctic sea ice

by David Spratt, Climate Code Red, September 19, 2013

Second in a series:  Arctic sea ice


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On 16 September 2012, Arctic sea-ice reached its minimum extent for the 2012 northern summer of 3.41 million square kilometres, the lowest seasonal minimum extent in the satellite record since 1979, and just half of the average area for the 1979–2000 period.  There was a loss of 11.83 million square kilometres of ice from the maximum extent on 20 March 2012.  This was the largest summer ice extent loss in the satellite record, more than one million square kilometres greater than in any previous year.

Two-thirds of the loss of sea-ice extent has happened in the 12 years since 2000, and the process appears to be accelerating.  From 1979 to 1983 in the Arctic, the sea ice summer minimum covered an average of just over 51% of the ocean.  It fell to just 24% of the Arctic ocean surface in 2012.

Not only does the sea ice cover a smaller area of ocean in summer, it is also thinning rapidly.  The sea-ice volume is now down to just one-fifth of what it was in 1979.  The PIOMAS project, which captures the process of sea-ice retreat far better than any other general climate models, finds a September 2012 minimum of 3,263 km3 of ice.  Contrasted with the figure of 16,855 km3 in 1979, more than 80% of ice volume has been lost.

Arctic sea-ice volume loss (based on PIOMAS)
It is now clear that the Arctic is heading quickly for summer periods free of sea ice.  A linear extrapolation of sea-ice mass loss suggests it may occur within a decade or so.  An exponential fit, which is a better fit for the current data, suggests it might occur within a few years . At time of publication, the minimum volume figure for 2013 was not available, but it may be a little higher than the record low of 2012, and similar to 2011.

Because climate models generally have been poor at dealing with Arctic sea-ice retreat , expert elicitations play a key role in considering whether the Arctic has passed a very significant and “dangerous” tipping point.  Here’s what leading figures in the research field say:
PIOMAS Arctic sea ice annual minimum volume (black) plus “best fit” trend (red)
  • Dr Tim Lenton of the University of Exeter told the March 2012 Planet Under Pressure
conference that sea ice since 2007 had departed from model predictions, and that disappearance of Arctic sea ice has crossed a “tipping point” that could soon make ice-free summers a regular feature across most of the Arctic Ocean.  This conclusion was drawn from a subsequently published paper  which finds that “an abrupt and persistent increase in the amplitude of the seasonal Arctic sea-ice cover in 2007 which we describe as a (non-bifurcation) ‘tipping point.’ ”  If 2007 is the crucial point on the Arctic sea-ice decline timeline, it is also important to note that global warming above pre-industrial was 0.76 ºC at that time. At equilibrium, a 0.76 ºC rise is equivalent to CO2 levels of 335 ppm, so the “safe boundary” of 350 ppm already looks too optimistic from this perspective.
  • The Australian Climate Commissioner, Professor Will Steffen, told The Age in September last year: “I’m pretty certain that we have now passed the tipping point for Arctic sea ice.”
  • Dr Seymour Laxon, of the Centre for Polar Observation and Modelling at University College London, says: “Preliminary analysis of our data indicates that the rate of loss of sea-ice volume in summer in the Arctic may be far larger than we had previously suspected…  Very soon we may experience the iconic moment when, one day in the summer, we look at satellite images and see no sea-ice coverage in the Arctic, just open water.”
  • Professor Carlos Duarte, Director of University of WA’s Oceans Institute, says an Arctic “snowballing” situation would prove as hard to slow down as a runaway train.  He says melting of the ice is accelerating faster than any of the models could predict and the prospect of an Arctic Ocean free of ice had been brought forward to 2015, compared with a prediction in 2007 that at least one-third of the normal extent of sea ice would remain in summer in 2100.  Duarte says that the Arctic region is fast approaching a series of imminent “tipping points” which could trigger a domino effect of large-scale climate change across the entire planet with “major consequences for the future of humankind as climate change progresses.”
  • US National Snow and Ice Data Centre Director Dr Mark Serreze told Climate Progress in 2010: “I stand by my previous statements that the Arctic summer sea-ice cover is in a death spiral.  It’s not going to recover.”   Without human intervention to drive recovery, the evidence is very clear that Serreze is right.
  • Professor Peter Wadhams, of Cambridge University and the Catlin Arctic Survey, and a leading authority on the polar regions, concludes in a research paper: “Has Arctic sea ice reached a tipping point? I believe that it has...”
Wadhams explains:
I have been predicting [the collapse of sea ice in summer months] for many years.  The main cause is simply global warming: as the climate has warmed there has been less ice growth during the winter and more ice melt during the summer… in the end the summer melt overtook the winter growth such that the entire ice sheet melts or breaks up during the summer months.  This collapse, I predicted would occur in 2015–16 at which time the summer Arctic (August to September) would become ice-free.  The final collapse towards that state is now happening and will probably be completed by those dates.  As the sea ice retreats in summer the ocean warms up (to +7 ºC in 2011) and this warms the seabed too.  The continental shelves of the Arctic are composed of offshore permafrost, frozen sediment left over from the last ice age.  As the water warms, the permafrost melts and releases huge quantities of trapped methane, a very powerful greenhouse gas so this will give a big boost to global warming.
Wadhams’ analysis relies in part on a new, more specialised regional climate model, acronym NAME, developed by Dr Wieslaw Maslowski and colleagues. NAME is head and shoulders above other models so far in projecting and replicating sea-ice losses.“The future of Arctic sea ice” found that: “Given the estimated trend and the volume estimate for October–November of 2007 at less than 9,000 cubic kms, one can project that at this rate it would take only 9 more years or until 2016 +/-3 years to reach a nearly ice-free Arctic Ocean in summer.”

The impacts of lengthening periods of sea-ice-free Arctic summers are significant and will, together with warming already “in the system,” push more climate elements past their tipping points. Our knowledge is limited because “a system-level understanding of critical Arctic processes and feedbacks is still lacking” (Maslowski, Kinney et al.) and “no serious efforts have been made so far to identify and qualify the interactions between various tipping points” (Schellnhuber).

However, we do know that the Arctic is warming quicker than the global average.  Duarte, Lenton et al. find that: “Warming of the Arctic region is proceeding at three times the global average, and a new ‘Arctic rapid change’ climate pattern has been observed in the past decade.” Reductions in the sea-ice cover are believed to be the largest contributor toward Arctic amplification. Maslowski, Kinney et al. note that: “a warming Arctic climate appears to affect the rate of melt of the Greenland ice sheet, Northern Hemisphere permafrost sea-level rise, and global climate change.”

The sea-ice cover in June is about 2% of the earth’s surface.  Replacing that during summer in the Arctic with darker, more heat-absorbing ocean waters is equivalent to about 20 years of human greenhouse emissions, or about +0.5 ºC of warming, according to Peter Wadhams.  This is consistent with a study by Stephen Hudson, which found that, if the Arctic were ice-free for one month a year plus associated ice-extent decreases in other months, then, without taking cloud changes into account, the global impact would be about +0.2 ºC of warming.  If there were no ice at all during the main three months of sunlight, the increase would be +0.5 ºC.

The consequences of the Arctic big melt and the subsequent regional amplification and global temperature increase will include:
  • Accelerated melting of the Greenland ice sheet, very likely pushing it past its tipping point;
  • Pushing Arctic temperatures into a range that will trigger large-scale Arctic carbon store releases of methane and CO2, a positive feedback which will drive further warming;
  • Further destabilisation of the Jet Stream and hence more northern hemisphere extreme weather; and
  • The destruction of the Arctic ecosystem, which is already well under way. This has been chronicled by many researchers and organisations, including the Center for Biological Diversity and Care for the Wild International.  In the Arctic, the rate of climate change is now faster than ecosystems can adapt to naturally, and the fate of many Arctic marine ecosystems is clearly connected to that of the sea ice (Duarte, Lenton et al.). I remember well attending an Academy of Science conference in Canberra in May 2008 where the international guest speaker was Dr Neil Hamilton, then head of the WWF Arctic Programme. He told a somewhat stunned audience that the WWF was not trying to preserve the Arctic ecosystem because “it was no longer possible to do so.”  Whilst the campaign to stop the development of an oil and gas industry in the Arctic is necessary (if only to prevent more global warming emissions), the claim that in so doing we can thereby “save the Arctic” seems wide of the mark.

Greenland Ice Sheet

Complex, non-linear systems typically shift between alternative states in an abrupt, rather than a smooth manner, so it is often difficult to identify tipping points in advance. Only a few Arctic specialists, including Ted Scambos, Mark Serreze and Ron Lindsay, said prior to 2007 that the sea ice was close to a phase change.

If it is sometimes hard to see tipping points coming, it is also too late to be wise after the fact. And that is precisely the case with the Greenland Ice Sheet (GIS).

Current-generation climate models are not yet all that helpful on GIS. They have a poor understanding of the processes involved, and acceleration, retreat and thinning of outlet glaciers are not represented.

Recent research (next post) puts a lower boundary of 0.8 ºC on GIS’s tipping point, a warming level we have already reached.  In July 2013, a new study found that stretches of ice on the coasts of Antarctica and Greenland are at risk of rapidly cracking apart and falling into the ocean: “rapid iceberg discharge is possible in regions where highly crevassed glaciers are grounded deep beneath sea level, indicating portions of Greenland and Antarctica that may be vulnerable to rapid ice loss through catastrophic disintegration.”

In 2012, GIS melting shattered the seasonal record; the duration of GIS melting was the longest yet observed; a rare, nearly ice sheet-wide melt event (covering as much as 97% of the ice sheet’s surface on a single day) occurred in July; and the reflectivity of GIS, particularly at the high elevations that were involved in the mid-July melt event, declined to record lows. Unfortunately, data from the GRACE satellite observation of GIS is not yet of sufficient duration to robustly describe the melt trend, but observations are that the rate of melting is increasing, and many glaciers are picking up speed. Since 2001, the Jakobshavn Glacier, the world’s fastest flowing glacier, has more than doubled its flow rate, and total GIS mass loss in 2011 was 70% larger than the 2003–2009 average annual loss rate.

Previously, studies have estimated that it would take centuries to millennia for new climates to increase the temperature deep within ice sheets such as GIS. But a new study finds that when the influence of meltwater (which drains through cracks in an ice sheet and can warm the sheet from the inside, softening the ice and letting it flow faster) is considered, warming can occur within decades and produce rapid accelerations. Lead author Thomas Phillips says this research “could imply that ice sheets can discharge ice into the ocean far more rapidly than currently estimated,” thus requiring a re-assessment of the rate of both future sea-level rises and the rate of mass loss of GIS. 
Has Greenland passed its tipping point?  What would be the impact of a sea-ice-free Arctic summer and the consequent amplified regional warming on the stability of the Greenland ice sheet? Research does not yet provide a robust framework for considering such questions, yet most scientists if asked for their expert elicitation would probably say that it is hard to imagine the GIS doing anything other than actively de-glaciating at an accelerating rate and passing a critical tipping point in such circumstances.

NASA climate research chief Dr James Hansen answered this question in the affirmative, in a peer-reviewed paper in 2007:
Could the Greenland ice sheet survive if the Arctic were ice-free in summer and fall? It has been argued that not only is ice sheet survival unlikely, but its disintegration would be a wet process that can proceed rapidly. Thus an ice-free Arctic Ocean, because it may hasten melting of Greenland, may have implications for global sea level, as well as the regional environment, making Arctic climate change centrally relevant to definition of dangerous human interference.”
In the same year, Hansen said that today’s level of CO2 was enough to cause Arctic sea-ice cover and massive ice sheets such as in Greenland to eventually melt away: “I think in most of these cases, we have already reached the tipping point.”

And last year, Hansen told Bloomberg that: “Our greatest concern is that loss of Arctic sea ice creates a grave threat of passing two other tipping points – the potential instability of the Greenland ice sheet and methane hydrates… These latter two tipping points would have consequences that are practically irreversible on time scales of relevance to humanity.”

Glaciologist Jason Box told reporters at the annual conference of the American Geophysical Union last December: “In 2012 Greenland crossed a threshold where for the first time we saw complete surface melting at the highest elevations in what we used to call the dry snow zone… As Greenland crosses the threshold and starts really melting in the upper elevations, it really won’t recover from that unless the climate cools significantly for an extended period of time, which doesn’t seem very likely.”
Next post: Dangerous impacts from the current implied temperature rise


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