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Summary and Conclusion

In summary, the decline in sea ice observed in the early twentieth century is not abnormal when considering past reductions which have been shown to oscillate, however it is the recent several decades decline that cannot be identified in the last couple of thousand years. Evidence of earlier oscillations and reductions is not of high enough resolution to make such a claim but there is no evidence in support of such a decline either. As orbital forcing and mechanisms acting today have been proven to cause a reduction alone the true magnitude of GHG loading on sea ice reduction is unknown however very evident. At current the GHG loading level, which is forecast to increase and the positive feedbacks operating in the process sea ice is surely to continue declining. All of the forecasts predict the seasonal disapearance of sea ice which has many negative impacts across many contexts.  

This study does not discuss political and economic agendas to decrease GHG emissions and ultimately reduce there abundance, but is aware. The models all adopt emissions scenarios with increasing GHG loadings; however the outcome and influence of aforementioned agendas is unknown.

This study posits that a tipping point has been reached and that sea ice will seasonally disappear. Even if GHG loading does stabilise or even decline, the rate at which it will do so coupled with the ice-albedo positive feedback it will be too late. Therefore also this study highlights the need for up to date models which take into account the GHG loading decline

Part 2: Sea ices' past, context for contemporary debate

From the geological data, reviewed by Polyak et al (2010) he concluded that the history of Arctic sea ice is closely linked with climate changes driven primarily by greenhouse and orbital forcing and associated feedbacks.  A link made when considering the Arctic amplification. Based on proxy records sea ice appeared as early as 47Ma, coinciding with a long term climatic cooling following the Paleocene-Eeocene Thermal Maximum which led to the formation of large ice sheets. At least 13-14Ma year round sea ice occurred in relation to the progression of the aforementioned cool period. This period has been described as similar to that of last decade. Sea ice then slowly declined until the onset of Quaternary glaciations in the Northern Hemisphere after about 3Ma. Quaternary interglacials and interstitials may have been seasonally ice free, as is the case at the moment. Evidence for the importance of precession as a determinant in sea ice extent has been established. Which also links into the previously mentioned alebedo feed back relationship. Orbital variation alone has been identified as not been a sole control revealing reductions as more complex than these variations alone. This is highlighted by high resolution Holocene proxies. There are many uncertainties as the proxies are sparse however several critical ideas can be carried forward furthering modern understanding of sea ice reduction.

Suborbital time scales and ice distributions varied in the Holocene, but no evidence has been found to suggest pan-Arctic fluctuations in sea ice extent. Oscillation in the extent have been identified, however reductions in the extent even close to what has been observed over the last several decades has not been experienced for at the least the last few thousand years. The magnitude and scale of contemporary ice loss can thus said to be anomalous in comparison submillenial evidence.

Part 1: Sea ices' past, methods and uses

Interpreting recent changes and modelling future changes in sea ice cover requires a longer term perspective. From which we can better understand the Arctic’s natural variability and response to external forcing over time. The past distribution of sea ice is recorded in sediments preserved on the sea floor and in archived deposits along the coasts. Indirect sea ice extent can also be inferred from terrestrial sources such as coastal vegetation and ice cores.

Marine sedimentary records are the most complete and spatially extensive records of past sea ice conditions and area affected physically, chemically and biologically by sea ice (Polyak et al, 2010). Thus allowing ice characteristic reconstruction, however generally sea ice deposition is low (Polyak et al, 2010) making short term characteristic are hard to capture. For example the September 2007 record minimum. Cores are preferably taken from deep sites as they are far less eroded by temporal erosion. Ice rafted sediment is the most direct proxy, which is the sediment that melts and drops out of the sea ice which has been transported from the site of entrainment. Other Marine proxies include skeletons of microscopic organisms in bottom sediments such as diatoms which may indicate the conditions of ice cover above the study site thus the climatic conditions can be inferred.

Costal records such as coastal sediments and coastal forms (eg. Plains, marine terraces) evidence of the past can be found. Change in sea level has exposed formerly marine environments and present easy opportunity for study. However such landforms only present a limited indication due to their limited time span. Information on past sea-surface and air temperatures, circulation and sea ice conditions can be obtained. Driftwood distribution can be used to infer the presence or absence of land fast ice. Ice is essential for the transport of driftwood, as it would otherwise become saturated, loose buoyancy, and sink after a year (Polyack et al, 2010). Whalebone and other mammal bones can be used to derive sea ice extents as such mammals have an affinity for sea ice. Seasonal migrations of certain species are known to follow breaks in the ice-pack and thus can be combined to give an indication of paleo sea ice distribution.

Terrestrial records including vegetation can be used to provide information on past environments for example past tree lines. Ice cores are taken to view more recent characteristics and the value for past climates is limited. So the overall goal is to identify key indicating chemicals (such as sea salt) in the composition which can then be used when viewing the aforementioned paleo sea ice proxies.

Historical records such as noted shipping routes have been compiled since 1870 (Walsh, 1978).

The Impacts of declining sea ice

The sea ice cover is part of a physical, biological, political and economic system so the impacts of declining sea ice are varied.

The impacts of sea ice loss are numerous and varied and have been summarised below, and I believe are mostly negative:

•   A Sharply warmer Arctic as a result of larger heat fluxes from ocean to atmosphere.
•  As sea ice declined winds have a larger fetch resulting in more wave action and therefore erosion. Eg in Alaska already.
•   Has affected traditional hunting techniques by indigenous cultures resulting in declining polar bear abundance.  
•   Potentially mid latitude storm tracks may be intensified resulting in increased European precipitation (Singarayer et al, 2006)
• The increased fresh water release may affect thermohaline circulation.
•   Increased total phytoplankton production in the Arctic (Pabi et al. 2008) which may have implication on marine ecosystem structuring.
• There has been an increased interest in shipping routes across the top of the world as the oceans are far easier to transverse.
•   Exploration of Arctic continental shelves for natural resources such as petroleum has been initiated.
• Tenure of economic zones had led to legal battles due to the Law of the Sea Treaty.
  ·         Changes in marine ecosystems.

The future of sea ice

The impact of GHGs on sea ice extent projections will be discussed and lead into the crux of this blog, sea ices’ future. Many authors review the models used in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) (Figure 1) when considering the role of GHG loading on sea ice extent. 

It is clear that the IPCC AR4 projections all exhibit a downward trend concerning sea ice extent. Zhang and Walsh (2006) showed that most of the models are within 20% of the observed sea ice extent over their adopted base period of 1979 to 1999 with good simulation of the seasonal cycle. Serreze et als (2007) extended bas period of 1979-2006 supports the aforementioned findings. For the past decade the September sea ice has been below the model ensemble mean and in the past several years more than 1 standard deviation lower. These results strongly posit that despite prominent contributions of natural variability, discussed in the previous blog, GHG loading has played a role. The models driven with the SRES A1B emissions scenario (CO2 reaches 720 ppm by 2100) show complete September sea ice  loss anywhere between 2040-well beyond 2100 (Stroeve et al, 2007). Note only the loss of summer sea ice is considered as winter sea ice is predicted to carry through this period (Perovitch and Menge, 2009). This large variability in model predictions can be attributed to factors such as simulated cloud conditions or natural variability in the model. Stroeve et al(2007) reasons that if the mean time series provides a true representation of forced change by GHG loading then 33-38% of the observed September trend from 1953-2006 is externally forced, growing to 47-57% from 1979-2006. Thus that the models underestimate the GHG response and the externally forced component may be larger. The importance of the ice-albedo feedback mechanisms across the models should be noted. Generally GHG loading resulting in higher and longer summer ocean ice melt. This results in more open water which has a lower albedo than ice (as it is darker) thus it absorbs solar radiation more readily. Perovich (2002) stated snow’s albedo is up to 85%, ice is up to 65% whereas melt ponds vary from 20-40%. The increased melting of sea ice resulting in rapidly increasing ice melt is a positive feedback.  This results in delayed ice growth in Autumn which leads to thinner ice during the next summer. However thinner ice grows more rapidly than thick ice when exposed to the same forcing is a negative feedback this potentially negating the positive feedback.