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Friday, February 20, 2009

4th International Polar Year research, Damocles: Arctic sea ice will probably not recover

Arctic sea ice will probably not recover

Environmental Research Web, February 19, 2009

As predicted by all IPCC models, Arctic sea ice will most likely disappear during summers in the near future. However, it seems like this is going to happen much sooner than models predicted, as pointed out by recent observations and data reanalysis undertaken during IPY and the Damocles Integrated Project.

On February 25, 2009, there will be a celebration in Geneva, Switzerland to officially close the 4th IPY that started on March 1, 2007, in Paris, France. It is not a surprise that one of the main topics of this 4th IPY was climate change, since the polar regions play a very important role in Earth’s climate. This role is magnified by the combined effect of two main processes: one is due to the presence of greenhouse gases in the atmosphere trapping longwave solar radiation, which keeps our atmosphere warm, and the other, called albedo, is due to the capacity of the Earth’s surface to either reflect (in particular over ice and snow) or absorb (in particular over the ocean) incoming shortwave solar radiation.

Unprecedented events have been reported during the past 20 years in the Arctic Ocean, mostly related to the Arctic sea ice summer minimum extent that retreated in September 2007, far beyond previous extreme minimum records. This is the first clear evidence of a phenomenon of importance on a planetary scale, forced by global warming, mainly caused initially by an Earth energy imbalance due to greenhouse gas concentrations increasing in the atmosphere.

The Earth now absorbs 0.85 watts per square meter more energy from the sun than it emits into space, raising the likelihood of an acceleration of sea ice melting, ice sheet disintegration and a rise in sea levels (Hansen et al. 2005).

The European integrated project Damocles is one of the major programs of the International Polar Year (2007-2009). It is dedicated to the Development of Arctic Modelling and Observing Capabilities for Long-term Environmental Studies (DAMOCLES). Damocles started in December 2005 and will end in June 2010. Damocles was proposed and selected by the European Union in response for a call addressing the development of observing systems for predicting extreme climate events.

Damocles was based on the fact that Arctic sea ice was retreating and thinning at an alarming pace. Projecting a disappearance of Arctic sea ice during summer in the near future could be considered as an extreme climate event. During international polar years 2007 and 2008, Damocles experienced two extreme climate events. Arctic sea ice retreated during both IPY years by more than 1 million km2 compared to a previous extreme case occurring in September 2005 before Damocles and IPY began.

It is quite instructive to compare the last three years (2005, 2007 and 2008) and also to compare the past 20-30 years, the period during which the satellites observing the planet Earth have been in existence.

Over the past 30 years, we have observed a gradual, long-term warming, mostly characterized by milder winter freezing seasons and longer summer melting seasons, evidencing strong albedo positive feedback effects. Less ice means more sea water being exposed to short-wave solar radiation that will be absorbed and transformed into heat by the ocean melting more ice and so on. Strong positive feedback accelerates the melting of Arctic sea ice, especially due to the sharp contrast of the high albedo for sea-ice areas covered with snow (>0.8) which reflect 80% of the incoming solar radiation back into space, in contrast with the very low albedo (0.2) of the ocean, absorbing 80% of incoming solar radiation.

Although long-wave and short-wave downwards solar radiation agreed rather well between models and observations, one of the biggest uncertainties in Arctic climate simulations still remains how albedo effects are affected by cloud cover and aerosols (Arctic haze). Warming amplification in the Arctic resulting in sea-ice thinning and retreat might also be attributed partly to atmospheric circulation (Graversen et al. 2008) and oceanic circulation (Zhang et al. 1998, Polyakov et al. 2005, Dmitrenko et al. 2008), but this is still controversial. A drastic retreat of the sea-ice minimum extent in summer has inevitably profound consequences during the following fall season. At that time, all the heat taken up by the ocean has to be evacuated by the atmosphere, delaying the onset of freezing and consequently the amount of sea ice formed during the following winter. Observations taken during the past 20 years indicate that sea ice is becoming thinner, younger, moves faster and retreats more and more in summer. The sea-ice extent, ice thickness, ice drift and ice age are all interrelated parameters best characterizing Arctic sea-ice evolution, and it is remarkable to realize that all these parameters have changed radically.

Surprisingly, the 2007 Arctic sea ice event was largely unpredicted, even though extreme sea-ice conditions were observed almost every September each year over the past 10 years (Perovich et al. 1999, 2003, Serreze et al. 2003, Stroeve et al. 2005). Premises for an Arctic sea ice thinning and an Arctic ocean warming were reported nearly 20 years ago by Wadhams (1990) and Quadfasel (1991). So why did the 2007 Arctic summer sea ice minimum extent come as a complete surprise if it was not an exceptional and extraordinary event?

During the summer of 2005, we did not observe any replenishment of the multi-year ice (MYI) lost during the previous months by first-year ice (FYI) because either the FYI melted entirely or because the FYI escaped from the Arctic Ocean through the Fram Strait or both. Thus, at the end of the summer of 2005, there was almost no FYI left and, consequently, second year ice could not form. Only MYI remained in the Arctic Ocean in 2005, and this is the reason for an extreme Arctic sea ice extent minimum being reached during that particular year.

During the summer of 2007, we observed a similar situation but one which was even more extreme simply due to a cumulative effect characterized by no replenishment of MYI by FYI during previous years. This explained why MYI was so depleted, leading to an extreme situation in September 2007, with an all-time minimum absolute sea-ice extent record. It is quite important to understand precisely why in 2005 and 2007 all FYI disappeared and could not replenish any of the MYI as it has in the past during normal years. There are several potential reasons, e.g., a polar amplification of global warming, in particular in summer during the melting season, due to strong albedo positive feedback.

During the summer of 2008, we observed a drastic decrease of the Arctic MYI and an exceptional replacement of MYI by FYI. Because this FYI resisted the summer melt in 2008, the record minimum sea-ice extent 2007 was not reached in 2008. It is quite important to understand precisely why FYI in summer 2008 resisted both the summer melt and the transpolar drift through Fram Strait.

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