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Cryospheric Sciences

Arctic

Image of the week — The warming effect of the decline of Arctic Sea Ice

Image of the week — The warming effect of the decline of Arctic Sea Ice

One of the most dramatic signals of Earth’s recent warming has been the precipitous decline of the Arctic sea ice. While the sea-ice decline is in response to warming ocean and atmosphere, it also has an important feed-back on the climate itself.

Solar radiation and albedo

Earth’s main energy source is solar radiation. This solar radiation is either absorbed in the atmosphere or at the surface of the planet, or it is reflected back into space. The measure of how reflective a surface is is called its albedo. Sea ice, being white, has an extremely high albedo, compared to the ocean. Therefore when the sea ice declines, more solar radiation is absorbed by the planet, leading to more warming.

Shrinking sea ice decreases the albedo of the Arctic

Our image of the week shows a figure based on data presented in Pistone et al (2014), produced by the NASA/GSFC Scientific Visualization Studio. It shows how the declining Arctic sea ice has decreased the average albedo in the Arctic, with darker colors indicating a declining albedo and therefore warming of the Arctic Ocean. Based on satellite estimates of the sea-ice extent since 1979, the authors were able to constrain that the impact of the sea-ice retreat alone has led to an amount of global warming that is more than 1/4 as strong as the effect due to increased C02 in the atmosphere. 

An animation of the annual Arctic sea ice minimum with a graph overlay showing the area of the minimum sea ice in millions of square kilometres.(Credit: NASA/GSFC Scientific Visualization Studio)

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Image of the Week – See How Seasonal Sea Ice Decline Differs!

Image of the Week – See How Seasonal Sea Ice Decline Differs!

Why do we care about sea ice in the first place?

  • Sea ice is important for several components of the climate system.
  • Due to its high albedo, sea ice reflects a high amount of the incoming solar radiation and is therefore relevant for the Earth’s energy budget.
  • Sea ice inhibits the exchange of heat, moisture and momentum between ocean and atmosphere, which usually occur at the sea surface.
  • Where sea ice forms, it releases heat and salt. When sea ice melts, it takes up heat and reduces the salinity of the surrounding water. Sea ice therefore redistributes heat and freshwater.
  • Sea ice provides habitat for plants and animals and hunting grounds for animals and indigenous populations.
  • Sea ice is an obstacle for shorter commercial shipping routes through the Arctic and oil and gas drilling.

The Arctic sea-ice cover is decreasing!

In recent decades, the Arctic sea-ice cover has been retreating rapidly. As we care about sea ice (see above!), scientists have been trying to understand this decline and to define a time span over which the sea-ice cover is expected to totally disappear (usually below 1 million km²). Up to now, research has mostly focused on the Arctic summer sea-ice cover, as this is expected to disappear much sooner than the winter cover. However, it is also of interest how winter sea-ice cover will evolve in the future and has evolved in the past.

What is meant by summer and winter sea-ice cover?

The Arctic sea-ice area follows a seasonal cycle with a maximum in late winter and a minimum in late summer (see figure below).

Figure 2: Arctic sea-ice concentration climatology from 1981-2010, at the approximate seasonal maximum (late winter) and minimum (late summer) levels based on passive microwave satellite data. (Credit : National Snow & Ice Data Center )

Figure 2: Arctic sea-ice concentration climatology from 1981-2010, at the approximate seasonal maximum (late winter) and minimum (late summer) levels based on passive microwave satellite data. (Credit : National Snow & Ice Data Center )

So, what about our Image of the Week?

In their study, Bathiany et al. (2016) compare the characteristics of the summer and winter sea-ice loss in the Arctic in general circulation models (GCMs). They investigate the changes in sea-ice area as a function of global annual mean surface air temperature. Summer sea-ice area (see red points) declines more linearly, “with no or a less pronounced change in slope”. Winter sea-ice area (see blue points), however, declines slowly at first and then more abruptly (this can still mean several years to decades, depending on the projection scenario used!). This abrupt decrease starts when ice volume is already very small.

How can this be?

Summer sea ice is distributed very heterogeneously over the Arctic, with very thick ice north of Greenland and Canada. It takes a given time (several years) until the thick multiyear ice (ice that has not melted during the previous summer) has melted. There can therefore still be ice in one location of the Arctic, while the rest of the area is ice-free. When these big “bunks” of ice have melted, then the summer sea-ice cover is gone. Large-scale abrupt shifts in sea ice therefore cannot occur in summer.

Winter sea ice, however, forms very homogeneously over the whole Arctic basin, when the ocean reaches the freezing temperature (the ocean temperature is relatively homogeneous over the basin). Warmer conditions in winter inhibit the growth of multiyear ice but a thin cover will always form on top of the ocean if the water is cold enough even if the ice melted in summer. Therefore, the sea-ice thickness and sea-ice volume decrease whereas the sea-ice area stays relatively constant and can still cover large areas (where the ocean is cold enough for ice to form). When the ocean does not reach the freezing temperature in winter, a large area of sea ice does not form any more and the sea-ice area declines abruptly.

What is the take-home message?

The explanation for the different behaviours in the retreat of summer and winter sea-ice is quite simple: the summer sea-ice cover disappears when all summer sea-ice has melted. The winter sea-ice cover disappears when no new ice forms in winter. As ice formation and ice melting are different processes governed by different mechanisms, the behaviour of the ice decline is different in both cases.

Note: These results are only relevant for the Arctic sea-ice cover as the Antarctic sea-ice cover is governed by different processes.

Further Reading

Acknowledgement: Thanks a lot to Sebastian Bathiany, who took the time to make sure I understood his paper well and helped me to make this blog entry understandable 🙂

Edited by Emma Smith and Sophie Berger

 

 

Image of the Week — Happy ValentICE’s day

Image of the Week — Happy ValentICE’s day

On the eve of 14 February, love and little hearts are everywhere, even trapped in lake ice!
The EGU Cryosphere blog team wishes you a happy Valentine’s Day 🙂

Behind this nice picture, there is also science

This picture was taken during a laboratory experiment that aimed to reproduce the bubbles observed in Arctic lake ice in the winter.

In this shot, we can see two types of gas bubbles in the ice. The elongated vertical bubbles are formed after the exsolution of gas at the water-ice interface. The gas present in “heart-shaped” bubbles originates from ebullition (i.e. it has been emitted as bubbles from the sediment) and it contains a large amount of methane, a significant greenhouse gas. In both cases, the gas is trapped in the ice during the downward evolution* of the freezing front but the shape and gas content of the bubbles largely depends on the velocity of the freezing front development.

The goal of this research is to better understand the origin of the methane emitted by Artic lakes and unravel the role of lake ice cover on the methane atmospheric burden.

*During the winter, the cold atmosphere cools down the water of the lake, when the freezing point is reached, a thin layer of layer of lake ice starts to form at the surface and extends downward.

Further reading/Reference

Boereboom, T., Depoorter, M., Coppens, S., and Tison, J.-L.: Gas properties of winter lake ice in Northern Sweden: implication for carbon gas release, Biogeosciences, 9, 827-838, doi:10.5194/bg-9-827-2012, 2012.

Sapart, C. J et al (in preparation).

Image of the Week — Future Decline of sea-ice extent in the Arctic (from IPCC)

Image of the Week — Future Decline of sea-ice extent in the Arctic (from IPCC)

The Arctic sea-ice extent has declined in the past 20 years and its future is uncertain. In the end, greenhouse gas emissions will determine the impact on the sea-ice from man-made climate change through radiative forcing (i.e. Representative Concentration Pathways or RCPs). The COP21 can determine the path we will follow and which course we will take to reduce emissions.

Reduction in sea-ice cover ranges from 43% (RCP 2.6) to 94% (RCP 8.5) in the period 2081-2100 compared to 1986-2005.

Why is sea important?

Decrease in sea-ice extent would:
– decrease the albedo of the Arctic ocean, therefore more heat would be absorbed by the ocean which would enhance the warming in this region.
– affect the global oceanic circulation as sea-ice formation influences the density of ice masses, which drives oceanic circulation.
– completely alter the ecosystem in the Arctic.

 

Further Reading

Stocker, T F, D Qin, G.-K. Plattner, L V Alexander, S K Allen, N L Bindoff, F.-M. Bréon, et al. 2013. “Technical Summary.” In Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, edited by T F Stocker, D Qin, G.-K. Plattner, M Tignor, S K Allen, J Boschung, A Nauels, Y Xia, V Bex, and P M Midgley, 33–115. Cambridge, United Kingdom and New York, NY, USA: Cambridge University Press. doi:10.1017/CBO9781107415324.005.

Read about sea ice and its importance on the NSIDC website

 

Previous blog posts featuring sea-ice science:

Do beers go stale in the Arctic?

Cruising for mud sediments from the ocean floor

Camping on the Svalbard coast

Image of the Week: Under the sea