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Science briefing - Antarctic and Arctic sea ice

Science briefing - Antarctic and Arctic sea ice
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Sea ice plays an important role in regulating the Earth’s climate system. Over the last few decades, satellites have detected changes to the amount of sea ice that covers the polar oceans. Understanding these changes, and what they mean for our future climate, is a key focus for scientists.

Why is sea ice important?
1. Reflection: Sea ice reflects solar energy. White surfaces are better than dark surfaces at reflecting light. As a result, white snow can reflect as much as 90% of the Sun’s heat, but the darker ocean can absorb over 90% of that same radiation. So, reducing the amount of sea ice causes more of the Sun’s energy to be absorbed by the ocean, potentially leading to warming and a further reduction in sea ice. This process is called the ice-albedo feedback mechanism.
2. Keeping a lid on the ocean: If the sea-ice cover is removed then the ocean and atmosphere are free to interact, allowing heat and gases like CO2 to be transferred much more easily between the two. This ‘lid’ of sea ice is important for a healthy polar ecosystem because organisms, such as plankton and krill, have adapted to these specialised conditions.
3. Ocean circulation: When seawater freezes the resultant sea ice is relatively fresh, and the additional salt is released as brine into the ocean, making the remaining ocean more salty. Increased levels of salt in seawater make the water denser causing it to sink, which helps to drive the circulation of the world’s oceans. Changes in the growth of sea ice could therefore affect the circulation of our oceans, altering an important component of the world’s climate system.

What is the difference between Arctic sea ice and Antarctic sea ice?
Sea ice at both poles is formed from the freezing of sea water. However geographical difference creates differences in the growth and melt processes. The Arctic is an ocean surrounded by land whilst the Antarctic is an ice-covered continent surrounded by oceans. Arctic sea ice is thus generally more confined than sea ice around Antarctica. Sea ice movement around the Arctic basin tends to be limited, with ice floes piling into one another creating thick ice which tends to stay frozen longer during the summer melt than sea ice in Antarctica.

The open ocean surrounding Antarctica allows the forming sea ice to drift more freely and quickly. Almost all of the sea ice that forms during the Antarctic winter melts during the summer.

Is sea ice decreasing globally?
Yes. Although the area of Antarctic sea ice has increased slightly in recent years, the loss in total area of sea ice in the Arctic outweighs that considerably. On average Arctic sea ice area has reduced by approximately 3.5 million km2 in the past 35 years, whereas Antarctic sea ice has increased by around 0.5 million km2 in the same time period. So – overall the world has lost approximately 3 million km2 in the past 35 years.

Is it good news that Antarctic sea ice is increasing?

Scientists do not fully understand the processes which affect Antarctic sea ice. The rate of decrease in sea ice extent observed on the western side of the Antarctic continent is similar to that seen in the Arctic. However, unlike the Arctic, this regional loss of sea ice cover is offset by a modest expansion around other parts of the continent. It is important for scientists to analyse the long-term records to determine the overall trends in sea-ice decrease and growth.

Recent research reveals a link between sea-ice extent and changes in the winds that blow around the Antarctic. It is not clear what has driven these changes, but a variety of causes such as natural variability, increasing greenhouse gases, or the effects of the Ozone Hole may be responsible. This issue is the subject of intense study by scientists from British Antarctic Survey, the National Oceanography Centre and the Met Office.

What is the Arctic sea-ice minimum day?
The summer melt season occurs between March and September. The Arctic sea-ice minimum marks the day – typically in September – when sea ice reaches its smallest extent at the end of the summer melt season.

In recent years the Arctic has experienced a longer melting season which has resulted in the sea-ice minimum occurring later. This change affects the amount of solar energy absorbed into the ocean and alters the Arctic’s overall energy balance.

In September 2013 the Arctic sea-ice area was greater than September 2012 – is this a sign of recovery?
Arctic sea ice has been decreasing in area and volume over time. Whilst it was seen to increase in extent during the 2013 summer it was still well below the long-term mean. The sea-ice area minimum for September 2013 was around 5 million km2 – 35 years ago there was typically 8 million km2 of sea ice every summer. The increase in area between 2012 and 2013 is well within the limits of natural year-to-year variability and does not necessarily mark the start of a long-term recovery.

Does sea ice add to sea-level rise when it melts?
No – as it is frozen sea water it has no real effect on sea level. When sea ice freezes it floats on top of the ocean, causing no change. Then when it melts back to its original state sea level remains the same.

The ice that contributes to sea-level rise originates from melted ice from glaciers and ice sheets like those of Greenland and Antarctica. That is because that freshwater ice has formed from snow that has fallen on land so when it melts and flows into the sea it adds its volume to the oceans. Changes in Antarctic sea ice impact on water column structure and ocean circulation that can then affect basal melting of ice shelves.

What is the difference between sea-ice extent, thickness and volume?

1. Sea-ice extent is the area of ocean covered by sea ice.
2. Sea-ice thickness is how thick the sea ice is. Young ice – called first-year ice – is that which has not survived through a summer melt season yet. It is relatively thin, usually less than 1.5m thick. Multi-year ice has survived at least one melt season and is around 2.5m thick.
3. Sea-ice volume is effectively the area times the average thickness, giving you a better overall gauge of the relative health of the sea ice. Scientists have observed that the overall volume of Arctic sea ice has decreased in recent decades.

Do climate models accurately account for ice loss and gain?
No model is perfect but our understanding of Arctic sea ice is quite robust. Climate model simulations carried out as part of the Intergovernmental Panel on Climate Change 5th Assessment Report reproduce the observed rapid decline in Arctic sea ice quite well. However, the same models do not reproduce the pattern of regional gains and losses that lead to the overall increase in Antarctic sea ice. Increasing our understanding of Antarctic sea ice processes and hence improving the performance of models is a focus of current research.

FACTFILE

  • Polar ice sheets and glaciers are formed from snow that falls on land. Icebergs break off from glaciers and fall into the ocean.
  • Sea ice is the thin layer of frozen ocean water that forms and grows during the winter and melts in summer.
  • Sea ice that lasts through more than one summer without melting is known as multi-year ice.
  • About 15% of the world’s oceans are covered by sea ice during part of the year.
  • The constant turbulence of ocean water creates various forms and shapes of sea ice – with names like frazil ice, pancake ice, brash ice
  • Ice shelves are the floating extensions of grounded ice. They can be hundreds of metres thick, whilst sea ice is only a few metres thick.

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