Sea ice is often used as an indicator for man-made climate change, so find out what it is and why it is important

This article is more than 11 years old

This article is more than 11 years old

What is sea ice?

Frozen sea water. Unlike the mighty ice sheets and glaciers that rest on the bedrock of Greenland and Antarctica, sea ice forms and melts on the surface of the ocean. The amount of sea ice waxes and wanes with the seasons. It forms in the winter and melts in the summer.

Where is it found?

In the deep south and the far north. Only the freezing conditions of a polar winter are harsh enough to freeze seawater on a large scale. Sea ice in the Arctic gets most attention because it covers the North Pole and much of the Arctic Ocean for miles around, even in the northern summer. Antarctic sea ice, along the continent's coast, tends to completely melt in the southern summer.

Why does it matter?

Giant swaths of sea ice in the Arctic help control the world's weather, as white ice reflects some 80% of sunlight. Sea ice also forms a natural barrier to navigation and trade in the frozen north, and generations of sailors have perished trying to find a way through. In recent years, changes in sea ice have become a powerful symbol of man-made climate change, not least because polar bears need the ocean to freeze to forage for food.

How is it measured?

The amount of ocean surface covered with sea ice has been measured by satellite since 1979. These images say little about ice thickness though, and for that information scientists have been forced to rely on patchy measurements taken by local people or Cold War submarines. In spring 2009, Arctic explorers led by Pen Hadow attempted to measure ice thickness during a trek across the ice on foot, but the effort was hampered by technical problems.

How is it changing?

Arctic sea ice seems to melt more each summer. While sea ice has always melted away in the spring, the speed and extent of the melt has accelerated in recent years. A series of record lows since 2000 peaked in summer 2007 when summer sea ice extent in the Arctic shrank to the lowest level recorded. Winter recovery of sea ice has also decreased.

What does that mean?

Shrinking sea ice is now commonly used to demonstrate the reality of man-made global warming, particularly in the absence of other visible short-term impacts. Scientists say the sharp acceleration in Arctic ice loss since 2000 could signal that the region has passed a tipping point, with man-made warming driving further ice loss as more summer heat is absorbed by open water. Other scientists caution that wind and currents can also influence ice loss from year to year, and that the influence of global warming can only be properly judged over a longer time frame. Dr Vicky Pope, head of climate change advice at the Met Office, has called on scientists and journalists to stop misleading the public with "claim and counter-claim".

What will happen in future?

It seems certain that the freezing darkness of the Arctic winter will continue to sustain sea ice, but the big question is when increased melting could cause the warming region to lose all of its ice in summer. Estimates range from 2070 to 2013, depending on whether experts believe the recent increased melt rate will continue.