Increased growth in Antarctic sea ice during the past 30 years is a result of changing weather patterns caused by the ozone hole, according to new research.
Reporting in the journal Geophysical Research Letters, scientists from British Antarctic Survey (BAS) and NASA say that while there has been a dramatic loss of Arctic sea ice, Antarctic sea ice has increased by a small amount as a result of the ozone hole delaying the impact of greenhouse gas increases on the climate of the continent.
Sea ice plays a key role in the global environment – reflecting heat from the sun and providing a habitat for marine life. At both poles sea ice cover is at its minimum during summer. However, during the winter freeze in Antarctica this ice cover expands to an area roughly twice the size of Europe. Ranging in thickness from less than a metre to several metres, the ice insulates the warm ocean from the frigid atmosphere above. Satellite images show that since the 1970s the extent of Antarctic sea ice has increased at a rate of 100,000 square kilometres a decade.
The new research helps explain why observed changes in the amount of sea-ice cover are so different in both polar regions.
Lead author Professor John Turner of BAS says: “Our results show the complexity of climate change across the Earth. While there is increasing evidence that the loss of sea ice in the Arctic has occurred due to human activity, in the Antarctic human influence through the ozone hole has had the reverse effect and resulted in more ice. Although the ozone hole is in many ways holding back the effects of greenhouse gas increases on the Antarctic, this will not last, as we expect ozone levels to recover by the end of the 21st Century. By then there is likely to be around one third less Antarctic sea ice.”
Using satellite images of sea ice and computer models the scientists discovered that the ozone hole has strengthened surface winds around Antarctica and deepened the storms in the South Pacific area of the Southern Ocean that surrounds the continent. This resulted in greater flow of cold air over the Ross Sea (West Antarctica) leading to more ice production in this region.
The satellite data reveal the variation in sea ice cover around the entire Antarctic continent. Whilst there has been a small increase of sea ice during the autumn around the coast of East Antarctica, the largest changes are observed in West Antarctica. Sea ice has been lost to the west of the Antarctic Peninsula – a region that has warmed by almost 3ºC in the past 50 years. Further west sea ice cover over the Ross Sea has increased.
Turner continues: “Understanding how polar sea ice responds to global change – whether human induced or as part of a natural process – is really important if we are to make accurate predictions about the Earth’s future climate. This new research helps us solve some of the puzzle of why sea-ice is shrinking is some areas and growing in others.”
Floating sea ice caps the ocean around the Antarctic and although it is mostly only 1-2 m thick, it provides effective insulation between the frigid Antarctic atmosphere and the relatively warm ocean below. The ice extent has a minimum in autumn, but by the end of the winter covers an area of 19 million square kilometres, essentially doubling the size of the continent. Instruments flown on polar orbiting satellites have been able to map the distribution and concentration of sea ice since the late 1970s and this study used a new data set of Antarctic sea ice extent created by NASA.
The ozone hole was discovered by BAS scientists in the mid-1980s and found to be a result of CFCs in the stratosphere that destroyed the ozone above the continent each spring. The loss of the ozone resulted in marked cooling in the Antarctic stratosphere, which increased the winds around the continent at that level. The effects of the ozone hole propagate down through the atmosphere during the summer and autumn so that the greatest increase in surface winds over the Southern Ocean has been during the autumn. CFCs have a long lifetime in the atmosphere and despite the Montreal Protocol, which has banned the use of CFCs, there is currently no indication of a recovery of springtime ozone concentrations. However, over approximately the next half century there is expected to be a return to the pre-ozone hole concentrations of ozone.
Strong winds are a major feature of the Southern Ocean with the remoteness of the Antarctic from other landmasses allowing active depressions to ring the continent. The Antarctic continent is slightly off-pole, which results in a large number of storms over the Amundsen Sea (the Amundsen Sea Low) giving average northerly winds down the Antarctic Peninsula and cold, southerly winds off the Ross Ice Shelf. The stronger winds around the continent in Autumn as a result of the ozone hole have deepened the Amundsen Sea Low, giving the positive and negative trends in sea ice over the Ross Sea and to the west of the Antarctic Peninsula respectively. Although there has been a loss of some sea ice to the west of the Antarctic Peninsula, this is negated by the larger increase of ice in the Ross Sea, giving a net increase in the amount of ice around the Antarctic.
There has been contrasting climate change across the Antarctic in recent decades. The Antarctic Peninsula has warmed as much as anywhere in the Southern Hemisphere, with loss of ice shelves and changes in the terrestrial and marine biota. The warming during the summer, which has the greatest impact on the stability of the ice shelves, has been linked to the ozone hole and increasing greenhouse gases. Recent research has suggested that the warming extends into West Antarctica. In contrast, East Antarctic has shown little change or even a small cooling around the coast, which is consistent with the small increase in sea ice extent off the coast. The increase in storm activity over the South Pacific sector is also consistent with the pattern of temperature change observed, with warming down the Antarctic Peninsula in the stronger northerly flow.
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