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NZ Scientists Link 'Wobbles' In Earth's Orbit



A New Zealand-led group of scientists has shown for the first time that long-term wobbles in the Earth's orbit have caused the East Antarctic ice sheet to expand and contract many times in the past.

Not only did the cyclical variations in the Earth's orbit cause rapid changes to the ice sheet, they also led to global climate changes and marked sea-level changes around the world.

The finding is the first direct evidence that confirms a 20 year-old hypothesis linking the expansion and contraction of the ice sheet to the Earth's orbital cycles.

It has been hailed internationally as a major contribution to the understanding of the behaviour of the ice sheet and it will enable scientists to make more accurate estimates about future ice sheet behaviour as global temperatures rise during the next 200 years.

The group, led by Tim Naish of the Institute of Geological & Nuclear Sciences Limited (GNS) and Professor Peter Barrett of Victoria University's Antarctic Research Centre, analysed sediment cores from under the Western Ross Sea to reach their finding. The cores, dating back to 34 million years, were recovered in three seasons of drilling by the multi-national Cape Roberts Project.

The findings have been published in the international scientific journal Nature this week.

It was already known that variations in Earth's orbit influenced the advance and retreat of the huge continental ice sheets that formed in the Northern Hemisphere during the ice ages over the past two and a half million years.

In contrast, the ice sheet that has covered Antarctica for the last 15 million years has been considered more stable because of its size, its insulation by the Southern Ocean, and its extremely low temperature.

Prior to 15 million years ago, however, when planetary temperatures were 3 to 4ºC warmer than today, drill cores from the Antarctic margin indicate that the ice sheet was more dynamic, with large and possibly frequent variations in area and volume.

However, the timing and scale of these variations, and their effect on the environment and climate has until now been poorly known. Analysis of Cape Roberts Project cores, jointly undertaken by GNS and Victoria University researchers with their international collaborators, has dramatically improved our knowledge of the nature and timing of these globally significant changes.

Lead author of the Naturearticle, Dr Tim Naish, said that "studies of ice sheet dynamics during the warmer past should help provide realistic estimates for their future behaviour following the increased levels of atmospheric carbon dioxide and temperature projected for the end of this century by the Intergovernmental Panel on Climate Change.”

The Cape Roberts cores, representing 1500m of strata from the mid-to-late Cenozoic era (34 million years ago to the present), provide the most detailed record of ice fluctuations for the Antarctic margin from this period.

Cape Roberts Project Chief Scientist, Professor Barrett, said the real importance of the cores lay in the ages determined from radiometric dating, which were far more precise than for any other Antarctic offshore sequence.

Scientists estimate that the fluctuations in the volume of the East Antarctic ice sheet could have driven global sea-level changes of up to 50m.

Dr Naish and fellow GNS scientist Stuart Henrys, and Prof Barrett, have recently been awarded a three-year grant of $390,000 from the government's Marsden Fund to undertake more in-depth research into the best-dated set of ice advance-and-retreat cycles from the Cape Roberts drill core.

The study is designed to quantify the frequency and magnitude of environmental changes recorded by the sediment cores. This work should identify the role of external climate forcing on past Antarctic ice sheet dynamics.

Dr Naish returned to Antarctica last week to lead an international team undertaking site surveys for a new drilling programme along the side of the East Antarctic ice sheet and on the Ross Ice Shelf in the southern McMurdo Sound region.

The new project, called ANDRILL, is designed to further improve the understanding of the impact of this huge ice sheet on global climate change. High-quality records from deep sediment cores from the Antarctic margin were rare compared to the Northern Hemisphere, Dr Naish said.

ANDRILL will consist of four drilling seasons between 2004 and 2008 and will be the most technically challenging and scientifically ambitious drilling programme ever undertaken on the edges of Antarctica.

It has the support of Antarctica New Zealand and involves scientists from New Zealand, Britain, the United States, Italy, and Germany.


Note: "Wobbles" in the Earth's orbit were first calculated early last century by Yugoslav mathematician Milutin Milankovich. He concluded that these long-term periodic changes produced corresponding variations in the amount of solar radiation received by the Earth. This led him to hypothesise that Earth's ice-ages and intervening warm periods were controlled by what became known as "Milankovich orbital cycles". The major cycles have durations of 20,000, 40,000 and 100,000 years. The hypopthesis that orbital cycles were linked to climate and sedimentation patterns was proven in the mid-1970s with publication of the first stable oxygen isotope records from deep-ocean sediments. These records of global temperature and ice volume information corroborated Milankovich's theory. The height of the last glaciation was 20,000 years ago and we are currently in the maximum of a warm interglacial period.

The Transantarctic Mountains divide Antarctica in to east and west ice sheets.

The larger east sheet is inherently more stable and less vulnerable to changes in climate. The volume of ice covering Antarctica is 30 million cubic km.

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