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POP - Pole-Ocean-Pole: global stratigraphy for millennial climate variability

Summary information

Funding:FP5 - Research project
Total cost:3265396
Ec contribution:1773968
Start date:2000-12-01
End date:2004-05-31
Duration:42 months
Coordinator:Jenny Walsham (
Organisation:Cambridge University, Department of Earth Sciences - United Kingdom
Regio:Arctic; North Atlantic
Keywords:Deep-sea sediments and ice cores; modelling, climatic change; high-resolution palaeoclimatic records for understanding the climate system
Project name:POP - Pole-Ocean-Pole: global stratigraphy for millennial climate variability
Project summary:Abstract
POP (Pole-Ocean-Pole) was proposed with the overarching objective of developing the methods for linking palaeoclimatic records from the polar ice cores and from deep ocean sediments. By examining sediments from the world oceans it is possible to make very valuable reconstructions of many aspects of the Earth’s climate system over the past million years and more. Methods for developing time scales for deep ocean sediment records have evolved over the past 30 years, largely based on work by Shackleton and colleagues in the 1970’s. On the other hand the polar ice cores have certain unique advantages. First, they contain bubbles of “fossil” air so that the natural record of changes in greenhouse gases can be obtained. Second, they are capable of yielding a more detailed record (in the Greenland ice sheet even at annual resolution, in the deeper parts of the Antarctic ice sheets better than century resolution) than marine cores that can very rarely yield a resolution better than about 200 years. Third, the factors controlling variations in snow accumulation are easier to understand than those causing changes in the accumulation of marine sediment, so that the conversion from depth to geological age can be carried out in a more refined manner.

Unfortunately Greenland is only able to provide records covering about 110,000 years (one glacial cycle) but these records are of superb quality and provide an ideal template for understanding the details of climatic variability in the North Atlantic region over that time interval. At the time that POP was proposed, the Vostok record from Antarctica provided the longest record, about 400,000 years. Hence the focus of POP was to develop an accurate and precise age scale for the Vostok record and for marine cores that could be precisely linked to it. There was to be a special focus on those cores that could substitute for the Greenland ice sheet by yielding highly detailed climatic records for the North Atlantic region.
Project outputs:Scientific achievements
For the last glacial cycle (the last seventy thousand years) POP developed a new age scale for ice cores in both the Greenland and Antarctic polar ice sheets as well as for marine sediments of the North Atlantic Ocean. This also permitted improved calibration of the radiocarbon time which assists in applying a consistent age scale to sedimentary records of rapid environmental change over this time interval in sediment cores from the other ocean basins, and in records that are collected on the continents.

For the 400,000-year time interval that is covered by the Vostok ice core that was collected in central Antarctica POP developed a new age scale that has been applied to the three long cores from central Antarctica that have so far been collected: Vostok, Dome Fuji, and Dome C.

Significant advances were made within POP in obtaining a reliable age scale for the air bubbles that have been analysed within the ice cores, so that we also have a more reliable age scale for the history of natural variations in the concentration of greenhouse gases (carbon dioxide and methane) in the atmosphere.

Records of changing sea surface temperature, and of conditions on the sea floor bathed by the deep ocean water masses, were collected from the subantarctic Indian Ocean, the tropical Indian Ocean, and both the East and the West of the North Atlantic Ocean. These records are extremely detailed and can be linked to the ice cores with unprecedented reliability. This will permit a more detailed understanding of the relative timing of changes in different components of the global climate system. New modelling in the POP project provides a considerably better understanding of the mechanism by which climatic changes propagate through the climate system with delays that can amount to over 1.000 years.

• The project has delivered new sediment cores, new analyses, new methodologies and new theoretical concepts.
• A suite of cores from the North Atlantic off Portugal was collected. Many types of analyses have already been performed on these cores, but they remain as a legacy for future researchers
• A very large number of oxygen isotope analyses; carbon isotope analyses; alkenone-based temperature estimates; fossil microfossil counts and other measurements have been collected. These will provide the basis both for publications by the POP scientists and by the broader community for whom these will constitute valuable data archives. Important new data, particularly regarding the concentration of methane in the air bubbles spanning the last 400,000 years, have been obtained.
• Significant advances have been made in the methods for interpreting records in ice cores. In particular methods for estimating the air temperature actually associated with the bubbles that are subjected to analysis have been refined.
• A new approach to the development of age scales in ice cores has led to a better understanding of the time frame for the records in these cores.
• New model results have been published that advance our understanding of the operation of the so-called ‘bipolar seesaw’ whereby temperature changes in one hemisphere can lead to changes in the opposite sense in the opposing hemisphere.

Socio-economic relevance
The POP findings will help to refine the reliability of the atmospheric and coupled atmosphere-ocean models that are used to predict future trends in global climate that will result from the continuing addition of greenhouse gases to the atmosphere that mankind is causing.

A broad conclusion that may be drawn from the findings of the POP project is that the ‘bipolar seesaw’ concept does constitute both a valuable means for describing the past record of rapid natural climate variability, and a useful route to making testable predictions that will further consolidate our confidence in climate model predictions.