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Global warming will not cause 'permanent El Niño'

Major climate shifts unseen in previous warm eras

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Boffins in Blighty have said that global warming and retreating Arctic sea ice cover is unlikely to result in a so-called "permanent El Niño" state in the Pacific, nor cause similar major weather-changing shifts in the behaviour of the Atlantic ocean.

The possibility of such major changes in the world's weather has been discussed in scientific circles for some time. Some climate researchers believe that a warming world with ice-free summers in the Arctic would cause periodic occurrences such as the well-known El Niño Southern Oscillation (ENSO) in the Pacific, or the Arctic Oscillation/North Atlantic Oscillation (AO/NAO), to become permanent conditions.

This would mean major permanent changes to climate and weather around the world, many of which - for instance droughts and massive rainfalls in different areas during ENSO events - would be seen as disasters in their local regions. But now scientists working at the UK's National Oceanography Centre in Southampton say that a warm world in coming decades with ice-free summers in the Arctic would still have periodic ENSOs and AO/NAOs positive and negative in the normal way.

"Based on our findings, it seems unlikely that man-made global warming would cause a permanent El Niño state," says Professor Alan Kemp, one of the boffins carrying out the new research.

Kemp and his colleagues determined their findings by analysing sediment cores taken from a ridge beneath the Arctic. They focused on layers of sediment deposited around 70 million years ago in the late Cretaceous, in which they found fossilised diatoms - tiny plankto-plants.

According to the NOC scientists, back then the world was in a naturally occurring greenhouse period and the Arctic was ice-free in the summer with some occasional ice cover in winter.

"The presence of diatom laminae testify to ice-free Arctic summers during the Late Cretaceous, although there is also evidence of ice rafting by intermittent winter ice," said Kemp.

Thus, if ice retreat led to permanent ENSO or AO/NAO conditions, one would expect to see evidence in the fossils. Some scientists say that in the past long periods of continuous ENSO conditions have definitely occurred.

Kemp and his colleagues say that no such shift from present-day climate conditions can be seen in the Cretaceous warm-time fossils, and cast doubt on the idea that permanent El Niño conditions have occurred in other greenhouse eras. They write:

Analysis of records spanning 1000 years reveals strong periodicities in the quasi-biennial oscillation and El Niño - Southern Oscillation (ENSO) band as well as a 14 year period, which all closely match periodicities typical of modern high latitude climate variability. This supports the view that an Arctic Ocean free of permanent sea ice would be driven by similar forcing to the present state... the prominent ENSO periodicities in our records argue against the hypothesized link between past warm climates and “permanent El Niño” states.

Details of the scientists' paper in Geophysical Research Letters can be viewed here. An accompanying statement from the National Oceanography Centre is here. ®

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