Global warming could disrupt the world's sea currents, sending Europe into a chill within 100 years and devastating tropical ocean life, a CSIRO scientist says.
Richard Matear, a Hobart-based marine researcher, said the oxygen content of deep ocean water between Australia and Antarctica had fallen 3 per cent since 1968.
If new research confirmed the decline was happening throughout the world's southern oceans, it would be a strong sign global warming was interfering with sea currents.
According to NASA "the thawing of sea ice covering the Arctic could disturb or even halt large currents in the Atlantic Ocean.
"Without the vast heat these currents deliver - comparable to the power generation of a million nuclear power plants - Europe's average temperature would likely drop 5 to 10 degrees."
While North America would not be as severely hit, the space agency said "such a dip in temperature would be similar to global average temperatures toward the end of the last ice age roughly 20,000 years ago".
While NASA said many scientists were sceptical, it quoted Dr Robert Gagosian, director of the private Woods Hole Oceanographic Institution, Massachusetts, as saying such a change in ocean currents could happen within 20 years.
Yesterday Dr Matear explained that if global warming continued melting Arctic ice and increasing evaporation, boosting rainfall in the North Atlantic, "the North Atlantic could be flooded with fresh water. The flood of fresh water reduces the density of the surface and prevents the water from sinking deep into ocean," he said, adding that the vertical sinking of sea water helped drive ocean circulation.
Officially called the thermo-haline circulation, the pattern is sometimes dubbed the Great Conveyer Belt.
Dr Matear said the decline in oxygen levels in deep water of the Southern Ocean was exactly what computer modelling suggested would be seen if global warming slowed this circulation, reducing the cold water flowing south of Australia. "Cold water is high in oxygen," he said.
Dr Matear said although it was too early to say what impact a shutdown of ocean currents would have on Australian temperatures, he agreed Europe could be sent into a severe chill.
"A decline of five degrees by end of this century . . . that wouldn't be unrealistic."
Dr Matear said a decline in oxygen levels would be particularly serious for sea life in the tropics, where the warmer water is already oxygen-poor.
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