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Swimmers in New Zealand waters would have enjoyed spa-like seawater temperatures of up to 30degC about 50 million years ago, according to a new study.
Scientists says the study could also mean it is possible that future warming projections are being underestimated and the world could get a lot warmer than previously thought.
The early Eocene, around 50 million years ago, has long been recognised as a time of pronounced global warming.
But until now, the degree of warmth was uncertain.
Now a group of scientists, led by paleontologist Chris Hollis, of GNS Science, have determined that surface sea waters around New Zealand exceeded 30degC and water at the sea floor hovered around 20degC during what they believe is a 2-3 million year episode of greenhouse gas-induced global warming.
"These temperatures are at the extreme end of modern tropical watermasses," Dr Hollis said.
In the study that will be published in the international scientific journal Geology next month, scientists have inferred warm climate conditions in New Zealand for this time period from a wide range of fossil evidence.
They used geochemical methods for extracting sea temperatures from sedimentary rocks exposed in the bed of the Waipara River in North Canterbury.
Tropical temperatures at this high latitude location presented a huge challenge for climate modellers, Dr Hollis said.
Computer models said New Zealand sea temperatures during the Eocene did not exceed 20degC.
"Anomalously warm conditions have also been reported for early Eocene records from high latitude regions in the Northern Hemisphere," Dr Hollis said.
"It now seems likely that some, as yet unknown, heat transport mechanism comes into play during times of extreme global warmth."
Co-author Matt Huber, of Purdue University in Indiana, said the new findings were at least 10 degrees warmer than scientists had previously thought for the time period.
"It suggests that existing temperature reconstructions for this period have been biased toward overly cold values.
"It also indicates that climate models tend to under-estimate temperatures during past climate warming episodes. It is possible that models are also under-estimating future warming projections."
Dr Hollis said further research into the causes of extreme climatic changes in New Zealand's geological past would benefit from New Zealand's recent decision to join the Integrated Ocean Drilling Programme - an international geoscience programme that operates a drilling ship that can recover sediment cores up to 2000m in length from beneath the ocean floor.
The ship will be conducting a research leg off the coast of Canterbury in November 2009.