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Thursday, 12 April, 2001, 19:27 GMT 20:27 UK
Ocean study points finger at mankind
Older computer models did not factor in the ocean
By BBC Science Correspondent Christine McGourty

Scientists in the United States have produced the strongest evidence yet that man-made global warming is responsible for a significant increase in the temperature of the world's oceans in the last 50 years.

The average temperature of the Atlantic, Pacific and Indian Oceans has risen 0.06 degrees C since 1955.

Two separate studies - both carried out using computer modelling techniques - have now linked that rise directly with global warming caused by human activity.

I believe our results represent the strongest evidence to date that the Earth's climate system is responding to human-induced forcing

Dr Sydney Levitus
"I believe our results represent the strongest evidence to date that the Earth's climate system is responding to human-induced forcing," said Dr Sydney Levitus of America's National Oceanographic and Atmospheric Administration in Silver Spring, Maryland.

Ocean simulation

Dr Tim Barnett of Scripps Institute of Oceanography, in La Jolla, California, who led the other research team, indicated the implications of the findings were not straightforward.

He said: "Warming in the oceans is bad news and good news. It really does add strength to the claims that global warming is here. But it also suggests that the immediate impact may not be as great, because the oceans may slow things down a little."

Each research team used a different computer model to simulate how ocean temperature should respond to current levels of greenhouse gases and other atmospheric conditions.

Both predicted a warming similar to that measured by scientists.

[This] will make it much harder for naysayers to dismiss predictions from climate models.

Dr Tim Barnett
Scripps Institute of Oceanography
The Scripps team also ran their model without the extra greenhouse gases and sulphate aerosols produced by human activity and found that without these the simulated ocean did not warm significantly.

'Big and bold' signal

"What we found is that the signal is so bold and big that you don't have to do any fancy statistics to beat it out of the data. It's just there, bang," said Dr Barnett.

"The odds are exceedingly good that the model did not trick us with this signal. It couldn't have done it by itself."

He added that the findings "will make it much harder for naysayers to dismiss predictions from climate models."

In their model, the team from NOAA also factored in the effects of the sun's changing intensity and the aerosol particles produced by volcanic eruptions over the last century.

'Robust' models

They too found a very close match to the actual measurements.

Dr Levitus said: "The fact that the model ocean warms by approximately as much as our observed estimate indicates that the models are very robust in simulating the observed changes in the Earth's climate system during the past 100 years."

Both sets of results are published in this week's Science magazine.

Computer models have come in for criticism in the past.

Earlier versions included measurements of atmospheric temperatures, but not ocean ones.

Revising predictions

As a result, it is thought that they frequently predicted that air temperatures would increase by more than they have - as the oceans absorb heat.

The findings have often been used by sceptics, who have argued that global warming predictions are exaggerated.

No doubt some will continue to argue that modellers can get any answer they want about climate change, simply by adjusting any of the numerous inputs into them.

The researchers themselves agree more work is needed to make computer models deliver more consistent and specific predictions.

But Dr Levitus says that one important benefit of the new research is that "in future, models will have to get the ocean right in order to be believable. These results raise the bar for sorting out the best models".

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