In hot water

GHGs raise ocean temperatures

Published: Thursday 31 May 2001

-- greenhouse gas emissions have caused the world's oceans to heat up significantly over the last 50 years, according to two latest studies. The Atlantic, Pacific, and Indian oceans have collectively warmed an average of 0.06 c since 1995. The two studies have correlated the ocean heating directly with global warming caused by humans.

"The parallel climate model, developed by the us d epartment of energy and the us National Science Foundation can reproduce the warming in the ocean witnessed over the last 50 years. This will make it much harder to dismiss predictions made using climate models," says Tim Barnett of Scripps Institution of Oceanography, usa, talking of the model developed for one of the two studies.

Covering 72 per cent of the Earth's surface, oceans are often called the 'memory' of the earth's climate system. They can absorb large amounts of heat and sequester it at depth for up to thousands of years before circulating it back to the atmosphere. Earlier climate models didn't include an ocean temperature component and therefore frequently predicted that modern air temperatures would increase more than they actually have. This discrepancy has been useful fodder for sceptics, who have argued the global warming wouldn't be as severe as commonly thought.

"I believe our results represent the strongest evidence to date that the Earth's climate system is responding to human-induced forcing,"says Sydney Levitus, researcher at the us National Oceanographic Data Center. Last year, Levitus and his colleagues determined how much the oceans had warmed on average. They compiled millions of deep ocean temperature measurements from periods between 1948 and 1995. But, it wasn't clear whether this heat came from greenhouse warming or was just a natural swing in the climate cycle. To investigate, Barnett and Levitus each used a different climate model to simulate how ocean temperature could respond to current level of ghg s and other modern day atmospheric conditions. Both models predicted an amount of warming quite similar to what scientists have measured.

To be sure that their model results weren't a fluke, Barnett and his colleagues ran five simulations and averaged their results. They also ran the model without the extra ghg s and sulphate aerosols produced by human activity. Without the anthroprogenic 'fingerprint', the simulated ocean did not warm significantly.

Using a different model, Levitus and his colleagues also factored in the effect of Sun's changing intensity and aerosol particles produced by volcanic eruptions over the last century. The simulation produced a very close match to the actual measurements.

Further work needs to be done to make computer models deliver more consistent and specific predictions. Barnett says, an important benefit of the two studies is that in the future, models will have to get the ocean-related parameters factored in to look reliable. These results raise the bar for sorting out the best models, believes Barnett.

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