Ocean study points finger at mankind
Older computer models did not factor in the ocean
BBC Science News, April 11, 2001
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," said Dr. Sydney Levitus of America's National Oceanographic and Atmospheric Administration in Silver Spring, Maryland.The study by the Levitus team was published in the journal Science.
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. The Barnett et. al. study was published in the same issue of Science
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.
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.
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.
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".
2 Studies Affirm Greenhouse Gases' Effects
The Washington Post,April 13, 2001
Two studies released yesterday provide the strongest evidence yet that greenhouse gases are causing the Earth's oceans to warm, further bolstering the case that global warming is real and is being caused at least in part by air pollution, researchers said.
Previous research had shown that the Atlantic, Pacific and Indian oceans -- covering 72 percent of the Earth's surface -- have collectively warmed on average of about one-tenth of a degree Fahrenheit since 1955. But whether that was caused by global warming has been far from clear.
The new studies, based on parallel computer climate models, show a direct connection between rising ocean temperatures and emissions of carbon dioxide and other gases that can trap heat within the atmosphere. The models showed that the ocean warming that has been measured over the last half-century is exactly what would be expected from the amount of greenhouse gases that have been emitted into the atmosphere.
"I believe our results represent the strongest evidence to date that the Earth's climate system is responding to human-induced forcing," said Sydney Levitus of the Commerce Department's National Oceanographic Data Center. He is the lead author of one of the studies.
"This will make it much harder for naysayers to dismiss predictions from climate models," added Tim Barnett of the Scripps Institution of Oceanography. He is the lead author of the other study, which was funded by the National Science Foundation.
The two studies, published in today's issue of the journal Science, come amid an international debate prompted by President Bush's recent decision to abandon the global warming treaty negotiated in Kyoto, Japan, in 1997.
Although administration officials have repeatedly described global warming as a real and serious problem, the president contends the treaty is unfair because it would seriously damage the U.S. economy if implemented and because it exempts China, India and developing countries from the tough strictures on industrial emissions.
Bush has ordered a Cabinet-level panel to draft proposals for combating global warming. They will be presented to U.S. allies later this summer. But the president's unilateral decision to pull out of the treaty has infuriated European, Japanese and Canadian leaders who fear Bush may have torpedoed an international negotiation process that has spanned the better part of a decade.
Underscoring the sensitivity of the issue, government scientists involved in the new global warming studies have been cautioned by a Commerce Department spokesperson to "stick with the science rather than delve into policy" in discussing their findings with reporters.
The two studies add to the wealth of recent data on global warming, which, scientists say may be causing changes in weather patterns and shrinking glaciers and permafrost. That, they added, could eventually touch off catastrophic climate changes.
A United Nations panel of scientists concluded earlier this year that the Earth's temperature could rise by as much as 10.4 degrees over the next 100 years -- the most rapid change in 10 millennia and more than 60 percent higher than what the group predicted less than six years ago.
William Patzert, a scientist with the NASA Jet Propulsion Laboratory in Pasadena, Calif., cautioned that although there is no doubt that the oceans are warming, "there is a lot of natural variability in the oceans."
"The trick is to extract the small warming or sea-level rise over the last 50 years and relate that directly to the greenhouse emissions, which have been significant," he added.
Previous studies on the effects of global warming have focused on surface temperatures -- largely because records of past air temperature are more numerous and reach further back in time than those for ocean temperatures. But those studies frequently were off in predicting future rises in temperature, which provided ammunition to skeptics who question the severity of the warming problem.
Levitus and his colleagues spent most of the past decade attempting to build up a comprehensive database of ocean temperatures. "We got tremendous cooperation from almost every nation," he said in an interview, "and the end of the Cold War was very positive. We received data from Russia that previously was classified. We also received help from the British and U.S. navies."
Last year, Levitus and his colleagues determined an average for how much the oceans had warmed by compiling millions of deep ocean temperature measurements from 1948 through 1995. But they couldn't say for sure whether the heat came from greenhouse warming or just a natural swing in the climate cycle.
To solve that riddle, Levitus and Barnett each used a different sophisticated computer model of the Earth's climate to simulate how ocean temperature should respond to current levels of greenhouse gases and other modern atmospheric conditions. Both models predicted an amount of warming similar to what scientists subsequently measured.
As a precaution, the scientists ran five simulations and averaged their results together. They also ran the model without the extra greenhouse gases and sulfate aerosols produced by human activity. Without the "fingerprint" of man-made gases, the simulated ocean did not warm significantly.
"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," Barnett said. "It's just there, bang."
© 2001 The Washington Post Company