n important piece of the global-warming picture has come into clearer focus with a confirmation by scientists that the world's oceans have soaked up much of the warming of the last four decades, delaying its full effect on the atmosphere and thus on climate.
The warming of the deep oceans had long been predicted, and the consequent delaying effect long thought to exist.
But until now the ocean's heat absorption had not been definitively demonstrated, and its magnitude had not been determined.
The finding, by scientists at the National Oceanographic Data Center in Silver Spring, Md., is based on an analysis of 5.1 million measurements, by instruments around the world, of the top two miles of ocean waters from the mid-1950's to the mid-1990's.
The analysis, the first on a global scale, is being published today in the journal Science.
As the earth warms, from either natural or human causes, or both, not all the extra heat goes immediately into the atmosphere, where its effect on climate is most direct.
Much of it is absorbed by the oceans, which store it for years or decades before releasing it.
This means that to whatever extent the planet is being warmed by emissions of greenhouse gases like carbon dioxide, which are produced by the burning of coal, oil and natural gas, only part of that heating has materialized so far at and above the earth's surface.
Some experts believe that about half the greenhouse warming is still in the oceanic pipeline and will inevitably percolate to the air in the decades just ahead.
The average surface temperature of the globe has risen by about 1 degree Fahrenheit over the last 100 years.
Over the last 25 years, the rate of surface warming has accelerated, amounting to the equivalent of about 3.5 degrees a century.
By comparison, the world is 5 to 9 degrees warmer now than in the depths of the last ice age, 18,000 to 20,000 years ago.
Scientists generally agree that it is unclear how much of the warming is attributable to greenhouse gases and how much to natural causes; many think both are involved.
The new study shows that the average warming of the seas over the 40-year study period amounted to about one-tenth of a degree Fahrenheit for the top 1.9 miles of ocean water as a whole, and more than half a degree in about the top 1,000 feet.
It is possible that the ocean may now be giving up to the atmosphere some of the heat it stored in the early part of the study period, but this has not been established, said Sydney Levitus, the chief author of the study. He is the director of the Ocean Climate Laboratory, part of the data center at Silver Spring, which in turn is part of the National Oceanic and Atmospheric Administration.
Likewise, Mr. Levitus said, it is possible but not established that more frequent appearances of the phenomenon known as El Niño, a semi-periodic warming of the eastern tropical Pacific that disrupts weather around the world, are related to the generally warming ocean.
The magnitude of the oceanic warming surprised some experts. One, Dr. Peter Rhines, an oceanographer and atmospheric scientist at the University of Washington in Seattle, said it appeared roughly equivalent to the amount of heat stored by the oceans as a result of seasonal heating in a typical year.
"That makes it a big number," he said.
Dr. James E. Hansen, a climate expert at the NASA Goddard Institute for Space Studies in New York, said the finding was important because, "in my opinion, the rate of ocean heat storage is the most fundamental number for our understanding of long-term climate change."
Three years ago, Dr. Hansen and colleagues used a computer model to calculate the amount of warming that should have been produced up till then by external influences on the climate system like greenhouse gases and solar radiation.
They found that because of the storage of heat in the ocean, only about half the surface warming should have appeared.
Mr. Levitus and his fellow researchers say in their paper that their findings support the Hansen conclusion.
Still, Mr. Levitus said the cause of the oceanic warming was not clear, although "I believe personally that some of it is due to greenhouse gases."
Some scientists believe that natural factors like recurring oscillations in ocean surface temperature in various parts of the world may play a role in the last century's warming. For example, studies by Dr. Gerard Bond of Columbia University's Lamont Doherty Earth Observatory found that the climate of the North Atlantic region, at least, had alternated between cooler and warmer every 1,500 years, more or less.
The world may be entering one of the natural warming cycles now, say Dr. Bond and Dr. Charles D. Keeling, a climate expert at the Scripps Institution of Oceanography in San Diego.
In a study published this week in the online edition of Proceedings of the National Academy of Sciences, Dr. Keeling suggested that a natural fluctuation in ocean tides over hundreds of years might contribute to these long-term cycles of warming and cooling.
Other possible causes
have also been suggested.