Skeptics, beware. One of the sturdiest pillars of the argument against global warming has crumbled under the weight of some 10 million newly compiled measurements of ocean temperature. These records, once scattered across the globe, together reveal a trend that many climatologists suspected, but could never prove: the world¿s oceans--the Pacific, Atlantic and Indian basins combined--have warmed substantially in the past 50 years.
Exactly what¿s causing the increase is still up in the air. The most likely culprits are greenhouse gases such as carbon dioxide, according to many climatologists, because they trap heat at the planet¿s surface. Indeed, James E. Hansen of the NASA Goddard Institute for Space Studies in New York and his colleagues reported in 1997 that the earth is absorbing about 0.5 watt per square meter of sunlight more than it is emitting back to space. Hansen speculated that this "missing heat" was probably in the oceans. But that wasn¿t enough to convince some doubters, who have ever since used the disparity to discount the potency of greenhouse gases.
The hitch is that the atmosphere's capacity to store heat is a factor of 20 less than that of the oceans, says Sydney Levitus of the National Oceanographic Data Center in Silver Spring, Md. "The oceans really are the memory of Earth¿s climate," Levitus says. The problem is that this is one memory that is destined to come back and haunt us: heat in the ocean eventually makes its way back into the atmosphere--delaying, but not preventing, a heat wave at the surface.
That knowledge inspired Levitus and his colleagues seven years ago to launch a United Nations-sponsored global search¿and-rescue operation for ocean data. The team¿s primary goal was to dig up every single temperature measurement they could find for the top 3,000 meters of the ocean, enough water to engulf a skyscraper seven times as tall as the World Trade Center. They knew that the deeper they looked, the more convincing any emerging temperature trends would be.
Numbers came in from all over the world. When the British Royal Navy told Levitus that it had hundreds of thousands of index cards of hand-written temperature profiles stashed away in a musty basement, the oceanographer jumped at the chance to add them to his growing database. He didn¿t have to go far. Officials loaded a ship with some 100 metal file drawers full of the cards and sent them across the Atlantic. His team also uncovered unexpected measurements from World War II at the Scripps Institution of Oceanography in San Diego and rare numbers from the icy seas around Russia and Norway in an obsure book tucked away in the New York Public Library.
When they put the millions of numbers they gathered together with several million more that already existed, they discovered that the Pacific, Atlantic and Indian oceans combined warmed an average of 0.06 degree Celsius between 1955 and 1995. What¿s more, that amount of warming accounts for a majority of Hansen¿s "missing heat."
"Everything points to the fact that the warming is due to increases in greenhouse gases," Levitus says. "But," he admits, "we still can¿t discount natural variability being involved." Indeed, scientists at Scripps recently suggested that 1,800-year cycles of ocean tides could drive a natural rise in global temperatures. If they¿re right, the planet has another five centuries of warming still to come.