A Chilling Effect

No one doubts that the 20th century is ending hotter than it began.

The 10 warmest years of the past century have occurred in the last 15 years, records show. Spring arrives a week earlier than 20 years ago, scientists say. Extremes of weather have become increasingly common, and even the butterflies appear to be fleeing northward to escape the heat.

Scientists are concerned that waste gases, such as carbon dioxide, from the burning of fossil fuels are acting like panes of glass in a solarium to keep the heat of the sun’s energy from escaping into space.

But many climate researchers have a hard time explaining why temperatures have not risen as quickly as many sophisticated computer models of global warming predicted. Computer models have predicted a worldwide warming of about 1.8 degrees Fahrenheit, but measurements since the turn of the century have found a temperature rise of about half that.

Some researchers have seized on the discrepancy as evidence that global warming is a scientific fiction. Others have attributed the difference to a failure to factor in the effect of clouds.

Now a new theory suggests that the Pacific Ocean may have slowed global warming, with a potentially unsettling effect on the weather in California and across the globe, which could range from unseasonable heat spells and drought to equally devastating storms and floods.

Researchers at Columbia University’s Lamont-Doherty Earth Observatory say a reexamination of this century’s sea surface temperature records shows a pattern of cooling along the equator in the eastern Pacific, even as surface temperatures elsewhere on Earth increased.

The unexpected cooling is convincing evidence that a powerful tropical Pacific Ocean current system, overlooked by the researchers who developed the computer models, has delayed the effects of global warming, the research team reported last month in Science.

Both their computer model and actual temperature records show that when global temperatures rise, only the western part of the Pacific Ocean gets warmer, and “the eastern part cools off,” said Lamont-Doherty climate expert Amy Clement. “If this mechanism is right, this pattern is a fingerprint of global warming.

“We speculate that this is one mechanism that can explain the discrepancy between the small observed warming over the last 150 years and the [computer] models, which don’t include this effect,” she said.

They believe that the warmer temperatures intensify the interaction between the eastern and western parts of the tropical Pacific.

Normally, prevailing trade winds push warm surface waters westward from Ecuador toward Indonesia, causing deep, cold waters off the coast of South America to well up in their wake. That, in turn, strengthens the temperature difference between the regions, which further strengthens winds from the east.

In broad stretches of the Pacific, warmer atmospheric temperatures result, somewhat paradoxically, in colder ocean surface temperatures, the researchers said.

When the atmospheric temperatures increase, the cycle gains strength and speed, cooling both the water and the air in the region.

“This is more or less the way it has gone in the 20th century,” said Mark Cane, a senior scientist at Lamont-Doherty. “We think there is something going on that these models have missed. There has been an impact of the greenhouse gases, and part of it has resulted in this change in the tropical Pacific.

“The world is still getting warmer but not as fast as it might have,” Cane said.