WASHINGTON -- Last year was the warmest on record for planet Earth, and for the first time federal climate researchers are willing to say people are at least partly to blame.
"I wouldn't have been willing to say this two years ago. I believe we are seeing evidence of global warming at least some of which is attributable to human activities," said Elbert W. Friday, research chief of the National Oceanic and Atmospheric Administration.
"Indeed, 1997 was the warmest year on record," added Tom Karl, a senior NOAA researcher. "The increasing trend of temperatures that we see, we believe, is at least partially attributed to human activities," such as pollution from cars and factories.
Karl said the Earth's average temperature last year was three-quarters of a degree Fahrenheit above normal. Normal is 61.7 degrees, the average for the years 1961-1990.
The 1997 reading tops the previous warmest year, 1990, by 0.15 of a degree.
Global warming has been a topic of sharp debate in recent years, culminating in the climate conference last month in Kyoto, Japan, where government officials from around the world sought ways to reduce the potential impact of climate change.
Many scientists believe that carbon dioxide and other gases released into the atmosphere by industrial activities are increasing the Earth's temperature by trapping heat from the sun, somewhat like a greenhouse.
Others disagree, and previously NOAA officials simply reported their findings without speculating on the cause of rising temperatures.
"We feel more comfortable now in saying there is a human effect because we have more data than before," said Friday, assistant NOAA administrator for oceanic and atmospheric research.
"The trend of global temperatures is never due to a single source," commented Karl, noting that factors can include periodic cycles of climate change, ocean circulation and even volcanoes.
But, he added, "the odds that we would be wrong, that there is no relation to human activity, is in the area of 5 to 10 percent."
The rising temperatures have occurred worldwide, Karl said, though the impact has been less in eastern North America, China and eastern parts of the Mediterranean.
This may be the result of sulfuric air pollution in those areas, he commented, with the hazy air reflecting sunlight and producing a local cooling effect much like that of the cloud generated when the volcano Pinatubo erupted in the Philippines.
That eruption shot sulfuric gases into the upper atmosphere, producing a brief worldwide cooling.
As far as the possible effects of global warming are concerned, Friday noted there have been reports of softening of the permafrost in Alaska and a 6-inch rise in overall ocean levels over the last century.
Karl said continued warming could produce "a lot of surprises," including changes in rain and snowfall patterns, though not necessarily a disaster.
"Based on the data we see, we certainly couldn't predict a catastrophic event," he said.
Karl said the El Nino phenomenon in the Pacific Ocean, which is affecting weather in large areas, contributed to the 1997 warming. El Nino is characterized by a large pool of warmer than normal water in the Pacific.
"But even without El Nino, 1997 would have been a very warm year," he added, noting that nine of the warmest years on record have occurred in the last 11 years.
"This continues a trend that we see in the records that date back to the 19th century, a trend of increasing global temperatures, both on land and in the ocean."
Land temperatures have risen about 20 percent faster than ocean readings, he added.
Karl's analysis was based on records of land temperatures back to 1880 and ocean readings since 1900.