Global land and sea records snow that 1997 was the warmest year on record, government climate researchers said Thursday.
The warmth continues a trend with 9 of the warmest years on record occurring in the past 11 years. Since 1900, the world's average temperature has risen a little over 1 degree Fahrenheit.
``We believe the increase is at least partially attributed to human activity,'' said Tom Karl, senior scientist at the National Climatic Data Center (NCDC). These activities include burning fossil fuels that add carbon dioxide to the air. This and other gases help warm the Earth.
Karl said natural climatic changes could be partly responsible for the warming, or even helping to slow human-induced warming. Climatic changes are ``never due to a single cause,'' he said.
Earth's average temperature last year was three-quarters of a degree Fahrenheit above normal. Normal is defined as 61.7 degrees, the average for the years 1961-1990. The 1997 reading tops the previous warmest year, 1990, by 0.15 degrees.
``We feel more comfortable now in saying there is a human effect because we have more data than before,'' said Elbert W. Friday, Assistant Administrator for Oceanic and Atmospheric Research at the National Oceanic and Atmospheric Administration, which includes the NCDC.
The current El Niño is partly responsible for 1997's warming, Karl said. ``But even without El Niño 1997 would have been a very warm year.''
In contrast to the land and sea records, satellite measurements of global temperatures from the surface up to about 20,000 feet show that 1997 ``was not the warmest year,'' says John Christy of the University of Alabama at Huntsville. The satellite records show that of the last 19 years 11 were warmer than 1997 and only seven were cooler. Christy and Roy Spencer of NASA compile the satellite records, which go back to 1979.
Christy says this doesn't mean the surface records are wrong. Computer models of the atmosphere show that as the surface warms, the air measured by satellites should also warm. ``Yet the record doesn't show this happening.'' Christy says. ``I wish I knew why.''
The differences probably arise because ``the atmosphere has subtle ways of losing heat to space'' that the computer models don't take account of, Christy says.
The NOAA researchers used land records going back to 1880 and sea records back to 1900. The land records are from weather stations that measure air temperatures right above the ground ``where we live and work and grow crops,'' Karl said. He added that they have been corrected for changes in measuring techniques and the growth of cities, which tends to increase temperatures in urban areas.
But Spencer of NASA says he has doubts about how good a job has been done, or even can be done, correcting for increases in temperatures caused by the addition of buildings and pavement even in rural areas.
The 1997 land record did not break the record set in 1990, but it was one of the five warmest years on land since 1880. But ocean warmth was enough to push global temperatures into the record books.
Karl doesn't expect the warming to cause any disasters any time soon, such as melting polar ice caps flooding the Earth. But, he said, ``we have to worry about surprises'' that could be caused by changes in ocean currents. Such surprises could include relatively quick warming or even cooling of parts of the world that would be harder to adjust to than a gradual warming over the next century.
During 1997, the eastern half of the USA averaged cooler than normal while the West was warm.