Conflicting Temperature Data Mar Global Warming Debate

Published February 1, 1998

Depending on which set of data one chooses to believe, 1997 was either the “warmest year on record” or “among the coolest years” in the last two decades. To further add to the confusion, the data underlying both assertions originated with agencies of the U.S. government.

On January 12, the National Oceanic and Atmospheric Administration (NOAA) announced that global temperatures last year were the warmest on record, averaging 62.45 degrees Fahrenheit–.15 degrees F higher than the previous highest mark, recorded in 1990. The NOAA data are based on land temperature readings dating back to 1880 and ocean temperature readings going back to 1900. NOAA scientists said their analysis confirmed earlier observations that the Earth’s climate had warmed on average by 1 degree F over the past century.

In releasing their report, NOAA scientists acknowledged that the weather pattern known as El Nino–a warm-water current originating in the eastern Pacific that profoundly affects global climate patterns–no doubt contributed to higher temperature readings. But Tom Karl, senior scientist with the National Climatic Data Center in Asheville, North Carolina, noted that “even without El Nino, we’re sure this past year was in the top ten warmest years on record.” He added that nine of the past eleven years have been the warmest on record.

The government scientists pointed out that the observed warming trend was by no means universal. Eastern North America, the eastern Mediterranean, and most of China registered below-average temperatures in 1997. In the eastern U.S., for example, temperatures last year averaged 2 to 4 degrees below normal, the NOAA data show. Nevertheless, noted Karl, “The data we’re seeing certainly follow the trend predicted by many of the climate models that are driven by man-made effects on the atmosphere, although the trends in global temperature are never due to any single cause,” Karl noted.

NOAA researcher Elbert “Joe” Friday went even further, telling the Associated Press that, “I wouldn’t have been willing to say this two years ago, [but] I believe we are seeing evidence of global warming, at least some of which is attributable to human activity.”

Now, the Rest of the Story

The picture of global warming painted by NOAA was clouded, however, by data gathered by researchers at the University of Alabama at Huntsville, released just a few days before the NOAA news conference. Based on temperature data collected by weather satellites, the Huntsville scientists found 1997 to be among the coolest years since satellite-based measurements began in 1979. The data show 1997 to be the seventh coolest of the 19 years covered by weather satellite measurements. Satellite readings continued to show the slight downward trend seen over the past two decades.

The discrepancy between ground-based and satellite measurements can be explained, climate scientists agree, at least in part by the so-called “urban heat-island” effect. Most land-based temperature readings are taken at airports located near urban areas, where they are subject to an artificial warming generated by all manner of human activity, from the heating of buildings to automobile traffic. This urban heat-island effect results in inflated temperature readings.

By contrast, measurements taken from weather satellites and weather balloons are free of such contamination and are generally considered the most reliable source of temperature data. Over the past two decades, weather satellites and weather balloons have failed to show any warming of the Earth’s climate.

While climate scientists will continue to measure and analyze temperature trends, their findings will, in one form or another, find their way into the political arena. Eager to shore up support for the Kyoto Protocol and its goal of reducing emissions of manmade greenhouse gases, the Clinton administration saw to it that the NOAA announcement received elaborate coverage in the media, complete with references to how the findings confirm fears of a human-induced warming of the climate. Given the high stakes involved in the outcome of the debate over the Kyoto Protocol, more such NOAA-style press events can be expected in the months, and even years, ahead.