The reasons to disbelieve that humans are causing global warming are many and varied, skeptics say. For example: Natural factors such as long-term variations in solar radiation are causing the rise in worldwide average temperature. The urban heat island effect is skewing modern weather data, so the warming observed in recent decades isn’t real. And besides, not long ago experts all believed the Earth was cooling, not warming.

Actually, research has shown that many such ideas are bogus. While changes in solar output have slightly increased global average temperature since the beginning of the Industrial Revolution, the planet-warming effect of man-made greenhouse gases is about 20 times larger (“Heated dispute” letter, SN: 10/27/07, p. 271). And although cities are warmer than neighboring rural areas, that phenomenon doesn’t mask recent warming trends in long-established cities (“Don’t blame the cities,” SN Online: 9/5/08).

Now, new research also skewers the global warming skeptics’ claim that, in the 1970s, scientists believed that an ice age was imminent. Researchers of the day had discovered that Earth had been cooling since the 1940s. Some believed that continued increases in the amount of planet-cooling aerosols kicked up or emitted by human activity — dust and smog, for example — could easily tip the planet into an ever-deepening cycle of cooling, skeptics have repeatedly pointed out. That wave of concern was obviously a false alarm, the skeptics note, so maybe today’s scientists are equally mistaken about global warming.

Not true, climatologist Thomas C. Peterson of the National Climatic Data Center in Asheville, N.C., and his colleagues report in the September Bulletin of the American Meteorological Society. The team’s survey of major journal papers published between 1965 and 1979 found that only seven articles predicted that global average temperature would continue to cool. During the same period, 44 journal papers indicated that the average temperature would rise and 20 were neutral or made no climate predictions.

The findings were “a surprise to us,” Peterson says. For decades the “skeptics had repeated their argument so often and so strongly that we misremembered the tenor of the times.”

When these skeptics mention previous concerns about global cooling, they typically cite media reports from the 1970s rather than journal papers —“a part of their tremendous smoke screen on this issue,” says Peterson. Among major magazines, Time and Newsweek ran articles expressing concern about the previous decades’ cooling trend, juxtaposing the specter of decreased food production with rising global population.

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CRYING WOLF?Global warming naysayers point to past media coverage of cooling trends to suggest the fallibility of today’s climatologists. But most evidence suggested a warmer future. Science News Archives, Courtesy of Newsweek

But even a cursory review of 1970s media accounts shows that there was no consensus about global cooling among journalists, either, Peterson says. In May 1975, the headline of a New York Times article warned that “major cooling may be ahead.” Three months later, another headline in the same paper — atop a feature written by the same reporter — stated that two recent journal articles “counter [the] view that [a] cold period is due.”

When skeptics do cite a research paper that predicted the possibility of global cooling, it is almost invariably a 1971 article in Science coauthored by Stanford University climatologist Stephen Schneider, then a graduate student at Columbia University. That paper suggested that a fourfold increase in atmospheric aerosols could increase worldwide cooling enough to trigger an ice age.

But soon after the paper was published, new information emerged, Schneider says. First, the global cooling effect of aerosols wasn’t as large as estimated, in part because the tiny particles appeared in high concentrations over only about one-fifth of the planet, primarily around major cities. Second, Schneider adds, scientists discovered that many other minor constituents of the atmosphere — including methane, ozone and man-made gases such as chlorofluorocarbons — have the same warming effect that carbon dioxide does.

By the late 1970s, these realizations, along with insights from studies of the cooling effects of aerosols spewed from an Indonesian volcano in 1963, helped climatologists better estimate the balance between greenhouse gas warming and aerosol-induced cooling. This rapid evolution of understanding, says Schneider, is a testament to the self-correcting nature of the scientific process — a question is posed, data are collected, analyses are performed and then opinions and theories are modified, if need be, based on results of the research.

When global warming skeptics draw misleading comparisons between scientists’ nascent understanding of climate processes in the 1970s and their level of knowledge today, “it’s absolute nonsense,” Schneider says. Back then, scientists were just beginning to study climate trends and their causes, and the probability of finding evidence to disprove a particular hypothesis was relatively high. Nowadays, he contends, “the likelihood of new evidence to overthrow the concept of global warming is small. Warming is virtually certain.”

Most climatologists have long shared a feeling that discussions in the 1970s about global cooling were common in the media but not in scientific journals, says Richard Somerville, a climatologist at Scripps Institution of Oceanography in La Jolla, Calif. Peterson’s research “is a levelheaded, not strident” documentation of that contention. Somerville says the new findings “will not stop the critics [of global warming] from repeating their myth, but for people who are willing to listen with an open mind, this is a nice piece of work.”

Despite the lopsided tally of journal articles that predicted global warming versus those that foretold a long-term cooling trend, the new findings may not sway many hard-core skeptics, says Alan Robock, an atmospheric scientist at Rutgers–New Brunswick in New Jersey. Peterson “is wasting his time by addressing these global warming critics,” he says. “There are only a few of them.”

Back Story | Ups and downs in global temperature

Click the timeline to view an enlarged version.

1870s

Efforts to collect global temperature records begin

1938

First analysis to show long-term warming trend

1960s

First recognition that Earth, on average, had been cooling for two-plus decades

1978

The balance between aerosols’ cooling and greenhouse gases’ warming effects is clear

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