Journalists covering the climate change issue for any period of time quickly run across arguments that the big concern just a few decades back had involved global cooling and not global warming.

They will do well to step back and look hard at those claims to see if they really hold up.

“There are ominous signs that the earth’s weather patterns have begun to change dramatically,” Newsweek reported in 1975. The influential newsweekly warned of a potential “drastic decline in food production.” These claims, spectacularly disproved by the rising global temperatures over the past 30 years, keep resurfacing in efforts to discredit current predictions of global warming.

For example, popular novelist Michael Crichton wrote in his novel State of Fear that, “in the 1970’s all the climate scientists believed an ice age was coming.”

Similarly, Senator James Inhofe (R-OK), the ranking Republican member of the Senate Environment and Public Works Committee, often points to what he calls the “global cooling scare” as a reason to distrust the current alarm over climate change.

But was there really a scientific consensus in the 1970s that global cooling was imminent? And is it comparable to current concerns about global warming? An examination of the scientific literature suggests that the 1975 Newsweek piece relied on highly speculative articles that did not reflect the consensus of the time, whereas the current concern over climate change is supported by the vast consensus of the scientific community.

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Northern hemisphere surface temperature trend as seen in 1968.

Falling global temperatures from 1940 to 1975 were one of the prime drivers of popular concern about global cooling. We now know, and suspected at the time, that the cooling was indeed strongly influenced by humans as a result of industrial emissions of aerosol particles (see Wikipedia).

These aerosols, primarily in the form of sulfur dioxide and nitrogen oxide, scatter and reflect incoming light, cooling Earth’s surface. As cloud condensation nuclei, aerosols also help increase the amount of clouds covering Earth’s surface and reflecting additional sunlight away from the planet. These aerosols, coupled with increased volcanic activity, led to a period of falling global temperatures.

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Attribution of observed 20th century climate change. Note the strong negative sulfate (aerosol) forcings from 1940 onward.

In 1971, S. Ichtiaque Rasool and Stephen H. Schneider published a paper in Science titled “Atmospheric Carbon Dioxide and Aerosols: Effects of Large Increases on Global Climate.” They predicted in the paper that rapidly increasing levels of sulfate in the atmosphere could cause the global temperature to continue to fall.

In retrospect, however, it is clear that they both overestimated the cooling effect of aerosols and underestimated the positive forcing of greenhouse gas emissions (primarily CO2 and methane), emissions of which were beginning their rapid rise at the time.

Additionally, Rasool and Schneider’s scenario for aerosol emissions proved overly pessimistic, as concern over local air quality and acid rain led to federal action requiring the widespread adoption of smokestack scrubbers, significantly reducing the sulfate emissions from many power plants.

In addition to concerns over aerosols, the global cooling worries were stoked by emerging research on the role of changes in orbital variations (called Milankovitch cycles) in driving periodic ice ages. Scientists noticed that, compared with past glacial and interglacial periods, the current warm period had lasted an unusually long time. Calculations at the time suggested that Earth was due for another ice age, though it was projected to be a gradual process over the next 10,000 years.

More recently, however, these projections have been considerably revised. As William M. Connolley writes, “Interpretations of future changes in the Earth’s orbit have changed somewhat. It now seems likely that the current interglacial, based purely on natural forcing, would last for an exceptionally long time: perhaps 50,000 years.”

In 1975, the U.S. National Academy of Sciences weighed in on the debate, arguing that “… we do not have a good quantitative understanding of our climate machine and what determines its course. Without the fundamental understanding, it does not seem possible to predict climate …”. While a few articles speculated on the cooling trend at the time, the potential role of aerosols in the atmosphere, and the long-term glacial cycles, there was little grounds for a firm consensus that Earth was poised to experience a new ice age in any time span relevant to human kind.

Since the 1970s, our understanding of the earth’s climate system has increased enormously. In 2001, the National Academy of Sciences released a report on another growing climate concern: global warming. The Academy’s 2001 report says:

Greenhouse gases are accumulating in Earth’s atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise. Temperatures are, in fact, rising. The changes observed over the last several decades are likely mostly due to human activities …

The current support for the theory of human-caused global warming is far more robust scientifically than anything seen in the 1970s. Surveys of the peer-reviewed literature have shown little fundamental disagreement with the consensus position.

While a few papers in the 1970s speculated on potential cooling, billions of dollars have been spent researching current warming, producing thousands of scientific papers and massive periodic synthesis reports by thousands of climate scientists.

The speculation on aerosol-driven cooling in the 1970s was legitimate in an atmosphere of vast uncertainty. Using it to cast doubt on projections of current warming in the face of mounting scientific understanding is an entirely different matter. Journalists should not be lulled into thinking otherwise.

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