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By the 1960s, aerosol pollution ("smog") had become a serious local problem in many cities, and some scientists began to consider whether the cooling effect of particulate pollution could affect global temperatures. Scientists were unsure whether the cooling effect of particulate pollution or warming effect of greenhouse gas emissions would predominate, but regardless, began to suspect that human emissions could be disruptive to climate in the 21st century if not sooner. In his 1968 book The Population Bomb, Paul R. Ehrlich wrote, "the greenhouse effect is being enhanced now by the greatly increased level of carbon dioxide... [this] is being countered by low-level clouds generated by contrails, dust, and other contaminants... At the moment we cannot predict what the overall climatic results will be of our using the atmosphere as a garbage dump."

Efforts to establish a global temperature record that began in 1938 culminated in 1963, when J. Murray Mitchell presented one the first up-to-date temperature reconstructions. His study involved data from over 200 weather stations, collected by the World Weather Records, which was used to calculate latitudinal average temperature. In his presentation, Murray showed that, beginning in 1880, global temperatures increased steadily until 1940. After that, a multi-decade cooling trend emerged. Murray’s work contributed to the overall acceptance of a possible global cooling trend.

In 1965, the landmark report, "Restoring the Quality of Our Environment" by U.S. President Lyndon B. Johnson’s Science Advisory Committee warned of the harmful effects of fossil fuel emissions:

The part that remains in the atmosphere may have a significant effect on climate; carbon dioxide is nearly transparent to visible light, but it is a strong absorber and back radiator of infrared radiation, particularly in the wave lengths from 12 to 18 microns; consequently, an increase of atmospheric carbon dioxide could act, much like the glass in a greenhouse, to raise the temperature of the lower air.

The committee used the recently available global temperature reconstructions and carbon dioxide data from Charles David Keeling and colleagues to reach their conclusions. They declared the rise of atmospheric carbon dioxide levels to be the direct result of fossil fuel burning. The committee concluded that human activities were sufficiently large to have significant, global impact—beyond the area the activities take place. “Man is unwittingly conducting a vast geophysical experiment,” the committee wrote.

Nobel Prize winner Glenn T. Seaborg, Chairperson of the United States Atomic Energy Commission warned of the climate crisis in 1966: "At the rate we are currently adding carbon dioxide to our atmosphere (six billion tons a year), within the next few decades the heat balance of the atmosphere could be altered enough to produce marked changes in the climate--changes which we might have no means of controlling even if by that time we have made great advances in our programs of weather modification."

Scientists increasingly predict warming, 1970s

Mean temperature anomalies during the period 1965 to 1975 with respect to the average temperatures from 1937 to 1946. This dataset was not available at the time.

In the early 1970s, evidence that aerosols were increasing worldwide and that the global temperature series showed cooling encouraged Reid Bryson and some others to warn of the possibility of severe cooling. The questions and concerns put forth by Bryson and others launched a new wave of research into the factors of such global cooling. Meanwhile, the new evidence that the timing of ice ages was set by predictable orbital cycles suggested that the climate would gradually cool, over thousands of years. Several scientific panels from this time period concluded that more research was needed to determine whether warming or cooling was likely, indicating that the trend in the scientific literature had not yet become a consensus. For the century ahead, however, a survey of the scientific literature from 1965 to 1979 found 7 articles predicting cooling and 44 predicting warming (many other articles on climate made no prediction); the warming articles were cited much more often in subsequent scientific literature. Research into warming and greenhouse gases held the greater emphasis, with nearly 6 times more studies predicting warming than predicting cooling, suggesting concern among scientists was largely over warming as they turned their attention toward the greenhouse effect.

John Sawyer published the study Man-made Carbon Dioxide and the “Greenhouse” Effect in 1972. He summarized the knowledge of the science at the time, the anthropogenic attribution of the carbon dioxide greenhouse gas, distribution and exponential rise, findings which still hold today. Additionally he accurately predicted the rate of global warming for the period between 1972 and 2000.

The increase of 25% CO2 expected by the end of the century therefore corresponds to an increase of 0.6°C in the world temperature – an amount somewhat greater than the climatic variation of recent centuries. – John Sawyer, 1972

The first satellite records compiled in the early 1970s showed snow and ice cover over the Northern Hemisphere to be increasing, prompting further scrutiny into the possibility of global cooling. J. Murray Mitchell updated his global temperature reconstruction in 1972, which continued to show cooling. However, scientists determined that the cooling observed by Mitchell was not a global phenomenon. Global averages were changing, largely in part due to unusually severe winters experienced by Asia and some parts of North America in 1972 and 1973, but these changes were mostly constrained to the Northern Hemisphere. In the Southern Hemisphere, the opposite trend was observed. The severe winters, however, pushed the issue of global cooling into the public eye.

The mainstream news media at the time exaggerated the warnings of the minority who expected imminent cooling. For example, in 1975, Newsweek magazine published a story titled “The Cooling World” that warned of "ominous signs that the Earth's weather patterns have begun to change." The article drew on studies documenting the increasing snow and ice in regions of the Northern Hemisphere and concerns and claims by Reid Bryson that global cooling by aerosols would dominate carbon dioxide warming. The article continued by stating that evidence of global cooling was so strong that meteorologists were having "a hard time keeping up with it." On 23 October 2006, Newsweek issued an update stating that it had been "spectacularly wrong about the near-term future". Nevertheless, this article and others like it had long-lasting effects on public perception of climate science.

Such media coverage heralding the coming of a new ice age resulted in beliefs that this was the consensus among scientists, despite this is not being reflected by the scientific literature. As it became apparent that scientific opinion was in favor of global warming, the public began to express doubt over how trustworthy the science was. The argument that scientists were wrong about global cooling, so therefore may be wrong about global warming has been called “the “Ice Age Fallacy” by TIME author Bryan Walsh.

In the first two "Reports for the Club of Rome" in 1972 and 1974, the anthropogenic climate changes by CO₂ increase as well as by waste heat were mentioned. About the latter John Holdren wrote in a study cited in the 1st report, “… that global thermal pollution is hardly our most immediate environmental threat. It could prove to be the most inexorable, however, if we are fortunate enough to evade all the rest.” Simple global-scale estimates that recently have been actualized and confirmed by more refined model calculations show noticeable contributions from waste heat to global warming after the year 2100, if its growth rates are not strongly reduced (below the averaged 2% p.a. which occurred since 1973).

copied from History of climate change science