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Students of ice ages often notice that the length of recent interglacials seems similar to the length of the preceding interglacial period. This implies that we might be nearing the end of this warm period.

The concern that an ice age may be imminent was particularly visible in the 1970s when the popular press began reporting that possibility. A cooling period began in 1945, and a thirty year cooling trend suggested a peak had been reached.

There was a paper by S. Ichtiaque Rasool and Stephen H. Schneider, published in the journal Science in July 1971. Titled "Atmospheric Carbon Dioxide and Aerosols: Effects of Large Increases on Global Climate," the paper examined the possible future effects of two types of human environmental emissions:

1. greenhouse gases such as carbon dioxide;
2. particulate pollution such as smog, some of which remains suspended in the atmosphere in aerosol form for years.

Greenhouse gases were regarded as likely factors that could promote global warming, while particulate pollution blocks sunlight and contributes to cooling. In their paper, Rasool and Schneider theorized that aerosols were more likely to contribute to climate change in the foreseeable future than greenhouse gases, stating that quadrupling aerosols "could decrease the mean surface temperature (of Earth) by as much as 3.5 degrees K. If sustained over a period of several years, such a temperature decrease could be sufficient to trigger an ice age!" As this passage demonstrates, however, Rasool and Schneider considered global cooling a possible future scenario, but they did not predict it.

There also was a study by the U.S. National Academy of Sciences about issues which needed more research. This heightened interest in the fact that climate can change. The 1975 NAS report titled "Understanding Climate Change: A Program for Action" did not make predictions, stating in fact 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." Its "program for action" consisted simply of a call for further research, because "it is only through the use of adequately calibrated numerical models that we can hope to acquire the information necessary for a quantitative assessment of the climatic impacts."

At the same time that these discussions were ongoing in scientific circles, a more dramatic account appeared in the popular media, notably an April 28, 1975 article in Newsweek magazine. Titled "The Cooling World," it pointed to "ominous signs that the Earth's weather patterns have begun to change" and pointed to "a drop of half a degree in average ground temperatures in the Northern Hemisphere between 1945 and 1968." However, the Newsweek article did not make "environmentalist" claims regarding the cause of that drop. To the contrary, it stated that "what what causes the onset of major and minor ice ages remains a mystery" and cited the NAS conclusion that "Not only are the basic scientific questions largely unanswered, but in many cases we do not yet know enough to pose the key questions." Rather than proposing environmentalist solutions, the Newsweek article suggested that "simple measures of stockpiling food or of introducing the variables of climatic uncertainty into economic projections of future food supplies" would be appropriate.

As the NAS report and the article in Newsweek both indicate, the scientific knowledge regarding climate change was more uncertain then than it is today. At the time that Rasool and Schneider wrote their 1971 paper, climatologists had not yet recognized the significance of greenhouse gases other than water vapor and carbon dioxide, such as methane, nitrous oxide and chloroflourocarbons. As a result, the climatological community was more concerned about possible cooling of the planet by the particulate pollution humanity was injecting into the atmosphere than about warming induced by greenhouse gases.

The term "global cooling" did not become attached to concerns about an impending glacial period until after the term "global warming" was popularized.

We don't know what has triggered past continental glacial periods, but recent studies indicate ice ages may start and end abruptly so it probably would be quickly apparent if a glacial period has begun. But in 1975 it seemed apparent that temperatures were going down.