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Global cooling is a concern that the Earth may be ending its current warm period, the climate will cool, and begin the glaciation of an ice age.

Beginning students of ice ages often notice that the length of the current interglacial temperature peak is similar to the length of the preceding interglacial peak (Sangamon/Eem), and from this conclude that we might be nearing the end of this warm period. However, this conclusion is mistaken: future orbital variations will not closely resemble those of the past. Concern peaked in the early 1970s, particularly because much less was then known about world climate and causes of ice ages. Regrettably, sometimes a single obvious example changes to a known fact as a concept spreads to the general public.

1970 Cooling Peak Piqued Interest

The concern that a glaciation may be imminent was particularly visible in the 1970s when the popular press began reporting that possibility. A cooling period began in 1945, and two decades of a cooling trend suggested a peak had been reached.

1971 Paper on Warming and Cooling Factors

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.

1975 NAS report

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."

1975 Newsweek article

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.

Climate science has improved

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. Thus a history of the discovery of global warming states that: While neither scientists nor the public could be sure in the 1970s whether the world was warming or cooling, people were increasingly inclined to believe that global climate was on the move, and in no small way. . However, at least by 1979 warming from CO2 attracted far more interest than cooling . 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 . Early in that decade, carbon dioxide was the only widely studied human-influenced greenhouse gas. The attention drawn to atmospheric gases in the 1970s stimulated many discoveries in future decades.

That was Now, Now that is Then

The term "global cooling" did not become attached to concerns about an impending glacial period until after the term "global warming" was popularized. In the 1970's compilation of records to produce hemispheric, or global, temperature records had just begun. Thus, at a WMO conference in 1979, F K Hare reported that:

"Fig 8 shows... 1938 the warmest year. They have since fallen by about 0.4 oC. At the end there is a suggestion that the fall ceased in about 1964, and may even have reversed.

Figure 9 challenges the view that the fall of temperature has ceased... the weight of evidence clearly favours cooling to the present date... The striking point, however, is that interannual variability of world temperatures is much larger than the trend... it is difficult to detect a genuine trend...

It is questionable, moreover, whether the trend is truly global. Calculated variations in the 5-year mean air temperature over the southern hemisphere chiefly with respect to land areas show that temperatures generally rose between 1943 and 1975. Since the 1960-64 period this rise has been strong... the scattered SH data fail to support a hypothesis of continued global cooling since 1938. "

See for further details.