There are two possible physical mechanisms that were most frequently advanced to cause cooling: aerosols and orbital forcing.
Human activity (mostly as a by-product of fossil fuel combustion; partly by land-use changes) increases the number of tiny particles (aerosols) in the atmosphere. These have a direct affect (they effectively increase the planetary albedo, thus cooling the planet by reducing the sunshine reaching the surface) and an indirect effect (they can affect the properties of clouds by acting as cloud condensation nuclei). At one time (early 1970's), it seemed possible to speculate that this cooling effect might dominate over the warming effect of the CO2 release: see discussion of Rasool and Schneider (1971), below. As a result of observations (aerosol concentrations may have increased, but not enormously) and a switch to cleaner fuel burning, this no longer seems likely: the overwhelming bulk of current scientific work concentrates on the forcing, prediction and understanding of possible global warming.
The other mechanism is orbital forcing (Milankovitch theory): slow changes in the tilt of the planets axis and shape of the orbit change the total amount of sunlight reaching the earth by a small amount and the seasonality of the sunshine by rather more. This mechanism is believed to be responsible for the timing of the ice age cycles, and understanding of it happened to be increasing rapidly in the mid-1970's. The idea that ice ages cycles were predictable appears to have become conflated with the idea that another one was due "soon" - perhaps because much of this study was done by geologists, who use "soon" to refer to periods of centuries to tens of millennia or more. A strict application of the Milankovitch theory does not allow the prediction of a "rapid" ice age (rapid being anything under a century or two) since the fastest orbital period is about 20,000 years. Some creative ways around this were found, notably Nigel Calder's "snowblitz" theory, but these ideas did not gain wide acceptance.
It is common to see it asserted 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, partly because of the cooling trend then apparent, and partly because much less was then known about world climate and causes of ice ages. Although there was a cooling trend then, it should be realised that climate scientists were perfectly well aware that predictions based on this trend was not possible - because the trend was poorly studied and not understood [1].
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.
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. [1].
The 1970 "Study of Critical Environmental Problems" [1] reported the possibility of warming from increased CO2, but no concerns about cooling, setting a lower bound on the beginning of interest in "global cooling".
There also was a study by the U.S. National Academy of Sciences about issues which needed more research [1]. 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."
The report further stated:
The [Washington Post] reports that in 1974 the National Science Board, the governing body of the National Science Foundation, stated:
Later in the decade, at a WMO conference in 1979, F K Hare reported that:
Thirty years later, the global warming is seen to have continued. The concern that the cooler temperatures would continue, and perhaps at a faster rate, can now be observed to have been wrong. More has to be learned about climate, but the growing records have shown the cooling concerns of 1975 to have been simplistic and not borne out.Aerosols
Orbital forcing
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 [1] suggested a peak had been reached.1970 SCEP report
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:
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 National Academy of Sciences report
This appears to be a clear rebuttal of those, such as SEPP who think that "the NAS
"experts" exhibited ... hysterical fears" in the 1975 report.National Science Board
This statement is correct (see Historical temperature record) although the Washington Post quotes it with disapproval. The Post says the Board had observed two years earlier:
This unsourced quote should be treated with caution, since its context cannot be verified. For example, the 1975 NAS report, on a similar topic, said:
That is, the NAS report makes it clear that the present warm period, viewed only from the Milankovitch perspective, would be expected to end (at some point), but qualifies it by pointing out that anthropogenic interference could change this.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.[1]1979 WMO conference
See [1] for further details.Climate cooling catastrophes
Concerns about nuclear winter arose in the early 1980s from several vague reports. Similar speculations have appeared over effects due to catastrophes such as asteroid impacts and massive volcanic eruptions. A prediction that massive oil well fires in Kuwait would cause significant effects on climate was quite incorrect.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. 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 [1]. 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. As the temperature pattern changed, by 1979 warming from CO2 attracted far more interest than cooling [1].The present level of knowledge