This article is about the climatological concept of global cooling. For the obsolete geophysical theory about the formation of natural features, see Geophysical Global cooling.
Global cooling in general can refer to a cooling of the Earth. More specifically, it refers to a conjecture during the 1970s of imminent cooling of the Earth's surface and atmosphere along with a posited commencement of glaciation. This hypothesis never had significant scientific support, but gained temporary popular attention due to press reports following a better understanding of ice age cycles and a slight downward trend of temperatures from the 1940s to the early 1970s.
Earth as a whole has not been cooling in recent decades, but is in a period of global warming.

Introduction: general awareness and concern
The cooling period is well reproduced by current (1999 on) Global Climate Models (GCMs) that include the effect of sulphate aerosol cooling, so it (now) seems likely that this was the dominant cause. However, at the time there were two physical mechanisms that were most frequently advanced to cause cooling: aerosols and orbital forcing.

Physical mechanisms
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 effect: 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. In the early 1970s some speculated 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. Although the temperature drops foreseen by this mechanism have now been discarded in light of better theory and the observed warming, aerosols are believed to have contributed a cooling tendency (outweighted by increases in greenhouse gases) and also have contributed to "Global Dimming".

Aerosols
The other mechanism was orbital forcing (Milankovitch cycles): slow changes in the tilt of the planet's 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-1970s.
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 onset (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. Firstly, because the lengths of previous interglacials were not particularly regular; see appended figure. Petit et al. note that interglacials 5.5 and 9.3 are different from the Holocene, but similar to each other in duration, shape and amplitude. During each of these two events, there is a warm period of 4 kyr followed by a relatively rapid cooling. Secondly, future orbital variations will not closely resemble those of the past.

Orbital forcing
The following sections discuss a variety of scientific papers and other sources in an attempt to trace the rise and fall of interest in this concept during the 1970s.

Concern in the Middle of the Twentieth Century
At a conference on climate change held in Boulder, Colorado in 1965, evidence supporting Milankovitch cycles triggered speculation on how the calculated small changes in sunlight might somehow trigger ice ages. In 1966 Cesare Emiliani predicted that "a new glaciation will begin within a few thousand years." In his 1968 book "The Population Bomb", Paul 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."

Pre-1970s
Concern peaked in the early 1970s, partly because of the cooling trend then apparent (a cooling period began in 1945, and two decades of a cooling trend suggested a trough had been reached after several decades of warming), 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 were not possible - because the trend was poorly studied and not understood (for example see reference

1970 SCEP report
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 C. 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.

greenhouse gases such as carbon dioxide;
particulate pollution such as smog, some of which remains suspended in the atmosphere in aerosol form for years. 1971 Paper on Warming and Cooling Factors
The Washington Post reports that in 1974 the National Science Board, the governing body of the National Science Foundation, stated:
During the last 20 to 30 years, world temperature has fallen, irregularly at first but more sharply over the last decade.
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:
Judging from the record of the past interglacial ages, the present time of high temperatures should be drawing to an end . . . leading into the next glacial age.
This quote is taken quite out of context, however, and is misleading as it stands. A more complete quote is:
Judging from the record of the past interglacial ages, the present time of high temperatures should be drawing to an end ... leading into the next glacial age. However, it is possible, or even likely, than human interference has already altered the environment so much that the climatic pattern of the near future will follow a different path. . .

1974 and 1972 National Science Board
There also was a study by the U.S. National Academy of Sciences about issues that needed more research..

1975 National Academy of Sciences report
While these discussions were ongoing in scientific circles, more dramatic accounts appeared in the popular media, notably an April 28, 1975 article in Newsweek magazine.

1975 Newsweek article
In the late 1970s there were several popular (and melodramatic) books on the topic, including The Weather Conspiracy: The Coming of the New Ice Age.
In popular culture The Clash's song London Calling is probably still the most widespread example, the first chorus is:
The ice age is coming, the sun's zooming in Meltdown expected, the wheat is growing thin Engines stop running, but I have no fear Cause London is drowning, and I live by the river Other than indicating the emergence of the idea within pop culture, this song makes little sense: the sun zooming in would cause warming and an ice age lowers sea levels rather than raising them.

Other 1970s Sources
Later in the decade, at a WMO conference in 1979, F K Hare reported that:
"Fig 8 shows [...] 1938 the warmest year. They [temperatures] have since fallen by about 0.4 °C. 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. [p 65]"

1979 WMO conference
Concerns about nuclear winter arose in the early 1980s from several 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. The idea of a global cooling as the result of global warming was already proposed in the 1990's.

Some other climate cooling catastrophes
Thirty years later, the concern that the cooler temperatures would continue, and perhaps at a faster rate, can now be observed to have been incorrect. 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.
As for the prospects of the end of the current interglacial (again, valid only in the absence of human perturbations): it isn't true that interglacials have previously only lasted about 10,000 years; and Milankovitch-type calculations indicate that the present interglacial would probably continue for tens of thousands of years naturally. Other estimates (Loutre and Berger, based on orbital calculations) put the unperturbed length of the present interglacial at 50,000 years. Berger (EGU 2005 presentation) believes that the present CO₂ perturbation will last long enough to suppress the next glacial cycle entirely.

Present level of knowledge
As the NAS report indicates, scientific knowledge regarding climate change was more uncertain 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 chlorofluorocarbons.

See also

Carslaw, K. S.. The Climate Record: The Last Several Centuries and Last Several Decades. Is the Climate Stable?. ENVI2150 Climate Change: Scientific Issues. Retrieved on November 17, 2005.
unknown. History of Continental Drift - Before Wegener. Retrieved on November 17, 2005.
http://openweb.tvnews.vanderbilt.edu/1978-2/1978-02-08-ABC-18.html