In the 1970s, there was increasing awareness that estimates of global temperatures showed cooling since 1945. Of those scientific papers considering climate trends over the next century, only 10% inclined towards future cooling, while most papers predicted future warming. The general public had little awareness of carbon dioxide's effects on climate, although Paul R. Ehrlich mentions climate change from the greenhouse gases in 1968. By the time the idea of global cooling reached the public press in the mid-1970s, the temperature trend had stopped going down, and there was concern in the climatological community about carbon dioxide's effects. In June 1976, in response to such reports, the World Meteorological Organization issued a warning that a very significant warming of global climate was probable. It was known that both natural and man-made effects caused variations in global climate.
Currently, there are some concerns about the possible cooling effects of a slowdown or shutdown of the thermohaline circulation, which might be provoked by an increase of fresh water mixing into the North Atlantic due to glacial melting. The probability of this occurring is generally considered to be very low, and the IPCC notes, "However, even in models where the THC weakens, there is still a warming over Europe. For example, in all AOGCM integrations where the radiative forcing is increasing, the sign of the temperature change over north-west Europe is positive.
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.
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 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."
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 seminal paper of Hays, Imbrie and Shackleton "Variations in the earths orbit: pacemaker of the ice ages" qualified its predictions with "forecasts must be qualified in two ways. First, they apply only to the natural component of future climatic trends - and not to anthropogenic effects such as those due to the burning of fossil fuels. Second, they describe only the long-term trends, because they are linked to orbital variations with periods of 20,000 years and longer. Climatic oscillations at higher frequencies are not predicted... the results indicate that the long-term trend over the next 20,000 years is towards extensive Northern Hemisphere glaciation and cooler climate"
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
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.
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 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."
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 1970s the 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.
In 1972 Emiliani warned "Man's activity may either precipitate this new ice age or lead to substantial or even total melting of the ice caps". By 1972 a group of glacial-epoch experts at a conference agreed that "the natural end of our warm epoch is undoubtedly near"; but the volume of Quaternary Research reporting on the meeting said that "the basic conclusion to be drawn from the discussions in this section is that the knowledge necessary for understanding the mechanism of climate change is still lamentably inadequate". Unless there were impacts from future human activity, they thought that serious cooling "must be expected within the next few millennia or even centuries"; but many other scientists doubted these conclusions.
The 1970 "Study of Critical Environmental Problems reported the possibility of warming from increased carbon dioxide, but no concerns about cooling, setting a lower bound on the beginning of interest in "global cooling".
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!"
The Board's report of 1974, Science And The Challenges Ahead , continued on this theme. "During the last 20-30 years, world temperature has fallen, irregularly at first but more sharply over the last decade." However discussion of cyclic glacial periods does not feature in this report. Instead it is the role of man that is central to the report's analysis. "The cause of the cooling trend is not known with certainty. But there is increasing concern that man himself may be implicated, not only in the recent cooling trend but also in the warming temperatures over the last century". The report can not conclude whether carbon dioxide in warming, or agricultural and industrial pollution in cooling, are factors in the recent climatic changes, noting; "Before such questions as these can be resolved, major advances must be made in understanding the chemistry and physics of the atmosphere and oceans, and in measuring and tracing particulates through the system."
There also was a study by the U.S. National Academy of Sciences about issues that 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."
The report further stated:
This is not consistent with claims like those of Science & Environmental Policy Project (SEPP) that "the NAS "experts" exhibited ... hysterical fears" in the 1975 report.
While these discussions were ongoing in scientific circles, other accounts 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 [Fahrenheit] in average ground temperatures in the Northern Hemisphere between 1945 and 1968." The article claimed "The evidence in support of these predictions [of global cooling] has now begun to accumulate so massively that meteorologists are hard-pressed to keep up with it." The Newsweek article did not state the cause of cooling; it stated that "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."
The article mentioned the alternative solutions of "melting the Arctic ice cap by covering it with black soot or diverting Arctic rivers" but conceded these were not feasible. The Newsweek article concluded by criticizing government leaders: "But the scientists see few signs that government leaders anywhere are even prepared to take the simple measures of stockpiling food or of introducing the variables of climatic uncertainty into economic projections of future food supplies...The longer the planners (politicians) delay, the more difficult will they find it to cope with climatic change once the results become grim reality." The article emphasized sensational and largely unsourced consequences - "resulting famines could be catastrophic", "drought and desolation," "the most devastating outbreak of tornadoes ever recorded", "droughts, floods, extended dry spells, long freezes, delayed monsoons," "impossible for starving peoples to migrate," "the present decline has taken the planet about a sixth of the way toward the Ice Age."
On October 23, 2006, Newsweek issued a correction, over 31 years after the original article, stating that it had been "so spectacularly wrong about the near-term future" (though editor Jerry Adler claimed that 'the story wasn't "wrong" in the journalistic sense of "inaccurate."') .
In the science series Cosmos: A Personal Voyage, physicist Carl Sagan warned of catastrophic cooling through the burning and clear cutting of forests. He postulated that the increased albedo of the earth's surface might lead to a new ice age. He also mentioned that this may be counteracted and overcome by the release of greenhouse gases. Cosmos was a popular series on public television and was often shown in elementary, junior and senior high schools in the United States.
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 inOther 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.
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
Later in the decade, at a WMO conference in 1979, F K Hare reported that:
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.
Currently, global cooling is of little interest; global warming is an important issue.
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.
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. 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, global cooling was of waning interest by 1979.