Global cooling
Global cooling was a conjecture, especially during the 1970s, of imminent cooling of the Earth culminating in a period of extensive glaciation, due to the cooling effects of aerosols or orbital forcing.
Some press reports in the 1970s speculated about continued cooling; these did not accurately reflect the scientific literature of the time, which was generally more concerned with warming from an enhanced greenhouse effect.
In the mid 1970s, the limited temperature series available suggested that the temperature had decreased for several decades up to then. As longer time series of higher quality became available, it became clear that global temperature showed significant increases overall.
Introduction: general awareness and concern
By the 1970s, scientists were becoming increasingly aware that estimates of global temperatures showed cooling since 1945, as well as the possibility of large scale warming due to emissions of greenhouse gases. In the scientific papers which considered climate trends of the 21st century, fewer than 10% were inclined towards future cooling, while most papers predicted future warming. The general public had little awareness of carbon dioxide's effects on climate, but Science News in May 1959 forecast a 25% increase in atmospheric carbon dioxide in the 150 years from 1850 to 2000, with a consequent warming trend. The actual increase in this period was 29%. Paul R. Ehrlich mentioned global warming from greenhouse gases as a counterforce to the cooling effect of aerosols in 1968. By the time the idea of global cooling reached the public press in the mid-1970s temperatures had stopped falling, and there was concern in the climatological community about carbon dioxide's warming effects. In response to such reports, the World Meteorological Organization issued a warning in June 1976 that "a very significant warming of global climate" was probable.Currently, there are some concerns about the possible regional cooling effects of a slowdown or shutdown of 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, "even in models where the THC weakens, there is still 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."
Physical mechanisms
The cooling period is reproduced by current global climate models that include the physical effects of sulfate aerosols, and there is now general agreement that aerosol effects were the dominant cause of the mid-20th century cooling. At the time there were two physical mechanisms that were most frequently advanced to cause cooling: aerosols and orbital forcing.Aerosols
Human activity — mostly as a by-product of fossil fuel combustion, partly by land use changes — increases the number of tiny particles in the atmosphere. These have a direct effect: they effectively increase the planetary albedo, thus cooling the planet by reducing the solar radiation reaching the surface; and an indirect effect: they 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, below. As a result of observations and a switch to cleaner fuel burning, this no longer seems likely; current scientific work indicates that global warming is far more likely. Although the temperature drops foreseen by this mechanism have now been discarded in light of better theory and the observed warming, aerosols are thought to have contributed a cooling tendency and also have contributed to global dimming.Orbital forcing
Orbital forcing refers to the slow, cyclical changes in the tilt of Earth's axis and shape of its orbit. These cycles alter the total amount of sunlight reaching the Earth by a small amount and affect the timing and intensity of the seasons. This mechanism is thought to be responsible for the timing of the ice age cycles, and understanding of the mechanism was increasing rapidly in the mid-1970s.The paper of Hays, Imbrie, and Shackleton "Variations in the Earth's Orbit: Pacemaker of the Ice Ages" qualified its predictions with the remark that "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 age 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 are accustomed to dealing with very long time scales and use "soon" to refer to periods of thousands of years. A strict application of the Milankovitch theory does not allow the prediction of a "rapid" ice age onset since the fastest orbital period is about 20,000 years. Some creative ways around this were found, notably one championed by Nigel Calder under the name of "snowblitz", but these ideas did not gain wide acceptance.
The length of the current interglacial temperature peak is similar to the length of the preceding interglacial peak, and so it could be concluded that we might be nearing the end of this warm period. This conclusion would be mistaken. Firstly, because the lengths of previous interglacials were not particularly regular; see 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.
Concern pre-1970s
In 1923, there was concern about a new ice age and Captain Donald Baxter MacMillan sailed toward the Arctic sponsored by the National Geographical Society to look for evidence of advancing glaciers.In 1926, a Berlin astronomer was predicting global cooling but that it was "ages away".
Concerns that a new ice age was approaching was revived in the 1950s. During the Cold War, there were concerns by Harry Wexler that setting off atom bombs could be hastening a new ice age from a nuclear winter scenario.
J. Murray Mitchell showed as early as 1963 a multidecadal cooling since about 1940. 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 ... 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."
Concern in the 1970s
1970s awareness
Concern peaked in the early 1970s, though "the possibility of anthropogenic warming dominated the peer-reviewed literature even then" . This peaking concern is partially attributable to the fact much less was then known about world climate and causes of ice ages. Climate scientists were aware that predictions based on this trend were not possible - because the trend was poorly studied and not understood. Despite that, in the popular press the possibility of cooling was reported generally without the caveats present in the scientific reports, and "unusually severe winters in Asia and parts of North America in 1972 and 1973 ... pushed the issue into the public consciousness".In the 1970s, the compilation of records to produce hemispheric, or global, temperature records had just begun.
Spencer R. Weart's history of The Discovery of Global Warming says 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" .
On January 11, 1970, The Washington Post reported that "Colder Winters Held Dawn of New Ice Age".
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".
Also in 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". George Kukla and Robert Matthews, in a Science write-up of a conference, asked when and how the current interglacial would end; concluding that, unless there were impacts from future human activity, "Global cooling and related rapid changes of environment, substantially exceeding the fluctuations experienced by man in historical times, must be expected within the next few millennia or even centuries", but many other scientists doubted these conclusions.