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Global cooling is (or perhaps more appropriately was) a concern that the Earth may be ending its current warm period, the climate will cool, and perhaps even begin the glaciation of an ice age.

Note: An obsolete geology meaning referring to the Earth's surface wrinkling due to contracting rock due to cooling is mentioned in a note at the end of this article.

Introduction

In the 1970s there was increasing awareness that estimates of global temperatures showed cooling since 1945. Textbooks showed paleoclimate records of natural variability during the past millions of years which included sudden drops toward an ice age, so popular concern was focused on that extreme. However, not long after the awareness reached the public press in the mid-1970s the temperature trend stopped going down.

Physical mechanisms

The cooling period is currently well reproduced by GCMs that include the affect of sulphate aerosol cooling, so it (now) seems likely that this was the cause. At the time there were two physical mechanisms that are 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 (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). 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.

Orbital forcing

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 in the Middle of the Twentieth Century

1970s Awareness

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 peak 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 was not possible - because the trend was poorly studied and not understood (for example:). However in the popular press the possibility of cooling was reported generally without the caveats present in the scientific reports.

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. .

1970 SCEP report

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".

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 National Academy of Sciences 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."

The report further stated:

The climates of the earth have always been changing, and they will doubtless continue to do so in the future. How large these future changes will be, and where and how rapidly they will occur, we do not know. .

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.

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 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.

1974 National Science Board

The 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 unsourced quote should be treated with caution, since its context cannot be verified. For example, the 1975 NAS report, on a similar topic, said:

If the end of the interglacial is episodic in character, we are moving toward a rather sudden climatic change of unknown timing, although as each 100 years passes, we have perhaps a 5% greater chance of encountering its onset. If, on the other hand, these changes are more sinusiondal in character, then the climate should decline gradually over a period of thousands of years.... These climatic projections, however, could be replaced by quite different future climatic scenarios due to man's inadvertent interference with the otherwise natural variation...

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.

1979 WMO conference

Later in the decade, 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.

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. The 2004 disaster movie The Day After Tomorrow, based on the book The Coming Global Superstorm, by Art Bell and Whitley Strieber, depicted a scientifically implausible assortment of climate disasters caused by global warming, including sudden freezing.

The present level of knowledge

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.

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 . 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 .

Historical geophysical meaning

Before the concept of plate tectonics, global cooling was a reference to a geophysical theory, The Contracting Earth by James Dwight Dana, that the Earth had been in a molten state, and features such as mountains formed as it cooled and shrank.

External link

• Discussion and quotes from various papers about the "1970s prediction of an imminent ice age"