Global cooling

Global cooling

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Global cooling was a conjecture
Conjecture
A conjecture is a proposition that is unproven but is thought to be true and has not been disproven. Karl Popper pioneered the use of the term "conjecture" in scientific philosophy. Conjecture is contrasted by hypothesis , which is a testable statement based on accepted grounds...

 during the 1970s
1970s
File:1970s decade montage.png|From left, clockwise: US President Richard Nixon doing the V for Victory sign after his resignation from office after the Watergate scandal in 1974; Refugees aboard a US naval boat after the Fall of Saigon, leading to the end of the Vietnam War in 1975; The 1973 oil...

 of imminent cooling of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's surface and atmosphere along with a posited commencement of glaciation. This hypothesis had mixed support in the scientific community
Scientific community
The scientific community consists of the total body of scientists, its relationships and interactions. It is normally divided into "sub-communities" each working on a particular field within science. Objectivity is expected to be achieved by the scientific method...

, but gained temporary popular attention due to a combination of a slight downward trend of temperatures from the 1940s to the early 1970s and press reports
Newspaper
A newspaper is a scheduled publication containing news of current events, informative articles, diverse features and advertising. It usually is printed on relatively inexpensive, low-grade paper such as newsprint. By 2007, there were 6580 daily newspapers in the world selling 395 million copies a...

 that did not accurately reflect the scientific understanding of ice age
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...

 cycles. In contrast to the global cooling conjecture, the current scientific opinion on climate change
Scientific opinion on climate change
The predominant scientific opinion on climate change is that the Earth is in an ongoing phase of global warming primarily caused by an enhanced greenhouse effect due to the anthropogenic release of carbon dioxide and other greenhouse gases...

 is that the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 has not durably cooled, but undergone global warming
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...

 throughout the twentieth century.

Introduction: general awareness and concern


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 21st 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, but Science News
Science News
Science News is an American bi-weekly magazine devoted to short articles about new scientific and technical developments, typically gleaned from recent scientific and technical journals. Science News has been published since 1922 by Society for Science & the Public, a non-profit organization...

 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
Paul R. Ehrlich
Paul Ralph Ehrlich is an American biologist and educator who is the Bing Professor of Population Studies in the department of Biological Sciences at Stanford University and president of Stanford's Center for Conservation Biology. By training he is an entomologist specializing in Lepidoptera , but...

 mentioned climate change from greenhouse gases 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
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

's warming effects. In response to such reports, the World Meteorological Organization
World Meteorological Organization
The World Meteorological Organization is an intergovernmental organization with a membership of 189 Member States and Territories. It originated from the International Meteorological Organization , which was founded in 1873...

 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
Shutdown of thermohaline circulation
Shutdown or slowdown of the thermohaline circulation is a postulated effect of global warming.There is some speculation that global warming could, via a shutdown or slowdown of the thermohaline circulation, trigger localised cooling in the North Atlantic and lead to cooling, or lesser warming, in...

, which might be provoked by an increase of fresh water
Fresh Water
Fresh Water is the debut album by Australian rock and blues singer Alison McCallum, released in 1972. Rare for an Australian artist at the time, it came in a gatefold sleeve...

 mixing into the North Atlantic due to glacial melting
Glacier
A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...

. The probability
Probability
Probability is ordinarily used to describe an attitude of mind towards some proposition of whose truth we arenot certain. The proposition of interest is usually of the form "Will a specific event occur?" The attitude of mind is of the form "How certain are we that the event will occur?" The...

 of this occurring is generally considered to be very low, and the IPCC
Intergovernmental Panel on Climate Change
The Intergovernmental Panel on Climate Change is a scientific intergovernmental body which provides comprehensive assessments of current scientific, technical and socio-economic information worldwide about the risk of climate change caused by human activity, its potential environmental and...

 notes, "even in models where the THC
Thermohaline circulation
The term thermohaline circulation refers to a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes....

 weakens, there is still a warming over Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

. For example, in all AOGCM
Global climate model
A General Circulation Model is a mathematical model of the general circulation of a planetary atmosphere or ocean and based on the Navier–Stokes equations on a rotating sphere with thermodynamic terms for various energy sources . These equations are the basis for complex computer programs commonly...

 integrations where the radiative forcing
Radiative forcing
In climate science, radiative forcing is generally defined as the change in net irradiance between different layers of the atmosphere. Typically, radiative forcing is quantified at the tropopause in units of watts per square meter. A positive forcing tends to warm the system, while a negative...

 is increasing, the sign of the temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 change over north-west Europe is positive."

Physical mechanisms


The cooling period is well reproduced by current (1999 on) global climate model
Global climate model
A General Circulation Model is a mathematical model of the general circulation of a planetary atmosphere or ocean and based on the Navier–Stokes equations on a rotating sphere with thermodynamic terms for various energy sources . These equations are the basis for complex computer programs commonly...

s (GCMs) that include the physical effects of sulphate aerosols, and there is now general agreement that aerosol
Aerosol
Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can...

 effects were the dominant cause of the mid-20th century cooling. However, 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
Fossil fuel
Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...

 combustion
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...

, partly by land use
Land use
Land use is the human use of land. Land use involves the management and modification of natural environment or wilderness into built environment such as fields, pastures, and settlements. It has also been defined as "the arrangements, activities and inputs people undertake in a certain land cover...

 changes — increases the number of tiny particles (aerosol
Aerosol
Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can...

s) in the atmosphere. These have a direct effect: they effectively increase the planetary albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

, thus cooling the planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

 by reducing the solar radiation reaching the surface; and an indirect effect: they affect the properties of clouds by acting as cloud condensation nuclei
Cloud condensation nuclei
Cloud condensation nuclei or CCNs are small particles typically 0.2 µm, or 1/100 th the size of a cloud droplet ) about which cloud droplets coalesce. Water requires a non-gaseous surface to make the transition from a vapour to a liquid. In the atmosphere, this surface presents itself as tiny...

. In the early 1970s some speculated that this cooling effect might dominate over the warming effect of the CO2
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 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; current scientific work indicates that global warming
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...

 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 believed to have contributed a cooling tendency (outweighted by increases in greenhouse gases) and also have contributed to "Global Dimming
Global dimming
Global dimming is the gradual reduction in the amount of global direct irradiance at the Earth's surface that was observed for several decades after the start of systematic measurements in the 1950s. The effect varies by location, but worldwide it has been estimated to be of the order of a 4%...

."

Orbital forcing



Orbital forcing refers to the slow, cyclical changes
Milankovitch cycles
Milankovitch theory describes the collective effects of changes in the Earth's movements upon its climate, named after Serbian civil engineer and mathematician Milutin Milanković, who worked on it during First World War internment...

 in the tilt of Earth's axis and shape of its orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

. 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 believed to be responsible for the timing of the ice age
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...

 cycles
Climate change
Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...

, and understanding of the mechanism was increasing rapidly in the mid-1970s.

The seminal paper of Hays, Imbrie and Shackleton Variations in the Earth's 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 period
Orbital period
The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...

s 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 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
Milutin Milankovic
Milutin Milanković was a Serbian geophysicist and civil engineer, best known for his theory of ice ages, suggesting a relationship between Earth's long-term climate changes and periodic changes in its orbit, now known as Milankovitch cycles. Milanković gave two fundamental contributions to global...

 theory does not allow the prediction of a "rapid" ice age onset (i.e., less than a century or two) since the fastest orbital period is about 20,000 years. Some creative ways around this were found, notably one championed by Nigel Calder
Nigel Calder
Nigel Calder is a British science writer.Between 1956 and 1966, Calder wrote for the magazine New Scientist, serving as editor from 1962 until 1966...

 under the name of "snowblitz", but these ideas did not gain wide acceptance.

It is common to see it asserted that the length of the current interglacial
Interglacial
An Interglacial period is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age...

 temperature peak is similar to the length of the preceding interglacial peak (Sangamon/Eem
Eem
The Eem is a river in the north of the Utrecht Province in the Netherlands with a length of approximately .The river is fed by the Vallei Canal and a number of Veluwe creeks, the most important of which are the Heiligenberger Beek, the Barneveldse Beek and the Lunterse Beek, all of which come...

), and from this conclude that we might be nearing the end of this warm period. This conclusion is supported by the fact that the lengths of previous interglacials were regular; see appended figure. Petit et al. note that "Interglacials 5.5 and 9.3 are different from the Holocene
Holocene
The Holocene is a geological epoch which began at the end of the Pleistocene and continues to the present. The Holocene is part of the Quaternary period. Its name comes from the Greek words and , meaning "entirely recent"...

, but similar to each other in duration, shape and amplitude." During each of these two events, there is a warm period of 4000 years followed by a relatively rapid cooling. As an objection, the future orbital variations will not closely resemble those of the past.

Pre-1970s


J. Murray Mitchell showed as early as 1963 a multidecadal cooling since about 1940. At a conference on climate change held in Boulder, Colorado
Boulder, Colorado
Boulder is the county seat and most populous city of Boulder County and the 11th most populous city in the U.S. state of Colorado. Boulder is located at the base of the foothills of the Rocky Mountains at an elevation of...

 in 1965, evidence supporting Milankovitch cycles
Milankovitch cycles
Milankovitch theory describes the collective effects of changes in the Earth's movements upon its climate, named after Serbian civil engineer and mathematician Milutin Milanković, who worked on it during First World War internment...

 triggered speculation on how the calculated small changes in sunlight might somehow trigger ice ages. In 1966 Cesare Emiliani
Cesare Emiliani
Cesare Emiliani was an Italian-American scientist, considered one of the greatest geologists and micropaleontologists of the 20th century and the founder of paleoceanography, developing the timescale of marine isotope stages, which despite modifications remains in very wide use today.He...

 predicted that "a new glaciation will begin within a few thousand years." In his 1968 book The Population Bomb
The Population Bomb
The Population Bomb was a best-selling book written by Paul R. Ehrlich and his wife, Anne Ehrlich , in 1968. It warned of the mass starvation of humans in the 1970s and 1980s due to overpopulation, as well as other major societal upheavals, and advocated immediate action to limit population growth...

, Paul R. Ehrlich
Paul R. Ehrlich
Paul Ralph Ehrlich is an American biologist and educator who is the Bing Professor of Population Studies in the department of Biological Sciences at Stanford University and president of Stanford's Center for Conservation Biology. By training he is an entomologist specializing in Lepidoptera , but...

 wrote "The greenhouse effect
Greenhouse effect
The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...

 is being enhanced now by the greatly increased level of carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

... [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
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

 as a garbage dump."

1970s awareness



Concern peaked in the early 1970s, though "the possibility of anthropogenic warming dominated
the peer-reviewed literature even then" (a cooling period began in 1945, and two decades of a cooling trend suggested a trough had been reached after several decades of warming). This peaking concern is partially attributable to the fact much less was then known about world climate and causes of ice ages. However, climate scientists were aware that predictions based on this trend were not possible - because the trend was poorly studied and not understood (for example see reference). 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.

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.

In 1972, George Kukla
George Kukla
George Kukla is a senior research scientist at the Lamont-Doherty Earth Observatory of Columbia University.Dr. Kukla was a member of the Czechoslovakian Academy of Sciences, prior to emigrating to the US, and a pioneer in the field of astronomical climate forcing...

 and Robert Matthews, in a Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....

 write-up of a conference, asked when and how the current integlacial would end; concluding that "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."

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
Stephen Schneider
Stephen Henry Schneider was Professor of Environmental Biology and Global Change at Stanford University, a Co-Director at the Center for Environment Science and Policy of the Freeman Spogli Institute for International Studies and a Senior Fellow in the Stanford Woods Institute for the Environment...

, 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
Pollution
Pollution is the introduction of contaminants into a natural environment that causes instability, disorder, harm or discomfort to the ecosystem i.e. physical systems or living organisms. Pollution can take the form of chemical substances or energy, such as noise, heat or light...

 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, they calculated, such a temperature decrease could be sufficient to trigger an ice age.

1972 and 1974 National Science Board


The National Science Board
National Science Board
The National Science Board of the United States is composed of 25 members appointed by the President and confirmed by the United States Senate, representing the broad U.S. science and engineering community. The Board establishes the policies of the National Science Foundation within the framework...

's Patterns and Perspectives in Environmental Science report of 1972 discussed the cyclical behavior of climate, and the understanding at the time that the planet was entering a phase of cooling after a warm period. "Judging from the record of the past interglacial ages, the present time of high temperatures should be drawing to an end, to be followed by a long period of considerably colder temperatures leading into the next glacial age some 20,000 years from now." But it also continued; "However, it is possible, or even likely, that human interference has already altered the environment so much that the climatic pattern of the near future will follow a different path."

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 period
Glacial period
A glacial period is an interval of time within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate within an ice age...

s 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
Particulates
Particulates – also known as particulate matter , suspended particulate matter , fine particles, and soot – are tiny subdivisions of solid matter suspended in a gas or liquid. In contrast, aerosol refers to particles and/or liquid droplets and the gas together. Sources of particulate matter can be...

 through the system."

1975 National Academy of Sciences report


There also was a study by the U.S. National Academy of Sciences
United States National Academy of Sciences
The National Academy of Sciences is a corporation in the United States whose members serve pro bono as "advisers to the nation on science, engineering, and medicine." As a national academy, new members of the organization are elected annually by current members, based on their distinguished and...

 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
Calibration
Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....

 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..


The Science & Environmental Policy Project
Science & Environmental Policy Project
The Science & Environmental Policy Project is a research and advocacy group financed by private contributions based in Arlington, Virginia in the United States. It was founded in 1990 by atmospheric physicist S. Fred Singer. SEPP disputes the prevailing scientific views of climate change and ozone...

 (SEPP), an organization which has no recognised scientific standing and has previously refuted the link between tobacco and cancer, claims that "the NAS "experts" exhibited ... hysterical fears" in the 1975 report.

1974 Time Magazine article


While these discussions were ongoing in scientific circles, other accounts appeared in the popular media. In their June 24, 1974 issue, Time
Time (magazine)
Time is an American news magazine. A European edition is published from London. Time Europe covers the Middle East, Africa and, since 2003, Latin America. An Asian edition is based in Hong Kong...

 presented an article titled Another Ice Age? that noted "the atmosphere has been growing gradually cooler for the past three decades" but noted that "Some scientists... think that the cooling trend may be only temporary"

1975 Newsweek article


An April 28, 1975 article in Newsweek
Newsweek
Newsweek is an American weekly news magazine published in New York City. It is distributed throughout the United States and internationally. It is the second-largest news weekly magazine in the U.S., having trailed Time in circulation and advertising revenue for most of its existence...

 magazine was 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."').

Other 1970s sources


In the late 1970s there were several popular books on the topic, including The Weather Conspiracy: The Coming of the New Ice Age.

1979 WMO conference


Later in the decade, at a WMO conference in 1979, F K Hare reported that:

1980s


Concerns about nuclear winter
Nuclear winter
Nuclear winter is a predicted climatic effect of nuclear war. It has been theorized that severely cold weather and reduced sunlight for a period of months or even years could be caused by detonating large numbers of nuclear weapons, especially over flammable targets such as cities, where large...

 arose in the early 1980s from several reports. Similar speculations have appeared over effects due to catastrophes such as asteroid impacts
Impact event
An impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Throughout recorded history, hundreds of minor impact events have been reported, with some occurrences causing deaths, injuries, property damage or other significant...

 and massive volcanic eruptions
Yellowstone Caldera
The Yellowstone Caldera is the volcanic caldera located in Yellowstone National Park in the United States, sometimes referred to as the Yellowstone Supervolcano. The caldera is located in the northwest corner of Wyoming, in which the vast majority of the park is contained. The major features of...

. A prediction
Kuwaiti oil fires
The Kuwaiti oil fires were caused by Iraqi military forces setting fire to 700 oil wells as part of a scorched earth policy while retreating from Kuwait in 1991 after invading the country but being driven out by Coalition military forces...

 that massive oil well fires in Kuwait
Kuwait
The State of Kuwait is a sovereign Arab state situated in the north-east of the Arabian Peninsula in Western Asia. It is bordered by Saudi Arabia to the south at Khafji, and Iraq to the north at Basra. It lies on the north-western shore of the Persian Gulf. The name Kuwait is derived from the...

 would cause significant effects on climate was quite incorrect.

1990s


The idea of a global cooling as the result of global warming
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...

 was already proposed in the 1990s. In 2003, the Office of Net Assessment
Office of Net Assessment
The United States Department of Defense's Office of Net Assessment was created in 1973. The Director of Net Assessment is the principal staff assistant and advisor to the Secretary and Deputy Secretary of Defense on net assessment matters...

 at the United States Department of Defense
United States Department of Defense
The United States Department of Defense is the U.S...

 was commissioned to produce a study on the likely and potential effects of a modern climate change, especially of a shutdown of thermohaline circulation
Shutdown of thermohaline circulation
Shutdown or slowdown of the thermohaline circulation is a postulated effect of global warming.There is some speculation that global warming could, via a shutdown or slowdown of the thermohaline circulation, trigger localised cooling in the North Atlantic and lead to cooling, or lesser warming, in...

. The study, conducted under ONA head Andrew Marshall
Andrew Marshall (foreign policy strategist)
Andrew W. Marshall is the director of the United States Department of Defense's Office of Net Assessment. Appointed to the position in 1973 by United States President Richard Nixon, Marshall has been re-appointed by every president that followed....

, modelled its prospective climate change on the 8.2 kiloyear event
8.2 kiloyear event
The 8.2 kiloyear event is the term that climatologists have adopted for a sudden decrease in global temperatures that occurred approximately 8,200 years before the present, or c. 6,200 BCE, and which lasted for the next two to four centuries...

, precisely because it was the middle alternative between the Younger Dryas
Younger Dryas
The Younger Dryas stadial, also referred to as the Big Freeze, was a geologically brief period of cold climatic conditions and drought between approximately 12.8 and 11.5 ka BP, or 12,800 and 11,500 years before present...

 and the Little Ice Age
Little Ice Age
The Little Ice Age was a period of cooling that occurred after the Medieval Warm Period . While not a true ice age, the term was introduced into the scientific literature by François E. Matthes in 1939...

. The study caused controversy in the media when it was made public in 2004. However, scientists acknowledge that “abrupt climate change initiated by Greenland ice sheet
Greenland ice sheet
The Greenland ice sheet is a vast body of ice covering , roughly 80% of the surface of Greenland. It is the second largest ice body in the world, after the Antarctic Ice Sheet. The ice sheet is almost long in a north-south direction, and its greatest width is at a latitude of 77°N, near its...

 melting is not a realistic scenario for the 21st century.”.

Present level of knowledge


Currently, the concern that cooler temperatures would continue, and perhaps at a faster rate, has been observed to be incorrect by the IPCC. More has to be learned about climate, but the growing records have shown that the cooling concerns of 1975 have not been 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.

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
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

, nitrous oxide
Nitrous oxide
Nitrous oxide, commonly known as laughing gas or sweet air, is a chemical compound with the formula . It is an oxide of nitrogen. At room temperature, it is a colorless non-flammable gas, with a slightly sweet odor and taste. It is used in surgery and dentistry for its anesthetic and analgesic...

, and chlorofluorocarbons
Chlorofluorocarbon
A chlorofluorocarbon is an organic compound that contains carbon, chlorine, and fluorine, produced as a volatile derivative of methane and ethane. A common subclass are the hydrochlorofluorocarbons , which contain hydrogen, as well. They are also commonly known by the DuPont trade name Freon...

. 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.

See also

  • Anti-greenhouse effect
    Anti-Greenhouse Effect
    The anti-greenhouse effect is a neologism used to describe two different effects that describe a cooling effect an atmosphere has on the ambient temperature of the planet. Unlike the greenhouse effect, which is common, an anti-greenhouse effect is only known to exist in one situation in our Solar...

  • Global dimming
    Global dimming
    Global dimming is the gradual reduction in the amount of global direct irradiance at the Earth's surface that was observed for several decades after the start of systematic measurements in the 1950s. The effect varies by location, but worldwide it has been estimated to be of the order of a 4%...

  • Global warming
    Global warming
    Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...

  • History of climate change science
    History of climate change science
    The history of the scientific discovery of climate change began in the early 19th century when natural changes in paleoclimate were first suspected and the natural greenhouse effect first identified. In the late 19th century, scientists first argued that human emissions of greenhouse gases could...

  • Volcanic winter
    Volcanic winter
    A volcanic winter is the reduction in temperature caused by volcanic ash and droplets of sulfuric acid obscuring the sun and raising Earth's albedo after a large particularly explosive type of volcanic eruption...


External links