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4.5 billion years ago

The Earth, newly formed, had the hottest climate in the planet's long history. Temperatures were hot enough to liquefy rock. Radioactive elements in Earth's core generated heat and pressure as they decayed, pushing molten rock toward Earth's surface. Volcanoes also brought molten rock to the surface, liberating heat. Volcanoes spewed carbon dioxide into the atmosphere, causing the Greenhouse Effect.

3.8 billion years ago

As the mass of radioactive elements in Earth's core diminished, the climate cooled and the first rock formed. The cooling of the atmosphere liquefied water vapor, which fell to Earth as rain.

3.5 billion years ago to 3 billion years ago

The origin of life enhanced the cooling of the climate, for among the first life were single-celled photosyn-thetic algae. Like plants, these algae consumed carbon dioxide and exuded oxygen. The reduction of carbon dioxide in the atmosphere weakened the Greenhouse Effect. With the reduction in carbon dioxide, temperatures dropped below freezing, causing the planet's first ice age 3 billion years ago.

2.9 billion years ago

The retreat of the glaciers inaugurated a long period of warm climate. The sun, burning steadily brighter, bathed Earth in its heat. Warm inland seas covered Earth, moderating the climate. Ocean currents circled the globe, spreading warm water from the equator to the poles.

800 million years ago to 550 million years ago

Glaciers covered the oceans as well as the land, killing photosynthetic algae that lived in the ocean. With algae in small numbers they were able to remove only a fraction of carbon dioxide from the atmosphere. With no check on its accumulation, carbon dioxide increased in the atmosphere, causing the Greenhouse Effect. The Greenhouse Effect ended the Late Pro-terozoic Ice Age roughly 550 million years ago, inaugurating a new warm period.

350 million years ago to 280 million years ago

The lush plant growth of the Carboniferous Era confirmed that the climate was warm and that carbon dioxide, essential for plant growth, was abundant.

230 million years ago

The continents gathered into a single landmass called Pangea. Because it was near the equator, Pangea's climate was tropical.

135 million years ago to 65 million years ago

Temperatures soared 20 degrees Fahrenheit warmer than today's temperatures during the Cretaceous Era.

Forests covered Antarctica. Ocean currents again carried warm water to the poles.

65 million years ago

An enormous meteor impacted Earth, ejecting a gigantic cloud of debris and dust. It ignited widespread fires, which pumped ash into the atmosphere. The debris, dust, and ash blocked out much of the sun's light, chilling the climate. So severe was the reversal in climate that the dinosaurs and a large number of marine species, unable to cope with the new conditions, perished.

55 million years ago to 35 million years ago

Temperatures declined 20 degrees F (11 degrees C). Glaciers formed on Antarctica.

130,000 years ago

The climate was again warmer than it is today. The water from melting glaciers flowed to the oceans, raising the sea level 60 ft. (18 m.) higher than it is today.

100,000 years ago

The climate cooled yet again and glaciers once more spread across the continents, plunging Earth into its most recent ice age.

16,000 to 13,000 years ago

The glaciers were in retreat, temperatures rose nearly 15 degrees Fahrenheit.

12,900 and 11,500 years ago

Temperatures during the Younger Dryas fell 50 degrees Fahrenheit in only a decade

7,000 years ago

Temperatures peaked at 2-3 degrees F (1-1.5 degrees C) above current temperatures. The climate remained warm and wet for another 3,000 years.

The Medieval Warm Period rewarded peasants with bountiful crops. With food in surplus, human population increased.

1400 to 1840

The Little Ice Age covered the globe with record cold, large glaciers, and snow. This massive climate change triggered disease, famine, and death. Today, many scientists around the world believe that global warming caused by the Greenhouse Effect will be the fastest warming of the Earth since the termination of the Little Ice Age.

1824

French mathematician and physicist Jean Baptiste Joseph Fourier established that a buildup of carbon dioxide in Earth's atmosphere warms the climate.

1859

Irish scientist John Tyndall discovered that some gases block infrared radiation. He suggested that changes in the concentration of the gases could bring climate change.

1863

Tyndall announced that water vapor is a greenhouse gas.

1875

British scientist James Croll established that ice and snow reflect sunlight into space and cool the Earth.

1896

Swedish scientist and Nobel laureate Svente Arrhenius coined the phrase Greenhouse Effect and predicted that the Earth's climate is slowly warming. Arrhenius published the first calculation of global warming from human-induced emissions of carbon dioxide.

1897

British scientist Thomas C. Chamberlin established the link between ice ages and low concentrations of carbon dioxide and between warm climates and high concentrations of carbon dioxide.

1920 to 1925

The opening of Texas and Persian Gulf oil fields inaugurated an era of cheap energy. The burning of petroleum releases greenhouse gases into the atmosphere, warming the climate.

1924

German climatologist and geologist Alfred Wegener posited that the continents move slowly across Earth. When they are near the equator their climate is warm, while near the poles their climate is cold.

1930

Serbian geophysicist Milutin Milankovitch proposed that changes in the eccentricity of Earth's orbit cause climate change, including ice ages.

1932

Meteorologist W.J. Humphreys elaborated the conditions for a return to an ice age. He believed that an increase in debris in the atmosphere and the reflection of sunlight by ice and snow might return Earth to an ice age.

1933 to 1935

The drought of the 1930s created dust storms on the Plains. The worst dust storm of the Dust Bowl gripped the Plains on what later becomes known as Black Sunday. President Franklin Roosevelt established the Soil Erosion Service in response to the devastation of the Dust Bowl and as a part of his New Deal programs to create jobs. The Soil Erosion Service was the predecessor of the Soil Conservation Service established in 1935, which is known today as the Natural Resources Conservation Service (NCRS).

1937

Royal Meteorological Society president George Simpson posited that an increase in solar radiation might cause an ice age. By warming the poles more than high altitudes, the increase in solar radiation would intensify the circulation of the atmosphere, carrying moisture to high latitudes, where it would fall as snow. If enough snow accumulated, a new ice age would ensue.

1938

Amateur scientist G.S. Callendar recorded an increase in temperatures in the Artic and posited the Greenhouse Effect as the cause.

1939

Simpson announced that the atmosphere seems to keep the climate nearly constant by regulating the amount of clouds. The more clouds, the lower the temperature, and the fewer clouds, the warmer the temperature.

1940

Many scientists dismissed Callendar's claims. However, in response to his theory scientists began to develop new ways to measure the history of and current conditions of Earth's climate.

1945

The U.S. Office of Naval Research began generous funding of many fields of science, some of them useful for understanding climate change.

1950

American scientist Charles F. Brooks announced that Artic ice might be melting and that, once started, the melting might shrink the ice to a vestige of its former size and raise sea levels.

1950s

The development of new technology led to an increased awareness of global warming and the Greenhouse Effect. Researchers began to show that the level of carbon dioxide in the atmosphere was rising each year and people became concerned about pollution.

1956

American scientists Maurice Ewing and William Donn posited that the last ice age had rapidly descended on Earth when the North Pole wandered into the Arctic Ocean, triggering the accumulation of snow and ice in this region. American scientist Norman Phillips produces a somewhat realistic computer model of the global atmosphere. Canadian physicist Gilbert Plass calculated that adding carbon dioxide to the atmosphere would affect the radiation balance.

1957

Launch of Soviet Sputnik satellite. Cold War concerns support the 1957-58 International Geophysical Year, bringing new funding and coordination to climate studies. U.S. oceanographer Roger Revelle warned that humanity is conducting a "large-scale geophysical experiment" on the planet by releasing greenhouse gases.

1958

Astronomers identified the Greenhouse Effect on Venus, where temperatures are far above the boiling point of water.

1960

A report found that global temperatures had declined since the early 1940s. American scientist Charles David Keeling set up the first continuous monitoring of carbon dioxide levels in the atmosphere. Keeling soon finds a regular rise in temperatures.

1961

Soviet meteorologist Mikhail Budyko warned that the burning of fossil fuels, and the attendant accumulation of greenhouse gases in the atmosphere, would warm the planet.

1963

Fritz Moller calculated that a doubling of carbon dioxide in the atmosphere might increase temperatures 50 degrees Fahrenheit.

1965

Climatologists gather in Boulder, Colorado, to discuss climate change. Edward Lorenz and others point out the chaotic nature of the climate system and the possibility of sudden shifts.

1966

Italian scientist Cesare Emiliani's analysis of deep-sea cores showed that the timing of ice ages was set by small orbital shifts, suggesting that the climate system is sensitive to small changes.

1967

The International Global Atmospheric Research Program was established, mainly to gather data for better short-range weather prediction. Computer modelers Syukuro Manabe and Richard Wetherald predicted that an increase in the number of clouds might hold heat in the atmosphere and so increase temperatures.

1968

Mikhail Budyko derived two mathematical models. One predicted an increase in temperatures due to the Greenhouse Effect. The other predicted the return of the ice age. Budyko favored the first model. Other models were also contradictory. Studies suggested that the Antarctic ice sheets might collapse, raising sea levels catastrophically.

1969

American climatologist William Sellers predicted that a 2 percent decrease in solar radiation, whether from a fluctuation in solar output or the result of debris in the air, might plunge Earth into a new ice age. Like Budyko, Sellers feared that the burning of fossil fuels might warm Earth. Nimbus III satellite begins to provide comprehensive global atmospheric temperature measurements.

1970

The First Earth Day. The environmental movement attains strong influence, spreading concern about global degradation. The creation of the U.S. National Oceanic and Atmospheric Administration was the world's leading funder of climate research. Aerosols from human activity were increasing in the atmo sphere. American scientist Reid Bryson claimed they counteracted global warming and may actually cool the Earth.

1971

The Study of Man's Impact on Climate (SMIC), a conference of leading scientists, reported a danger of rapid and serious global change caused by humans and called for an organized research effort. The American Mariner 9 spacecraft found a great dust storm warming the atmosphere of Mars along with indications of a radically different climate in the planet's past.

1972

Budyko predicted that a 50 percent increase in greenhouse gases in the atmosphere might raise temperatures enough to melt all the ice on Earth, whereas a 50 percent reduction might plunge Earth into an ice age. Budyko favored the first scenario and predicted that temperatures might rise enough to melt all the ice by 2050. Ice cores and other evidence showed that the climate changed in the past in the space of 1,000 years or so, especially around 11,000 years ago.

1972 to 1974

Serious droughts and other unusual weather since 1972 increased scientific and public concern about climate change, with cooling from aerosols suspected to be as likely as warming. Journalists wrote about ice ages.

1975

Concern about the environmental effects of airplanes led to investigations of trace gases in the stratosphere and the discovery of danger to the ozone layer. Manabe and collaborators produced complex but plausible, computer models, which predicted an increase of several degrees Fahrenheit for a doubling of carbon dioxide.

1975 to 1976

Studies showed that chlorofluorocarbons (CFCs) (1975) and also methane and ozone (1976) contribute to the Greenhouse Effect. Deep-sea cores show a dominating influence from 100,000-years ago. Mila-nkovitch's prediction of orbital changes emphasized the role of feedbacks. Deforestation and other ecosystem changes were recognized as major factors in the future of the climate. American meteorologist Amos Eddy showed that the absence of sunspots in past centuries corresponded with cold periods.

1977

Scientific opinion tended to converge on global warming, not cooling, as the chief climatic risk in next century.

1978

Attempts to coordinate climate research in United States ended with an inadequate National Climate Program Act, accompanied by rapid, but temporary, growth in funding. American scientist James Hansen predicted that the accumulation of aerosol particles in the atmosphere might reflect sunlight back into space and so reduce temperatures.

1979

The second oil energy crisis. A strengthened environmental movement encouraged the development of renewable energy sources and the reduction of technologies that burn fossil fuels. The U.S. National Academy of Sciences estimated that a doubling of carbon dioxide might increase temperatures 35 to 40 degrees Fahrenheit. The World Climate Research Program was launched to coordinate international research on climate change.

1980

The election of President Ronald Reagan caused a backlash against the environmental movement. Political conservatism is linked to skepticism about global warming. Some scientists predicted greenhouse warming should be measurable by about the year 2000.

1982

Greenland ice cores revealed temperature oscillations over a single century in the distant past. Strong global warming since the mid-1970s was reported, with 1981 the warmest year on record.

1983

Reports from U.S. National Academy of Sciences and Environmental Protection Agency spark conflict, as greenhouse warming becomes prominent in mainstream politics.

1984

Theories about global warming and the Greenhouse Effect became more prevalent, gaining attention from the mass media. However, many people believe the threat is not imminent and some doubt that global climate change is a danger.

1985

The Center for Atmospheric Science Director Veer-abhadran Ramanathan and collaborators announced that methane and other trace gases together could bring as much global warming as carbon dioxide itself. The Villach conference in Indonesia declared consensus among experts that some global warming seems inevitable and called on governments to consider international agreements to restrict emissions of greenhouse gases. Antarctic ice cores show that carbon dioxide and temperature went up and down together through past ice ages, pointing to powerful biological and geochemical feedbacks. American scientist Wallace Broecker speculated that a reorganization of North Atlantic Ocean circulation could bring swift and radical climate change.

1987

This was the warmest year since humans began to keep records. The 1980s were the hottest decade on record, with seven of the eight warmest years recorded up to 1990. Even the coldest years in the 1980s were warmer than the warmest years of the 1880s. The Montreal Protocol of the Vienna Convention imposed international restrictions on the emission of ozone-destroying gases.

1988

Global warming attracts worldwide headlines after scientists at Congressional hearings in Washington, D.C., blamed the U.S. drought on its influence. A meeting of climate scientists in Toronto subsequently called for 20 percent cuts in global carbon dioxide emissions by 2005. The United Nations set up the Intergovernmental Panel on Climate Change (IPCC) to analyze and report on scientific findings. News media coverage of global warming leapt upward following record heat and droughts. The Toronto conference called for strict, specific limits on greenhouse gas emissions. British Prime Minister Margaret Thatcher is the first major leader to call for action. Ice-core and biology studies confirmed that living ecosystems give climate feedback by way of methane, which could accelerate global warming. The level of carbon dioxide in the atmosphere reached 350 parts per million.

1989

Fossil-fuel suppliers and other industries formed the Global Climate Coalition in the United States to lobby politicians and convince the media and public that climate science is too uncertain to justify action.

1990

American meteorologist Richard Lindzen predicted that an increase in carbon dioxide in the atmosphere might not cause a concomitant increase in water vapor. Consequently, the Greenhouse Effect might be less severe than some were forecasting. The first IPCC report stated that the world has been warming and continued warming seems likely in the future. Industry lobbyists and some scientists disputed the tentative conclusions.

1991

Mount Pinatubo erupted. Hansen predicted that the eruption would cool Earth, verifying (by 1995) computer models of aerosol effects. Global warming skeptics emphasized research indicating that a significant part of 20th-century temperature change was due to solar influences. Studies from 55 million years ago show the possibility of the eruption of methane from the seabed causing enormous warming.

1992

A conference in Rio de Janeiro produced the United Nations Framework Convention on Climate Change, but the United States blocked calls for serious action. The study of ancient climates revealed climate sensitivity in the same range as predicted by computer models.

1993

Greenland ice cores suggested that great climate changes (at least on a regional scale) could occur in the timespan of a single decade.

1995

The second IPCC report detected the "signature" of human-caused Greenhouse Effect warming, declaring that serious warming is likely in the coming century. Reports of the breaking up of the Antarctic ice sheets and other signs of current warming in polar regions began to affect public opinion.

1997

Japanese automobile manufacturer Toyota introduces the Prius in Japan, the first mass-marketed electric hybrid car. Engineers progressed in the design of large wind turbines and other energy alternatives. An international conference in Japan produced the Kyoto Protocol, setting targets to reduce greenhouse gas emissions—if enough nations would approve and sign the treaty.

1998

The warmest year on record globally averaged (1995, 1997, and 2001-2006 were near the same level). Borehole data confirmed extraordinary warming trend. Qualms about arbitrariness in computer models diminish as teams model ice-age climate and dispense with special adjustments to reproduce current climate.

1999

A National Academy Panel dismissed criticism that satellite measurements showed no warming. V. Ramanathan detected massive "brown cloud" of aerosols from South Asia.

2000

The Global Climate Coalition dissolved as many corporations grappled with the threat of warming, but the oil lobby convinced the U.S. administration to deny the problem. Various studies emphasized variability and the importance of biological feedbacks in the carbon cycle that are liable to accelerate warming.

2001

The Third IPCC report stated that global warming, unprecedented since the end of last ice age, is "very likely," with possible severe surprises. The National Academy panel marked a "paradigm shift" in scientific recognition of the risk of abrupt climate change (decade-scale). Warming is observed in ocean basins. These observations match computer models, giving a clear signature of Greenhouse Effect.

2002

Studies found surprisingly strong "global dimming," due to pollution. This factor had retarded greenhouse warming, but dimming is now decreasing.

2003

Numerous observations raised concern that the collapse of ice sheets (West Antarctica, Greenland) might raise sea levels faster than most had believed. A deadly summer heat wave in Europe deepens divergence between European and U.S. public opinion.

2004

In a controversy over temperature data covering the past millennium, most scientists concluded that climate variations were substantial, but not comparable to post-1980 warming. The first major books, movies and artwork feature global warming.

2005

The Kyoto Treaty, signed, by all major industrial nations except the United States, took effect. Work to retard greenhouse emissions accelerated in Japan, Western Europe, U.S. regional governments, and corporations. Hurricane Katrina and other major tropical storms spurred debate over the impact of global warming on storm intensity.

2006

An Inconvenient Truth premiered at the 2006 Sundance Film Festival and opened in New York and Los Angeles on May 24, 2006, earning $49 million.

2007

The fourth IPCC report warned that serious effects of warming have become evident. The cost of reducing emissions would be far less than the damage they will cause. Al Gore shared the Nobel Peace Prize for his efforts to spread knowledge about global warming. The level of carbon dioxide in the atmosphere reaches 382 parts per million.

Christopher Cumo Independent Scholar Fernando Herrera University of California, San Diego

Abrupt Climate Changes throughout recorded history, and in studies of geological and other records from much earlier periods of the Earth's history, there have been a number of abrupt climate changes. These significant and widespread shifts in climate heavily impacted many parts of the world. Current studies of these changes draw from details revealed from ice core samples, especially from Greenland and northern Canada, and also from records compiled showing signs of geological changes. More recent information has emerged from examinations of the fluctuations in the size of tree rings, and also from historical accounts.

The quantity of these abrupt climate changes has led some academics, often labelled climate change skeptics, to explain the current global warming in terms of these trends. They suggest or state that the current changes are, or could be, merely a part of a cycle of global warming and cooling, similar to those that have occurred over hundreds of thousands of years.

Explaining global warming in the 1990s and 2000s by studying these abrupt climate changes gained much publicity around the world through the article "A Pervasive Millennial-Scale Cycle in

North Atlantic Holocene and Glacial Climates," which was written by ten leading scientists, Drs. G. Bond, W. Showers, M. Chesby, R. Lotti, P. Alamsi, P. de Menocal, P. Priore, H. Cullen, I. Hajdas, and G. Bonani, and published in the journal Science in November 1997. This article, and related work, led to a substantial body of research on abrupt climate changes—when the Earth's temperature has either significantly increased or decreased over a short period of time—and also the possible causes of these changes.

A development of this theory came from two other scientists associated with those skeptical of global warming, S. Fred Singer of the University of Virginia, and Dennis Avery of the Hudson Institute in New York. They raised the possibility of a 1,500-year climate cycle, especially in the North Atlantic region, with hot temperatures every 1,500 years, The present global warming could be a part of this cycle. Certainly a part of this theory clearly rests on the extreme weather patterns experienced around the world during the years 535-36 c.E. They are recorded by the Byzantine historian Procopius, and also in Irish annals, as well as in records kept in China—all showing that the climate change occurred across a large number of areas.

The Little Ice Age, which lowered world temperatures from about 1600 to 1750, froze large rivers and canals in Europe. This engraving depicts a fair held on the ice of the frozen River Thames in London in 1683.

These historical accounts are confirmed by a tree ring analysis undertaken by the dendrochronolo-gist Mike Baillie from Queen's University of Belfast, Northern Ireland, which showned little growth in Irish oak trees during this period. Similar data emerging from a study of tree ring samples conducted on trees from Scandinavia, California, and Chile. The rise in temperature during these years seems to have led to a widespread series of famines around the world, and the collapse or destruction of a number of empires including that of the Persians, and the Mesoamerican city of Teotihuacan.

There is no accepted cause of this instance of abrupt climate change, although there has been the suggestion that it could have been caused by the large-scale eruption of a volcano, such as Kraka-toa, off the coast of Java (modern-day Indonesia); this idea formed a central part of the work of David Keys in his book Catastrophe: A Quest for the Origins of the Modern World (1999). It should, however, be noted that other scientists have been critical of these conclusions.

HISTORY

Going back further in history, there was a prolonged drought in the 22nd century B.c.E., which had a dramatic effect on the Old Kingdom of Egypt. It led to about 40 years of famines and social dislo cation, which produced the emergence of the unified Kingdom of Egypt as a new political identity capable of financing irrigation projects and the like to overcome these problems. There have also been more recent changes in temperature, with suggestions that the collapse of the Mayan Empire in Mexico in the 8th and 9th centuries was possibly caused by a regional abrupt climate change, although others have pointed to the greater likelihood that this stemmed from overpopulation, foreign invasions, epidemics, and internal insurgencies. The idea of localized problems is reinforced by the fact that there is little evidence of an abrupt climate change elsewhere in the world at that time.

There have also been studies of what has been deemed the Little Ice Age, which took place from about 1600 until about 1750, with the freezing of rivers such as the River Thames in England, of canals in the Netherlands (shown in contemporaneous paintings), and the southern section of the Bos-phorus in 1622. As the process seems to have been gradual, it might also have been responsible for the end of the Viking colonies in Greenland during the 15th century.

Certainly the concept of abrupt climate change goes back far further than recorded history. The last Ice Age in the Pleistocene period, which ended in about 10,000 b.c.e., ended a period of cold weather that is believed to have started with a long glacial advance from about 70,000 b.c.e., reaching a peak in 18,000 b.c.e., when most of northern Europe and considerable portions of modern-day Canada were covered with glaciers.

THE DEBATE

As a result of these studies, there are scientists who argue that global warming is just a phenomenon that has taken place before, and will take place again. They believe that global warming is not caused by the emission of increasing amounts of carbon dioxide and other greenhouse gases. There are also many who suggest that while warming is a regular phenomenon, it has been exacerbated by increasing greenhouse gas emissions.

Although abrupt climate changes have happened throughout history, scientists who argue that global warming is extremely serious, point to the rapidity of recent climate changes, and that they are occurring at an increasing rate throughout the world. In contrast to the changes that have been studied, measured, examined, and analyzed over the last 20 years, events such as the Little Ice Age took place gradually over at least 120 years, if not longer. Some date the start of the period back to 1250, when the Atlantic Pack Ice started to grow, and suggest it was a cause of the Great Famine of 1315-17, with the serious glacial expansion taking place only since 1550, and the first significant climatic changes in non-Arctic Europe beginning in the early 17th century.

By contrast, similar fluctuations in temperature (rising now, instead of falling as in the 1600s) in recent decades, have happened over a very short time, and the loss of large amounts of Antarctic ice have been evident over a 10-year period. The ability to track many of these changes from satellites has allowed geographers and scientists to identify other problems, such as the "Ozone Hole." .

The theory of abrupt climate change clearly indicates that there are many forces that contribute to global warming around the world, in addition to human-caused carbon dioxide and other emissions. However, researchers have been increasingly able to monitor these changes since the early 1990s, and the fact that the symptoms of climate change and global warming are currently being recorded around the planet, and are accelerating, has led many scientists to suggest global warming cannot be solely explained by the theory of abrupt climate change. It has further led to the theory that these man-made causes might be coinciding with a cyclical period of abrupt climate change, leading to a worsening of problems being faced by people in most parts of the world.

SEE ALSO: Attribution of Global Warming; Climate Change, Effects; Climatic Data, Historical Records; Climatic Data, Ice Observations; Climatic Data, Nature of the Data; Climatic Data, Tree Ring Records; Global Warming; Little Ice Age.

BIBLIOGRAPHY. Committee on Abrupt Climate Change and National Research Council, Abrupt Climate Change: Inevitable Surprises (U.S. National Academy of Sciences, 2002); Richard Alley, The Two-Mile Time Machine: Ice Cores, Abrupt Climate Change, and Our

Future (Princeton University Press, 2000); R.B. Alley, J. Marotzke, W.D. Nordhaus, J.T. Overpeck, D.M. Peteet, R.A. Pielke Jr., R.T. Pierrehumbert, P.B. Rhines, T.F. Stocker, L.D. Talley, J.M. Wallace, "Abrupt Climate Change," Science (v.299/5615, March 28, 2003); G. Bond, et al., "A Pervasive Millenial-Scale Cycle in North Atlantic Holocene and Glacial Climates," Science 278 (November 1997); William H. Calvin, A Brain for All Seasons: Human Evolution and Abrupt Climate Change (University of Chicago Press, 2002); John D. Cox, Climate Crash: Abrupt Climate Change and What It Means for Our Future (Joseph Henry Press, 2007); K.M. Cuffey, and G.D. Clow, "Temperature, Accumulation, and Ice Sheet Elevation in Central Greenland Throughout the Last Deglacial Transition," Journal of Geophysical Research (v.102, 1997); Joel D. Gunn, ed., Years without Summer: Tracing A.D. 536 and its Aftermath (Archaeopress, 2000); David Keys, Catastrophe: A Quest for the Origins of the Modern World (Ballan-tine Books, 1999).

JUSTIN CORFIELD Geelong Grammar School, Australia

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