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December 9, 2003 -- A new hypothesis suggests that humans began altering greenhouse-gas concentrations and global climate thousands of years ago, long before the 1800s date widely assumed. In a paper to be presented at the December American Geophysical Union meeting in San Francisco and published in the December issue of Climatic Change, climate scientist Bill Ruddiman, professor emeritus of environmental sciences at the University of Virginia, concludes that human-caused emissions of carbon dioxide (CO2) since 8,000 years ago, and methane (CH4) since 5,000 years ago have combined to prevent a significant natural cooling of Earth’s climate.

Cyclic changes in these two greenhouse gases and in the size of ice sheets have occurred over hundreds of thousands of years for natural reasons. These natural cycles are driven by small variations in Earth’s orbit that cause rhythmic changes in the amount of solar radiation received at every location on the planet. The changes in solar radiation in turn cause predictable changes in climate that drive the greenhouse-gas cycles. But within the last several thousand years, these natural cycles were over-ridden by human activities that resulted from the early spread of agriculture in Eurasia, Ruddiman said.

Highlights of the new study include:

  • Beginning 8,000 years ago, humans reversed an expected decrease in CO2 by clearing forests in Europe, China, and India for croplands and pasture (page 2).
  • Beginning 5,000 years ago, humans reversed an expected decrease in methane by diverting water to irrigate rice and by tending large herds of livestock (page 3).
  • In the last few thousand years, the size of the climatic warming caused by these early greenhouse emissions may have grown large enough to prevent a glaciation that climate models predict should have begun in northeast Canada (page 4).
  • Abrupt reversals of the slow CO2 rise caused by deforestation correlate with bubonic plague and other pandemics near 200-600, 1300-1400 and 1500-1700 A.D. Historical records show that high mortality rates caused by plague led to massive abandonment of farms. Forest re-growth on the untended farms pulled CO2 out of the atmosphere and caused CO2 levels to fall. In time, the plagues abated, the farms were reoccupied, and the newly re-grown forests were cut, returning the CO2 to the atmosphere (page 5).

Humans Reversed the Natural Methane Trend 5,000 Years Ago

Natural controls on methane.
Bubbles of ancient air preserved in cores drilled from the Antarctic ice sheet show that atmospheric methane concentrations over the last 350,000 years varied at a cycle of 22,000 years. The reason for this rhythm is well understood. Changes in Earth’s orbit at this cycle cause variations in summer solar radiation. When a stronger summer sun heats the land, hot air rises, and air flowing in from the ocean to replace the rising air carries moisture that falls in summer monsoons. The monsoon rains then fill up natural wetlands that emit methane. The result is a predictable 22,000-year methane cycle resulting from tropical monsoons driven by summer solar radiation.

Breakdown of Natural Controls.
Near 11,000 years ago, methane concentrations reached a natural maximum predicted by the most recent peak in summer solar radiation. The subsequent drop in methane until 5,000 years ago is expected from the slow decrease in solar radiation. But the reversal and slow increase of methane after 5,000 years ago is anomalous, because solar radiation continued to decrease and tropical monsoon sources were drying out. This methane increase violates the ‘rules’ followed for 350,000 years.

Human Origin of the Methane Increase.
Irrigation was first used to grow wet-adapted strains of rice in Southeast Asia about 5,000 years ago. Because flooded rice fields are a major source of methane, irrigation is probably the main factor behind the anomalous methane increase that began at that time. Later, irrigation spread to hillside rice paddies. Other methane emissions resulted from growing human populations, increased tending of livestock and biomass burning.

Humans Reversed the Natural CO2 Trend 8,000 Years Ago
Natural controls on CO2.

Air bubbles in ice show that natural CO2 concentrations in the atmosphere have varied at regular cycles related to changes in Earth’s orbit. The main CO2 cycle is at 100,000 years. In each cycle, peak CO2 values occurred late in the intervals when ice sheets were rapidly melting, and just as warmer interglacial intervals began. For the three CO2 peaks that occurred prior to the most recent one, CO2 concentrations then fell for at least the next 15,000 years during the early stages of interglacial warmth.

Breakdown of Natural Controls.
Near ten thousand years ago, at the end of the most recent ice-melting interval, CO2 levels rose to the same kind of natural peak they had reached in the three previous cycles. The CO2 drop from 10,500 to 8,000 years ago also resembles the earlier decreases. But the reversal of this trend and the slow increase to higher concentrations after 8,000 years ago is anomalous compared to the earlier trends.

Human Origin of the CO2 Increase.
The start of clearance and burning of European forests for agriculture 8,000 years ago is documented by the slow westward spread of remains of wheat, barley, peas and lentils from source areas in the ‘fertile crescent’ area of the Near East. Agriculture also began in naturally forested areas of China near 9,500 years ago and in India after 8,500 years ago. By 2,000 years ago, extensive clearance of forests from hill slopes in Mediterranean Europe allowed erosion of mud that clogged river mouths and forced relocation of coastal port cities. CO2 emissions from early forest clearance in Eurasia are large enough to explain the anomalous increase in atmospheric CO2 since 8,000 years ago.

Human Greenhouse Emissions May Have Stopped a Glaciation

Natural Controls on Ice Volume.
For almost three million years, changes in the size of Northern hemisphere ice sheets have occurred at the natural cycles of Earth’s solar orbit. Changes in solar radiation received in summer at high northern latitudes control the rate of growth and melting of ice sheets. Models have been devised to mimic the way solar radiation controls the size of ice sheets over thousands of years, including the delayed response of the ice to solar heating. The ‘target signal’ used to evaluate the models is an index of ice volume found in ocean sediments (the marine oxygen-isotope signal).

Breakdown of Natural Controls.
The models successfully match most features in the target ice-volume signal over the last several hundred thousand years. But several models indicate that ice should have again begun accumulating in the last 5,000 years as a delayed response to the summer radiation decrease that has been underway since 11,000 years ago. The failure of ice to reappear suggests that the natural controls on ice sheets broke down in the last few thousand years.

Humans as the Cause of Overdue Glaciation.
A plausible explanation for the overdue glaciation is the build-up of CO2 and methane resulting from human activities during the last several thousand years. Releases of these gases should have warmed global climate by ~0.8oC, and high-latitude climate by ~2oC, enough to have kept snow and ice from accumulating in extensive areas of high terrain in northeast Canada.

Diseases Explain Short-Term CO2 Drops

Natural Controls on Short-Term CO2.
Over shorter intervals of centuries to decades, variations in climate have been linked to factors such as changes in Sun strength and sporadic volcanic eruptions. Several large drops in CO2 have been detected in ice-core layers that span the last 2,000 years. The cause of these drops has been evaluated with models that include physical, chemical and biological parts of the climate system. These models provide estimates of the associated changes in temperature and CO2 that should occur in response to changes in solar radiation and volcanic eruptions.

Inadequacy of Natural Controls.

To match the full amplitude of the observed drops in CO2, the models require such large decreases in northern hemisphere temperature that they violate reconstructions of actual temperature trends. The implication is that ‘natural’ factors (solar and volcanic changes) cannot explain the size of these CO2 decreases.

Human Origin of the Abrupt CO2 Decreases.
The first two low CO2 pulses correlate with bubonic plague pandemics. Historical records document mortality rates of 25-45 percent in western Eurasia during these pandemics, with massive abandonment of farms and rural villages for decades to centuries. Ecologists estimate that forests can reclaim farmland within just 50 years. Rapid reforestation and sequestration of carbon in trees and shrubs growing on abandoned farms can pull enough CO2 out of the atmosphere to explain both the speed and amount of the CO2 declines during plague pandemics. For the interval between 1500 and 1750 A.D., 90 percent mortality caused by first exposure of native American populations to European diseases appears to have been to the major cause of the last CO2 drop.

Contact: William Ruddiman, Professor Emeritus of Environmental Sciences, University of Virginia
Phone: 540-348-1963 Email:,
Author staying at Powell Hotel in San Francisco, Dec. 8-12; Phone# 415-398-3200
AGU Session PP22D-01 (Emiliani Lecture) on Dec. 9 at 5 p.m. in Room MCC 3001-3003

FOR ADDITIONAL INFORMATION: Contact the Office of University Relations at (434) 924-7116. Television reporters should contact the TV News Office at (434) 924-7550.

SOURCE: U.Va. News Services


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