Millennia of global warming

A new study suggests that cutting down trees over the last few thousand years has increased global warming BENEFICIALLY -- staving off a new ice age.

Scientific, economic, and political discussions about global warming caused by human activity have tended to focus on the emissions of carbon dioxide (CO2) by the burning of fossil fuels, a process that became significant only 200 years ago. But deforestation, the conversion of forest land to agricultural or pasture land, also increases CO2 as carbon stored in trees is released to the atmosphere. Indeed, a new study by William F. Ruddiman of the University of Virginia indicates that human agricultural and deforestation activities have been increasing greenhouse gases and inducing global warming for thousands of years (Climatic Change 2003, 61, 261; Nature 2004, 427, 582) and may have prevented the return of Ice Age climates.

Ruddiman's analysis begins with the well-accepted theory that the cyclical alternation of Ice Ages with brief interglacial periods, such as the present, is controlled by regular oscillations in Earth's orbit. The amount of sunlight received by the planet in summer and winter varies by as much as ñ10% as Earth's orbital eccentricity (ellipticity) changes, as the point in Earth's orbit nearest the sun moves around the orbit, and as Earth's axis wobbles (precesses). This 20% oscillation-a combination of 100,000-, 41,000-, and 23,000-year cycles-sets in motion changes in Earth's climate that amplify the variation in solar radiation. In the end, the oscillation determines the advance and disappearance of the giant ice sheets that have periodically covered much of the northern hemisphere for the last 2.75 million years.

Samples of atmospheric gases trapped in ice in Greenland and Siberia show that the levels of two greenhouse gases, CO2 and methane, closely track solar-radiation cycles, with the gases increasing as the radiation and temperature rise and declining when they fall. But this close correlation, valid over hundreds of thousands of years, breaks down in the most recent period.

Although solar radiation started to decline 10,000 years ago, CO2 in the atmosphere began to rise 8,000 years ago and methane started to rise 5,000 years ago, rather than falling as expected. The anomaly amounts to a rise of one-sixth in CO2 and nearly one half in methane over the levels that would be expected by the radiation cycle alone. After ruling out possible nonhuman causes for the rise in greenhouse gases, Ruddiman showed that deforestation, which began with the development of agriculture in the Eastern Mediterranean some 8,000 years ago, could account for the observed rise of CO2. Deforestation during the last 8 millennia has resulted in clearing nearly 13 million square kilometers of land and the release of some 320 billion tons of carbon into the atmosphere. This is about twice the carbon released by the burning of fossil fuels. Also, beginning about 5,000 years ago, East Asian farmers began widespread rice farming with irrigated paddies, which would emit roughly enough methane, in Ruddiman's view, to account for the methane anomaly.

The gases released by deforestation and agriculture may have pushed back the onset of a new Ice Age. In the past, ice caps in North America started to form 5,000 years after solar radiation began dropping, which would mean some 3,000 to 6,000 years ago. Ruddiman estimates that the additional CO2 released by human activities would have elevated temperatures at high latitudes by the 2 øC needed to prevent glaciation. His estimates assumed that the deforestation alone affected climate through CO2 release, and ignored the effects of reduced cloud cover caused by fewer trees recycling water to the atmosphere.

"Although the conclusion that humans have been warming the climate for thousands of years seems startling, my colleagues have generally been quite supportive," Ruddiman reports. There have been disputes over the possible magnitude of the effect but general acceptance of its reality. For the present, the knowledge that deforestation has already caused substantial climatic modification serves as a warning because export-driven deforestation in tropical areas is now proceeding at a record pace. Although staving off the growth of Earth's ice sheets is certainly beneficial, melting them clearly would not be.

Source

The closing observation -- that the cutting down of trees may have gotten out of hand in recent years -- fails to take account of INCREASED forest cover in North America and Scandinavia in recent years

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Many people would like to be kind to others so Leftists exploit that with their nonsense about equality. Most people want a clean, green environment so Greenies exploit that by inventing all sorts of far-fetched threats to the environment. But for both, the real motive is to promote themselves as wiser and better than everyone else, truth regardless.

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