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Prior posts, occasionally with tongue planted firmly in cheek, have noted that human impact on carbon loading in the atmosphere goes back to well before the industrial era.
A recent assessment has calculated that over the eight millennia before 1850, clearing land (for both hunting and for farming) removed trees that had stored or would store CO2, adding approximately 350 billion metric tons (1 mt = 1,000 kg or 2204.62 lbs) of carbon into the atmosphere by 1850. For purposes of comparison, it should be noted that it has been estimated that humans have added 440 billion metric tons of carbon since 1850, mostly from burning fossil fuels.
As population grew over the 8 millennia before 1850, the Earth's forests were converted to other uses. However, it is not a linear relationship. With irrigation, fertilizer, multicropping, and new tools, crop yields increased and the per capita rate of land use fell.
This new estimate (350 billion metric tons) is much larger than several other estimates published within the last year, and reflects assumptions regarding how carbon is stored in the soil when forests are replaced by grasslands. Critics have noted that the chemical composition of carbon dioxide bubbles in ice cores do not reflect changes that might be expected if forests were removed as hypothecated. In response, researchers have stated that rapidly expanding peatlands could counter-balance some of the impacts.
Other researchers have undertaken more regional studies, and have found that land use often varies significantly. For example, the use of wet-grown rice systems in China required fewer acres to feed people than the cereal grains grown in Mesopotamia. Because of population distribution across the globe, preindustrial emissions goes up for countries like India and China, which emitted a lot of carbon millennia ago. In contrast, North America remained relatively pristine forests until the last couple of hundred years. Thus, the balance between pre-1850 emissions and post-1850 emissions is more complex and nuanced.
The study can be found at http://hol.sagepub.com/content/early/2010/12/24/0959683610386983.short?rss=1&ssource=mfr.