Thursday, September 24th 2009, 3:50 AM EDT
Early humans also burned fuels such as wood, peat, and, eventually, coal. Scientists have wondered whether natural forces played the dominant role in boosting CO2 or whether humans had a hand in the phenomenon even that far back. To find out, a European team analyzed nearly 200 samples of ancient air extracted from Antarctic ice cores that span the time period between the end of the ice age and the beginning of industrialization. Then they measured the ratio of the heavier carbon-13 and the lighter carbon-12 isotopes in the CO2. That ratio can be used to identify the specific sources of the gas, because biological processes have a penchant for the lighter, more mobile carbon-12 isotope. Thus, the ratio of carbon-13 to carbon-12 is lower in plant matter and other carbon from land-based sources than it is in the carbon compounds found in seawater.
So what caused the preindustrial bump? As the researchers report tomorrow in Nature, it was predominantly natural, a combination of vegetation buildup after the ice age and, more prominently, the slow reaction to this change by ocean chemistry. But humans, the team concluded, played a small part.
The findings confirm the workings of the carbon cycle in the climate system, says climate physicist and co-author Thomas Stocker of the University of Bern in Switzerland. The study shows that neither vegetation alone nor human-generated CO2 was primarily responsible for the preindustrial buildup, he says.
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