There's no doubt that the burning of fossil fuels over the past 2 centuries has caused a huge spike in the amount of carbon dioxide in the atmosphere. But CO2 levels increased gradually over the preceding millennia, too, and scientists did not know how much of that rise was caused by human activity. Now, an isotopic analysis of ancient air trapped in Antarctic ice shows that humans caused little if any of the preindustrial buildup of CO2. The findings negate previous thinking about the role of early humans in the process, experts say, and the research should help scientists develop better baseline models to forecast future climate.
After the last ice age ended about 11,000 years ago, the amount of atmospheric CO2 dropped for about 4 millennia, reaching a low of about 260 parts per million (ppm)--or 0.026% by volume--around 7000 years ago. Scientists know this from ice-core samples taken from Greenland and Antarctica, as well as other sources. The cores contain tiny bubbles of trapped air, so by analyzing samples from deep within the ice, scientists can trace the composition of the atmosphere back to 800,000 years ago.
About 7000 years ago, CO2 levels started rising. Over the next 6800 years, CO2 levels rose by about 20 ppm, to a level of 280 ppm--versus 387 ppm now. For more than a decade, scientists have been trying to isolate the factors that contributed to that buildup. Among the major players in producing CO2 were biological processes on land, such as animal respiration; soil chemistry; the death and decay of vegetation; the release of CO2 from the oceans, triggered in part by the growth of coral reefs; and the burning of forests and plains, either by wildfires or by people clearing land for agriculture.
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.
"It's a much better picture than we previously had," says geochemist Edward Brook of Oregon State University, Corvallis. Many mechanisms have been proposed to explain the post-ice-age CO2 buildup, he says, "including the controversial idea that human land use caused CO2 to rise." Although "it's still hard to resolve all of the competing processes," Brook says, "the conclusion that the land biosphere played a relatively minor role seems fairly robust."