Man's activities are releasing carbon dioxide faster than ever, while the natural processes that normally slow its build up in the atmosphere appear to be weakening, suggests a new study in the Proceedings of the National Academy of Sciences. The report states that "together, these effects characterize a carbon cycle that is generating stronger-than-expected climate forcing sooner than expected." In the last five years, human activities such as burning fossil fuels and tropical deforestation contributed an average of 4.1 billion metric tons of carbon to the atmosphere each year. "This is the highest since the beginning of continuous monitoring in 1959," says the report. The growth rate of atmospheric carbon dioxide is significantly larger than those for the 1980s and 1990s, respectively.
While the acceleration in carbon dioxide emissions had been previously noted, the current analysis provides insights into its causes. "The new twist here is the demonstration that weakening land and ocean sinks are contributing to the accelerating growth of atmospheric CO2," says study co-author Chris Field, director of the Carnegie Institution's Department of Global Ecology.
The report cites changes in wind patterns over the Southern Ocean resulting from human-induced global warming that have brought carbon-rich water toward the surface, reducing the ocean's ability to absorb excess carbon dioxide from the atmosphere. On land, where plant growth is the major mechanism for drawing carbon dioxide out of the atmosphere, large droughts have reduced the uptake of carbon.
Fossil fuels constituted the largest source of anthropogenic carbon, releasing an average of 7.6 billion metric tons each year between 2000 and 2006, a significant jump from 6.5 billion tons in the 1990s. Emissions generated by land-use changes such as deforestation have remained constant, but shifted in geographic focus.
Perhaps the gloomiest aspect of the study is that carbon intensity of the global economy (kilograms of carbon per dollar of economic activity) has increased since 2000 at about 0.3 percent per year, reversing a 30-year decline of about 1.3 percent per year. Because practically all proposed scenarios for managing future emissions postulate improvements in carbon intensity in the global economy, this deterioration presents a serious challenge in stabilizing atmospheric carbon dioxide and mitigating climate change.
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Source: Carnegie Institution
