The increase in CO2 emissions, the study said, is consequently leading to a weakening of the natural carbon sink.
A combination of these two effects, the study said, is characterizing "a carbon cycle that is generating stronger-than-expected climate forcing sooner than expected."
The study said that between 2000 to 2006, human activities such as burning of fossil fuels, manufacturing cement, and tropical deforestation contributed an average of 4.1 billion metric tons of carbon to the atmosphere each year, yielding an annual growth rate for atmospheric carbon dioxide of 1.93 parts per million (ppm).
The study authors reported that the present atmospheric concentration of carbon dioxide, 381 ppm, was the largest concentration in the last 650,000 years, and probably in the last 20 million years. The authors said changes in wind patterns over the Southern Ocean resulting from human-induced global warming had 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 was the major mechanism for drawing carbon dioxide out of the atmosphere, large droughts had reduced the uptake of carbon, they said.
"The new twist here is the demonstration that weakening land and ocean sinks are contributing to the accelerating growth of atmospheric CO2," said co-author Chris Field, director of the Carnegie Institution's Department of Global Ecology.
The scientists said emissions from the burning of fossil fuels had 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 had remained constant, but shifted in geographic focus, the study said.
The study also showed that the 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% per year.
It said as "practically all proposed scenarios for managing future emissions postulated improvements in carbon intensity in the global economy, this deterioration of carbon intensity presented a serious challenge in stabilizing atmospheric carbon dioxide and mitigating climate change"