Rate Of CO₂ Rise Accelerates

Release of CO₂ due to human activity is increasing faster than ever while "carbon sinks"—CO₂ uptake by oceans and land—are weakening, according to a new report (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0702737104). These conclusions are based on an analysis of data from CO₂-monitoring stations around the world.

Human activities such as burning fossil fuels, cement production, and tropical deforestation added a net average of 4.1 billion metric tons of CO₂ (measured as carbon) to the atmosphere each year between 2000 and 2006, the study says. As a consequence, the CO₂ concentration in the atmosphere rose 1.93 parts per million per year during this period to reach 381 ppm. In contrast, average CO₂ growth rates were 1.58 ppm per year during the 1980s and 1.49 ppm per year during the 1990s.

The current growth rate "is the highest since the beginning of continuous monitoring in 1959," according to the report, whose lead author is Christopher B. Field, director of the Carnegie Institution's department of global ecology at Stanford University. Recent emission increases have been "above the spectrum of any of the scenarios described by the United Nations Intergovernmental Panel on Climate Change," he says.

The rapid rise in CO₂ levels is primarily a result of three things, Field continues. First, the carbon intensity of the global economy has increased. Contrary to expectations, the amount of carbon, in kilograms, produced per dollar of global economic activity has risen recently, in contrast to the 1980s and 1990s, when the intensity fell. The intensity has gone up because most new power plants that have come on-line since 2000 are fueled by coal, the most carbon-intensive fuel, Field says.

Second, the ability of the oceans, especially the Southern Ocean near Antarctica, to absorb CO₂ has declined, Field says. This conclusion is based on an analysis of data from Antarctic waters, which now have a higher concentration of carbon than in prior decades. And third, large droughts have reduced the uptake of CO₂ by forests and other plants, he says.

"The new twist here is the demonstration that weakening land and ocean sinks are contributing to the accelerating growth of atmospheric CO₂," Field says.