North America may sop up a whopping 1.7 petagrams of carbon a year--enough to suck up all the carbon discharged annually by fossil fuel burning in Canada and the United States, according to a report in tomorrow's Science . Some scientists are concerned that this controversial finding could be used to argue that the United States and other countries shown to harbor substantial carbon "sinks" do not need to offset their CO2 emissions.
Critics have already thrown up a fistful of red flags, attacking the study for everything from its methodology to its implications. "There's a huge amount of skepticism about the result," says ecologist David Schimel of the National Center for Atmospheric Research in Boulder, Colorado.
The study takes a look at the so-called "missing sink" problem, which boils down to a simple math problem: Less than half of the 7.1 petagrams of carbon produced by human activity each year stays in the atmosphere. Although about 2 petagrams go into the oceans, another 1.1 to 2.2 petagrams--the missing sink--appears to vanish into the land, likely taken up by plants during photosynthesis. To get at how much carbon the different land masses are absorbing, Jorge Sarmiento of Princeton University and his colleagues first gathered data on atmospheric CO2 levels taken from 1988 to 1992 at 63 ocean-sampling stations. Then they fed the CO2 data into two mathematical models: one that estimates how much carbon the oceans absorb and release, and another that gauges how CO2 is spread across the globe by wind currents. Surprisingly, the model indicated that CO2 levels dropped off slightly from west to east across North America--even though fossil fuel emissions should boost levels in the east. That meant there must be a big carbon sink in North America.
Some experts find the sink's estimated magnitude--1.7 petagrams of carbon per year, plus or minus 0.5 petagrams--hard to swallow. "It's hard for me to know where that much carbon could be accumulating in North America," says biogeochemist Richard Houghton of the Woods Hole Research Center in Massachusetts.
Schimel and others worry that groups opposed to the climate treaty signed in Kyoto last December will seize on the estimate to argue that the United States doesn't need to reduce its emissions to comply with the accord. "We're all really concerned that many people will find it convenient to accept the result," Schimel says.