6 December 2012

Oops. Greenhouse gas levels appear to be significantly underestimated

by Will Parker

Measuring greenhouse gas emissions has traditionally relied on estimating emissions from all the activities and processes that might generate the gases, but Berkeley Lab scientists who instead took actual atmospheric measurements found that levels of nitrous oxide (N2O) may be up to 3 times greater than previous estimates.

Their paper, "Seasonal variations in N2O emissions from central California" in Geophysical Research Letters also suggests that levels of methane, another potent greenhouse gas, may be twice as much as previous estimates.

"If our results are accurate, then it suggests that N2O makes up not 3 percent of California's total effective greenhouse gases but closer to 10 percent," said lead researcher Marc Fischer "And taken together with our previous estimates of methane emissions, that suggests those two gases may make up 20 to 25 percent of California's total emissions. That's starting to become roughly comparable to emissions from fossil fuel CO2."

Nitrous oxide, better known as laughing gas, is an especially potent greenhouse gas because it traps far more infrared radiation than both carbon dioxide and methane. "It's present in the atmosphere at tiny concentrations but it is very potent," Fischer explained. "It has a global warming potential of approximately 300, meaning it is 300 times more active than CO2 per unit mass. And it's 10 to 15 times more potent than methane."

Worldwide levels of N2O have been rising rapidly for decades, and the major culprit was recently confirmed to be the heavy use of nitrogen fertilizers to grow the world's food. Other less significant sources of N2O emissions include wetlands, animal and industrial waste and automobiles.

Fischer said that the standard method for estimating emissions levels in the past has been to do what is called a "bottom-up inventory." This process involves listing all the activities that emit N2O, assigning an emission factor for each activity, and then tallying up the emissions. However, this method can result in large uncertainties because of the way N2O is produced.

"The biogeochemical processes that produce N2O are sensitive to environmental conditions and very small changes in things like temperature, moisture, the type of soil and when the fertilizer is applied," Fischer said. "All those factors can result in big differences in the amount of N2O that's produced. If you try to use a single number for a given patch of land, you're almost certainly going to get a variable result."

The method that Fischer and his colleagues describe in their paper compares measurements taken from a 2,000-foot tower (pictured) in Walnut Grove, California to the standard predictions of expected N2O levels based on the bottom-up inventory. "This is the first study of its kind to look at a full annual cycle of emissions from a large region of California that includes the sources that we believe are most important," Fischer said. "In general, we found that the measured signals were much bigger than the predicted signals."

As a way to verify the sampling method, Fischer and his team are currently comparing measured and predicted fossil fuel CO2 signals at the tower since there is a much better understanding of how much fossil fuel is burned in California. If the predicted signal and measured signal are close, then that would be a good indication that the method is sound. "Initial comparisons of measured and predicted fossil fuel signals agree at the 10 to 20 percent level, suggesting that it is unlikely there are much larger errors in the transport model," he said. "But we haven't ruled it out."

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Source: Lawrence Berkeley National Laboratory