Keeping Greenhouse Emitters Honest

14 May 2012 3:00 pm

A new study has a message for any country claiming to limit its emissions of greenhouse gasses: don't cheat. Using data gathered by sensors scattered around an urban area, researchers say they can track changes in a city's carbon dioxide output. That means that when a nation says it's complying with an emissions-limiting treaty, scientists may soon be able to see whether it's telling the truth.

The Kyoto Protocol, which was adopted in December 1997, has been signed and ratified by more than 190 countries, including 37 industrial or developing nations that agreed to reduce their emissions of carbon dioxide and three other greenhouse gases by 5.2% each year, on average, between 2008 and 2012. Although the United States signed the treaty, it stands alone as the only signatory to have not ratified the deal.

But signing a treaty is one thing. Actually following through is something else. So how can nations keep each other honest? One possibility is to use satellites to peer down on the atmosphere over a country and measure its carbon dioxide emissions directly. No probes now in orbit can do that, however, so researchers are looking into ground-based systems. And because most carbon dioxide emissions come from urban areas, cities offer tempting targets for observation.

A team led by Kathryn McKain, an atmospheric scientist at Harvard University, recently analyzed data gleaned by a network of carbon dioxide sensors in and around Salt Lake City where atmospheric concentrations of carbon dioxide have been measured since 2002. Six of the sensors are located in the city and its suburbs, and one is located atop a peak upwind of the city, giving a baseline measurement of carbon dioxide concentrations of the air flowing into the city. In particular, the researchers looked at data gathered during four intervals between mid-June and late December in 2006. The team compared actual carbon dioxide concentrations across the region with those produced by their computer simulation. The modeling predicted how wind and weather patterns during those intervals would have distributed the city's estimated carbon dioxide emissions. It also calculated how trees and other vegetation in the region would have affected carbon dioxide concentrations on an hour-by-hour basis.

Patterns of carbon dioxide concentrations produced by the team's simulations are similar to those seen in real life, says McKain. Concentrations are higher at night, when the air is stable and emissions are typically trapped near ground level, than they are in the daytime when sunlight heats the ground, triggering mixing in the atmosphere. Also, average carbon dioxide concentrations are lower in the summer, when plants are growing and absorbing carbon dioxide, than they are in the winter. Overall, the combination of a few measured carbon dioxide concentrations and modeling can discern month-to-month changes in emissions of 15% or more.

The team's results, reported online today in the Proceedings of the National Academy of Sciences, "are a very important first step," says Riley Duren, a systems engineer at NASA's Jet Propulsion Laboratory in Pasadena, California. "This [technique] is a critical tool in our toolbox," he adds. "We can't really diagnose and assess the efficacy of emissions reductions if we don't measure what's going on in the atmosphere."

However, Duren notes, researchers need to develop techniques to track the changing emissions of other important greenhouse gases, particularly methane, which on a molecule-by-molecule basis traps heat much more effectively than carbon dioxide does. Also, he suggests, using this technique in other cities—many of which cover a much larger area and have a more complicated set of emission sources, including seaports—may prove more challenging.

An even bigger complicating factor might be vegetation, says John Miller, an atmospheric scientist with the National Oceanic and Atmospheric Administration in Boulder, Colorado. For example, off the Atlantic seaboard in winter, the amount of carbon dioxide generated by decomposition of fallen leaves onshore overwhelms the emissions from coastal cities, he notes. "You'd make a huge error assuming all of the carbon dioxide is coming from burning fossil fuels." One way to determine the fraction of the greenhouse gas coming from rotting leaves would be to measure its levels of carbon-14. While recently living plants contain a certain proportion of that carbon isotope, petroleum products and coal—and therefore the emissions generated by burning them—contain none.

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