Two new satellites will soon launch, each with the specific purpose of counting concentrations of carbon dioxide and methane in the atmosphere. Japan’s Greenhouse Gases Observing Satellite, or GOSAT, will monitor the greenhouse gases present at up to 56,000 locations around the world. Later this winter, NASA plans to launch its own carbon-counting satellite, called the Orbiting Carbon Observatory. These two space-based platforms have the potential to revolutionize our understanding of emissions.

Though many countries have long tracked their greenhouse gas emissions, that data comes from ground-based sensors that provide spotty coverage. As a result, large swaths of Africa and South America are currently not being monitored. Earlier earth-observing satellite missions have also contributed atmospheric data relevant to the climate–and one, DSCOVR, which would have measured the earth’s reflectivity, was iced by a Republican Congress in 1999–but none have been able to capture how greenhouse gas concentrations change through the carbon cycle, with carbon uptake and release in constant flux across the planet. GOSAT will be the first space project entirely dedicated to greenhouse gas observation.

The hope is that the missions’ data will sharpen carbon-trading programs and influence climate policy, in particular in the design of a successor to the Kyoto Protocol, which expires in 2012. The data could be used to address unanswered questions about how CO2 densities in the atmosphere change over time. For example, the oceans and land both absorb a certain amount of carbon dioxide from the atmosphere, but scientists still don’t know exactly how much. Some climatologists contend that the presence of higher CO2 concentrations has spurred more plant growth, which in turn sequesters more of the greenhouse gas. Nonetheless, the exact nature of CO2 absorption remains elusive.

Measurements will work like this: The satellites will carry two instruments that infer greenhouse gas concentrations in regions across the planet by observing the infrared light in columns of atmosphere. The essential principle of greenhouse warming is that certain gases absorb infrared radiation and trap heat in the atmosphere. Because carbon dioxide and methane absorb light at certain wavelengths, the instruments can track how much infrared light is present in a given column of atmosphere. The light measurements in turn reveal the concentrations of the different molecules in each column that the instruments observe. Measuring and mapping the gas concentrations across the planet should produce a reasonable estimate of the global distribution of CO2.

If carbon accounting ever gets truly serious, the major players will demand fair play through accurate data and assurances that the world’s carbon dioxide is doing what we think it’s doing. These satellites represent a big step towards true carbon accountability.

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