快猫短视频

A PAIR of satellites will soon take on the politically charged task of measuring carbon dioxide emissions. They could eventually help determine whether nations are meeting their emission targets and guide future climate-change negotiations.

The total amount of CO₂ in the atmosphere is calculated from ground-based measurements taken at the 100 or so CO₂-monitoring stations scattered around the world. These measurements show that about half of the CO₂ emitted from burning fossil fuels is absorbed by carbon sinks such as forests and oceans. But it is unclear exactly where and how large the sinks are. And if they shut down for some reason, climate change could accelerate dramatically.

To get a handle on these issues, NASA鈥檚 Orbiting Carbon Observatory (OCO) and Japan鈥檚 Greenhouse Gases Observing Satellite (GOSAT), designs of which were presented at the annual meeting of the American Geophysical Union in San Francisco earlier this month, will launch within 45 days of each other in late 2008. The satellites will detect CO₂ levels by monitoring the sunlight reflected from the Earth鈥檚 surface. Carbon dioxide absorbs specific wavelengths of light, so examining the spectra of the reflected light from space will reveal the abundances of CO₂ in different regions of the atmosphere. This information will then be combined with carbon transport models to figure out how much of the gas is emitted or absorbed over a given region.

While NASA鈥檚 primary interest is in the scientific question, GOSAT project manager Takashi Hamazaki says the Japanese team would like to use its satellite to drive policy. 鈥淭he current Kyoto protocol doesn鈥檛 have any good measure to verify the effort of each country,鈥� says Hamazaki. 鈥淓ach country reports its inventory, but we are not sure if it is correct or not.鈥�

Such efforts will have to wait for more advanced versions of the satellites, however. The first-generation probes will not have the resolution to determine the carbon fluxes of individual nations, except for extremely large countries such as India and China. Also, the measurements will be confounded by clouds and aerosols, so extensive ground and aircraft measurements will be needed to verify the accuracy of the satellites.

Even with improved resolution, the satellites may not be useful in enforcing the Kyoto protocol as it stands. Trees count as carbon credits only if they have been planted specifically to absorb carbon, something the satellites cannot verify. But regulations may change, says Peter Rayner of the Laboratory of Sciences of the Climate and Environment in Saclay, France. Some countries are pushing for regulation on the basis of net carbon emissions alone, which would be easier for the satellites to determine.