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O-CO2, Can You See by the Dawn’s Early Light?

July 11, 2014

The A-train constellation of earth observing satellites. Included is the recently added OCO-2 mission spacecraft (leading the train at far right), which will focus on improving our understanding of the global carbon cycle by mapping Earth’s CO2 distribution approximately once every 2 weeks for the next 2 years or more. (NASA)

The following is this week’s edition of the USGS Hawaiian Volcano Observatory‘s Volcano Watch:

This year, two days before America’s Independence Day fireworks celebrations began, a remarkably different “rocket’s red glare” was seen at Vandenberg Air Force Base, California. This event , which actually occurred a couple of hours before dawn, marked the successful launch of NASA’s first spacecraft dedicated to the study of carbon dioxide (CO2) in the Earth’s atmosphere.

The centerpiece of the ambitious mission of the Orbiting Carbon Observatory-2, or OCO-2, is essentially the mapping of CO2 distribution in Earth’s atmosphere on a continuing basis, for at least two years. While the “OCO-2” acronym nicely describes the target of the mission, there was also an OCO-1 launch back in 2009, but things went awry before the spacecraft reached orbit.

To carry out its mission, OCO-2 will orbit Earth at its poles about every hundred minutes or so as a member of a constellation of half-a-dozen satellites, all of which will be moving together as the “A-Train.” Each “car” in this satellite train is equipped with sensors that collect data that complement each other. For example, while OCO-2 collects CO2 data, two other satellites, CALIPSO and CloudSat, will be collecting information about clouds and aerosol in the atmosphere at the same time, helping to improve the quality of OCO-2’s findings.

Although measuring CO2 in Earth’s atmosphere from space does require some rocket science to accomplish, it isn’t the simple detection of CO2 that’s so difficult, since there’s lots of it in the atmosphere to measure. What makes OCO-2’s job so challenging is that this mission’s chief goal is to help us better understand Earth’s carbon cycle—a global-scale topic with many variables.

In general, human activity and some processes of nature act as CO2 sources, pumping huge amounts of this greenhouse gas into the atmosphere, while other processes occurring in the oceans and on land act as sinks, removing it. Studies have shown convincingly that before the industrial age, human sources of CO2 were small, compared with the carbon-removing influence of vegetation, oceans, and other natural processes. Modern carbon inventories indicate that the balance has been tipped the other way: we’re putting more CO2 into the air than Earth processes can practically remove.

OCO-2 will attempt to unravel this mystery through its mapping of the amount and location of CO2 in Earth’s atmosphere at an unprecedented level of precision and sensitivity. This feat, if accomplished, will produce a better quantitative understanding of regional CO2 sources and sinks. To do this, OCO-2 will be using a sophisticated infrared spectrometer system that measures sunlight energy passing through the atmosphere. The sunlight is absorbed in proportion to the amount of CO2 in the light’s path as it reaches the surface and is reflected back up to the satellite. At the same time, OCO-2 will measure light energy emitted during photosynthetic processes associated with CO2 removal.

The potential to identify CO2 sources and sinks is of great interest to volcanologists, as well. Volcanoes can release significant amounts of CO2 during large eruptions, even though, averaged over time, they add little CO2 to Earth’s atmosphere on a global scale (less than 0.5 % of that contributed by human activity on average, including large eruptions). Furthermore, volcanic CO2 is associated with magma supply, so the possibility of measuring CO2 from space everywhere on the planet, twice a month, could be very helpful for detecting volcanic restlessness at remote volcanic settings worldwide.

Even if the OCO-2 mission is wildly successful, challenges will remain for applying these CO2 maps to volcanoes. Although the OCO-2 should, theoretically, be able to see fairly small CO2 concentration differences on the Earth’s surface, the detection footprint (as small as 1 square mile or 3 square km) is of the same order as the central crater of many volcanoes where CO2 is emitted. And once volcanic CO2 is released to the atmosphere, it begins to disperse and dilute, making it harder to detect.

A successful OCO-2 mission, one that helps us better understand the quantitative balance between CO2 sources and sinks on regional and global scales, could help guide efforts to effectively decrease human-made CO2 emissions. The volcano science community is looking forward to fruits of this mission, as well.

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