The NASA Orbiting Carbon Observatory–2: The Next Step torward an Operational Greenhouse Gas Satellite Network

Fossil fuel combustion, deforestation, and other human activities are currently adding more than 30 billion tons of carbon dioxide (CO2) to the atmosphere each year. These CO2 emissions are superimposed on active natural carbon cycle, that emits more than 20 times as much CO2 into the atmosphere each year as human activities, and then reabsorbs a comparable amount, along with about half of the human contributions. Existing ground-based measurements provide a strong global constraint on both human and natural CO2 fluxes into the atmosphere. However, a far more comprehensive measurement network is needed to identify and quantify the strongest natural sources and sinks, and to discriminate the human CO2 emissions from the natural background. Such measurements will be essential to monitor compliance with CO2 emissions regulations or to assess the effectiveness of CO2 emission reduction strategies.

One way to improve the spatial and temporal sampling of CO2 is to retrieve precise, spatially-resolved, global measurements of the column-averaged CO2 dry air mole fraction, XCO2 from space. Surface weighted estimates of XCO2 can be retrieved from high resolution spectroscopic observations of reflected sunlight in near infrared CO2 and O2 bands. This is a challenging space based remote sensing observation because even the largest CO2 sources and sinks produce changes in the background XCO2 distribution no larger than 1%, and most are smaller 0.25%. The European Space Agency (ESA) EnviSat SCIAMACHY and Japanese Greenhouse Gases Observing Satellite (GOSAT) TANSO-FTS were the first satellite instruments designed to exploit this measurement approach. SCIAMACHY collected column averaged CO2 and methane (XCH4) measurements over the sunlit hemisphere from 2002 to 2012. TANSO-FTS has been collecting XCO2 and XCH4 observations since April 2009. These data have provided an excellent proof of concept, and are beginning to yield new insights into the carbon cycle, but improvements in sensitivity, resolution, and coverage will be needed to fully exploit this approach.

The NASA Orbiting Carbon Observatory – 2 (OCO-2) is the next dedicated CO2 monitoring satellite. This mission is currently scheduled to launch from Vandenberg Air Force Base on a Delta-II 7320 launch vehicle in July 2014. After a series of orbit-raising maneuvers, the satellite will be inserted at the head of the 705-km Afternoon Constellation (A-Train). OCO-2 will fly along a ground track that is displaced 217.3 km to the east of the World Reference System-2 (WRS-2) track followed by the NASA Aqua platform, such that it overflies the CloudSat radar and CALIPSO lidar ground footprints, in a sun-synchronous orbit with an equator crossing time near 1:30 PM. The OCO-2 spacecraft will carry and point a 3-channel, imaging, grating spectrometer that collects high resolution spectra of reflected sunlight in the 765 nm O2 A-band and in the 1610 and 2060 nm CO2 bands. Each spectrometer channel will collect 24 spectra per second, yielding up to a million soundings per day over the sunlit hemisphere.

For routine science operations, the instrument's bore sight will be pointed to the local nadir or at the “glint spot,” where sunlight is specularly reflected from the Earth's surface. Nadir observations provide the best spatial resolution and are expected to yield more cloud-free XCO2 soundings. Glint observations will have much better signal-to-noise ratios (SNR) over dark, ocean surfaces. The nominal plan is to alternate between glint and nadir observations on successive 16-day ground-track repeat cycles, so that the entire sunlit hemisphere is sampled by both observing modes at 32-day intervals. The satellite can also target selected surface calibration and validation sites and collect thousands of observations as the spacecraft flies overhead. The instrument's rapid sampling, small (< 3 km2) sounding footprint, and high SNR, combined with the spacecraft's ability to point the instrument's bore sight toward the glint spot over the entire sunlit hemisphere, are expected to provide more complete coverage of the ocean, cloudy regions, and high latitude continents than earlier missions.

Even with these assets, OCO-2 is still a more of a “research” satellite, designed to validate the space-based CO2 measurement approach than a CO2 monitoring mission. A coordinated network of satellites, similar to the existing operational weather satellite network will be needed to accurately quantify the CO2 emissions from human activities in the context of the natural carbon cycle. No such network is yet on the drawing boards, but a series of greenhouse gas satellite missions are under development. OCO-2 will be followed by the Chinese TanSat in mid-2015. Other CO2 monitoring satellites including the CNES MicroCarb, Japanese GOSAT-2, and ESA CarbonSat missions are in the planning stages. These dedicated missions will be joined by the proposed NASA OCO-3 instrument on the International Space Station, and then by active CO2 monitoring sensors, such as the proposed NASA Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) mission. While each of these missions has unique measurement capabilities that will contribute to our understanding of the atmospheric CO2 distribution, much greater benefits could be realized if they can be coordinated as part of a global network of surface and space-based CO2 sensors. Their data could then be cross calibrated, cross-validated, and assimilated into source-sink inversion models. This would be the first step in the implementation of a CO2 monitoring system proposed above.