The Evolving Atmospheric Carbon Dioxide Monitoring Constellation
Jet Propulsion Laboratory, California Institute of Technology, UNITED STATES
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. Greenhouse gas measurements from a global network of surface stations show that these CO2 emissions are superimposed on an active carbon cycle, driven by natural processes in the land biosphere and oceans. These natural processes emit more than 20 times as much CO2 into the atmosphere each year as human activities, and then reabsorb a comparable amount, along with about half of the human contributions. While existing ground-based measurements provide a strong global constraint on both human and natural CO2 fluxes into the atmosphere, a far more comprehensive measurement network is needed to identify and quantify the strongest natural sources and sinks, or to discriminate the human CO2 emissions from the natural background. Such measurements are 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 is to retrieve precise, spatially-resolved, global measurements of the column-averaged CO2 dry air mole fraction, XCO2, from space-based measurements of reflected sunlight in near infrared CO2 and O2 bands. Estimates of XCO2 from these measurements are ideal for this application because the amount of sunlight absorbed by these bands is most sensitive to CO2 concentrations near the Earth surface, where most CO2 sources and sinks are located. This is still one of most challenging space based remote sensing observations ever attempted 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%. Because CO2 is a long-lived gas, these small CO2 perturbations are embedded in the time-varying atmospheric circulation, producing “CO2 weather.” The ESA EnviSat SCIAMACHY instrument pioneered solar remote sensing observations of the dry air mole fractions of CO2 and CH4 (XCH4), but stopped returning data in early 2012, after ten years of operation. The Japanese Greenhouse Gases Observing Satellite, GOSAT (nicknamed Ibuki), was the first satellite optimized specifically to retrieve estimates XCO2 and XCH4. GOSAT was successfully launched in January 2009 and has been returning data since April 2009 that are beginning to yield new insights into the carbon cycle. The Orbiting Carbon Observatory – 2 (OCO-2) is NASA’s first dedicated CO2 monitoring satellite. OCO-2 is currently being integrated and tested in preparation for a launch in mid-2014. OCO-2 measures only CO2, but is expected to provide substantial improvements in coverage, resolution, and precision. Other CO2 monitoring satellites, including the Chinese TanSat, CNES MicroCarb, and Japanese GOSAT-2 missions are currently under development, in preparation for launches in the second half of this decade. NASA also proposes to deploy the OCO-2 flight spare instrument on the International Space Station as the OCO-3 mission as early as 2017. Then, in the early 2020’s this suite of missions could be joined by the European Space Agency’s wide-swath CarbonSat mission and by the first space based CO2 lidar mission, NASA’s proposed Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS). If all goes as planned, a succession of partially-overlapping missions with a range of CO2 measurement capabilities are expected to fly within the next decade. Each mission has been conceived with unique measurement capabilities that could make unique contributions to our understanding of the role of CO2 in the carbon cycle. However, much greater benefits could be realized if these missions could be coordinated with the each other and with the surface CO2 monitoring network to produce a true, global, CO2 monitoring system. In particular, if these measurements could be cross calibrated and the retrieved XCO2 estimates could be cross validated against recognised standards, all of these data could be assimilated into flux inversion models to quantify CO2 sources and sinks on regional scales over the globe. A coordinated space-based constellation would also provide continuity and resiliency to losses of individual satellites. The objective is to implement a CO2 monitoring system with the spatial and temporal coverage, resolution, and reliability of the current weather monitoring satellite system. Here, we summarize the specific assets of each of these systems, and describe the actions needed and benefits expected from their integration into an ad-hoc CO2 monitoring constellation.