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Satellite Observations to Benefit Science and Society: Recommended Missions for the Next Decade ASCENDS ACTIVE SENSING OF CO2 EMISSIONS OVER NIGHTS, DAYS, AND SEASONS LAUNCH: 2013–2016 MISSION SIZE: Medium ORBIT: LEO, SSO AGENCY: NASA ESTIMATED COST: $400 million AREAS OF INTEREST: Ecosystems, Health INSTRUMENTS: Multifrequency laser BENEFITS: Improved climate models and predictions of atmospheric CO2 Identification of human-generated CO2 sources and sinks to enable effective cabon trading Closing of the carbon budget for improved policy and prediction Fossil fuel burning, deforestation, and other human activities have driven a 40 percent increase in atmospheric CO2 since the mid-1800s. Of the CO2 released by human action, about half remains in the atmosphere. Land ecosystems and oceans absorb the remainder in roughly equal amounts. However, this process varies over time and space in ways that remain poorly understood. Direct measurements of carbon and CO2 fluxes across the land and sea surface are expensive, scarce, and difficult to extrapolate. ASCENDS will provide long-term, accurate global CO2 data using a multifrequency laser to measure the total amount of CO2 in atmospheric columns. ASCENDS data will allow for weekly mapping of CO2 sources and sinks at 1° longitude and latitude. These data will assist society in predicting future CO2 concentrations, monitoring CO2 sequestration efforts, and supporting future carbon and energy policies. ASCENDS will provide measurements by day, when photosynthesis occurs, as well as at night, when plant respiration dominates. Simply going from a single daily measurement to two readings, one taken by day and the other at night, can provide a greatly improved picture of CO2 fluxes. The time frame for ASCENDS was chosen to allow overlap with the Orbiting Carbon Observatory. Simultaneous lidar-based measurement of oxygen (O2) would correct for variations in atmospheric pressure. Ideally, coordinated observations from a carbon monoxide (CO) sensor would provide additional data to distinguish the CO2 emitted by plant life from that produced by fires and fossil fuels. Extensive aircraft flights will be needed to test the CO2 and O2 sensing techniques under a variety of conditions.