The Orbiting Carbon Observatory (OCO) mission

Abstract

The Orbiting Carbon Observatory (OCO) mission will make the first global, space-based measurements of atmospheric carbon dioxide (CO₂) with the precision, resolution, and coverage needed to characterize CO₂ sources and sinks on regional scales. The measurement approach and instrument specifications were determined through an analysis of existing carbon cycle data and a series of observing system simulation experiments. During its 2-year mission, OCO will fly in a 1:15 PM sun-synchronous orbit with a 16-day ground-track repeat time, just ahead of the EOS Aqua platform. It will carry a single instrument that incorporates three bore-sighted high-resolution spectrometers designed to measure reflected sunlight in the 0.76-μm O₂ A-band and in the CO₂ bands at 1.61 and 2.06 μm. Soundings recorded in these three bands will be used to retrieve the column-averaged CO₂ dry air mole fraction (X_CO2). A comprehensive validation program was included in the mission to ensure that the space-based X_CO2 measurements have precisions of ∼0.3% (1 ppm) on regional scales. OCO measurements will be used in global synthesis inversion and data assimilation models to quantify CO₂ sources and sinks. While OCO will have a nominal lifetime of only 2 years, it will serve as a pathfinder for future long-term CO₂ monitoring missions.

Introduction

Carbon dioxide (CO₂) is an efficient greenhouse gas, whose atmospheric concentration has increased from 280 to 370 parts per million (ppm) since the beginning of the industrial age (Fig. 1(a); Cicerone et al., 2001). These rapid increases have raised concerns about global climate change. For more than 20 years, data collected from a global network of surface stations indicate only about half of the CO₂ that has been emitted into the atmosphere by fossil fuel combustion and biomass burning has remained there (Fig. 1(b); cf. Schnell et al., 2001; Etheridge et al., 1996). The terrestrial biosphere and oceans have apparently absorbed the rest. The nature and geographic distribution of these CO₂ sinks is not well understood. Specifically, while data from the Globalview-CO₂ database (GV-CO2; cf. Gloor et al., 2000) provide compelling evidence for a Northern Hemisphere terrestrial carbon sink, this network is too sparse to resolve North American and Eurasian contributions to this sink, or to estimate fluxes over the southern oceans (Battle et al., 2000; Bousquet et al., 2000; Ciais et al., 1995; Conway and Tans, 1999; Denning et al., 1995; Keeling and Shertz, 1992; Morimoto et al., 2000; Pacala et al., 2001; Tans et al., 1989; Fan et al., 1998; Rayner and O'Brien, 2001; Enting, 1993). Existing measurements and models also cannot fully explain why the atmospheric CO₂ increase has varied from 1 to 7 gigatons of carbon (GtC) per year in response to steadily rising fossil fuel emission rates (Fig. 1(b); Randerson et al., 1997, Randerson et al., 1999; Lee et al., 1998; Le Quéré et al., 2000; Keeling et al., 1995; Houghton, 2000; Frolking et al., 1996; Langenfelds et al., 2002). Because the present-day behavior of these CO₂ sinks is not understood, predictions of their response to future climate or land use changes have large uncertainties. If their efficiency decreases over time, the atmospheric CO₂ buildup could accelerate (Cox et al., 2000; Friedlingstein et al., 2001).

Global simulations with source–sink synthesis inversion models (Rayner and O'Brien, 2001) indicate that uncertainties in the atmospheric CO₂ balance could be reduced substantially if data from the existing ground-based CO₂ network were augmented by spatially resolved, global, measurements of the column-integrated dry air mole fraction (X_CO2) with precisions of ∼1 ppm (0.3% of 370 ppm). This information would also facilitate monitoring compliance with future CO₂ emissions treaties that offer credits for CO₂ sequestration as well as emissions reductions. The Orbiting Carbon Observatory (OCO) has been designed to provide these measurements.