Observing System Simulations for Carbon–Climate Feedbacks

Human activity has altered the carbon cycle, but after decades of research, feedbacks between the carbon cycle and the climate system remain mysterious. Anthropogenic emissions of CO₂ and CH₄ are partitioned between the atmosphere, where they affect climate, and poorly understood “natural” sources, sinks, and feedbacks. If these change, then the climate effect of the human enterprise will change. New space-based observations show coupling between carbon and climate, with positive and negative feedbacks from terrestrial sources and sinks for carbon. New satellite observations are beginning to reveal and quantify feedbacks critical for climate prediction, but next generation observing constellations will be needed to address remaining unknowns. Carbon scientists, recognizing the challenge of a carbon and climate observing system have long used simulation, using Observing System Simulation Experiments (OSSEs) to design optimal observing strategies. Planning greenhouse gas missions is especially challenging as the actual retrieval, the vertically integrated mass of the gas in question, is not the quantity of scientific interest. Instead, the surface fluxes, deduced from the time-space patterns of concentration in combination with atmospheric transport, are the focus of research. Carbon cycle OSSEs begin by assuming or simulating a set of surface fluxes, the “truth”, creating a simulated global pattern of concentrations using an atmospheric transport model to translate fluxes into concentrations and then translating those concentrations into observations using a model of the proposed instrument, and its interaction not only with CO₂ but also factors that interfere with the observation, such as clouds, aerosols, surface albedo and viewing geometry. The set of simulated observations is used to estimate fluxes, and the estimated fluxes compared to truth. OSSEs allows quantifying the uncertainty of the observing system and diagnosing problems to use in iterative designs of the proposed instrument. OSSEs have played a significant role in the design of NASA’s Orbiting Carbon Observatory missions, have aided NASA and ESA in designing follow-on missions and were critical to NASA’s decision to select a geostationary (GEO) carbon observatory for X_CO2 and X_CH4. Recent OSSEs document the advantages of persistent observations from for resolving both small scale biogeochemical and regional urban processes, particularly in cloudy environments. Other studies quantify the need for winter measurements in the high latitudes where low solar angles and short days limit the use of reflected sunlight spectroscopic measurements, and either airborne in situ or active remote sensing using laser measurements are required. Carbon science has been critically limited by data, with data effectively characterizing only the largest and the local scales, but critical scales where climate variation produces globally-significant carbon cycle fluxes have been all but impossible to observe directly, and satellite observations if correctly designed and analyzed can fill this gap. Carbon cycle-climate feedbacks influence future climate uncertainty and can confuse emission reduction targets. Feedbacks alter the magnitude of reductions required to achieve any given temperature target, and could make mitigation either excessively expensive, if negative, stabilizing feedbacks are over-estimated, or reduce their effectiveness, if positive, amplifying feedbacks prove to be stronger than expected. The anticipated global costs of mitigation are such effort to design and optimal observing system to reduce carbon cycle-climate feedback uncertainty is a cost-effective step towards planning efficient and effective climate action.