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Letter Report on the Orbiting Carbon Observatory NATIONAL RESEARCH COUNCIL OF THE NATIONAL ACADEMIES Board on Atmospheric Sciences and Climate http://dels.nas.edu/basc 500 Fifth Street, N.W., Keck WS603 Washington, D.C. 20001 Phone: 202-334-3512 Fax: 202-334-3825 July 28, 2009 Major General Charles F. Bolden, Jr. Administrator National Aeronautics and Space Administration 300 E Street, SW Washington, DC 20546 Dear General Bolden: A National Research Council committee is conducting a study on how well greenhouse gas emissions can be measured for treaty monitoring and verification. The committee’s analysis suggests that NASA’s Orbiting Carbon Observatory (OCO), which failed on launch in February 2009, would have provided proof of concept for spaceborne technologies to monitor greenhouse gas emissions, as well as baseline emissions data. This letter focuses on the capabilities of an OCO and currently deployed satellites that measure atmospheric carbon dioxide (CO2) and their potential role in monitoring and verifying a greenhouse gas treaty.1 The committee’s study is focused on emission estimates of the greenhouse gases resulting from human activities (e.g., fossil fuel burning, deforestation, agriculture) that have the greatest potential to warm the planet and in particular on CO2 (see Attachment B for the committee charge). The committee is currently in the analysis and writing phase, with the expectation that its report will be delivered in December 2009. We are writing you now because a decision on replacing OCO will be made in the coming months,2 before our final report is completed. Current proposals for an OCO reflight focus on the original scientific objectives of studying natural CO2 sources and sinks.3 In addition, it is important to consider the potential contribution of an OCO-like instrument for treaty monitoring and verification. Such capabilities may be an important consideration in treaty discussions at the December 2009 Copenhagen meeting of the United Nations Framework Convention on Climate Change. If a treaty is negotiated in the coming months, monitoring and verification will initially have to rely on current capabilities and on measurement enhancements that can be deployed quickly. As the committee’s final report will describe in more detail, current methods for estimating greenhouse gas emissions have limitations for monitoring a climate treaty. National emission 1 This report reflects the consensus of the committee and has been reviewed in accordance with standard NRC review procedures (see Attachment C). 2 For example, see testimony by Michael Freilich, Director, NASA’s Earth Science Division, before the House Committee on Science and Technology, on April 22, 2009. 3 Boland, S., H. Bösch, L. Brown, P. Ciais, B. Connor, D. Crisp, S. Denning, S. Doney, I. Fung, D. Jacob, B. Johnson, J. Martin-Torres, A. Michalak, C. Miller, D. O’Brien, I. Polonsky, C. Potter, P. Rayner, R. Salawitch, M. Santee, P. Wennberg, D. Wunch, and Y. Yung, 2009, The need for atmospheric carbon dioxide measurements from space: Contributions from a rapid reflight of the Orbiting Carbon Observatory, White paper to NASA, April 2, 2009, 48 pp.
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Letter Report on the Orbiting Carbon Observatory inventories, required under the United Nations Framework Convention on Climate Change, are self-reported and are not required regularly for all countries. Verification requires checking these self-reported emissions estimates. However, independent data against which to verify the statistics used to estimate CO2 emissions, such as fossil fuel consumption, are not available. Existing instruments and methods for remote monitoring of atmospheric CO2 are not able, with useful accuracy, to distinguish fossil fuel emissions from natural fluxes or to verify trends in fossil fuel emissions, such as reductions against a baseline. Atmospheric CO2 measurements by ground stations, aircraft, and satellites can be combined with atmospheric circulation models to infer emissions from the land surface, a method known as tracer-transport inversion. The principle is that an emission source located between two sites will cause the abundance of the gas to be higher at the downwind site than at the upwind site by an amount proportional to the source strength. However, estimated changes in atmospheric CO2 abundance due to fossil fuel sources are confounded by errors in the reconstruction of atmospheric transport, by sparse CO2 observations, and by the much larger changes due to biological sources and sinks.4 Because of these complications, the tracer-transport inversion method is currently able to estimate emissions with a useful accuracy only for some large continents. The method’s accuracy could be improved by expanding the CO2 sampling network on the ground and from space, and OCO was in fact designed to improve tracer-transport inversions. A complementary approach to tracer-transport inversion is to measure the increased atmospheric abundance on top of large local sources such as cities or power plants. The majority of fossil fuel emissions emanate from such sources and would likely be a target of mitigation measures. These large sources increase the local CO2 abundance in the atmosphere by 1-10 ppm, a signal large enough to overwhelm the signal from natural sources and sinks, reducing this source of uncertainty.5 Because the increased abundances are largest over the source of emissions and disperse within a few tens of kilometers, they can usually be attributed unambiguously to their country of origin. Statistical or systematic sampling of CO2 from large local sources would thus support treaty verification by providing independent data against which to compare trends in emissions reported by countries, at least for the fossil fuel emissions from cities and power plants. The existing atmospheric CO2 sampling network of ground stations, aircraft, and satellites is not well designed for estimation of emissions from large local sources distributed around the globe. Ground stations and aircraft were purposefully deployed away from large fossil fuel sources to better detect natural sources and sinks, but could be deployed to monitor CO2 emitted from selected cities and power plants. However, this would require international cooperation and such nationally operated stations would still have the verification challenges associated with self-reporting. Satellites obviate these problems. As shown in Attachment A, Japan’s GOSAT is the 4 Fossil fuel emissions from the United States change the average abundance of atmospheric CO2 by only ~0.7 parts per million (ppm; less than 0.2 percent) as air moves across the U.S. continent. Depending on season, analogous changes from biological sources will be two to five times larger. The signals produced by most countries are significantly smaller than these. See Tans, P.P., P.S. Bakwin, and D.W. Guenther, 1996, A feasible global carbon cycle observing system: A plan to decipher today’s carbon cycle based on observations, Global Change Biology, 2, 309-318. 5 Riley, W.J., D.Y. Hsueh, J.T. Randerson, M.L. Fischer, J.G. Hatch, D.E. Pataki, W. Wang, and M.L. Goulden, 2008, Where do fossil fuel carbon dioxide emissions from California go? An analysis based on radiocarbon observations and an atmospheric transport model, Journal of Geophysical Research, 113, G04002, doi:10.1029/2007JG000625.
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Letter Report on the Orbiting Carbon Observatory best available spaceborne measurement of CO2, although it is not optimal for monitoring emissions by large fossil fuel sources. It has lower uncertainty and higher spatial resolution than SCIAMACHY, AIRS, or IASI, and it senses near the surface where emission signals are largest, unlike AIRS and IASI. However, the CO2 signal produced by the emissions of a large power plant is typically too small to measure with GOSAT.6 In contrast, OCO would have enabled monitoring of CO2 emissions from such local sources.6 No other satellite has its critical combination of high precision, small footprint, readiness, density of cloud-free measurements, and ability to sense CO2 near the earth’s surface (Attachment A). In particular, its 1- to 2-ppm accuracy and 1.29 × 2.25-km sampling area would have been well matched to the size of a power plant.6 OCO would have had limitations for monitoring CO2 emissions from large sources in the context of a climate treaty. It would have sampled only 7-12% of the land surface7 with a revisit period of 16 days, and its lifetime would be only 2 years (Attachment A). However, many metropolitan areas are large enough to be sampled by OCO, and OCO would have provided a sample of a few percent of the power plants. Monitoring urban and power plant emissions from space is challenging and has not been demonstrated. A replacement OCO could demonstrate these capabilities. Nevertheless, it would be valuable to explore changes in the orbit and other parameters so that a greater fraction of large sources is sampled. For example, consider a precessing orbit covering ~100% of the surface but with only two measurements per year of each location. With 100-500 large local sources in high-emitting countries, it might be possible to obtain a statistical sample of hundreds of measurements of plumes of CO2 being emitted by the large sources in each of these countries. The trade-offs in optimizing monitoring capabilities while meeting scientific objectives would have to be examined by a technical advisory group. Because of its two-year mission life, OCO would not by itself have been able to track emission trends. However, it would have provided the first few years of measurements (a baseline) necessary to verify a decadal trend for the large local sources within its footprint, and served as a pathfinder for successor satellites designed specifically to support treaty monitoring and verification. Even with the data and lessons learned from a replacement OCO, a successor mission is unlikely to be ready for almost a decade.8 Space-based monitoring of emissions to support a greenhouse gas reduction treaty has received little attention by U.S. scientists and the government. The committee’s analysis suggests that existing measurement methods alone are insufficient to independently verify reported emissions trends. Although OCO was not designed for treaty monitoring and verification, it 6 Assume that a 500 MW pulverized coal power plant emits ~0.13 t s−1 of CO2 (e.g., 4 Mt CO2 yr−1) and that the wind speed is 3 m s−1. This would produce a perturbation of approximately 0.5 percent (~1.7 ppm) in the abundance of CO2 within an OCO sample, which is consistent with the design’s estimation error of 1-2 ppm and significantly larger than the ground-tested value of 1 ppm. In contrast, because a GOSAT sample covers a larger area than an OCO sample, the CO2 perturbation within a GOSAT sample would be approximately 0.1 percent (~0.4 ppm). This is an order of magnitude smaller than GOSAT’s estimation error of 4 ppm. 7 Miller, C.E., D. Crisp, P.L. DeCola, S.C. Olsen, J.T. Randerson, A.M. Michalak, A. Alkhaled, P. Rayner, D.J. Jacob, P. Suntharalingam, D.B.A. Jones, A.S. Denning, M.E. Nicholls, S.C. Doney, S. Pawson, H. Boesch, B.J. Connor, I.Y. Fung, D. O’Brien, R.J. Salawitch, S.P. Sander, B. Sen, P. Tans, G.C. Toon, P.O. Wennberg, S.C. Wofsy, Y.L. Yung, and R.M. Law, 2007, Precision requirements for space-based XCO2 data, Journal of Geophysical Research, 112, D10314, doi:10.1029/2006JD007659. 8 For example, the Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) mission has been recommended for launch in the 2013-2016 time frame. See National Research Council, 2007, Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond, The National Academies Press, Washington, D.C., 456 pp.
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Letter Report on the Orbiting Carbon Observatory would have provided baseline emission data from large fossil fuel sources as well as essential tests of the engineering designs and measurement concepts required to develop a robust capability for monitoring emissions from space. The committee hopes this report helps to inform NASA’s upcoming decision on flying a replacement OCO. Sincerely, Stephen W. Pacala, Chair Committee on Methods for Estimating Greenhouse Gas Emissions Attachments cc: Todd Stern, Special Envoy for Climate Change, State Department John Holdren, Director, Office of Science and Technology Policy