Birdsey, R., M. A. Mayes, P. Romero-Lankao, R. G. Najjar, S. C. Reed, N. Cavallaro, G. Shrestha, D. J. Hayes, L. Lorenzoni, A. Marsh, K. Tedesco, T. Wirth, and Z. Zhu, 2018: Executive summary. In Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report [Cavallaro, N., G. Shrestha, R. Birdsey, M.A. Mayes, R. G. Najjar, S. C. Reed, P. Romero-Lankao, and Z. Zhu (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 21-40, https://doi.org/10.7930/SOCCR2.2018.ES.
The conclusions from this report underscore the significant advances made in the understanding of the North American carbon cycle in the decade since SOCCR1 (CCSP 2007). Results show that emissions from the burning of fossil fuels for energy and other technological systems still represent the largest single source of the North American carbon budget. About 43% of these emissions are offset by terrestrial and coastal ocean sinks of atmospheric CO2. A better understanding of inland waters is among the major scientific advances since SOCCR1 that are highlighted in this report. In contrast to SOCCR1, SOCCR2 clearly identifies a significant source of CO2 from inland waters, as well as a similarly sized sink in the coastal ocean. This report also describes progress in documenting key elements of the CH4 budget, which were largely absent in SOCCR1. Improved consistency between bottom-up inventories and top-down atmospheric measurements is encouraging for the design of future monitoring, reporting, and verification systems. Such systems will be enhanced greatly if uncertainties in the two approaches continue to decline as new measurement systems are deployed and as integrated analysis methods are developed. Importantly, understanding of the main causes of observed changes in the carbon budget has improved over the last decade, helping to establish a strong foundation for assessing options for reducing atmospheric carbon concentrations and for developing and using carbon management choices. Reducing carbon emissions from existing and future sources and increasing carbon sinks will need to involve science-informed decision-making processes at all levels: international, national, regional, local, industrial, household, and individual.
Despite improvements in calculating the carbon budget since SOCCR1, some regions and ecosystems still have highly uncertain estimates compared with others and thus need significant improvements in research and monitoring. Among these areas are Arctic and boreal regions, grasslands, tropical ecosystems, and urban areas. Also needed is a better overall understanding of the CH4 cycle. The continued advancement of cross-disciplinary and cross-sectoral carbon cycle science to fill these gaps and to address the research challenges and opportunities identified in this report will be important for the third SOCCR to assess a decade from now.
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