Fossil-Fuel CO2 Emissions at Various Spatial and Temporal Scales

Annual values of fossil-fuel carbon emissions (FFCE), estimated directly from fuel combustion data or fuel- supply data, exist at global and national scales and have been used in studies of the global carbon cycle. Annual averages at national levels are useful in tracking emissions and developing mitigation strategies. However, such large-scale long-term averages contain no information about small-scale, short-term processes influencing FFCE. Recent development of a monthly FFCE database for the United States (US) facilitates correlations of seasonal FFCE with other seasonal variables. An increasing fraction of FFCE has occurred during the warmest months, and appears to be related to population shifts to warmer regions, general climate warming, and increased use of air conditioners. These results incorporate effects of even smaller-scale variables. FFCE from electric-generating facilities occur on space scales represented by individual stacks, and depend on regional electricity demand, which depends on temperature and other regional variables which change from hour to hour. Such variables also influence FFCE from urban areas, which, like electric generating facilities, contribute a large fraction of global FFCE but occupy a small fraction of the earth's surface. Preliminary studies in US cities indicate elevated winter CO2 concentrations from local emissions due to combustion of natural gas for heating, but summer concentrations are much closer to global background levels. Energy for air conditioning in summer is likely to be provided by electric-generating facilities at distant locations; distance from the emissions sources, along with increased convection in summer would reduce urban CO2 concentrations. Greater understanding of processes affecting local and urban FFCE, as well as any spatial homogeneities arising from reliance on common resources and energy demands, transportation links, etc., will be necessary to project emissions changes in response to future developments. Global analysis requires incorporation of 3 to 4 orders of magnitude in space and time, and must combine accurate numerical data with prior knowledge about the processes involved and their intra- and inter-scale interactions. Our interest here is in obtaining accurate FFCE estimates over the widest possible range of spatial and temporal scales, incorporating the processes involved as much as possible. Available large-scale estimates, based on fuel-combustion or fuel-supply data, can constrain less precise estimates for smaller space and time scales, obtained from variables such as vehicle traffic patterns, population distribution, temperature, etc. Available hourly fuel-combustion-based FFCE estimates for US electric generating facilities can help constrain FFCE estimates from other sources when integrated to scales where total emissions are relatively well known. Isotopic analysis can also provide information about relative magnitudes of FFCE from specific sources.