A successful mitigation strategy must include the means to quantify emissions and their changes and include the ability to attribute changes in emissions to specific processes, be they natural or anthropogenic. Emissions are estimated using different methodologies. • Inventory-based, 'bottom-up' accountings of activities resulting in emissions are multiplied by appropriate emission factors. GHG emissions reported to the United Nations Framework Convention on Climate Change are derived from inventory-based accountings that have a range of accuracies. These inventories are based on an incomplete understanding of emissive processes, have uncertainties that are difficult to quantify and typically are updated infrequently. • Atmosphere-based studies involve inferring flux magnitudes on local to global scales on the basis of measured atmospheric mixing ratio changes and gradients, their time-dependent variations, and covariations with meteorological parameters or other trace gases. These 'top-down' approaches have historically provided estimates of total global fluxes and can provide i ndependent assessments of global, inventory-based emission magnitudes. Significant discrepancies have been identified761'63'91 between bottom-up and top-down global estimates, highlighting uncertainties in both methods. Recent advances in measurement technologies and high-resolution transport models have made it possible to calculate regional estimates of emissions from top-down analyses43'4492"96. These approaches may provide an independent means to assess or verify regional or national bottom-up emission inventories but at present have substantial uncertainties.
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