Uncertainty in Emissions Estimates

The emissions numbers presented in this report are estimates based on estimated activity data and estimated emission factors. As such, they have an element of uncertainty, given that the activity data and emission factors on which the emission estimates are based also have a range of possible values. The activity data and emission factors can themselves be characterized by systematic biases and/or random errors. In 2000, EIA employed a Monte Carlo analysis to estimate the range of uncertainty, at a 95-percent confidence level, around estimated emissions of carbon dioxide, methane, and nitrous oxide (HFCs, PFCs, and SF₆ were not part of the analysis).¹¹

The Monte Carlo simulations revealed that uncertainty varies by type of gas. There is less uncertainty around the simulated mean for carbon dioxide (-1.4 percent to +1.3 percent) than for methane (-15.6 percent to 16.0 percent) or nitrous oxide (-53.5 percent to +54.2 percent). The simulations also showed that the uncertainty around the simulated mean of total greenhouse gas emissions (excluding HFCs, PFCs, and SF₆) is -4.4 percent to +4.6 percent.

The reliability of emissions data varies by category and by source. In general, the estimates for carbon dioxide emissions are more reliable than the estimates for other gases. It is likely that the estimate of carbon dioxide emissions is accurate to within 5 percent. Estimates of methane emissions are much more uncertain, with a level of uncertainty that may exceed 30 percent. Estimates of methane emissions are also likely to understate actual emissions as a result of the exclusion of sources that are unknown or difficult to quantify, such as abandoned coal mines and industrial wastewater. Nitrous oxide emissions estimates are much less reliable than those for carbon dioxide or methane emissions, in part because nitrous oxide emissions have been studied far less than emissions of the other greenhouse gases and in part because the largest apparent sources of nitrous oxide emissions are area sources that result from biological activity, which makes for emissions that are highly variable and hard to measure or characterize. The uncertainty for nitrous oxide emissions may exceed 100 percent.

Notes and Sources