After calculating GHG aggregates, we identified the key categories (emission sources that contribute to 95% of the total tons of CO2-eq released) in order to determine the major impact sources. Key categories were quantified by dividing the GHG emissions related to every specific activity (in decreasing order) by total GHG emissions from the IWMS. The uncertainty associated with each category was evaluated from the accuracy attributed to the activity
data and emission factors chosen for the calculations. The accuracy of these parameters had three levels of precision: low (degree of uncertainty between 3% and 5% e data collected at local or regional scale), medium (degree of uncertainty between 5% and 20% e data published on statistical national reports), high (degree of uncertainty between 20% and 50% e extrapolated or qualitative world average data), as reported in IPCC, 2000 and 2006. A 95% confi- dence interval was also calculated applying the Monte Carlo technique (IPCC, 1996, 2000).