Biofuels are frequently portrayed as “clean fuel” and considered to be carbon neutral, since CO2 emitted through combustion of motor fuel is re-absorbed by growing more sugarcane rendering the balance practically zero. Numerous articles advocate  an increase in biofuels production and consumption as an environmentally friendly option. Sugarcane ethanol is considered an efficient way of reducing CO2 emissions of energy production and can significantly contribute to greenhouse gas mitigation, since the net CO2 released per unit of energy produced is significantly lower compared to fossil fuels; sugarcane bioenergy systems stand out as promising candidates for GHG mitigation. Feedstock for ethanol production, in this particular case, sugarcane, grows by transforming CO2 from atmosphere and water into biomass, which is, as mentioned before the reason why such fuel is called carbon neutral. Nevertheless, fossil fuel emissions are always associated with any agricultural activity.
It has been a popular misconception that bioenergy systems have no net CO2 emission. Considerable amounts of fossil fuel inputs are required for plant growth and transportation, as well as for ethanol distribution, and therefore CO2 emissions are present during the process of ethanol production. Fertilisers, herbicides and insecticides have net CO2 emissions associated with their production, distribution and application. Sugarcane production also results in emissions of other GHG, namely methane and nitrous oxide. Based on Lima et al. (1999), CH4 and N2O emissions from sugarcane correspond to 26.9 and 1.33 kg per hectare respectively. Such emissions correspond to, based on Schlesinger (1997), 672 kg and 399 kg respectively of CO2 equivalent. Theoretically, there are no GHG emissions associated with distillery operations. All the energy required comes from the burning of bagasse, which is a residue of the milled sugarcane. In fact, the burning of bagasse generates more energy than the distillery requires, resulting in some surplus of energy. Conceptually CO2 emissions associated with bagasse burning are not accounted for, since it is sequestered during sugarcane growth and will be re-absorbed in the next season. The same rationale applies to the ethanol burning in mother vehicles. For accounting purposes a complete combustion is assumed in both cases.
Based on an average production of 80 tons per ha which is representative of Brazil (In Guyana it ranges from 60 – 82 t/ha) and ethanol conversion efficiency of 80 L per ton of sugarcane processed; the amount of ethanol resulting from one ha or sugarcane plantations is 6.4 m3. Consequently for production of one m3 of ethanol, GHG emissions account for 457 kg of CO2eq production and distribution; this corresponds to approximately 19 kg of CO2 per gigajoule (kg/GJ) of fuel. Estimating the potential for GHG reduction from the use of ethanol derived from sugarcane requires a comparison with the fossil fuel displaced. In Brazil, the automobile fleet has basically three fuel options: natural gas, ethanol and gasoline, the last option is actually a mixture of gasoline and ethanol. The proportion of each fuel varies slightly according to government decisions, currently it is 75% gasoline and 25% ethanol. Natural gas running automobiles are not manufactured in Brazil, but automobiles can be converted to natural gas at a price ranging from US$ 1200 to US$ 2100.Although conversion to natural gas continues to rise in Brazil stimulated by its fuel economy, currently such vehicles represent only about 5% of the automobile fleet. The main attention in this work will be devoted to the impacts of ethanol substitution for gasoline.
In 2003, Brazil began to produce flex fuel cars, which can run with both gasoline and ethanol in any proportion using the same tank. In that year about 40,000  of such automobiles were produced, corresponding to only 2.6% of the new cars. In 2006, flex fuel cars corresponded to almost 60% of the new cars with 1.25 million units. This augment is directly related to a strategy for increasing biofuel consumption in Brazil, where the consumer is stimulated to use ethanol as an environmentally responsible option. The differences in price between ethanol and gasoline also contribute to the scenario. Presently in Brazil, ethanol is about 49% cheaper than gasoline, mostly due to heavier incidence of taxes over gasoline. The advantage of flex fuelled cars is that owners can switch back and forth between ethanol and gasoline according to the prices at the pump.
It is undeniable that the use of ethanol from sugarcane represents reduction in CO2 emissions when compared with gasoline. Nevertheless, the importance of such an option regarding its role in global warming has been disproportionately optimistic and leads to neglect of important environmental and social aspects. Biomass plantations can produce carbon neutral fuels for power plants or transportation, but photosynthesis has too low a power density for biofuels to contribute significantly to climate stabilisation. Based on UNFCCC, GHG emissions in tropics are mainly related to deforestation and agricultural intensification, while in temperate regions GHG comes from the combustion of fossil fuel in the transportation and industry sector. Agricultural intensification and deforestation are exactly the possible outcomes from significant increases of ethanol production in Guyana. The idea of reducing fossil fuel consumption from temperate areas by using sugarcane ethanol is unpracticable. In order to contribute to reduction of fossil fuel used in developed countries, the amount of ethanol that Guyana would have to produce would require a significant increase of the agricultural area devoted for such crops.