The emissions of green house gases contribute to global warming. Consequently, renewable energies such as solar, wind power, hydropower and bio energy appear as alternatives for reducing these emissions. One promising source of renewable energy is forest biomass, for it is considered to have neutral CO₂ balance. The problem is to quantify the amount of CO₂ that is emitted for a certain amount of energy generated, compared to the CO₂ emitted to generate the same amount of energy from a non-renewable source. In this paper we calculate biomass production and carbon dioxide balance of an eight-year-old Populus deltoides short rotation forestry (SRF) under two cutting cycles (harvesting every two years, biennial and every three years, triennial). We also compare the amount of energy generated per kg of CO₂ emitted in the use of fossil fuels versus the amount of energy generated per kilogram of CO₂ emitted. On the one hand, total biomass yield was 12.6 ± 0.9 Mg ha^-1 year^-1 and 15.1 ± 1.5 Mg ha^-1 year^-1 for biennial and triennial cutting cycles respectively. On the other hand, total emissions of CO₂ for growing a poplar SRF with a cycle of eight year and biennial cutting sequence reached a value of 738.8 kg ha^-1 year^-1; however, triennial short total emissions are equivalent to 695.5 kg ha^-1 year^-1. We concluded that the use of biomass makes a positive contribution to the reduction of greenhouse gases, in particular CO₂, reducing emissions almost five times compared with fossil fuels.