Global Deforestation and Co₂ Emissions: Past and Present. A Comprehensive Review

The present study makes a comprehensive review on the causes of the deforestation process and their consequent CO₂ emissions both in the past and present. Our primary goals are: (i) identify the range of current deforestation estimates. (ii) analyse global deforestation in an historical context which will consider both temperate and tropical forests. We particularly consider those factors upon which carbon emissions from tropical forests are dependent, such as area, biomass content and deforestation rates. A methodological approach shows that (i) the best available estimates on rates and areas of deforestation are mainly remote sensing-based studies, (ii) new approaches for the calculations of the biomass of tropical forests have narrowed the range of the estimates and (iii) the most unknown factor determinant of biotic carbon emissions is the burning efficiency. The fluxes of carbon from the vegetation to the atmosphere due to deforestation and land use changes have been calculated by the use of models that simulate alterations of the stored carbon as different disturbances are provoked on the forested land. The conclusions that arise from these models should be considered just a first approach to the question of biotic carbon emission, once (i) several assumptions are made so as to allow the model to work and (ii) there are uncertainties concerning several basic data that feed the models. A historical approach show us that past temperate deforestation, at its most explosive moment (from 1860 to 1890), may have contributed more than one and a half times the average figure for the carbon released per year from current tropical deforestation (3.7 Gt versus 2.4 Gt of carbon/year, respectively). The total amount of carbon released by the temperate deforestation in the referred 3 decades (1860–1890) was also one and a half times larger than the total amount released from 1850 to 1980 due to the tropical deforestation (110 versus 75 Gt of carbon). In the eighties, the biotic carbon emissions in the tropics ranged from 0.4 to 2.8 Gt of carbon per year. The Brazilian Amazon deforestation is currently responsible for the release of 0.27 to 0.45 Gt of carbon per year. Tropical deforestation is currently responsible for 33 per cent of the global annual CO₂ emissions. These biotic emissions are crucial because (i) they are taking place concomitantly with the massive burning of fossil fuels and (ii) the present atmospheric CO2 concentration is much increased. Moreover, mechanisms of the planet have already been requested to control the atmospheric CO₂ build-up. CO₂ emissions from current tropical deforestation may play an important role in the greenhouse effect, contributing to around 16% of the total effect.