Major results for assessing emissions due to land-use change were received using inventory-based approaches or models of different type. Inventory-based models consider all or some of the basic processes: (1) the immediate release of carbon to the atmosphere from organic matter burned at the time of clearing, (2) postdisturbance flux of carbon from decay of slash, (3) accumulation of carbon during regrowth, and (4) changes in soil carbon.
Table 2 Annual carbon emissions from tropical deforestation
aRange due to available estimates.
^Emissions are calculated based on the average estimate for 1990-2000.
Table 2 contains data on carbon emissions caused by deforestation in the Tropics. The estimates differ substantially: the average annual carbon emissions for 1990-2005 are estimated in the range 0.8-2.2 PgCyr (15-35% of the annual global emissions from fossil fuels approximately during this period) with the overall average at about 1.5 PgCyr-1. This estimate corresponds well to the estimate of the third IPCC assessment of 1.6 ± 0.8 PgCyr for the period 1987-98 and to recent estimates for 2000-06. Simulations done with the model IMAGE 2.1 estimated C emissions from deforestation from 0.83 PgCyrin 1995, 1.04 in 2000, 1.58 in 2005, to 2.16PgCyr in 2015. Several estimates of aggregated carbon fluxes from tropical land given by inverse modeling vary from 1.2 to1.5PgCyr-1, if both fluxes to the atmosphere and hydrosphere are accounted for.
These estimates do not include carbon emissions from wildfire which could be very high, particularly during years of severe droughts. For instance, recent estimates put global carbon emissions from fires during 1997-98 El Nino event at 2.1 ± 0.8 PgC, particularly in Indonesia.
The carbon stocks in forests may change without a change in forest area (e.g., selective harvest, forest fragmentation, non-stand-replacing disturbances, shifting cultivation, browsing, and grazing) and accumulation of biomass in growing and recovering forests. During the last two decades, the area of primary natural forests decreased or modified through human intervention by 6 x 106ha yr-1. Due to FAO estimates, degraded and secondary forests in Africa, America, and Asia covered about 850 x 106ha in 2002. While deforestation can be measured from space with relatively high accuracy, this is not the case for degradation and secondary regrowth; usually regrowth is spectrally indistinguishable from mature forests as early as after 15-20 years. Forest inventories, as a rule, do not contain any specific data on forest degradation. FAO (2000) estimated the area of disturbances that can be labeled as forest degradation at 24 x 106hayr-1 in the period 1990-2000; another recent estimate is at 10 x 10 hayr-1. Estimates of carbon emissions from the degradation of forests (expressed as a percentage of the emission from deforestation) vary greatly - from 5% for the world's humid Tropics to 25-42% for tropical Asia
Total emissions in 1990-2005 (PgC)
Carbon emissions due to deforestation (Pg Cyr 1990sa 2000-05b and above 100% for tropical Africa. Another study reports the global net emissions from land-use change in the Tropics including emissions from conversion of forest to other land use (71%) and loss of soil carbon after deforestation (20%), emissions from forest degradation (4.4%), emissions from the 1997-98 fires (8.3%), and sinks from regrowth (-3.7%).
Uncertainties of the above data are high. A number of reasons impact reliability of carbon emissions from deforestation and forest degradation: (1) accuracy of recognizing the areas of tropical deforestation and degradation; (2) weak knowledge of the amount of biomass and soil carbon on areas impacted by the land-use change; (3) fate of deforested land, that is, how much is reverting to secondary forests; (4) how much forests are burnt; and (5) how forest disturbance is affecting soil and forest floor carbon stores. In a number of studies, uncertainties on the amount of CO2 released are estimated to be 25-50%. For the Brazilian Amazon, for example, a range of 150280 Mt C yr~ was reported.
The greenhouse impact of deforestation is greater than the difference in carbon stock between the forested and replacement landscapes due to releases of other GHGs, basically methane (CH4) and nitrous oxide (N2O) (ozone, carbon monoxide, and some other gases which are produced by deforestation are not direct GHGs; nevertheless, they impact concentrations of CO2 and CH4 in the atmosphere). The emissions of these gases do not occur directly with deforestation, but basically with the following land use such as rice cultivation, cattle breeding, application of fertilizers, etc. IPCC-2001 assesses the following contribution of the major GHGs to the enhanced greenhouse effect in 1750-2000: CO2 -60%, CH4 - 20%, and N2O - 6% (the other 14% are caused by halocarbons which are not produced by the biosphere). The contribution of deforestation to the global greenhouse effect is estimated in the range of 25-35%. Of this total, the contribution of CO2 is about 15% (or about one-fourth of the global CO2 emissions), CH4 911% (40-50% of the global methane emissions), and N2O 2% (from one-fifth to one-third of the global nitrous oxide emissions). Available regional estimates are of a similar magnitude. In the case of Brazilian Amazonia, for example, gases other than CO2 increase the greenhouse effect by about 35%.
For decades, deforestation and degradation were considered as an almost exceptional phenomenon of the Tropics and arid lands. However, recent years have brought much evidence of possible damage to forests due to ongoing and expected global change in the boreal biome. Forest degradation and deforestation here mostly relate to the increase in frequency and severity of large-scale disturbances, change of hydrological regimes mostly related to permafrost destruction, industrial pressure on landscapes, pollution, and unsustainable logging. For instance, wild vegetation fires enveloped 23 x 106ha (of which 17 x 106ha on forest land) in Russia in 2003; during the first years of this century, outbreaks of dangerous insects in boreal forests exceeded 20 x 106ha in the circumpolar boreal zone, ofabout the same area in American and Asian continents. The increase in the area of 'green desertification' in the Russian taiga zone is estimated to be about 5 x 106ha during the last two decades. The direct carbon emissions due to a fire in 2003 are estimated to be about 200TgCyr~\ Very likely, the expected dramatic warming in high latitudes (up to 6-10 ° C) will substantially accelerate processes of northern deforestation and degradation.
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