“Greenhouse gases” are atmospheric gases which have been accumulating in an unprecedented fashion in recent years. They have been cited as major factors in the increase in mean global temperature which appears to be occurring at present. Most important among these gases are carbon dioxide (CO2) and methane (CH4). How is this related to tropical forests? Changes occurring during deforestation include the movement of carbon (as CO2) to the atmosphere from vegetation which is burned to make agricultural fields, the release of carbon from decaying vegetation (“slash”) remaining after logging, and the loss of carbon when wood products are removed from the site. On the other hand, if and when forests regrow, carbon is returned to terrestrial systems by the incorporation of CO2 from the atmosphere into new plant material.
The data on the release of carbon to the atmosphere from deforestation is quite variable. According to recent studies in the Brazilian Amazon, during land conversion, 20% of forest biomass is burned, 70% remains on site as slash, 8% becomes wood products, and 2% is released as CO2 by burning (Houghton, 1995; Houghton, et al., 2000). Approximately 0.18 – 2.0 petagrams (1 petagram [Pg] = 1015 g) of carbon are released annually by deforestation in this region, but the rapid regrowth of young forest on abandoned agricultural land absorbs about the same amount. (As a reference, the global aboveground plant biomass is estimated as between 568 and 651 Pg of carbon [Potter, 1999].) In this case, the absorption of carbon by the natural ecosystem is able to overcome the release of carbon to the atmosphere by respiration, fire, and logging. Other studies suggest that deforestation is a substantial source of atmospheric carbon, in fact, the second largest source of atmospheric CO2 after fuel burning. Tropical deforestation has been estimated to add (net) from 0.4 – 4.2 Pg of carbon per year to the atmosphere (Palm, Houghton and Melillo, 1986), while Detwiler and Hall (1988) gave an estimate of the same order, 0.4 to 1.6 Pg per year. Scholes and Noble (2001) estimate 1.6 Pg annually. Potter (1999) estimated that 1.44 Pg of carbon is released to the atmosphere annually by deforestation, offset somewhat by 0.29 Pg/yr accumulated in vegetation and soil by regrowth and expansion of forests. The net release of carbon to the atmosphere by deforestation amounts to approximately 20-30% of global carbon emissions (Kremen, et al., 2000). Slash-and-burn agriculture also adds to the production of CO2 released to the atmosphere because farmers burn fields every few years. Such methods add approximately 1.6 metric tons of carbon annually to the atmosphere, as well as other greenhouse gases such as nitrogen oxides and hydrocarbons (Kaiser, 1997). Nitric oxide is another greenhouse gas released during forest burning. It is not only the immediate effects of deforestation which provide a source of carbon dioxide; there are long-term effects as well. Since organic material below ground decomposes more slowly than surface biomass, disturbed soils will continue to release carbon for decades after the forest has been removed.
Recent calculations by Cox, et al., (2000) indicate that land temperatures may rise by as much as 8oC because of increased CO2 in the atmosphere (an additional 600 Pg by the year 2100, or 6 Pg annually) due to a reduction in the ability of the oceans to absorb this gas and because of the deforestation and collapse of the Amazon rainforest. Collapse would be a consequence of increased aridity (as discussed above) and an enhancement in the respiration of soil organic matter because of higher temperatures. In other words, the earth would be losing six Pg of “carbon sinks” per year directly by rainforest removal and indirectly by the effects of global warming (much caused by deforestation) on still-standing forest. Rainforests must suffer the double insult of being the cause of their own destruction! They also calculate that mitigating the loss of these sinks would cost about $200 per ton of potential carbon emissions, or US$1.2 trillion per year.
Thus, the removal of forests, at least partly through its effects on atmospheric gases, may lead to destabilization of the global climate. These climatic changes may reduce agricultural and natural productivity, and will certainly adversely affect tourism, transportation and forestry activities. There have already been shifts in boreal forests, which are retreating northward because of global warming. Many animals have already altered their migration times and reproductive seasons. This may have negative consequences when their life cycles are out of synchrony with their sources of food and shelter. Growing seasons for plants have increased by several days in some places. Increasing temperatures have allowed the invasion of nonnative species into some ecosystems, as well as alterations in the species composition of these ecosystems (Walther, et al., 2002). At the very least, the complex interrelationships which have evolved over millions of years are being and will be further disrupted by continued warming.
The enhancement of photosynthesis and growth (and, so, carbon uptake) which occurs with higher temperatures has been mentioned as a possible mitigating factor in climate change. However, there are limits to how much additional growth can be expected by warming. Most plants have either physiological or physical limits on growth, and, also, as plants age, they grow more slowly and consume less carbon dioxide.