Land Use Conversion

The earth's great carbon dioxide stonehouses, its tropical forests, stretch in a belt along the equator from Brazil and the Amazon, through Zaire, to the Democratic Republic of the Congo, Malaysia, and Indonesia. The areas involved are immense. The term "land-use conversion" is a circumlocution, a euphemism, for deforestation: the replacement of natural forests with roads, cattle ranches, cropland, and lumber production. As with limestone, these forests are yet another of our great COi repositories—ripe for the ravaging and release of CO2. What does it mean to say that tens of thousands of square miles of forest disappear annually?

In Peru, 500,000 acres of forest, 780 square miles, 12 times the size of Washington, DC, were cleared for cocoa crops to satisfy the world's craving for cocaine. Deforestation contributes to additional warming in several ways. The cleared areas become more reflecting of infrared back into the greenhouse, and there is a concomitant loss of CO2 absorptive capacity with the loss of trees. Cutting trees also removes moisture, which causes clouds to develop, playing a major role in the earth's radiation budget. With moisture and cloud cover gone over large areas, these losses could mean adding additional weeks or months to already dry seasons. The large contribution of CO2 to the greenhouse notwithstanding, the combination of other gases may be equal to, if not greater than, a cause of observed global warming. So, for example, methane (CH4), the simplest carbon-containing molecule, can now be measured and its concentration accurately determined by the "scanning imaging absorption spectrophotometer for atmospheric chartography" (SCIAMACHY) on board the European Space Agency's research satellite, ENVISAT [11].

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Figure 5.3. Atmospheric methane concentration during the years 1000-2000. [Figure reproduced with the kind permission of the Intergovernmental Panel on Climate Change (IPCC).]

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Figure 5.3. Atmospheric methane concentration during the years 1000-2000. [Figure reproduced with the kind permission of the Intergovernmental Panel on Climate Change (IPCC).]

Although the simplest carbon-containing molecule, it is the second most important anthropogenic GHG. More than 50% of current global methane emissions are the work of human pursuits. Curiously enough, although the presence of CO2 became known by the late eighteenth century, methane was not detected until 1948. Like CO2, methane, "marsh gas," or, as miners refer to it, "fire damp," methane is not only a strong IR absorber but is also, in fact, 20 times more absorbing than CO2. But for its presence in the atmosphere at a minuscule 1.7-2.0%, it would be the major IR absorber. From its first analytic measurements in the late 1960s to the most current, it has been found, as shown in Figure 5.3 - to be rising steadily. Records in the Antarctic ice cores indicate that since 1750 its concentration in the atmosphere has increased over 150%, and as the trendline so clearly shows, its rise has been almost exponential since the 1920s. Rice cultivation, the prime contributor, has more than doubled since 1950, and will surely continue to increase as the demand for rice keeps pace with population growth. The anaerobic conditions indigenous to rice paddy flooding results in the formation of methane. More than 60% of all the world - s rice paddies are found in China and India, where unfortunately data on emissions are unavailable.

In addition to rice production, anaerobic microbial metabolism in the gut of ruminant cattle, sheep, goats, buffalo, and camel, of which there are well over a billion in the world, are heavy emitters of methane. Management of ruminant diets is a high-priority item for those concerned with cutting greenhouse emissions, but, as you may suspect, it is a complex and uncertain undertaking. Yet other contributions come from termites, whose gut is also filled with methane-producing bacteria. Land-use changes, including deforestation, increases land available for incursion by termites, whose populations have also been increasing. In addition, methane emissions arise from landfills, coal mining, and oil and gas extraction. If that were not enough, methane magnifies the effect of CO2 when chemically reacting with the hydroxyl radical OH, to yield CO- and water. Between 1999 and 2002, CH- appears to have been increasing at a steady state of 1751 parts per billion (ppb). Indeed, methane remains a prime climate forcer even with modifications in rice farming (cultivar choice, irrigation management, and fertilization) as well as efforts to capture methane at landfills and in mining operations [10] .

Moreover, as we shall see, methane, along with the other non-CO2 GHGs, may emerge as the primary cause of global warming. However, reductions in these gases will require international cooperation at a level that may be extremely unlikely, unless warming ratches up a notch or two and uncertainty recedes. Even if international cooperation were to occur, there is a methane source that, if emitted, could nullify calculations and all reduction efforts.

Methane trapped in marine sediments as hydrates can become an immense energy reservoir, and also a warming time bomb if suddenly released by natural forces [12].

Gas hydrate is a crystalline solid consisting of methane gas surrounded by water molecules, looking much like water ice, and is stable in ocean floor sediments at depths around 1000 feet. The global levels of carbon bound in gas hydrates is conservatively estimated at twice the total amount of carbon in all known fossil fuels on earth. Colossal would not be an overstatement. Consequently, extraction of methane from hydrates could provide an enormous energy resource. Furthermore, methane bound in hydrates appears to be some 3000 times the volume of methane in the atmosphere. There's the rub. There's a great lack of information to judge what geologic processes might affect the stability of hydrates and the possible release of methane to the atmosphere. Methane released as a result of landslides caused by sea-level fall could easily and seriously warm the earth, as would methane released in arctic sediments as they become warmed during sea-level rise. Such releases could exacerbate climatic warming and surely destabilize the climate.

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