ATMOSPHERIC ROLE OF FORESTS

Rainforests play the important role of locking up atmospheric carbon in their vegetation via photosynthesis. The vegetation and soils of the world's forests contain about 125% of the carbon found in the atmosphere. When forests are burned, degraded, or cleared, the opposite effect occurs: large amounts of carbon are released into the atmosphere as carbon dioxide along with other greenhouse gases (nitrous oxide, methane, and other nitrogen oxides). The burning of forests releases almost one billion tons of carbon dioxide into the atmosphere each year.

The buildup of carbon dioxide and other gases in the atmosphere is known as the "greenhouse effect." The buildup of these gases is believed to have altered the earth's radiative balance meaning more of the sun's heat is absorbed and trapped inside the earth's atmosphere, producing global warming. The largest anthropogenic contributor to the greenhouse effect is carbon dioxide gas emissions, 73-92% of which comes from the combustion of fossil fuels, 13-23% of which is attributed to deforestation, and the final 1% coming primarily from energy costly production activities like the manufacture of concrete, steel, and aluminum. Preindustrial atmospheric concentration of carbon dioxide was 280 ppm, though today levels have risen to 360 ppm, a 28.6% increase. Climatologists estimate that a level 450 ppm may result in an eventual 2.5-3C (4.3-5.4F) increase in temperature. Even though we appear to be coming to an end of an interglacial period, some climatologists predict that global warming will produce a sharp upswing in global temperatures followed by a deep plunge into a glacial period several thousands years from now.

The extent and effect of global warming has been long debated by scientists, industries, and politicians. In 1995 leading scientists, the Intergovernmental Panel on Climate Change (IPCC) concluded that global warming had been detected and that "the balance of evidence suggests a discernible human influence of global climate." Their evidence included a 0.5-1F (0.3 to 0.6C) increase in average global temperature, a 4.5F (2.5C) degree increase at the Earth's poles, the breaking up of the Antarctic ice sheets, the receding of glaciers worldwide, the longest El Niño ever recorded, a record number of hurricanes in 1995, a record number of heat waves, and an increase of epidemics attributed to global climate change including dengue fever, malaria, hanta virus, and the plague. According to scientists at the National Oceanic and Atmospheric Administration 1997 was the warmest year on record . . . until 1998, which was even warmer. A British study at the University of East Anglia suggested that 1998 may be the warmest year in over 800 years. The 1990s have been the warmest decade of the millennium and the past decade has witnessed nine of the eleven hottest years this century. In the 900 years before the twentieth century, temperatures dropped an average of 0.02 degrees C (0.04 degree F) per century.

In the past 150 years, the carbon dioxide level has increased from 280 ppm to 360 ppm (28.6% increase) and may double within the next 60 years. Although the effect of global warming can only be anticipated, the consequences are believed to include increased violent weather like hurricanes, melting of polar ice translating in a decline in krill populations which are critical to the ocean food chain, a rise in global sea levels which would inundate low elevation cities like Cairo, Lagos, New Orleans, and Amsterdam, the migration to cooler regions or extinction of species sensitive to changes in temperature like amphibians, the heightened danger from human pollutants like ozone, and the spread of tropical disease into cooler climates.

The projected rise in sea level from ocean water expansions and ice melt (sea ice in the Artic is shrinking by an average of 14,000 square miles per year and is strongly correlated to greenhouse gas and aerosol emissions) corresponding to global warming varies, though there is a good chance that oceans will rise from 10" (25 cm) to 20" (50 cm) within the next century if greenhouse gas emission rates continue at present levels. Such a rise in sea level does not sound like much, but it would have profound effects on both humankind and natural systems. Note that any sea level increase would be magnified during tides, storm surges, and hurricanes and have a devastating impact. Island nations like the Maldives and scattered South Pacific republics face extinction. The sea is a tremendously important resource for man: our greatest cities lie along the coast for trade and commercial fishing. Any rise in sea level would directly effect these metropolises, causing flooding and the failure of sewage and transit systems, along with inundating neighboring agricultural plots. A change in sea levels will also affect coastal ecosystems like river deltas, wetlands, swamps, and low-lying forests, which play an important role in providing services for mankind, in addition to housing biological diversity. Though sea levels have been higher in the past, today there is less room for species affected by flooding since buildings and concrete now occupy the areas that were once extensions of their environment. Modern humankind is so dependent on existing conditions, that a change in sea level, even if it is 10-20" (25-50 cm) will have a drastic effect on our species. Global warming is as much a social problem as it is an environmental one.

Global Warming - The Impact on Agriculture

Forest Fragmentation and Global Warming

The good news is that some carbon emissions can be canceled by the planting of trees which absorb carbon into their vegetation through photosynthesis. Tropical forests have the best potential for the mitigation of greenhouse gases since have the greatest capacity to store carbon in their tissues as they grow. Reforestation of 3.9 million square miles (10 million square km) could sequester 100-150 billion metric tons of carbon dioxide over the next 50-100 years. Already several tree-planting projects specifically designated for carbon emissions mitigation have been initiated around the world including the INFAPRO project in Sabah, Malaysia (see chapter 10 for more information). However the bad news is that even if we reversed the progress of global warming today there is a lag time before the effects can be slowed because of the ocean's ability to store heat. Thus the effects from past emissions are not entirely apparent today.

Tropical rainforests also are responsible for adding oxygen to the atmosphere as a by-product of photosynthesis. It is estimated that 20% of the the planet's oxygen is produced by the world's rainforests. Cutting the rainforest diminishes the capacity of the global system to supply oxygen reserves.