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Aerial view of sections of rainforest felled for subsistence agriculture. (Photo by R. Butler)
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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 percent 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 about two billion metric tons of carbon dioxide into the atmosphere each year, or about 22 percent of anthropogenic emissions of carbon dioxide.
The buildup of carbon dioxide and other gases in the atmosphere is known as the "greenhouse effect." The accumulation 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. Greenhouse gases like carbon dioxide are transparent to incoming shortwave solar radiation. This radiation reaches the earth's surface, heats it, and re-radiates it as long-wave radiation. Greenhouse gases are opaque to long-wave radiation and therefore, heat is trapped in the atmosphere. As greenhouse gases build up, this opacity is increased and more heat is trapped in the atmosphere.
The largest anthropogenic contributor to the greenhouse effect is carbon dioxide gas emissions, about 77 percent of which comes from the combustion of fossil fuels and 22 percent of which is attributed to deforestation. The final 1 percent comes primarily from energy-costly production activities like the manufacture of concrete, steel, and aluminum. The preindustrial atmospheric concentration of carbon dioxide was 280 ppm, though today levels have risen to 375 ppm, a 30 percent increase. Climatologists estimate that a level pf 450 ppm—as projected for 2050—may result in an eventual 1.8-3 degrees Celsius (3.2-5.4 degrees Fahrenheit) increase in temperature. Some scientists 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. However, there are still a lot of unknowns about the impact of climate change.
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Global carbon-dioxide emissions from energy use and production, 1748-2002.
Variations in Earth's surface temperature, 1000-2000.
Projected carbon-dioxide emissions by country, 1790-2025.
Global carbon-dioxide concentrations with anthropogenic emissions, 1750-2000.
Atmospheric CO2 Record from Mauna Loa, 1958-2004.
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The extent and effect of global warming has been long debated by scientists, industries, and politicians. In 1995 leading scientists and 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 on global climate." Their evidence included a 0.5-1F (0.3 to 0.6C)
increase in average global temperature since 1960, 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, 1998 was the warmest year on record, although 2005 was a close second. 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 degrees F) per century.
Since 1960 atmospheric carbon-dioxide levels have increased from 313 ppm to 375 ppm (20 percent increase), according to measurements from Mauna Loa observatory, and carbon-dioxide levels are now 27 percent higher than at any point in the last 650,000 years. The Intergovernmental Panel on Climate Change (IPCC) projects that atmospheric carbon-dioxide levels could reach 450-550 ppm by 2050, possibly resulting in higher temperatures and rising sea levels, along with a myriad of potential impacts ranging from increased storm and hurricane intensity; melting of polar ice, Arctic permafrost, and glaciers; changes in ocean currents including the Gulf Stream; a rise in global sea levels which could inundate
low-elevation cities like Cairo, Venice, Lagos, New Orleans, and Amsterdam and cause problems for low-lying nations; increased coral bleaching and mortality of reef ecosystems; changes in ecosystems; species migration and mass extinction, especially among cold climate species; heightened danger from human pollutants like
ozone; health impacts including the spread of tropical disease into cooler climates and range expansion of other pathogens; and water shortages.
Rising sea levels
The projected rise in sea level from ocean-water expansion and ice melt varies depending on estimates of global warming (sea ice in the Arctic is shrinking by
an average of 14,000 square miles per year and is strongly correlated with greenhouse-gas and aerosol emissions). But there is a good chance that oceans will rise from 10 inches (25 cm)
to 20 inches (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.
Any sea-level increase would be magnified during tides, storm surges, and hurricanes and could have a devastating
impact as shown by Category 3 Hurricane Katrina in 2005 . Island nations like the Maldives and scattered South Pacific republics face extinction. The sea is a tremendously
important resource for man, and some of the world's largest cities lie along the coast for trade and commercial fishing. Any rise
in sea level would directly affect these metropolises, causing flooding and the potential disruption 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-inch (25-50 cm) will have a drastic effect on our society. Global warming is as
much a social problem as it is an environmental one.
Changes in ecosystems
Scientists expect climate change to cause major shifts in species distribution and ecosystems, though there is still considerable debate over how climate change will affect specific ecosystems. Moderate climate warming simulations show that coral reefs will decline significantly over the next 50 years due to higher water temperatures and increased ocean acidity, and a similar fate will befall many organisms that form the base of the oceanic food chain. On land, permafrost across frozen landscapes may melt and give way to forest vegetation, while agricultural belts may move polewards. In the Amazon, temperatures are expected to climb, resulting in drier forests and expanded savanna. In Africa, climate change may disrupt regular seasonal weather patterns over large regions of the continent, reducing rainfall in some areas while producing more rainfall in the drought-stricken Sahel region.
Climate risks to global agriculture are underestimated
Climate change will impact U.S. economy
Climate change may decimate Indonesia's food supplies, worsen fires
Past climate change triggered wars, population decline
40 percent of the Amazon could be grassland by 2050
Global warming will cause local climates to shift and disappear
Climate change brought tropical forest to Wyoming
Global Warming - The Impact on Agriculture
Forest Fragmentation and Global Warming
More news on the impact of climate change
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 a proposal by a coalition of developing countries at the 2005 UN climate conference in Montreal to trade forest conservation for carbon and ecosystem-services credits [Latest news on avoid deforestation, carbon finance, and REDD]. While such schemes could provide ways for poor tropical countries to capitalize on their natural assets without destroying them, the bad news is that even if carbon emissions are reversed today there is a lag time of around 50 years before the effects can be slowed, because of ocean thermal inertia, or their capacity to store heat. Thus the effects from past emissions are not entirely apparent today.
Lungs of the Earth
While the role of rainforests in oxygen generation is often overstated—more oxygen is produced by microorganisms in the world's oceans—tropical rainforests do add oxygen to the atmosphere as a by-product of photosynthesis. Some scientists estimate that 20 percent of the planet's oxygen is produced by rainforests.
Clearing rainforests diminishes the capacity of the global system to supply oxygen reserves.
Review questions:
- How does deforestation affect global warming?
- Why are rainforests called "the lungs of the world"?
[print version | spanish | french | portuguese
| chinese | japanese]
Continued: Extinction
This article was written by Rhett A. Butler [bibliographic citation for this page] and was last updated on the most recent date listed in the column on the right side.
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