Rainforests play the important role of locking up atmospheric carbon in their vegetation via photosynthesis. 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 clearing and burning of tropical forests and peatlands releases more than a billion metric tons of carbon (3.7 billion tons of carbon dioxide) into the atmosphere each year, or about more than ten percent of anthropogenic carbon emissions.
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, more than 85 percent of which comes from the combustion of fossil fuels (roughly one percent of emissions result from 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 400 ppm, a 43 percent increase. Climatologists estimate that a level of 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.
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.
The projected rise in sea level from ocean-water expansion and ice melt varies depending on estimates of global warming. 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.
The good news is that some carbon emissions can be canceled out by planting trees, which absorb carbon into their tissue 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 3.7-5.5 billion metric tons of carbon dioxide over the next 50-100 years.
Already a number of 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 seek compensation in the form of carbon payments for forest conservation. This proposal has since developed into the so-called Reducing Emissions from Deforestation and Degradation or REDD+ mechanism, which is expected to mobilize tens of billions of dollars in carbon finance for tropical forest conservation. [Latest news on avoid deforestation, carbon finance, and REDD].
While schemes like REDD+ 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.
Aerial view of sections of rainforest felled for subsistence agriculture. (Photo by R. Butler)
Review questions:
How does deforestation affect global warming?
Why are rainforests called "the lungs of the world"?
The burning of forests releases almost one billion tons of carbon dioxide into the atmosphere each year according to T.E. Lovejoy in "Biodiversity: What is it?" in Biodiversity II, Reaka-Kudla, Wilson, Wilson, eds.., Washington D.C.: Joseph Henry Press, 1997. The role of deforestation in global warming is further discussed in Peters, R.L. and Lovejoy, T.E., eds. Global Warming and Biological Diversity, New Haven: Yale University Press 1992 and Shukla, J., Nobre, C., Sellers, P., "Amazon Deforestation and Climate Change," Science; 247: 1322-25, 1990.
In their paper, "Carbon Dioxide Fluxes in Moist and Dry Arctic Tundra during the Snow-free Season: Responses to Increases in Summer Temperature and Winter Snow Accumulation" (Arctic and Alpine Research Vol. 30, No. 4 (373-380), November 1998), Jones, M. H., J. T. Fahnestock, D. A. Walker, M. D. Walker, and J. M. Welker warn that higher temperatures resulting from global warming could result in higher levels of carbon dioxide being released into the atmosphere from arctic tundra.
E.J. Barron in "Climate Models: How Reliable are their Predictions?" Consequences Vol. 1 No. 3, 1995 describes the phenomenon of the cooling of the stratosphere during warming events.
Global carbon reserviors are given in Kasting, J.F., "The carbon cycle, climate, and the long-term effects of fossil fuel burning," Consequences Vol. 4, No. 1, 1998.
W.F. Laurance discusses die-off in forest fragments and the possibly effect on global climate in "Forest Fragmentation May Worsen Global Warming," Science 298: 1117-1118 1/5/98.
In "Tropical forestry practices for carbon sequestration: a review and case study from southeast Asia," Ambio Vol. 25 No. 4, June 1996, P.M. Costa notes that forest fragments store less carbon per unit of area than contiguous forest because fragments are often comprised of fast-growing tree species which store less carbon per volume than longer-lived trees.
M. McKloskey ("Note on the Fragmentation of Primary Rainforest," Ambio 22 (4), June: 250-51, 1993) provides the two-thirds figure for global fragmented rainforest.
In 1995 the Intergovernmental Panel on Climate Change (IPCC) released its report on climate change (Watson, R. T. et al., eds., Climate Change 1995: Impacts, Adaptations, and Mitigation of Climate Change: Scientific-Technical Analyses: Contribution of Panel on Climate Change) concluding "the balance of evidence suggests a discernible human influence of global climate."
Mann, M.E., Bradley, R.S. and Hughes, M.K. ("Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations." Geophysical Research Letters, Vol. 26 (759-760), 1999) reported the NOAA's findings that 1998 was the warmest year on record. The same paper (picked up by the national press in "Report: 1990s warmest decade of millennium" Reuters 3/3/99) also reported that the 1990s have been the warmest decade of the millennium. J. Warrick in "Scientists See Weather Trend as Powerful Proof of Global Warming," The Washington Post 1/9/98 reported that the past decade has witnessed nine of the eleven hottest years this century.
The National Research Council of the National Academies (J.M. Wallace et al. Reconciling Observations of Global Temperature Change, National Research Council 2000) examined the apparent conflict between surface temperature and atmospheric temperature, which has led to the controversy over whether global warming is actually occurring and concluded that strong evidence exists to show that surface temperatures in the past two decades have risen at a rate substantially greater than average for the past 100 years. Angell, J.K. further discusses the discrepancies in "Comparison of surface and tropospheric temperature trends estimated from a 63-station radiosonde network, 1958-1998," Geophysical Research Letters, Vol. 26, No. 17 (2761-2764), Sep. 1, 1999.
L.D. Hatfield provided an excellent overview of the worldwide effects of el Niño in "An Ill Wind Blows in Again," San Francisco Examiner, 9/4/1997.
The National Oceanic and Atmospheric Administration 1997-2000 reports on the history, frequency, and duration of past el Niño (ENSO) events.
D.T. Rodbell looks at the history of ancient ENSO events in "An ~15,000-Year Record of El-Nino Driven Alluviation in Southwestern Ecuador," Science, Vol. 283 (516-519), 22-Jan-99.
Leighton, M. and Wirawan, N found a direct correlation between ENSO events and drought in Eastern Borneo in "Catastrophic Drought and Fire in Borneo Rain Forests Associated with the 1982-83 El Niño Southern Oscillation Event," in G.T. Prance, ed., Tropical Rain Forests and the World Atmosphere., Westview: Boulder, Colorado, 1986.
The Large Scale Biosphere-Atmosphere Experiment in Amazonia (LBA) - sponsored by INPE (the Brazilian Institute for Space Research) 1997 - provided data for the global carbon emissions breakdown.
D. Holt-Biddle in "The Heat is On," Africa-Environment and Wildlife May/June Vol. 2 No. 3. 1994 notes the increase in atmospheric carbon dioxide levels over the past 150 years.
Martin and Lefebvre discuss the spread to tropical diseases into cooler climes in "Malaria and climate: sensitivity of malaria potential transmission to climate," Ambio Vol. 24 No. 4, June 1995.
Based a studies of ice cores from Greenland, Steig et al. ("Synchronous Climate Changes in Antarctica and the North Atlantic." Science October 2; 282: 92-95. 1998.) proposed that a chaotic temperature change in Greenland occurred at the end of the last Ice Ages. J. P. Severinghaus and E. J. Brook followed up with similar findings in "Abrupt Climate Change at the End of the Last Glacial Period Inferred from Trapped Air in Polar Ice," Science 1999 October 29; 286: 930-934.
K.Y. Vinnikov et al. ("Global Warming and Northern Hemisphere Sea Ice Extent," Science 1999 December 3; 286: 1934-1937) found ice in the Artic is shrinking by an average of 14,000 square miles per year and shrinkage is strongly correlated to greenhouse gas and aerosol emissions.
Mitigating carbon emissions by reforestation is reviewed in E.O. Wilson's The Diversity of Life (Belknap Press, Cambridge, Mass 1992.), Biotic Feedbacks in the Global Climatic System: Will the Warming Feed the Warming? ( New York: Oxford University Press 1995) by G.M. Woodwell and R.A. Mackenzie, eds., and Phillips, O.L. at al. "Changes in the carbon balances of tropical forests: Evidence from long-term plots." Science Vol. 282. October 1998. However this proposition has come under criticism of late by several important agencies including the International Geosphere-Biosphere Programme (IGBP) (B. Scholes, "Will the terrestrial carbon sink saturate soon?" Global Change NewsLetter No. 37:2-3, March 1999) and the Intergovernmental Pannel on Climate Change (R. Watson et al. IPCC Special Report on Land Use, Land Use Changes, and Forestry, 1999).
Parry, M. et al. ("Adapting to the Inevitable," Nature Vol. 395 22-Oct-1998 "(741)) conclude the cuts under the Kyoto Protocol would only shave off 0.1°F by 2050.
In "Bogging Down in the Sinks" (Worldwatch Nov/Dec 1998) A.T. Mattoon discusses some of the problems with forestry sinks under the Kyoto protocol.
Agricultural changes brought on by climate change are considered by R.C. Rockwell in "From a carbon economy to a mixed economy: a global opportunity," Consequences Vol. 4 No. 1, 1998 and at the Global Change and Terrestrial Ecosystems Focus 3 Confrence (1999). Several studies presented at this confrence suggest that grain grown under carbon dioxide enriched conditions maybe less nutritious than than grain grown under current conditions. This conference was arranged under the International Geosphere-Biosphere Programme (IGBP).
R. Monastersky in "Acclimating to a Warmer World," (Science News, Vol. 156. 28-Aug-99) reviews some of the pitfalls and windfalls from a warmer climate including an increase in number of "hot" days, sewage and transit problems, and lower heating bills.
A.E. Waibel et al. ("Arctic Ozone Loss Due to Denitrification," Science Vol. 283 No. 5410 (2064-2069), March 26, 1999) showed that global warming could slow the recovery of the ozone layer.
Houghton (Houghton, R.A. "Tropical deforestation and atmospheric carbon dioxide," in: Tropical Forests and Climate, ed. N. Myers., Dordrecht: Kluwer Academic Publishers, 1992 and Houghton, R.A., "Role of forests in global warming," in: World Forests for the Future: Their Use and Conservation, ed K. Ramakrishna and G.M. Woodwell, New Haven: Yale Univseristy Press, 1993) and Myers (Myers, N., "The world's forests: problems and potentials," Environmental Conservation. 23 (2), 1996) estimate carbon sequestration by the reforestation of 3.9 million square miles (10 million square km).
