Overall, we do not have a great deal of quantitative data on the effects of rainforests on climate, but it is known that rainforests have an effect on heat balance, influence air currents, release a great deal of moisture into the air via evapotranspiration (which moderates temperature), and absorb some CO2 from the atmosphere (the amount is presently being hotly debated).
There is currently considerable controversy over whether the earth’s average temperature is increasing or not (“global warming”), although most scientists are convinced that it is. An increase of approximately 0.6oC has occurred during this past century, and temperature ranges are decreasing (Walther, et al., 2002). Kremen, et al., (2000), estimate that global temperatures will increase between 1o and 4oC over the next century; Töpfer (2001) and Harvell, et al., (2002) suggest 1.4 to 5.8oC, predictions corroborated by Houghton, (1995), who calculated a rate of temperature increase of 0.3oC per decade, or 3oC by the year 2100. While this subject is beyond the scope of this essay, we should note that there is a natural “greenhouse” effect caused by the emission of gases of various kinds into the atmosphere – from plant and animal respiration (carbon dioxide, CO2), from animal digestion (methane, CH4), from volcanic activity, from evaporation (water vapor) and others. Secondly, human activity is increasing the amounts of many of these gases in the atmosphere. For example, carbon dioxide concentrations have increased by 30% in the past 150 years, and ozone levels have more than tripled over the past century (Percy, et al., 2002). Thirdly, the earth’s surface temperature depends upon the amount and types of these gases contained in the atmosphere and the albedo of the surfaces which can absorb heat. (Albedo is the fraction of solar radiation reflected from a surface, here mainly from plant bodies and, to a lesser extent, soil.) There is some evidence that energy emissions from the tropics have increased to a substantial degree during the past 20 years, which appears to have some relationship to cloud cover and tropical air circulation shifts. In some areas the air masses near the equator moved upwardly more strongly than usual, which decreased cloud cover and humidity in these areas. Whether or not these alterations are part of the normal climatic cycles is not known (Hartmann, 2002; Chen, Carlson and Del Genio, 2002; Wielicki, et al., 2002).
a. water vapor: Albedo depends upon the type of land cover (desert, forest, grassland, etc.). Forests absorb a higher proportion of the energy impinging on the earth’s surface than do other types of ecosystems. A forest canopy can absorb up to 93% of the solar radiation (energy) falling on it, of which less than 2% is utilized for photosynthesis. Most of this energy is absorbed in the evaporation of water from the forest vegetation. The resultant water vapor disperses heat into the atmosphere, and so heat (energy) is transferred from the surface of the earth to the atmosphere. Water vapor, as clouds, can also reflect heat energy. So it can be cooling or heating, depending upon the type and volume of cloud cover. These forest effects are not restricted to the tropics. As warm moist air moves north from tropical forested areas, heat is transferred to northern regions, and humidity and rainfall increase. Thus, tropical water balance/heat balance is linked to that of temperate areas.
b. carbon dioxide: There has been considerable controversy about whether or not rainforests act as carbon “sinks,” by net absorption of carbon dioxide from the atmosphere. This phenomenon is known as sequestration. Plants do absorb huge amounts of carbon dioxide from the atmosphere in photosynthesis, during which is converted to sugars which are oxidized or “burned” to support growth and metabolism. However, the end result of oxidation (respiration) is carbon dioxide, which is released as a waste product into the air, and a great deal of carbon dioxide enters the atmosphere in this way. Carbon dioxide is one of the “greenhouse” gases which contribute to the insulating effects of the atmosphere, and so the question of whether or not forests provide net sequestration of carbon dioxide (that is, whether forests absorb more carbon dioxide during photosynthesis than they give off during respiration) is extremely important, given current rates of deforestation and the fact that mankind is emitting huge amounts of carbon annually into the atmosphere (partly from deforestation).
The concentration of carbon dioxide in the atmosphere in the atmosphere has increased by 30% in the past 300 years, with half of that increase occurring during the past 40 years. Seven petagrams (one petagram = 1015 grams, or two billion tons) are exuded annually into the atmosphere by the combustion of fossil fuels, and 1-2 petagrams from burning of forests in the tropics (Prentice and Lloyd, 1998). Palm, et al., (1986) gives a higher estimate – between 0.4 and 4.2 petagrams of carbon emitted annually from the combustion of tropical forests, with between 0.15 and 0.43 petagrams coming from Southeast Asia alone. Schimel et al., (2001) estimate that land-use changes (mainly tropical deforestation) result in the emission of 0.6 to 2.5 gigatons of carbon to the atmosphere annually. (For comparison, United States per capita emissions are more than 5 metric tons per year, in comparison with an average of 0.6 metric tons in developing countries [Baer, et al., 2000].) Kremen, et al., (2000), state that 20% to 30% of total carbon emissions is due to tropical deforestation. Half of emitted carbon remains in the air, a quarter is absorbed by the ocean, and the remainder, approximately two petagrams, must be being taken up (sequestered) by terrestrial systems.
As much as 40% of this sequestration might be due to uptake by tropical forests (Adam, 2001). However, estimates as to the extent of uptake vary greatly. At present it is not possible to establish a figure with any exactitude because of the difficulty of obtaining accurate measurements on such a vast scale, because of the diversity of tropical forests, and because carbon absorption varies with climate, soil type, species composition, whether the forest is primary of secondary, age of the forest, etc. As an estimate, tropical forests sequester on the order of 0.7 billion tons of carbon annually (as much as 200 tons per hectare per year), approximately one-and-a half to two times as much as temperate forests (Moffat, 1997). Prentice and Lloyd (1998) calculate that Amazonia has a net sequestration rate of 0.1 Pg of carbon per year (i.e., excess uptake of carbon by plants over carbon released by deforestation). Schimel, et al., (2001) suggest that tropical forests absorb more carbon than they emit, a net “sink” of approximately 0.4 Gt per year. Pimentel, et al., (1997) give a figure of 2.5 metric tons of carbon sequestered per hectare per year for tropical forests. The large trees of the central Amazon alone have been estimated to sequester 0.2-0.3 petagrams of carbon annually (Chambers, et al., 2001). Increased net carbon dioxide sequestration by forests may be due in part to current higher growth levels. (Paradoxically, elevated growth rates occur because of increased quantities of atmospheric CO2 and the higher global temperatures due to greenhouse gases.)
Secondary forests, because of their rapid growth rates, accumulate carbon more rapidly than primary forests. However, it is difficult to calculate this rate, because many factors are involved – the age distribution of trees in the forest, for one, is very significant (Nelson, et al., 2000). Another factor in this equation seems to be that intact tropical forest trees, at least, continue to accumulate carbon in wood for as much as a century after there has been a surge in productivity (growth). The rate of storage as wood depends, however, on the size of the tree – large-diameter trees (that is, older ones) store more carbon, relatively, than smaller ones. An interesting article by Percy, et al., (2002) demonstrates that, in temperate trees such as aspens, at least, higher levels of atmospheric carbon dioxide increase tree diameter (i.e., growth). High plant diversity, such as is characteristic of tropical rainforests, also encourages CO2 sequestration (see Biodiversity Part II), although this has so far been demonstrated only in temperate forests. Thus, the high usage of atmospheric carbon by forests may allow them to buffer climate change by regulation, but their ability to absorb CO2 is limited by a number of factors: the finite nature of plant growth; the rapid reduction of tropical forest land because of deforestation; the fact that most forests are now found in regions of lesser fertility, where their growth capacity (and therefore ability to sequester carbon) is problematic (Oren, 2001), and the fact that deleterious climate changes (such as El Niño) can depress forest growth. Schimel, et al., (2001) warn that ongoing climate change as well as the maturation of secondary forests will reduce sinks, and that the terrestrial sink may vanish.
There is no question, however, that deforestation and burning of tropical forest releases huge quantities of carbon dioxide into the atmosphere (see above for quantity). It is rather ironic that humans now expect forests to absorb the carbon dioxide released when they are cut down and/or burnt!
c. methane: The atmospheric concentration of methane (CH4), another major greenhouse gas, is rising at a rate of approximately 1% per year. About 60% of methane emissions are anthropogenic (from human activities), mainly from landfills, coal mining, oil and natural gas systems, domestic ruminants (cattle, sheep, goats), animal wastes and wastewater, rice cultivation, and burning of biomass (Hogan, Hoffman, & Thompson, 1991). Methane released from wet tropical forests comprises perhaps 6-8% of total global methane emissions (Lodge, et al., 1996). Termites, which are ubiquitous in tropical forests, interestingly, account for a great deal of this methane. But most tropical methane release comes from the denitrification (breakdown of nitrogen-containing compounds) of organic material by microbes. Soils can act as “sinks” for methane, and, to some extent, offset methane emissions from termites.
d. nitrogen: The addition of nitrogen to the soil by human activities (See Section 1D) equals that added by natural sources. This includes the mobilization of more than 550 metric tons of nitrogen by land conversion, more than 80 million metric tons by fertilizer usage, and more than 20 million metric tons from the burning of fossil fuels (Vitousek, et al., 1997). The consequences are an increase in the greenhouse gas nitrous oxide (NO) as well as other reactive nitrogen compounds such as ammonia (NH3) and nitric oxide (N2O), components of acid rain and smog. In addition, the disproportionate presence of nitrogen in soils and water causes eutrophication.
(For discussions of these issues see Lawton, et al., 2001; Peñuelas and Filella, 2001; Potter, 1999; Scholes and Noble, 2001; Chambers, et al., 2001; Adam, 2001; Oren, et al., 2001; Sarmiento, 2000; Ferber, 2001; Prentice and Lloyd, 1998; Phillips, et al., 1998; Hogan, Hoffman and Thompson, 1991; Houghton, 1995; Houghton, 2000; Palm, Houghton and Melillo, 1986; von Storch and Stehr, 2000; Töpfter, 2001; Clancy, 1998; Baer, et al., 2000; Detwiler and Hall, 1988; Shaver, et al., 2000; Moffat, 1997; Vitousek, et al., 1997. There are many other articles in the literature, as well.)
e. Case in point: The Brazilian Amazon is about five million km2 in extent, of which four million km2 is forested. By 1988, 5.6% of this area had been deforested (230,000 km2), and approximately 10-15,000 km2 of primary forest are cut annually, although estimates vary considerably. Some say that as much as 30% of Brazilian forests is now gone. Carbon released as a consequence of deforestation, mainly from burning but also from the decomposition of forest vegetation after logging, provides Brazil with the credential of having the fourth greatest carbon emissions in the world (after the USA, countries of the former Soviet Union, and China) (Nelson, et al., 2000). Overall, rainforest destruction is the cause of 20%- 30% of global greenhouse gas emissions (Kremen, et al., 2000).
2) Air currents
Forests modulate air currents and reduce the turbulence of the air above them.
This is related to the effects on temperature. [See Sections I, and J.]
To summarize, rainforests are of crucial importance because they recycle approximately 50% of impinging rainwater. A reduction of the forest cover will reduce this recycling activity, which in turn restricts rainfall, leads to a longer dry season, and reduces cloud cover. All of these factors will increase temperature and lower humidity, which have a negative feedback action on rainfall, reducing it yet further. These events will not only affect the forest directly – eliminating or reducing species which depend on lower temperatures and high humidity, but will alter weather conditions elsewhere on the earth (see Deforestation, Part III).