BIOGEOCHEMICAL PRIMER

In order to understand the important role that existing tropical forests have on the earth’s atmosphere and climate, some background information on the carbon cycle may help. Terrestrial sinks of carbon, such as forests, are in a dynamic state. They are a biological recycling system. Carbon, in the form of CO₂ is "captured" from the atmosphere by plants in the process of photosynthesis. This chemical reaction provides green plants with energy to fuel their growth and reproduction. About half of the carbon uptaken by plants is rapidly returned to the surrounding air through respiration. The remaining carbon is stored in plant tissues in the form of carbohydrates. Microbial respiration -- the cashing in of the stored energy -- releases another 45% of the original carbon uptaken. The remaining carbon is allocated to plant tissues that decay with varying speed. Ultimately, once vegetation decomposes or burns, the carbon which was "borrowed" from the atmosphere is returned.

Some above-ground plant tissues live very long and/or decompose very slowly. These tissues can essentially hold on to the carbon for decades, centuries, or even millennia in rare cases. Ecological factors such as moisture, PH, and temperature interact with specific plant tissues resulting in these various rates of carbon recycling. While some of the plant material is stored above-ground in bark, leaves, and branches, other carbon is kept below ground in roots and humus material. Below-ground carbon is frequently broken into organic carbon and calcium carbonate (limestone). Organic carbon is carbon from living organisms and is considerably more active than calcium carbonate — a mineral. Soil carbon is an important terrestrial component of the carbon pool, accounting for between 700 and 3000 GT C. ⁽¹⁶⁾ Unlike boreal forests of the north, tropical forests have roughly equal amounts of carbon stored above-ground and below-ground. (Boreal forests actually have more carbon stored in the soils below the trees than in the trees themselves.) Following is a table that shows the typical fate of carbon after its assimilation by an ecosystem.

Process	% of CO2 assimilated	Time
Photosynthesis	100	Days
Growth	50	days-years
Fast Decay	< 5	Decades
Slow Decay	< 0.5	up to centuries

Adopted from GACGC (see footnote 9)

Currently, forests worldwide are estimated to be a net source of emissions, due largely to the conversion and degradation of forests in lower latitude regions ⁽¹⁷⁾. Tropical deforestation is responsible for approximately 1.6 +/- 0.5 GT of carbon emissions per year, while fossil fuel emissions account for approximately 5.5 GT. Together, tropical forests account for 37 percent the global forest carbon, which in turn accounts for 46 percent of the global terrestrial carbon stock ⁽¹⁸⁾.

In general, the conversion of tropical forests to cropland results in between 56 T and 212 T of carbon per hectare being released into the atmosphere. ⁽¹⁹⁾ This variability represents the diverse types of forest that are included in the term "tropical forest".

 

 

THE CONSERVATION OF CLIMACTIC ECOSYSTEM SERVICES

There are three greenhouse gas mitigation strategies involving LUCF: conservation and management of existing forests, increasing the amount of forest area, and management of forest products as fossil fuel replacements or as long-term carbon stores. ⁽²⁰⁾ We have chosen only to explore the first LUCF strategy in this paper. This is because protecting existing forests, especially in tropical regions, creates a significant set of positive externalities. ⁽²¹⁾ Some of the ecosystem services maintained by forests include:

• habitat protection for a vast number of species and populations

• soil conservation
• water purification
• maintenance of watersheds and hydrological cycles (including flood and drought control)
• nutrient storage and cycling
• maintenance of local, regional, and global climate

Ecosystem services that pertain to climate regulation are explored in more detail below. The loss of these services through deforestation has immediate feedback on the climate. Together, they make a powerful case for concluding that a ton of carbon entering the atmosphere because of deforestation has a greater impact on climate change than a ton of carbon from fossil fuel emissions. This biological evidence suggests stopping or slowing tropical deforestation should be a policy priority.

Changes in Albedo ⁽²²⁾

Increased albedo ratios in deforested lands are an example of a direct biological feedback that impacts climate. Forests play an essential role in regulating the energy balance of the planet. ⁽²³⁾ Surface albedo, a measure of a surface’s relative "shininess", modifies the amount of solar radiation received in that area. The higher the albedo ratio, the less energy is absorbed in a region. Solar radiation which heats the earth’s surface converts moisture into water vapor. This in turn causes water to evaporate, rise, and fall as rain. When forests are degraded and albedo ratios rise, less surface moisture is converted into water vapor. This leads to lower rainfall, sparser vegetation, and decreased cloud cover. As a result, less water is circulated and local and regional hydrologic cycles are impacted. If all of Amazonia were deforested and converted to crops and grasslands, the surface albedo would rise from 0.11 to 0.19 and annual rainfall would decline by as much as 25 cm. ⁽²⁴⁾ Most disturbing is that this cycle is self-reinforcing. When rainfall declines, the amount of vegetation declines. At one point in the earth’s history, when total precipitation was approximately 14% lower than today, deserts expanded and the total amount of vegetation on earth was significantly reduced. ⁽²⁵⁾ Cloud cover, which produces a net cooling effect on the climate, is also reduced in this self-reinforcing process. ⁽²⁶⁾

Changes in the Hydrological Cycle

As noted above, when forests are converted or degraded, their role as water regulators is altered. Changes in land use on changes in hydrological cycles are particularly acute in lower altitude countries. In the Ivory Coast, rivers flowing from a primary forest released between three and five times as much water at the end of the dry season, as did rivers from coffee plantations. ⁽²⁷⁾ Even some of the most sophisticated terrestrial/climate models do not consider the role of changes in the hydrological cycle on climate. ⁽²⁸⁾ The link between decreased forest cover and decreased water availability may also produce a positive (self-reinforcing) feedback similar to the albedo effect. With decreased water availability after deforestation, vegetation may be water-stressed, leading to further reductions in forest cover. In a tropical moist forest plot in Panama, a long-term drying trend is pushing 16 species of shrubs and treelets toward extinction. ⁽²⁹⁾ These researchers found their study-forest "remarkably sensitive to a subtle climate shift." ⁽³⁰⁾

The Insurance Value of Biodiversity

Tropical forests contain a disproportionately large number of the world’s species of plants and animals. Using very conservative estimates of the number of species in rainforests, current rainforest destruction destroys between 4,000 and 6,000 species per year. ⁽³¹⁾ Genetically distinct populations are at an ever-greater risk. In tropical forests, 1,800 populations per hour may be going extinct due to deforestation. ⁽³²⁾ The planet is losing species at a rate between 100 and 1,000 the normal background rate of extinction. ⁽³³⁾ This lost genetic variability may substantially alter the impact that climate change has on humans and ecosystems. Under climate change, new breeds of crops, drugs, and chemical applications will be needed to counter the negative impacts that a warmer planet has on human health, agriculture, and the economy. Currently, one in four commercial pharmaceuticals are at least partially derived from tropical plants. ⁽³⁴⁾

Biological diversity — concentrated in tropical forests — serves as a pool of just the type of novel biochemical formulas (genes) that will be required to limit climate’s effect on humans and natural systems. On a warmer, more crowded planet (such as might result from sea level rises and the subsequent climate refugees), it is possible that disease outbreaks will become more common. ⁽³⁵⁾ Certain major crops may become vulnerable to new fungi and pests. In the 1970s, billions of dollars of rice crops were eventually salvaged — and an unknown number of food emergencies averted — when a gene from a wild, forest rice was found to be resistant to the virus. Should climate change occur, especially at a rapid non-linear rate, humanity will need novel genetic combinations to combat climate-related problems. Without active intervention, these answers may not be there when we need them.

Probability of Non-Linearity in Ecosystems

Climate change will likely proceed under enhanced CO₂ in a non-linear fashion. That is, marginal increase in CO₂ concentrations will produce varying degrees of climatic response. For example, increased CO₂ may be uptaken by enhanced plant growth for a certain amount of time. Below certain levels of CO₂ concentrations, there may be no obvious manifestation of a ‘problem’. This may not continue indefinitely. Above a certain threshold level, the assimilative capacity of the biosphere may be exhausted. Similarly, changes in ocean currents may occur in step-wise fashion, producing serious, unpredictable change. ⁽³⁶⁾

Several non-linear climate change possibilities exist. Some non-linear responses were noted above in the albedo factor and with water cycles. These biogeochemical responses create conditions where change begins to "feed" on itself. Other possibilities exist. Some scientists have postulated that if large ice sheets melt, surface albedo at the poles will decrease, heating up the land, and causing more ice to melt. Other scientists have begun to evaluate how plant physiology will respond to increased CO₂ levels and higher temperatures. It is estimated that certain plant communities will be unable to adapt to these changes and may die off. Although this impact will be greatest in temperate and boreal zones, tropical biomes are also at risk. Forests will play a large buffering role in these fluxes. Annual soil carbon turnover alone accounts for more than 10 times the throughput of total fossil fuel emissions. Biomass, largely in the form of forests and grasslands, is the dominant regulator of soil carbon. It is likely that as we destroy the large green swath of forests around the equator, the planet’s climate will respond unpredictably.

The Possibilities of Carbon Conservation in the Tropics ⁽³⁷⁾

Topical countries offer the most potential for forestry-based carbon conservation programs. Countries in low latitudes are estimated to be able to conserve or sequester between 45 and 72 GT of carbon between 1995 and 2050. This is equal to the total carbon emissions of the world for approximately 7-12 years. Slowing deforestation is estimated to be able to conserve 10.8 to 20.8 GT of carbon for the same period. Clearly, forest conservation has considerable potential as a mitigation strategy.