Reforestation
Fall 2000
Presented by: Brett Vander Velden and Shawn Nauman
52:163 Atmospheric Chemistry & Physics
Department of Chemical & Biochemical Engineering
The University of Iowa
Contents
Biological and Chemical Processes
Quantity of Trees
The objective of this project is to look at reforestation as a method that will counter the Earth’s warming and reduce the average global temperature with minimal negative environmental and economical impacts. This can be accomplished by preventing rainforest depletion and reforesting vast areas of land. Trees act as a carbon dioxide sink, thus removing CO2 from the atmosphere. Removing CO2 aids in decreasing the amount of energy absorbed by the atmosphere and will in turn reduce temperatures. The following information has been determined in support of the objective:
Benefits of preventing rainforest depletion
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Carbon Dioxide as a Greenhouse Gas
The Earth’s atmosphere is unique because it is able to naturally absorb infrared waves directly from the sun and also those that are radiated from the planet’s surface. Gases found in the atmosphere, mainly water vapor, carbon dioxide (CO2), and methane (CH4), also known as greenhouse gases, are responsible for the absorption thermal energy. Without this effect, the average global temperature would be lower and the heat would bounce off the Earth’s surface, unimpeded into space. Water vapor is the most abundant component of the atmosphere and has a lifetime of approximately eight days. As the atmosphere warms, more water vapor is produced, thus increasing the greenhouse effect. This results in a cyclical ef fect. Carbon dioxide is the second most abundant greenhouse gas. The main contribution of CO2 to the atmosphere by man is the burning of fossil fuels. Carbon dioxide has a lifetime of over one hundred years. This means that the addition of CO2 to the atmosphere has a devastating effect on increased warming.
Figure 1: Global warming is causing glaciers and icebergs to melt at an extraordinary rate.
Impacts of Increased Global Temperature
The average global temperature has been increasing since the dawn of the idustrial age. Many scientific models predict an increase of 2-4 degrees Celsius in the next century for a doubled amount of CO2. In order to maintain balance and achieve a sustainable environment for all life forms, it is necessary to reduce the average global temperature. Increases in the average global temperature result in drastic effects on the environment, including the melting of polar ice caps, glaciers, and icebergs. This causes the ocean levels to rise, forcing coastal populations to move inland. Meteorological impacts include greater occurrences of severe weather, such as tornadoes, hurricanes, and thunderstorms, and shifts in regional climates, which may cause agricultural regions to become less productive. Pervasiveness of particular diseases is directly related to warm climates. Therefore, as the temperature of areas increase there is a greater threat to human health. The ecosystem is also threatened by the loss of species due to intense changes in habitats.
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Biological and Chemical Processes
Carbon dioxide poses an extreme global warming problem in today’s world. Due to the burning of fossil fuels and other various man-made activities, the natural carbon cycle has been disrupted and does not balance. More CO2 is being produced than is being consumed by either plants and animals or the oceans.
Figure 2: A typical schematic of carbon dioxide cycling featuring net production and consumption.
Carbon dioxide is consumed readily by most green plants and the leaves of trees. Trees uptake the CO2 in specific cells called chloroplasts and give off oxygen. These chloroplasts produce energy for the plant and make glucose. This occurs through a process called photosynthesis. Normally, the equation for which photosynthesis occurs is as follows:
CO2 + H2O + sunlight ® glucose + oxygen
This is reversed in the process of respiration, performed by both plants and animals:
Glucose + oxygen ® CO2 + H2O
.
Figure 3: Trees, such as the ones in this rain forest, are responsible for photosynthetic reactions and uptake of CO2.
Trees use more CO2 through photosynthesis than they release through respiration. CO2 that is not used in the photosynthetic process and is not released by respiration is then stored in the cells of the tree. This CO2 is not released again until the tree decomposes or stops growing. As a matter of fact, one growing tree can consume about 50 pounds of CO2 in one year. For every ton of wood grown, trees uptake about 1.5 tons of carbon dioxide and give off a little more than a ton of oxygen for up to about 40 years. For example, one acre of pine will grow about 3 tons of wood and give off 3.2 tons of oxygen. This means that the trees will consume close to 4.4 tons of CO2 in photosynthetic processes and in their cells in a period of only one year! This surprising statistic is even greater for deciduous trees. So, how much of a volume of air do trees effect? Statistically, a forest growing at the rate of 10 m3 of wood per hectare per year is absorbing the CO2 from 14 million m3 of air. Because of these surprising facts, trees increase the rate at which greenhouse gases are removed from the atmosphere.
These facts sound very pleasing on paper, however trees can also release carbon dioxide and uptake oxygen. This happens most commonly in the decay process. Trees will not experience this type of situation until they are much older, stop growing, or are left to rot. The decay process requires oxygen and gives off relatively high levels of methane. Methane is also a greenhouse gas and absorbs almost 20 times more radiation than does carbon dioxide. Therefore, in order to maintain a proper balance and to reduce global warming, steps should be taken to harvest the trees. This harvesting would ensure a continuous cycle of birth, growth, and death. This would also ensure proper reforestation of harvested lands.
Approximately one-half of the world’s forests are temperate, and one-half are tropical. While the temperate forests have recently been increasing in size each year, the tropical forests continue to be deforested at an alarming rate. Nearly 214,000 acres (slightly larger than New York City) are destroyed each day resulting in approximately 78 million acres (about the size of Poland) being destroyed each year. Rainforests cover about 2 percent of the earth’s surface. Scientists believe that less than half of the original rainforests currently remain. The following figure shows areas where rainforests currently exist and areas where they once existed.
Figure 4: The area of existing tropical rainforest versus the former area of rainforests reveals approximately a 50 percent reduction.
Along with being one of the most productive and complex ecosystems, rainforests provide oxygen and absorb carbon dioxide from the atmosphere. The destruction of the rainforests cause several problems that can be linked to global warming. Not only is less CO2 removed from the atmosphere, deforestation results in increased levels of CO2 added to the atmosphere. One of the major reasons for deforestation is to obtain land suitable for cattle ranching. To prepare land for raising cattle, the land is clear cut, the trees are left to dry, and then the wood is burned. This slashing and burning process accounts for 20-25 percent of man-made CO2 emissions. Therefore, in order to reduce the effects of global warming, deforestation must be prevented.
Deforestation hurts our planet.
Figure 5: Rainforests like this one are being cut down around the world at an alarming rate.
In order to determine the quantity of trees needed to lower the carbon dioxide levels to those in 1990, assuming no more emissions are released, the total CO2 removal from the atmosphere must first be calculated. It is estimated that the CO2 concentration has risen from 350ppm to 370ppm. The CO2 removal is calculated bel ow:
Based on the types of trees being used, the number of trees that can be planted per acre can be determined. The average amount of space needed by these types of hardwoods is about 32ft2. Thus, 1,361 trees can be planted on one acre of land. If each tree can remove 50lb of CO2 from the atmosphere each year, then the total amount of CO2 removed by each acre of trees each year is 68,050lb or 30,867kg. If the amount of CO2 calculated is to be removed in 10 years, then the amount of land needed for reforestation would be about 593 milliion acres, or about 1.5 times the size of Alaska. However, there are certain factors that need to be considered when looking at the size of usable areas for reforestation. Step 1. ppm is a volumetric measurement and must be converted to a mass measurement. For CO2 (MW=44) at standard conditions, 1ppm(v/v) is equal to 1.8x10-6 kg/m3. If this number is multiplied by 370ppm, the total CO2 concentration can then be determined to be 6.66x10-4 kg/m3.
Step 2. Calculate the volume of the atmosphere. Only the volume of the troposphere is con sidered because that is the only part of the atmosphere that trees can uptake CO2. This can be done by taking the volume of the troposphere and earth, and subtracting the volume of the earth. The troposphere is estimated to be 10 km high from the earth. The volume of the earth and troposphere is found by using V = (4/3)pr^3, where r is the radius of the earth plus 10 km. Then the volume of the earth is found using the same formula. These volumes are subtracted to give the volume of the troposphere, which is 5.1x1018m3.
Step 3. Determine the value of the CO2 concentration. Multiplying the total volume of the atmosphere by the total CO2 concentration yields the mass of CO2 in the troposphere. The mass in 2000 is 3.39x1015kg, while the mass in 1990 was 3.213x1015kg. The difference between the two, 1.83x1014kg, is the amount of CO2 that must be removed from the atmosphere to reach 1990 levels.
Figure 6: In order to reach 1990 levels of CO2, many acres of trees must be planted.
Size of Usable Areas for Reforestation
There are approximately 36.5 billion acres of land on the Earth. However, much of it cannot be used to plant hardwood trees. Several factors must be considered when analyzing how much land can be used for reforesting:
The first factor to consider in determining usable areas for reforestation is what climates maintain the best opportunities for growth of these types of trees. Some of the colder climates, north or south of about 48o latitude, will not support the growth of the recommended hardwood trees. Also, dry, desert-like conditions cannot support the growth of trees. Together, this narrows the available land for reforestation by ~50%.
Another factor to consider in finding usable areas is the effect of the winter months on CO2 removal by trees. The leaves fall off deciduous trees above and below certain latitudes on the Earth. This would result in less CO2 removal for the trees planted in these regions. This would narrow the available land for reforestation by another ~20%.
The third factor in considering the usable areas for reforestation includes analyzing heavily populated areas. Trees cannot be planted in these types of regions throughout the world. However, there are several areas that are open and would support the growth of trees very well. The depleted rainforests are about 2% of the total land area of the Earth. This would be almost 1 billion acres of land to use. Other places conditions would be suitable include Central and South America, the southern United States, southern Asia and the Middle East. It is quite possible that as much as 5 billion acres of land throughout the world could be used to plant trees. If 30,867kg of CO2 are removed from the atmosphere per acre per year, then 5 billion acres would decrease the CO2 concentration by 1.54x1014kg/yr. When considered in the problem above, although this is highly unfeasible because this would require all suitable available land for reforestation, the 1.83x1014kg of carbon dioxide could be removed in a little over a year!
Figure 7: Although the earth has roughly 36.5 billion acres of land, only about 5 billion acres could be used for reforestation.
