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Like a greenhouse, the earth has an atmosphere that traps the radiant energy from the sun and warms the planet, making it suitable for life. Without this protective atmosphere, the planet would lose heat into space, with the result being a drastic cooling of the planet's surface. The gases that trap the heat, like the glass of a greenhouse, are called "greenhouse gases." Consisting of carbon dioxide, methane, nitrous oxides, and chlorofluorocarbons, these gases account for less than 1% of the atmosphere, yet are crucial to the sustenance of life on earth.
It is difficult to conceive that human activity could alter the atmosphere of the earth. But ever since the beginning of the Industrial Revolution some 200 years ago, greenhouse gas levels, especially carbon dioxide, have risen significantly and are projected to increase even more in the next century. This transformation of earth's atmosphere is taking place by the burning of fossil fuels and by destroying the vegetational cover of the earth through massive deforestation. How do scientists know that greenhouse gases are increasing? By drilling into the ice caps at the poles and taking ice-core samples that are then dated, trapped air bubbles in ice layers hundreds of years old can be analyzed and compared to recent atmospheric levels of greenhouse gases. What these studies show is that the level of carbon dioxide has dramatically increased from about 280 ppm (parts per million) in 1800 to roughly 360 ppm in 2001, with a projected doubling of CO2 to 700 ppm by the middle to the end of the next century. This represents a yearly rate of change of approximately 1.8 ppm.
Why should we be concerned with increases in carbon dioxide and other greenhouse gases? There are several reasons. First, climate scientists tell us that with doubling of CO2 levels, the earth's surface temperature will increase an average of between 1.5°C and 4.5°C. With warming occurring on a global scale, snow fields and glaciers will begin to melt and contribute to a rise in sea level that will inundate thousands of miles of coastal areas and force vast numbers of people to seek shelter inland. Sea-level rise will also occur by ocean water expanding as it warms. Coastal areas that now support extensive agriculture will be unfarmable because of flooding. Secondly, an increased CO2 level will have effects on plant life that could be detrimental to world agriculture. Plant biologists studying the effects of an enriched CO2 atmosphere tell us that some plants will grow more robust because of a "CO2 fertilizing" effect. However, to sustain this increased plant productivity will require sufficient nutrients, light, and water—commodities that will be available only at a high price. Because farmers in most of the world cannot afford to increase fertilizer applications and buy irrigation water, plant growth in a CO2-enriched environment will be limited to current production levels. A warmer climate may cause more frequent droughts and, therefore, reduce crop yields in today's major grain-producing regions of the world, notably the Great Plains of the United States. Extreme climate fluctuations would limit the yields of certain crops, thus negating the potential for greater productivity through the "CO2 fertilization" effect.
How is carbon dioxide released into the atmosphere? Although all living organisms give off CO2 during respiration, this does not affect global CO2 levels significantly. The primary culprit is the burning of fossil fuels—oil, natural gas, and coal. Deforestation and burning of timber are other sources of CO2 emissions. On a per capita basis, the United States is the leading nation in CO2 emissions (19.7 tons per person), followed by Canada (17.4) and Saudi Arabia (13.1). China (1.9), Egypt (1.4), and India (0.7) have significantly lower per capita CO2 emissions. These figures clearly reveal the United States' and Canada's dependence upon fossil fuels, while less industrialized countries such as China, Egypt, and India consume much less energy per capita.
Plants and algae created our present atmosphere by absorbing CO2 and releasing O2 during photosynthesis. Because of the oxygen-generating abilities of plants, the gradual buildup of an oxygen atmosphere occurred some 600 million years ago and facilitated the development of large animals on earth. Plants have maintained the delicate balance of our atmosphere for millions of years. Yet this balance is now threatened by human activity. Plant response to these perturbations of the atmosphere is one of the most important reasons for studying plants today.
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Except in insect-trapping plants, digestion takes place within plant cells where the carbohydrates, fats, or proteins are stored, while in animals, digestion usually occurs outside of the cells in the digestive tract. Apart from the location, the process is essentially similar in plants and animals.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.