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Emissions of carbon dioxide (CO2) occur when carbon dioxide is released into the atmosphere, either naturally or through human activities such as burning fossil fuels. Carbon dioxide is a chemical compound composed of one carbon atom and two oxygen atoms that exists in the phase of a gas in Earth’s atmosphere. The atmosphere is a series of layers of gases that surround the Earth. These gases are bound to the planet and prevented from floating into space by Earth’s gravity. The atmosphere is mainly composed of nitrogen and oxygen, but many other gases, including a small amount of carbon dioxide, exist in the atmosphere as well.
Man-made emissions of carbon dioxide and other greenhouse gases, however, have upset this balance. A greenhouse gas is an atmospheric gas that lets sunlight pass through the atmosphere to reach the Earth. As the light hits the Earth, some of it bounces back toward space as infrared radiation, or heat. Greenhouse gases trap this heat in the atmosphere, thus making the world warmer in a phenomenon known as the greenhouse effect. A certain amount of greenhouse gases is necessary to control Earth’s climate and make the planet warm enough for life, but too much warming can also threaten life.
Natural processes occur which emit and remove carbon dioxide from the atmosphere in a complex ongoing exchange known as the carbon cycle. The processes responsible for the emissions of carbon dioxide are called sources and those that remove carbon dioxide are called sinks. Natural sources of CO2 include plant and animal respiration, such as human exhalations, and volcanic eruptions. Natural sinks include plant photosynthesis, during which plants remove carbon dioxide from the air to make sugars and store in plant biomass. Oceans are both a sink and a source, with tropical waters typically releasing carbon dioxide and colder ocean waters absorbing carbon dioxide.
In a healthy carbon cycle, the emissions of carbon dioxide are canceled out by the removals of carbon dioxide so that there is no net change in the atmosphere’s carbon dioxide concentration. Human activities such as metal production, mineral production, and burning of fossil fuels have increased emissions of carbon dioxide. Meanwhile, ocean pollution and deforestation, the cutting down of trees and other plants for timber, agriculture, and urban development, has impeded the natural processes of carbon dioxide removal. In the time between the Industrial Revolution of the 1700s and 2005, the concentration of carbon dioxide in the atmosphere rose 35%, indicating that the sources in the carbon cycle have outstripped the sinks.
When fossil fuels, such as coal, gas, or oil, are burned to create energy, they release carbon in the form of CO2. These fossil fuels are commonly burned to generate residential and industrial electricity, create heat, and move vehicles such as airplanes, trains, automobiles, and boats. Mineral production processes of lime, cement, and soda ash involve chemical reactions that transform chemicals within the raw materials to create emissions of carbon dioxide. CO2 emissions occur by similar means in the production of metals such as iron, zinc, and lead. The degradation of petroleum-based products and the production of chemicals such as ammonia also contribute to emissions of carbon dioxide.
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