Four representations chemists use for carbon dioxide. In the colored models, carbon is light gray and oxygen is red.
Credit: Randy Russell (©2006 NESTA)
Carbon dioxide is a colorless and non-flammable gas at normal temperature and pressure. Although much less abundant than nitrogen and oxygen in Earth's atmosphere, carbon dioxide is an important constituent of our planet's air. A molecule of carbon dioxide (CO2) is made up of one carbon atom and two oxygen atoms.
Carbon dioxide is an important greenhouse gas that helps to trap heat in our atmosphere. Without it, our planet would be inhospitably cold. However, an increase in CO2 concentrations in our atmosphere is causing average global temperatures to rise, disrupting other aspects of Earth's climate.
Carbon dioxide is the fourth most abundant component of dry air. Today it has a concentration of over 400 ppm (parts per million) in the atmosphere. Before industrial activity, there was around 270 ppm in the atmosphere. Carbon dioxide levels in our atmosphere have thus risen about 40% since the start of the Industrial Revolution, which is raising global temperature.
Since 1958, scientist Charles Keeling and others have measured the amount of carbon dioxide in Hawaii's atmosphere. The yearly fluctuations in carbon dioxide are due to seasonal plant growth, while the overall rise in carbon dioxide over many years is due to a combination of fossil fuel burning, deforestation, and cement production.
L.S. Gardiner/UCAR
Atmospheric carbon dioxide concentrations have varied substantially through Earth history, which had profound impacts on climate and life.
Carbon dioxide plays a key role in Earth's carbon cycle, the set of processes that cycle carbon in many forms throughout our environment. Volcanic outgassing and wildfires are two significant natural sources of CO2 in Earth's atmosphere. Respiration, the process by which organisms liberate energy from food, emits carbon dioxide. When you exhale, it is carbon dioxide (amongst other gases) that you breathe out. Combustion, whether in the guise of wildfires, as a result of slash-and-burn agricultural practices, or in internal combustion engines, produces carbon dioxide.
Photosynthesis, the biochemical process by which plants and some microbes create food, uses up carbon dioxide. Photosynthetic organisms combine CO2 and water (H2O) to produce carbohydrates (such as sugars) and emit oxygen as a by-product. Places such as forests and areas of the ocean that support photosynthetic microbes, therefore, act as massive carbon "sinks", removing carbon dioxide from the atmosphere via photosynthesis. Earth's early atmosphere had much higher CO2 levels and almost no oxygen; the rise of photosynthetic organisms led to an increase in oxygen which enabled the development of oxygen-breathing creatures such as us!
Burning generates CO2, although incomplete combustion due to limited oxygen supply or an excess of carbon can also produce carbon monoxide (CO). Carbon monoxide, a dangerous pollutant, eventually oxidizes to carbon dioxide.
Small canisters containing pressurized CO2 are used to inflate bicycle tires and life jackets and to power paintball guns. The "fizz" in soda pop is supplied by carbon dioxide. Carbon dioxide is also released by yeast during fermentation, giving beer its head and making champagne bubbly. Because it is not flammable, CO2 is used in some fire extinguishers. Carbon dioxide forms a weak acid, called carbonic acid (H2CO3), when dissolved in water.
Carbon dioxide is the most abundant gas in the atmospheres of Mars and Venus. Solid, frozen carbon dioxide is called "dry ice". The polar ice caps of Mars are a mixture of normal water ice and dry ice. Liquid CO2 only forms at pressures higher than about 5 times the atmospheric pressure on Earth at sea level, so in many situations dry ice does not melt into a liquid form. Instead, it goes directly from a solid state to a gaseous state in a process called sublimation.
The greenhouse effect happens when certain gases, which are known as greenhouse gases, accumulate in Earth’s atmosphere. Greenhouse gases include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), ozone (O3), and fluorinated gases.
Greenhouse gases allow the sun’s light to shine onto Earth’s surface, and then the gases, such as ozone, trap the heat that reflects back from the surface inside Earth’s atmosphere. The gases act like the glass walls of a greenhouse—thus the name, greenhouse gas.
According to scientists, the average temperature of Earth would drop from 14˚C (57˚F) to as low as –18˚C (–0.4˚F), without the greenhouse effect.
Some greenhouse gases come from natural sources, for example, evaporation adds water vapor to the atmosphere. Animals and plants release carbon dioxide when they respire, or breathe. Methane is released naturally from decomposition. There is evidence that suggests methane is released in low-oxygen environments, such as swamps or landfills. Volcanoes—both on land and under the ocean—release greenhouse gases, so periods of high volcanic activity tend to be warmer.
Since the Industrial Revolution of the late 1700s and early 1800s, people have been releasing larger quantities of greenhouse gases into the atmosphere. That amount has skyrocketed in the past century. Greenhouse gas emissions increased 70 percent between 1970 and 2004. Emissions of CO2, rose by about 80 percent during that time.
The amount of CO2 in the atmosphere far exceeds the naturally occurring range seen during the last 650,000 years.
Most of the CO2 that people put into the atmosphere comes from burning fossil fuels. Cars, trucks, trains, and planes all burn fossil fuels. Many electric power plants do as well. Another way humans release CO2 into the atmosphere is by cutting down forests, because trees contain large amounts of carbon.
People add methane to the atmosphere through livestock farming,
landfills, and fossil fuel production such as coal mining and natural gas processing. Nitrous oxide comes from agriculture and fossil fuel burning. Fluorinated gases include chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs). They are produced during the manufacturing of refrigeration and cooling products and through aerosols.
All of these human activities add greenhouse gases to the atmosphere. As the level of these gases rises, so does the temperature of Earth. The rise in Earth’s average temperature contributed to by human activity is
known as global warming.
The Greenhouse Effect and Climate Change
Even slight increases in average global temperatures can have huge effects.
Perhaps the biggest, most obvious effect is that glaciers and ice caps melt faster than usual. The meltwater drains into the oceans, causing sea levels to rise.
Glaciers and ice caps cover about 10 percent of the world’s landmasses. They hold between 70 and 75 percent of the world’s freshwater. If all of this ice melted, sea levels would rise by about 70 meters (230 feet).
The Intergovernmental Panel on Climate Change states that the global sea level rose about 1.8 millimeters (0.07 inches) per year from 1961 to 1993, and about 3.1 millimeters (0.12 inches) per year since 1993.
Rising sea levels cause flooding in coastal cities, which could displace millions of people in low-lying areas such as Bangladesh, the U.S. state of Florida, and the Netherlands.
Millions more people in countries like Bolivia, Peru, and India depend on glacial meltwater for drinking, irrigation, and hydroelectric power. Rapid loss of these glaciers would devastate those countries.
Greenhouse gas emissions affect more than just temperature. Another effect involves changes in precipitation, such as rain and snow.
Over the course of the 20th century, precipitation increased in eastern parts of North and South America, northern Europe, and northern and central Asia. However, it has decreased in parts of Africa, the Mediterranean, and southern Asia.
As climates change, so do the habitats for living things. Animals that are adapted to a certain climate may become threatened. Many human societies depend on predictable rain patterns in order to grow specific crops for food, clothing, and trade. If the climate of an area changes, the people who live there may no longer be able to grow the crops they depend on for survival. Some scientists also worry that tropical diseases will expand their ranges into what are now more temperate regions if the temperatures of those areas increase.
Most climate scientists agree that we must reduce the amount of greenhouse gases released into the atmosphere. Ways to do this, include:
- driving less, using public transportation, carpooling, walking, or riding a bike.
- flying less—airplanes produce huge amounts of greenhouse gas emissions.
- reducing, reusing, and recycling.
- planting a tree—trees absorb carbon dioxide, keeping it out of the atmosphere.
- using less electricity.
- eating less meat—cows are one of the biggest methane producers.
- supporting alternative energy sources that don’t burn fossil fuels.
Fast Fact
Manmade Gas
Chlorofluorocarbons (CFCs) are the only greenhouse gases not created by nature. They are created through refrigeration and aerosol cans.
CFCs, used mostly as refrigerants, are chemicals that were developed in the late 19th century and came into wide use in the mid-20th century.
Other greenhouse gases, such as carbon dioxide, are emitted by human activity, at an unnatural and unsustainable level, but the molecules do occur naturally in the Earth's atmosphere.
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