The gases that heat up Earth's atmosphere are called greenhouse gases. They create an atmospheric layer that prevents the energy of reflected sunlight from escaping into space. A number of compounds contribute to this effect, including carbon dioxide, methane and nitrous oxide, and they are all produced by natural as well as industrial processes. The No. 1 greenhouse gas, however, is one that most people don't normally consider a pollutant. It is water vapor.
The Greenhouse Effect
In the absence of atmospheric gases, all the sunlight reflected off the surface of Earth would bounce into space. When certain gases are present, however, they absorb the sunlight, and their molecules enter a state of higher energy. When they return to their normal state, they release some of that energy back to the ground in the form of heat. The effect is like that of the glass roof covering a greenhouse, which traps heat energy inside the structure and makes the air warmer. The effect is desirable in a greenhouse, and it's desirable in the atmosphere as well, at least to a point.
Greenhouse Gases as Pollution
Without the ability of greenhouse gases to trap reflected light and radiate heat energy to the ground, Earth would probably be too cold for humans to survive. The moon is roughly the same distance from the sun as Earth, but because it is too small to have an atmosphere, its temperature varies from 110 degrees Celsius (230 degrees Fahrenheit) on the sunny side to minus 144 degrees Celsius (minus 290 degrees Fahrenheit) on the dark side. However, a high concentration of greenhouse gases creates serious problems. The atmosphere of Venus is 90 times as dense as Earth's and is mostly carbon dioxide. Because of this, Venus experiences a runaway greenhouse effect producing a surface temperature of 462 degrees Celsius (864 degrees Fahrenheit). That's hot enough to melt lead.
Top Greenhouse Gases
Water vapor is the No. 1 heat-trapping gas in the atmosphere, but it isn't considered a pollutant. Tops on the list of polluting greenhouse gases is carbon dioxide, which has many natural sources but is also a byproduct of modern industry. Methane is another abundant greenhouse gas -- it comes from livestock and from human agricultural activities, such as rice cultivation. Nitrous oxide, which is used in the rocket industry and for supercharging vehicles, and ozone, another industrial byproduct, are also important heat-trapping gases, as are certain man-made chemicals, such as hydrofluorocarbons. Some of these last in the atmosphere for thousands of years.
Rising Carbon Dioxide Levels
Because it is such an important greenhouse gas, scientists keep a close watch on the levels of carbon dioxide in the atmosphere. A research team led by oceanographer David Keeling has recorded daily levels at Mauna Loa since the late 1950s. Keeling died in 2005, but his team was still compiling the data in 2013. The Keeling curve, a graph produced from the data, shows clearly that carbon dioxide levels have risen steadily since they started the project. Data also shows that Earth's average surface temperature has risen by 0.75 degrees Celsius (1.33 degrees Fahrenheit) since the 1880s. The effects on Earth's climate aren't completely understood and are the subject of research.