What Are Greenhouse Gases?

Airplane traces or contrails
High-elevation aircraft contrails. freebigpictures.com

Greenhouse gases absorb reflected solar energy, making the Earth's atmosphere warmer. A lot of the sun’s energy reaches the ground directly, and a portion is reflected by the ground back into space. Some gases, when present in the atmosphere, absorb that reflected energy and redirect it back to Earth as heat. The gases responsible for this are called greenhouse gases, as they play a similar role as the clear plastic or glass covering a greenhouse.

Recent Increases Tied to Human Activities

Some greenhouse gases are emitted naturally through wildfires, volcanic activity, and biological activity. However, since the industrial revolution at the turn of the 19th century, humans have been releasing increasing amounts of greenhouse gases. This increase accelerated with the development of the petro-chemical industry after World War II.

The Greenhouse Effect

The heat reflected back by greenhouse gases produces a measurable warming of the Earth’s surface and oceans. This global climate change has wide-ranging effects on the Earth’s ice, oceans, ecosystems, and biodiversity.  

Carbon Dioxide

Carbon dioxide is the most important greenhouse gas. It is produced from the use of fossil fuels to generate electricity (for example, coal-fired power plants) and to power vehicles. The cement manufacturing process produces a lot of carbon dioxide. Clearing land from vegetation, usually in order to farm it, triggers the release of large amounts of carbon dioxide normally stored in the soil.


Methane is a very effective greenhouse gas, but with a shorter lifespan in the atmosphere than carbon dioxide. It comes from a variety of sources. Some sources are natural: methane escapes wetlands and oceans at a significant rate. Other sources are anthropogenic, which means man-made. The extraction, processing, and distribution of oil and natural gas all release methane. Raising livestock and rice farming are major sources of methane. The organic matter in landfills and waste-water treatment plants releases methane.

Nitrous Oxide

Nitrous oxide (N2O) occurs naturally in the atmosphere as one of the many forms nitrogen can take. However, large amounts of released nitrous oxide contribute significantly to global warming. The main source is the use of synthetic fertilizer in agricultural activities. Nitrous oxide is also released from during the manufacturing of synthetic fertilizers. Motor vehicles release nitrous oxide when operating with fossil fuels like gasoline or diesel.


Halocarbons are a family of molecules with a variety of uses, and with greenhouse gas properties when released into the atmosphere. Halocarbons include CFCs, which were once widely used as refrigerants in air conditioners and refrigerators. Their manufacture is banned in most countries, but they continue to be present in the atmosphere and damage the ozone layer (see below). Replacement molecules include HCFCs, which act as greenhouse gases. These are being phased out as well. HFCs are replacing the more harmful, earlier halocarbons, and they contribute much less to global climate change.


Ozone is a naturally occurring gas located in the upper reaches of the atmosphere, protecting us from much of the damaging sun rays. The well-publicized issue of refrigerant and other chemicals creating a hole in the ozone layer is quite separate from the issue of global warming. In the lower parts of the atmosphere, ozone is produced as other chemicals break down (for example, nitrogen oxides). This ozone is considered a greenhouse gas, but it is short-lived and although it can contribute significantly to warming, its effects are usually local rather than global.

Water, a Greenhouse Gas?

How about water vapor? Water vapor plays an important role in regulating climate through processes operating at lower levels of the atmosphere. In the upper parts of the atmosphere, the amount of water vapor appears to vary a lot, with no significant trend over time.

There are things you can do to reduce your greenhouse gas emissions.


Observations: Atmosphere and Surface. IPCC, Fifth Assessment Report. 2013.

Watch Now: How to Reduce Your Carbon Footprint