Explainer: CO2 and other greenhouse gases | Science News for Students

Explainer: CO2 and other greenhouse gases

Carbon dioxide is just one of several chemicals that contribute to the greenhouse effect
Sep 20, 2018 — 6:30 am EST
an photo of offshore natural-gas platform burning off methane

As this offshore natural-gas platform burns off one greenhouse gas — methane — it produces another, carbon dioxide.


Many different gases make up Earth’s atmosphere. Nitrogen alone accounts for 78 percent. Oxygen, in second place, makes up another 21 percent. Many other gases comprise the remaining 1 percent. Several (such as helium and krypton) are chemically inert. That means they don’t react with others. Other bit players have the ability to act like a blanket for the planet. These have come to be known as greenhouse gases.

Like the windows in a greenhouse, these gases trap energy from the sun as heat. Without their role in this greenhouse effect, Earth would be quite frosty. Global temperatures would average around -18° Celsius (0° Fahrenheit), according to the National Oceanic and Atmospheric Administration (NOAA). Instead, the surface of our planet averages around 15 °C (59 °F), making it a comfy place for life.

Since about 1850, though, human activities have been releasing extra greenhouse gases into the air. This has slowly propelled a rise in average temperatures across the globe. Overall, the 2017 global average was 0.9 degree C (1.6 degrees F) higher than it had been between 1951 and 1980. That’s based on calculations by NASA.

Stephen Montzka is a research chemist with NOAA in Boulder, Colo. There are four main greenhouse gases to worry about, he says. The best known is carbon dioxide (CO2). The others are methane, nitrous oxide and a group that contains chlorofluorocarbons (CFCs) and their replacements. (CFCs are refrigerants that have played a role in thinning the planet’s protective high-altitude ozone layer. They are being phased out as part of a global agreement begun in 1989.)

Many chemicals influence climate. However, Montzka notes, these four greenhouse gases are the ones “that we [humans] have direct control over.”

Climate-warming chemicals

Each greenhouse gas, once emitted, rises into the air. There, it helps the atmosphere hold onto heat. Some of these gases trap more heat, per molecule, than do others. Some also stay in the atmosphere longer than others. This is because each has different chemical properties, Montzka notes. They also are removed from the atmosphere, over time, by different processes.

Excess CO2 comes mainly from burning fossil fuels — coal, oil and natural gas. Those fuels are used for everything from powering vehicles and generating electricity to manufacturing industrial chemicals. In 2016, CO2 accounted for 81 percent of the greenhouse gases emitted in the United States. Other chemicals are more effective at trapping heat in the atmosphere. But CO2 is the most abundant of the ones released by human activities. It also sticks around longest.

a pie chart showing percentages of US greenhouse gas emissions in 2016
Carbon dioxide accounted for most of U.S. greenhouse gas emissions in 2016.

Some CO2 gets removed each year by plants as they grow. However, much CO2 is released during colder months, when plants aren’t growing. CO2 also can be pulled from the air and into the ocean. Organisms in the sea can then convert it into calcium carbonate. Eventually that chemical will become an ingredient of limestone rock, where its carbon can be stored for millennia. That rock-forming process is really slow. Overall, CO2 can linger in the atmosphere for anywhere from decades to thousands of years. So, Montzka explains, “even if we stopped emitting carbon dioxide today, we would still see warming from that for a very long time.”

Methane is the main component of natural gas. It’s also released from a host of biological sources. These include rice production, animal manure, cow digestion and the breakdown of wastes put into landfills. Methane accounts for about 10 percent of U.S. greenhouse-gas emissions. Each molecule of this gas is much better at trapping heat than is one of CO2. But methane does not remain in the atmosphere as long. It gets broken down as it reacts in the atmosphere with hydroxyl radicals (neutrally charged OH ions made from bound atoms of oxygen and hydrogen). “The timescale for methane removal is about a decade,” Montzka notes.

Nitrous-oxide (N2O) made up 6 percent of greenhouse gases emitted by the United States in 2016. This gas comes from agriculture, the burning of fossil fuels and human sewage. But don’t let its small quantity make you disregard N2O’s impact. This gas is hundreds of times more effective than is CO2 at trapping heat. N2O also can linger in the atmosphere for nearly a century. Each year, only about 1 percent of airborne N2O gets converted by green plants into ammonia or other nitrogen compounds that plants can use. So this natural N2O removal “is really slow,” Montzka says.

CFCs and their more recent replacements are all manufactured by people. Many have been used as refrigerants. Others are used as solvents for chemical reactions and in aerosol sprays. Together, these made up only about 3 percent of U.S. greenhouse gas emissions in 2016. These gases are only removed when they get locked up in a high layer of the atmosphere. In this stratosphere, high-energy light bombards the chemicals, breaking them apart. But that can take decades, Montzka says.

Fluorine-based chemicals, such as CFCs, he notes, “are potent greenhouse gases, on a per molecule basis.” But releases of them are so low that compared to CO2, their overall impact is quite small. Reducing emissions of methane, N2O and CFCs will help slow climate change, Montzka notes. “But if we’re going to solve this [greenhouse gas] problem, we need to take care of CO2,” he says. “It’s contributing the most … and it has this extremely long residence time in the atmosphere.”

Power Words

(more about Power Words)

aerosol     A group of tiny particles suspended in air or gas. Aerosols can be natural, such as fog or gas from volcanic eruptions, or artificial, such as smoke from burning fossil fuels.

agriculture     The growth of plants, animals or fungi for human needs, including food, fuel, chemicals and medicine.

ammonia     A colorless gas with a nasty smell. Ammonia is a compound made from the elements nitrogen and hydrogen. It is used to make food and applied to farm fields as a fertilizer. Secreted by the kidneys, ammonia gives urine its characteristic odor. The chemical also occurs in the atmosphere and throughout the universe.

atmosphere     The envelope of gases surrounding Earth or another planet.

average     (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.

calcium carbonate     The main chemical compound in limestone, a rock made from the tiny shells of ancient marine organisms. Its formula is CaCO3 (meaning it contains one calcium atom, one carbon atom and three oxygen atoms). It’s also the active ingredient in some antacid medicines (ones used to neutralize stomach acids).

carbon dioxide (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.

chemical     A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

chemical reaction     A process that involves the rearrangement of the molecules or structure of a substance, as opposed to a change in physical form (as from a solid to a gas).

climate     The weather conditions that typically exist in one area, in general, or over a long period.

climate change     Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.

component     Something that is part of something else (such as pieces that go on an electronic circuit board or ingredients that go into a cookie recipe).

electricity     A flow of charge, usually from the movement of negatively charged particles, called electrons.

fossil fuel     Any fuel — such as coal, petroleum (crude oil) or natural gas — that has developed within the Earth over millions of years from the decayed remains of bacteria, plants or animals.

greenhouse     A light-filled structure, often with windows serving as walls and ceiling materials, in which plants are grown. It provides a controlled environment in which set amounts of water, humidity and nutrients can be applied — and pests can be prevented entry.

greenhouse effect     The warming of Earth’s atmosphere due to the buildup of heat-trapping gases, such as carbon dioxide and methane. Scientists refer to these pollutants as greenhouse gases. The greenhouse effect also can occur in smaller environments. For instance, when cars are left in the sun, the incoming sunlight turns to heat, becomes trapped inside and quickly can make the indoor temperature a health risk.

greenhouse gas     A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.

helium     An inert gas that is the lightest member of the noble gas series. Helium can become a solid at -272 degrees Celsius (-458 degrees Fahrenheit).

inert     Inactive or having no chemical or physical effects.

ion     (adj. ionized) An atom or molecule with an electric charge due to the loss or gain of one or more electrons. An ionized gas, or plasma, is where all of the electrons have been separated from their parent atoms.

krypton     An inert gas that's a member of the noble gases, used in some types of electric lights.

landfill     A site where trash is dumped and then covered with dirt to reduce smells. If they are not lined with impermeable materials, rains washing through these waste sites can leach out toxic materials and carry them downstream or into groundwater. Because trash in these facilities is covered by dirt, the wastes do not get ready access to sunlight and microbes to aid in their breakdown. As a result, even newspaper sent to a landfill may resist breakdown for many decades.

limestone     A natural rock formed by the accumulation of calcium carbonate over time, then compressed under great pressure. Most of the starting calcium carbonate came from the shells of sea animals after they died. However, that chemical also can settle out of water, especially after carbon dioxide is removed (by plants, for instance).

manure     Feces, or dung, from farm animals. Manure can be used to fertilize land.

methane     A hydrocarbon with the chemical formula CH4 (meaning there are four hydrogen atoms bound to one carbon atom). It’s a natural constituent of what’s known as natural gas. It’s also emitted by decomposing plant material in wetlands and is belched out by cows and other ruminant livestock. From a climate perspective, methane is 20 times more potent than carbon dioxide is in trapping heat in Earth’s atmosphere, making it a very important greenhouse gas.

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

NASA     Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It also has sent research craft to study planets and other celestial objects in our solar system.

National Oceanic and Atmospheric Administration (or NOAA) A science agency of the U.S. Department of Commerce. Initially established in 1807 under another name (The Survey of the Coast), this agency focuses on understanding and preserving ocean resources, including fisheries, protecting marine mammals (from seals to whales), studying the seafloor and probing the upper atmosphere.

natural gas     A mix of gases that developed underground over millions of years (often in association with crude oil). Most natural gas starts out as 50 to 90 percent methane, along with small amounts of heavier hydrocarbons, such as propane and butane.

nitrogen     A colorless, odorless and nonreactive gaseous element that forms about 78 percent of Earth's atmosphere. Its scientific symbol is N. Nitrogen is released in the form of nitrogen oxides as fossil fuels burn.

organism     Any living thing, from elephants and plants to bacteria and other types of single-celled life.

oxide     A compound made by combining one or more elements with oxygen. Rust is an oxide; so is water.

oxygen     A gas that makes up about 21 percent of Earth's atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).

ozone     A colorless gas that forms high in the atmosphere and at ground level. When it forms at Earth’s surface, ozone is a pollutant that irritates eyes and lungs. It is also a major ingredient of smog.

ozone layer     A layer in Earth’s stratosphere. It contains a lot of ozone, which helps block much of the sun’s biologically damaging ultraviolet radiation.

potent     An adjective for something (like a germ, poison, drug or acid) that is very strong or powerful.

radical     An charged molecule having one or more unpaired outer electrons. Radicals readily take part in chemical reactions. The body is capable of making radicals as one means to kill cells, and thereby rid itself of damaged cells or infectious microbes.

sewage     Wastes — primarily urine and feces — that are mixed with water and flushed away from homes through a system of pipes for disposal in the environment (sometimes after being treated in a big water-treatment plant).

solvent     A material (usually a liquid) used to dissolve some other material into a solution.

stratosphere     The second layer of the Earth’s atmosphere, just above the troposphere, or ground layer. The stratosphere stretches from 10 kilometers to 50 kilometers (about 6.2 to 31 miles) above sea level.

sun     The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.