What are Greenhouse Gases? And Where do they Come From?

Carbon dioxide (CO₂): This greenhouse gas is present in relatively low concentrations in the atmosphere, 0.03%. Despite its low levels, it is the major driver of global warming. There are currently around 3 trillion tonnes of CO₂ in the atmosphere; this is 27% higher than the level prior to the industrial revolution. In the late 1800s levels of carbon dioxide were 280 parts per million (ppm) now concentrations are around 380 ppm. Some scientists warn that if the amount of carbon dioxide in the atmosphere goes above 450 ppm the Earth's climate could spiral out of control.

Natural sources of carbon dioxide include rotting plant and animal matter, natural forest fires and volcanoes. The major human sources of CO₂ are from the burning of fossil fuels (coal, oil and gas) and from deforestation. Scientists attribute the increased concentration of CO₂ in the atmosphere to these human sources.

Methane (CH₄): Methane is a very strong greenhouse gas. Over 100 years, a tonne of methane would heat the globe 23 times more than 1 tonne of carbon dioxide would. The atmosphere has a methane concentration of 1,774 parts per billion (ppb); this is a 59% increase from the methane concentration prior to the industrial revolution. There is, however, around 220 times less methane than carbon dioxide in the atmosphere, so overall carbon dioxide has a far greater effect in the atmosphere.

Methane is created by the decay of organic matter. Large amounts come from landfills, cattle and the rest of the livestock sector (chicken and pigs) in general.

Methane hydrates, a frozen combination of methane and water, are found in vast quantities on the sea floor. It is possible that continued climate change could release these frozen stores of methane and cause a sudden, very large addition of methane to the atmosphere. This would massively magnify the greenhouse effect, causing global warming to reach unprecedented levels.

Nitrous Oxide (N₂O): Nitrous oxide is the only one of the nitrogen oxides (NO_x) that acts as a greenhouse gas. Nitrous oxide has a global warming effect roughly 300 times that of carbon dioxide over 100 years. However, like methane, nitrous oxide exists in much lower concentrations than carbon dioxide in the atmosphere. Nitrous oxide concentrations in the atmosphere are currently 319 ppb, 18% higher than what they were prior to the industrial revolution.

Nitrous oxide is emitted by bacteria in soil. Agriculture and the use of nitrogen based fertilizers, along with the handling of animal waste, increase the production of nitrous oxides. Industries, such as the nylon industry, and the burning of fossil fuels in internal combustion engines also release nitrous oxide into the atmosphere.

Black carbon (BC): Black carbon, or soot, is not an actual greenhouse gas as it is a solid and warms the atmosphere differently to a gas. However, it has a significant warming effect on the atmosphere. BC is microscopic particles that result from the incomplete burning of organic matter, especially fossil fuels. Black carbon warms both in the atmosphere and when deposited on lighter surfaces, like snow and ice.

Black carbon may be responsible for as much as 25 percent of observed global warming. The overall contribution of black carbon to global warming may be substantial, perhaps second only to that of carbon dioxide.

Because it can accelerate the melting of snow and ice, black carbon may play a particularly important role in Arctic climate change. In fact, black carbon may be responsible for over 90 percent of Arctic warming. Because the Artic has warmed at around twice the rate of the rest of the world over the last 100 years, controlling and reducing black carbon emissions is particularly important.