The international community has identified seven anthropogenic greenhouse gases (GHGs) that are harmful to the Earth’s atmosphere and, as a consequence, harmful to the planet’s finely tuned climate system. The gases are:
- carbon dioxide (CO2)
- methane (CH4)
- nitrous oxide (N2O)
- hydrofluorocarbons (HFCs)
- perfluorocarbons (PFCs)
- sulphur hexafluoride (SF6), and
- nitrogen trifluoride (NF3)
Over 30 billion tonnes of these pollutants are discharged around the globe annually. The human activities responsible for these emissions include: the burning of fossil fuels to produce energy; the alteration and destruction of the Earth’s natural habitat for economic development; and the production and use of fluorinated gases for various industrial applications.
Burning fossil fuels
Fossil fuel combustion produces carbon dioxide (CO2) in exhaust gases. The heat of combustion oxidizes some nitrogen in the air to form nitrous oxide (N2O). The methods and processes involved in mining, refining, and transporting fossil fuels discharge methane (CH4). Additional CO2 is produced where fuel combustion is required for heat and mechanical energy in the fossil fuel supply chain.
Devices that burn fossil fuels are ubiquitous fixtures of the world economy. Over a billion internal combustion engines power automobiles, motor cycles, locomotives, airplanes, ships, electric generators, mowers, and hand-held tools. Tens of thousands of industrial boilers and gas turbines generate electricity for the grid and for use in factories.
An uncounted number of stoves, furnaces, kilns, refinery distillers and smelters provide heat for homes and industrial processes such as steel making, refining crude oil, and cement production. Cement production produces an extra measure of GHG because, in addition to burning fossil fuels to heat kilns, it emits CO2 when transforming limestone, a fossil mineral, into calcium oxide (clinker).
Fossil fuel production and combustion accounts for about 75 percent of global greenhouse gas pollution.
Altering and destroying natural habitat
Burning trees and other plants, or leaving them to rot, releases CO2 that had been removed from the atmosphere during the life of those plants. Normally, this is a net-zero emissions equation, but when more plants are destroyed than grow, there is a net increase in atmospheric CO2.
Most deforestation now occurs in tropical regions where industrial-scale agriculture, such as cattle farming and palm oil production, is eliminating jungle habitat.
Cultivation of soil through tillage releases CO2 stored by organic matter in the soil, and fertilizer enhances emissions of N2O from normal bacterial activity.
Industrial-scale farming of cattle, goats, sheep, poultry, pigs, and other animals produces CH4, both from manure and from the digestive tracts of domesticated ruminants.
Rice paddies produce CH4 and CO2 in a way similar to reservoirs (see below).
Natural lakes and river systems produce CH4 and CO2. Man-made reservoirs, including those created by hydro electric dams, behave in a similar manner, but CH4 emissions can spike or increase overall when trees and other vegetation are left to rot in flooded areas.
Use of fluorinated gases
Industrial chemicals containing fluorine (HFCs, PFCs, SF6 and NF3) are used as refrigerants, as fire extinguishing materials, as solvents, and in the manufacture of plastic foams. Aluminum production is the largest source of PFC emissions. Pollution from these chemicals is small in terms of volume, but as greenhouse gases, they are up to 23 thousand times more powerful than carbon dioxide and persist in the atmosphere for thousands of years.
Pollution response and clean up
The simplest way to understand and deal with GHG pollution is to treat it in the same manner as the discharge of any harmful substance: that is, stop it at source and remove it from the environment. In practice, stopping GHG pollution means replacing energy technologies that burn fossil fuels, and restoring natural habitat. Removal from the environment means developing technologies to remove CO2 directly from the atmosphere.
The global scale of GHG pollution, and its already devastating impacts, calls for focused, cooperative and rapid action from government, industry and private individuals worldwide.