How human activities produce greenhouse gases
Most important human activities emit greenhouse gases (GHGs). Emissions started to rise dramatically
in the 1800s due to the Industrial Revolution and changes in land use. Many greenhouse gas-emitting
activities are now essential to the global economy and form a fundamental part of modern life.
Carbon dioxide from the burning of fossil fuels is the largest single source of greenhouse gas emissions
from human activities. The supply and use of fossil fuels accounts for about 80 percent of mankind's
carbon dioxide (CO2) emissions, one fifth of the methane (CH4), and a significant
quantity of nitrous oxide (N2O). It also produces nitrogen oxides (NOx), hydrocarbons
(HCs), and carbon monoxide (CO), which, though not greenhouse gases themselves, influence chemical cycles in
the atmosphere that create or destroy other greenhouse gases, such as tropospheric ozone. Meanwhile,
fuel-related releases of sulphate aerosols are temporarily masking part of the warming effect of greenhouse
Most emissions associated with energy use result when fossil fuels are burned. Oil, natural gas, and
coal (which emits the most carbon per unit of energy supplied) furnish most of the energy used to produce
electricity, run automobiles, heat houses, and power factories. If fuel burned completely, the only
by-product containing carbon would be carbon dioxide. But combustion is often incomplete, so carbon monoxide
and other hydrocarbons are also produced. Nitrous oxide and other nitrogen oxides are produced because fuel
combustion causes nitrogen in the fuel or air to combine with oxygen in the air. Sulphur oxides
(SOx) result when sulphur (primarily from coal and heavy fuel oil) combines with oxygen; the
resulting sulphate aerosols have a cooling effect on the atmosphere.
Extracting, processing, transporting, and distributing fossil fuels also releases greenhouse gases.
These releases can be deliberate, as when natural gas is flared or vented from oil wells, emitting mostly
carbon dioxide and methane, respectively. They can also result from accidents, poor maintenance, and small
leaks in well heads, pipe fittings, and pipelines. Methane occurring naturally in coal seams as pockets of
gas or "dissolved" in the coal itself is released when coal is mined or pulverized. Hydrocarbons
enter the atmosphere as a result of oil spills from tanker ships or small losses during the routine fueling
of motor vehicles.
Deforestation is the second largest source of carbon dioxide. When forests are cleared for agriculture
or development, most of the carbon in the burned or decomposing trees escapes to the atmosphere. However,
when new forests are planted the growing trees absorb carbon dioxide, removing it from the atmosphere. Recent
net deforestation has occurred mainly in the tropics. There is a great deal of scientific uncertainty about
emissions from deforestation and other land-use changes, but it is estimated that from 800 million to 2.4
billion tonnes of carbon are released globally every year.
Producing lime (calcium oxide) to make cement accounts for 3% of CO2 emissions from industrial
sources. Like the CO2 emitted from fossil fuels, the carbon dioxide released during cement
production is derived from limestone and is thus of fossil origin, primarily sea shells and other biomass
buried in ancient ocean sediments.
Domesticated animals emit methane. The second-most important greenhouse gas after carbon dioxide,
methane is produced by cattle, dairy cows, buffalo, goats, sheep, camels, pigs, and horses. Most
livestock-related methane emissions are produced by "enteric fermentation" of food by bacteria and
other microbes in the animals' digestive tracts; another source is the decomposition of animal manure.
Livestock account for 30% of the methane emissions from human activities.
Rice cultivation also releases methane . . . "Wetland" or "paddy" rice farming
produces roughly one-fifth to one-quarter of global methane emissions from human activities. Accounting for
over 90 percent of all rice production, wetland rice is grown in fields that are flooded or irrigated for
much of the growing season. Bacteria and other micro-organisms in the soil of the flooded rice paddy
decompose organic matter and produce methane.
. . . as does the disposal and treatment of garbage and human wastes. When garbage is buried in a
landfill, it sooner or later undergoes anaerobic (oxygen-free) decomposition and emits methane (and some
carbon dioxide). Unless the gas is captured and used as a fuel, the methane eventually escapes to the
atmosphere. This source of methane is more common near cities, where garbage from many homes is brought to a
central landfill, than in rural areas where garbage is typically burned or left to decompose in the open air.
Methane is also emitted when human waste (sewage) is treated anaerobically, for example in anaerobic ponds or
Fertilizer use increases nitrous oxide emissions. The nitrogen contained in many mineral and organic
fertilizers and manures enhances the natural processes of nitrification and denitrification that are carried
out by bacteria and other microbes in the soil. These processes convert some nitrogen into nitrous oxide. The
amount of N2O emitted for each unit of nitrogen applied to the soil depends on the type and amount
of fertilizer, soil conditions, and climate - a complex equation that is not fully understood.
Industry has created a number of long-lived and potent greenhouse gases for specialized uses.
Developed in the 1920s, chlorofluorocarbons (CFCs) have been used as propellants in aerosol cans, in the
manufacture of plastic foams for cushions and other products, in the cooling coils of refrigerators and air
conditioners, as fire extinguishing materials, and as solvents for cleaning. Thanks to the 1987 Montreal
Protocol on Substances that Deplete the Ozone Layer, atmospheric concentrations of many CFCs are stablizing
and expected to decline over the coming decades. Other halocarbons that are being used as ozone-safe
replacements for CFCs – notably hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs) –
contribute to global warming and so are targeted for reduction under the 1997 Kyoto Protocol. The Protocol
also targets sulphur hexafluoride (SF6), used as an electric insulator, heat conductor, and
freezing agent; molecule for molecule, its global warming potential is thought to be 23,900 times greater
than that of carbon dioxide.