Greenhouse Gases: Sources, Sinks & Concentrations
Since the Industrial Revolution, the atmospheric concentrations of many greenhouse gases have been increasing, primarily due to human activities. During the last fifty years there has additionally been an input to the atmosphere of halocarbons such as CFCs, which as well as depleting ozone, also act as greenhouse gases. With more greenhouse gases in the atmosphere, the natural greenhouse effect is being enhanced artificially, and this could bring about global warming.
Sources, Sinks and Atmospheric Lifetimes
In considering any atmospheric pollutant, including greenhouse gases, it is important to be able to identify its sources, sinks and lifetime within the atmosphere. A source is a point or place from which a pollutant is released or emitted. In the case of greenhouse gases this will mean the point where the gas enters the atmosphere. An atmospheric sink is a point or location where the gas is removed from the atmosphere, either by chemical reaction or absorption into other parts of the climate system, including the oceans, ice sheets and land. The atmospheric lifetime of a pollutant is denoted by the average period of time a molecule of that pollutant resides within the atmosphere. This is determined by the relative rates of emission (from the source) and removal (via the sink).
Increased concentrations of greenhouse gases in the atmosphere are enhancing the Earth's natural greenhouse effect by trapping a greater proportion of the terrestrial (infrared) energy (or radiation) leaving the Earth. This is called climate forcing (or radiative forcing), and causes a temperature rise at the Earth's surface. Climate forcing is commonly known as the enhanced greenhouse effect. The enhanced greenhouse effect can be quantified in terms of the amount of extra energy trapped in the atmosphere (measured in Watts per square metre (Wm-2)). The "forcing" potential of a greenhouse gas is determined by a combination of factors including how well the gas absorbs infra-red energy, how much of it is already in the atmosphere, how much is being added to the atmosphere, and how long it survives in the atmosphere (the atmospheric lifetime). This potential of a greenhouse gas is sometimes expressed as a Global Warming Potential (GWP).
Carbon Dioxide (CO2)
Carbon dioxide is produced naturally through respiration, decay of plant / animal matter and natural forest fires. Anthropogenic or man-made sources of CO2 include fossil fuel combustion, land use changes (primarily deforestation), biomass burning and the manufacture of cement. The main removal processes for carbon dioxide is absorption by the oceans and terrestrial biota, especially forestry.
Since the Industrial Revolution concentrations have increased from 280 parts per million by volume (ppmv) to 367ppmv in 1999. This is higher than at any other time in the last 160,000 years. CO2 emitted into the atmosphere today will influence its atmospheric concentration in the years to come, since the time taken for atmospheric CO2 to adjust to changes in sources or sinks is in the order of 50-200 years. CO2 is the largest individual contributor to the enhanced greenhouse effect, having a positive forcing value of about 1.56Wm-2 (or 60% of the total) over the period 1765 to 1999. That is to say that an additional 1.56Wm-2 of energy is trapped in the atmosphere as a result of the build up of CO2 since 1765. [Compare with 1368Wm-2, the amount of energy arriving from the Sun hitting the top of the atmosphere.] To stabilise concentrations at present day levels would require a massive 60% reduction of global CO2 emissions.
Methane is formed naturally in wetland regions during the decay of organic material and by termites. Additionally, there are many man-made sources of CH4 that have contributed to an increase in the global average atmospheric concentration, including rice cultivation, biomass burning, fossil fuel combustion and disposal of domestic refuse in landfill sites. The major sink for CH4 is the chemical reaction with hydroxyl (OH) radicals in the lower atmosphere. The global atmospheric concentration of CH4 in 1998 was 1.75 ppmv, more than double the pre-industrial value of about 0.8ppmv. Over the period 1765 to 1998, CH4 has contributed a forcing value of +0.48Wm-2. In order to stabilise concentrations of CH4 at present day levels, an immediate reduction in global emissions by 15-20% would be needed.
Nitrous Oxide (N2O)
Nitrous oxide is naturally produced by oceans and rainforests. Man-made sources of N2O include nylon and nitric acid production, agricultural practices, cars with catalytic converters and biomass burning. The major sinks for N2O are photolytic reactions (i.e. in the presence of light) in the atmosphere. The global atmospheric concentration of N2O in 1998 was measured at about 314ppbv (parts per billion by volume), about 16% greater than the pre-industrial level. N2O has been responsible for a forcing of about +0.15Wm-2 since 1765. In order to stabilise concentrations at present day levels an immediate reduction of 70-80% of N2O from man-made sources would be necessary.
Chlorofluorocarbons (CFCs) are a group of man-made compounds containing chlorine, fluorine and carbon. The production of CFCs began in the 1930s for the purpose of refrigeration; since then they have been extensively utilised as propellants in aerosols, as blowing agents in foam manufacture, in air conditioning units and in various other applications. There are no sinks for CFCs in the lower atmosphere. As a result they are transported to the stratosphere (10 to 50km altitude) where they are broken down by UV radiation, releasing free chlorine atoms which cause significant ozone depletion. In 1998 global atmospheric concentrations of on of the CFCs, CFC-11 was 268pptv. Over the past few decades CFCs 11,12 and 113 have increased more rapidly (on a percent basis) than any other greenhouse gas, but there is now clear evidence that growth rates of CFCs have slowed significantly in the aftermath of the Montreal Protocol (1985) to prevent ozone depletion. In fact, the 1998 atmospheric concentration of CFC-11 was lower than the concentration 5 years earlier. The total forcing value for Chlorofluorocarbons is +0.3Wm-2. This includes CFC-11, 12, 113, 114, 115, methylchloroform and carbon tetrachloride. Under the Montreal Protocol, the production of CFCs 11, 12 and 113 has been successfully phased out since 1995. However, despite these measures, the concentration of CFCs in the atmosphere will remain significant into the next century because of the relatively long lifetimes associated with these compounds.
The total increase in the enhanced greenhouse effect since the Industrial Revolution is about +2.45Wm-2. Clearly, CO2 has contributed most to the enhancement of the Earth's natural greenhouse effect. More recently, the CFCs have contributed a greater proportion to this, although their effect is expected to decline in the aftermath of the Montreal Protocol to limit CFC emissions for the protection of the ozone layer. This fact sheet has not considered the man-made sulphate aerosols emitted during the burning of fossil fuels. These are believed to act as global cooling agents, and recent climate models have incorporated their effects into the simulation of future climate. Consequently, estimates of future global warming have been revised downwards from those calculated solely from the effects of increased greenhouse gases.
Radiative forcing of greenhouse gases