Greenhouse Gas – a Definition

A greenhouse gas (sometimes abbreviated GHG) is a gas in the atmosphere that absorbs and emits radiant heat energy. Natural greenhouse gases in the atmosphere are essential for maintaining the earth at a temperature suitable for life on the planet. Averaged over the globe, the surface temperature is +15 °C. Without these naturally occurring greenhouse gases the earth would be a frozen wasteland, with a temperature of -18 °C. Greenhouse gases in the atmosphere are regulated naturally as part of the water, carbon and nitrogen cycles. This natural regulation has however been disrupted by human activities which cause the emission of vast additional quantities of greenhouse gases into the atmosphere. These anthropogenic emissions (i.e., due to human activities) are overwhelming the natural regulation mechanisms.
The primary greenhouse gases in the Earth's atmosphere are carbon dioxide, water vapour, methane and nitrous oxide.

1. Carbon Dioxide
In nature carbon dioxide is produced by plant and animal respiration and by the decay of organic matter. Its concentration in the atmosphere is controlled by absorption in plants in the process of photosynthesis. Since carbon dioxide is soluble in water the oceans play an important role in removing excess concentrations from the atmosphere, and in releasing it back to the atmosphere to maintain a stable concentrations.
Since the beginning of the Industrial Revolution (taken as the year 1750), the burning of fossil fuels has contributed to more than a 40% increase in the atmospheric concentration of carbon dioxide, from 280 to 400 parts-per-million (ppm) in 2016. The current level of carbon dioxide in the atmosphere is the highest in over 800 thousand years. This increase has occurred despite the uptake of a large portion of the emissions by various natural "sinks" involved in the carbon cycle.  
Anthropogenic carbon dioxide (CO2) emissions come from combustion of carbon based fuels, principally coal, oil, natural gas and wood. Additional carbon dioxide produced by human activities will remain in the atmosphere for hundreds to thousands of years.

2. Water Vapour
Water vapour in the atmosphere makes a significant contribution to the natural planetary greenhouse heating effect. There is however a limit to the concentration of water vapour in the atmosphere, as it is a function of temperature. Also, it is a condensing gas, and if too much water vapour is present in the atmosphere it will condense as clouds, and fall as rain or snow. Of its own accord, water vapour cannot initiate atmospheric warming – a non-condensing greenhouse gas must also be present. Carbon dioxide does not condense, and when present in the atmosphere produces a steady warming effect which produces water vapour, which then greatly assists the natural warming. As more water vapour enters the atmosphere clouds form, which reflect sunlight, ensuring that the warming effect is controlled. Water in the atmosphere eventually falls as rain or snow as part of the hydrological cycle. In the absence of human emissions, were the earth to heat marginally beyond its ideal natural temperature, increased plant growth would absorb carbon dioxide, which reduces the steady heating effect, which in turn reduces water vapour, and the temperature is returned back to the natural set-point. Essentially carbon dioxide is the main control gas for planetary temperature regulation.

3. Methane
Methane is a naturally occurring greenhouse gas, produced in the process of decay of organic matter through the action of microorganisms. It is also produced in the digestive tracts of ruminant species such as cattle and sheep. While it is present in low concentrations in the atmosphere, it has nearly thirty times the warming effect of carbon dioxide per unit mass. It is naturally controlled in the atmosphere by oxidation to carbon dioxide and water, and has an average lifetime of about 12 years.
Human activity has greatly increased the concentration of methane in the atmosphere. This has happened as a result of emissions of natural gas from fossil fuel exploration and processing, and from intensive agriculture, in particular rice production and cattle farming. Since the industrial era the methane concentration in the atmosphere has increased by 150%.
One of the dangers of a warming world is that a runaway methane emission process could occur. As temperature rises, there is a risk that methane which is currently locked away in permafrost regions could be released. In addition methane which is currently stored in the deep oceans in solid form as methane hydrates (clathrates) could potentially be released in large quantities in future centuries. This would lead to rapid further planetary warming, which would provoke further methane releases, and a runaway temperature increase.

4. Nitrous oxide
Nitrous oxide is a naturally occurring greenhouse gas which plays an important part in the biophysical nitrogen cycle. Soil bacteria convert atmospheric nitrogen to nitrates in the soil which are essential for plant growth. Decay of plants and manures in the soil results in generation of nitrous oxide gas which returns to the atmosphere where it has a lifetime of about 120 years. Nitrous oxide is present in very low concentrations in the atmosphere, but has over 267 times the warming effect compared with carbon dioxide per unit mass.
Human activities result in greatly increased emissions of nitrous oxide to the atmosphere. A major source is the production and use of artificial nitrogenous fertilizers. It is also released in all fuel combustion processes where nitrogen in the air and nitrogen in the fuel are oxidised. Its concentration in the atmosphere has increased by 20% compared with the pre-industrial era.