GREENHOUSE GAS (GHG) (Encyclopedia)

Greenhouse gases are components of the atmosphere. They can be both anthropogenic and natural gases. The principal greenhouse gases (GHGs) are carbon dioxide (CO2), water vapour (H2O), nitrous oxide (N2O), methane (CH4) and ozone (O3). The Kyoto Protocol also includes sulphur hexafluoride (SF6), hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs). 
GHGs have the peculiarity to absorb and release the infrared radiation emitted by clouds, the atmosphere, and the Earth surface. This process impacts on the radiative balance and results in the greenhouse gas effect, which warms the Earth’s surface. A great concentration of these gases in the atmosphere impedes the natural process of absorption and release of infrared radiation.
While is it clear that emissions of greenhouse gases have a warming effect on climate, the final net warming effect is not likewise clear as it also depends on the cooling impact of sulphate aerosol emissions which have moderated the increase in temperatures we are currently experiencing. Indeed, a prediction of the overall global warming effect is not as simple as it can be imagined, as other factors interfere and interact.
After water vapour, which contributes 36%-72% to the GHG effect, CO2 is the main anthropogenic greenhouse gas (it contributes 9-26%). Water vapour concentrations are not significantly affected by human activity (with the exception of local scales), while CO2 concentrations are. Moreover, by causing  the main GHG effect, water power and CO2 generate primary positive feedbacks inducing further warming of the earth’s surface.
The climate-forcing strength of different greenhouse gases is based on the gas Global Warming Potential (GWP), which is described relatively to that of carbon dioxide (used as numerator: it is defined to have a GWP of 1 over all periods). This measure, the carbon dioxide equivalent, allows us to compare emissions from various greenhouse gases and it can be derived by multiplying the quantity of tons of a GHG and its associated GWP. For instance, Table 2.14 in the Fourth IPCC Assessment Report states that Methane has a GWP of 72 over 20 years, 25 over 100 years and 7.6 over 500 years. This shows a higher warming potential of methane as compared to carbon dioxide.
Both natural and anthropogenic activities may change the GHG concentrations in the atmosphere. Natural sources of carbon dioxide are far greater than sources coming from human activity but, in the long run, the former are almost balanced by the carbon uptake of natural sinks such as vegetation growth and photosynthesis process, oceans, etc. (IPCC AR4 WGI, 2007). 
During the pre-industrial era, the concentrations of existing gases were approximately constant. Human activities have subsequently contributed to the "changes in atmospheric concentrations of greenhouse gases and aerosols, land cover and solar radiation" which "alter the energy balance of the climate system", (IPCC AR4, 2007).
Furthermore, the concentration recently increases at a higher rate as compared with the past: in the 1960s, the average annual increase was only 37% of what it was in 2000 (IPCC, 2001).
Human activity has a relevant impact on the levels of nitrous oxide, methane and carbon released. Nitrous oxide emissions are mainly associated with agriculture, the rice cultivation being the primary source; while methane emissions refer to the human activity associated with ruminant livestock (Raupach et al., 2007). Man-made CO2 emissions mostly derive from the burning of fossil fuels, such as gasoline, fuel oil, natural gas, coal, etc. (IPCC AR4 WGII, 2007), the cement and lime production, and the land use management (e.g. deforestation, forest degradation, etc.) 
The major greenhouse gas contributing sectors are industry and transportation, followed by construction, commerce and agriculture; while the main sources of individual greenhouse gas emission are energy waste, electricity consumption, heating and cooling, and transportation.
Raises in anthropogenic gas concentrations seem "to have caused most of the increases in global average temperatures since the mid-20th century" (IPCC AR4, 2007) and warming is in turn expected to affect freshwater resources, food availability and health, and the economic situation among other phenomena (Steinfeld et al., 2006).
Given that concentrations of these gases are strongly correlated with temperature, the United Nations Framework Convention on Climate Change (UNFCCC), opened for signature in the Rio de Janeiro Summit in 1992 and entered into force in 1994, assumed the objective of stabilizing the GHG concentrations in the atmosphere at a level that "would prevent and reduce dangerous human-induced interference with the climate system". 
Today, the most
significant international policy step to control emissions of GHGs is represented by the Kyoto Protocol, entered into force in 2005. It fixes both a general reduction target compared to 1990 values (1990 is the baseline for all emissions reduction targets in the Protocol, with few exceptions), and country-specific emission reduction targets, to be reached in the commitment period 2008-2012. According to the Kyoto Protocol, "Annex B Parties" can reduce emissions either by (i) limiting fossil fuel consumption or (ii) increasing the net carbon sequestration in terrestrial carbon sinks (IGBP, 1998). With the aim of monitoring and estimating the current and future levels of greenhouse gas (GHG) emissions and removals, articles 4 and 12 of the UNFCC Convention, in accordance with the Conferences of the Parties (COPs), states that Annex B countries must present the national inventories of anthropogenic greenhouse gases distinguishing between emissions by sources and removals by sinks. The inventories are technically reviewed annually.
H. Steinfeld, P. Gerber, T. Wassenaar, V. Castel, M. Rosales and C. de Haan (2006). Livestock’s Long Shadow: Environmental Issues and Options. LEAD/FAO. 
IGBP Terrestrial Carbon Working Group (1998). The Terrestrial Carbon Cycle: Implications for the Kyoto Protocol. Science, pp. 1393 – 1394
IPCC, 2001: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Edited by J. T. Houghton et al. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA
IPCC, 2007: Climate Change 2007: Synthesis Report. Contribution of Working Group I, II, III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. [Core Writing Team, Pachauri, R.K and Reisinger, A. (eds.)]. IPCC, Geneva, Switzerland, 104 pp.
Raupach MR, Marland G., Ciais P., Le Quéré C. , Canadell J. G., Klepper G., and Field C. B. (2007). Global and regional drivers of accelerating CO2 emissions. Proceedings of the National Academy of Sciences of the United States of America 104 (24): 10288-10293. 
The Kyoto Protocol. Downloadable at:
United Nations Framework Convention on Climate Change (UNFCCC)

Editor: Melania MICHETTI