We hear about greenhouse gases and the damage to earth that is a consequence of their rapid increase on a regular basis. But which gases are actually to blame for climate change, and what are they caused by? While nitrogen and oxygen make up the majority of the atmosphere’s volume at around 99% [1], greenhouse gases such as water vapour, carbon dioxide, methane, nitrous oxide and ozone are all found in the atmosphere, and play an important part in regulating the earth’s climate. While the gases have both natural and man made sources, the increase in anthropogenic emissions since pre-industrial times has been rapid enough to change the natural greenhouse effect, which is known now as global warming.
What are the major greenhouse gases?
Water vapour (H20) is the most abundant of the greenhouse gases and comes mostly from natural sources, though human contributions (e.g. irrigation, artificial dams and lakes) also play a small part. Water vapour does not directly cause climate change, but rather responds to and amplifies it, so often receives less attention than other gases such as carbon dioxide. More water can be held in a warmer atmosphere, so as global temperatures rise, concentrations of water vapour are also likely to increase, which is turn leads to a rise in global temperatures.
Carbon dioxide (CO2) is produced naturally as part of the carbon cycle during processes such as the respiration and decompositions of organisms, as well as from volcanic eruptions and forest fires. Carbon dioxide is the second most abundant greenhouse gas after water vapour, and is stored naturally in carbon sinks such as soil, plants, the ocean surface and the deep ocean. However, carbon dioxide atmospheric concentration has risen from 280ppm in pre-industrial times to 397ppm as of July 2013 [2], with estimates that ‘about 2/3rds of anthropogenic CO2 emissions have come from fossil fuel burning and about 1/3rd from land use change’ [3].
CO2 is extremely stable, and so stays in the atmosphere for approximately 100 years, meaning even if CO2 levels were reduced as of 2013, the gas would still play a part in global warming for decades to come. While emissions from developed countries are now fairly stable, there is a great deal of concern over emissions from developing countries, especially China, which are rising at an alarming rate.
Methane (CH4) is produced by both natural sources (e.g. oceans, natural wetlands and hydrates) and from anthropogenic sources (e.g. waste treatment, rice cultivation, ruminant livestock and landfill) [4]. Since pre-industrial times methane atmospheric concentrations has more than doubled to 1774ppb in 2005, though there appears to have been a decrease in growth rate since the early 1990s [5]. Methane is composed of carbon and hydrogen, and has a fairly short lifetime (12 years), although atmospheric concentrations of methane are almost five times that of carbon dioxide.
Nitrous oxide (N2O) can be produced by natural sources such as processes in soils and oceans, and the oxidation of ammonia in the atmosphere, as well as from man made sources such as biomass burning, management of livestock manure, nitrogenous fertiliser use and fossil fuel combustion [6]. Concentrations in the atmosphere have increased since pre-industrial times to 319ppb, roughly 18%, with this increase thought to be primarily due to human activities [7].
As part of the nitrogen cycle nitrogen is a stable molecule, however when in the form of nitrous oxide, it plays a part in the formation of acid rain and photochemical smog, as well as causing the destruction of stratospheric ozone. Nitrous oxide has an average lifetime of 114 years, and has a short term (20 year) global warming potential 289 times that of carbon dioxide. However over a much longer time period (500 years), the global warming potential decreases to 153 times that of carbon dioxide [8].
Finally, ozone (O3) is mostly found in the stratosphere (90%), while the troposphere contains the a smaller amount (10%) [9]. Stratospheric ozone is referred to as the ‘ozone layer’, and is formed by chemical reactions that involve the Sun’s ultraviolet radiation breaking down an oxygen molecule into two separate oxygen atoms (O), which then combine with an oxygen molecule (O2). Ozone depletion in the stratosphere may lead to global warming, due to more ultraviolet radiation entering the Earth’s atmosphere.
References
[1] National Environmental Education Foundation. Earth Gauge – Climate. 2013. http://www.earthgauge.net/climate/climate-q-a/climate-q-a-3
[2] CO2 Now. Earth’s CO2 Homepage. 2013. http://co2now.org/
[3] & [5] Solomon, S., D. Qin, M. Manning, Z.Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds). IPCC 2007a. Climate Change 2007: Mitigation of Climate Change. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge: Cambridge University Press.
[4] & [6] Garnaut, R. 2008. The Garnaut Climate Change Review. Melbourne: Cambridge University Press IPCC.
[7] & [8] Solomon, S., D. Qin, M. Manning, Z.Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds). IPCC 2007a. Climate Change 2007: Mitigation of Climate Change. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge: Cambridge University Press.
[9] NOAA. Science – Ozone Basics. 2008. http://www.ozonelayer.noaa.gov/science/basics.htm
