Greenhouse gases By S.M. Enzler MSc
Atmospheric concentrations of greenhouse gases that absorb infrared light lead to additional global warming, causing climate change (figure 1). Past and future emissions of the greenhouse gases carbon dioxide, methane and nitrous oxide and the impact on climate are described on this page.
Figure 1: integrated framework of climate change by IPCC
The impact of greenhouse gases on the enhanced greenhouse effect is determined by their residence time in the atmosphere. When atmospheric residence time is greater, the total impact of a greenhouse gas on global warming is larger. Atmospheric residence time represents the average amount of time the molecules of a greenhouse gas exist in air before it is somehow removed. The average residence time of the greenhouse gases carbon dioxide and nitrous oxide is more than a century. Consequentially, these greenhouse gases will impact global warming long after emission cut-backs are achieved. Contrastingly, methane only has a residence time of one decade.
Carbon dioxide production is not the same for every country. Still, we all need to reduce our greenhouse gas emissions because CO2 is a transboundary air pollutant. This basically means that pollution is not limited to the country where it is emitted. This is the same for all greenhouse gases and air pollutants.
Figure 2 shows the atmospheric CO2 concentration in the past century. CO2 is the greenhouse gas with the highest anthropogenic emission. The main cause of anthropogenic CO2 emissions to the atmosphere is fossil fuel combustion. Fossil fuels are oil, coal and natural gas. These are exploited by humans to gain energy.
It is clear that a sustainable society will obtain more energy from renewable energy sources than a society based on increasing wealth. This results in lower eventual carbon dioxide emissions in both sustainable IPCC scenarios (figure 3). The global scenario scores better for carbon dioxide emissions than does the regional approach.
Figure 3: carbon dioxide emissions according to the SRES
CO2 is the main contributor to anthropogenic radiative forcing because of changes in concentrations from pre-industrial times. Whereas CO2 emissions are by-and-large attributable to two major sources; energy consumption and land-use change, other emissions arise from many different sources and a large number of sectors and applications. Projections of emissions of CO2 in 2100 range from more than 40 to less than 6 billion tons of elemental carbon, meaning a range from almost a sevenfold increase to roughly the same emissions level as in 1990.
There are six different sources of atmospheric methane. In order of importance these are wetlands, fossil fuels, landfills, ruminant animals, rice paddies and biomass combustion. Methane has a larger global warming potential than carbon dioxide. Still, emissions are lower than those of carbon dioxide. It is estimated that methane produces about one-third the amount of global warming of carbon dioxide.
After the industrial revolution there was an increase in global atmospheric methane concentrations. In the past 200 years methane concentrations have increased from approximately 620 to approximately 1700 ppb (figure 4). The radiative forcing of methane has also increased.
Methane (CH4) emissions are expected to increase most in the two regional scenarios. Increases are most pronounced in the regional wealth scenario where emissions may rise to between 549 and 1069 Mt CH4 by 2100, compared to 310 Mt CH4 in 1990. In the globalized scenarios, the CH4 emissions level off and subsequently decline sooner or later in the 21st century.
The National Oceanic and Atmospheric Climate Administration (NOAA) reports that the atmospheric build-up of methane has slowed greatly (figure 5). It is claimed that if current trends continue, it may reach zero within a few decades (Dlugokencky et al., 1998). This finding was however not reported on major news broadcasts or websites, for some reason.
Figure 5: contrast between findings of the IPCC and NOAA
The author of the IPCC chapter dealing with future methane concentrations has admitted that the assumptions about methane of the IPCC were based on an understanding of methane that was five to 15 years old. This may explain the differences between IPCC and NOAA scenarios.
The greater part of the natural supply of nitrous oxide gas is released by oceans. Processes occurring in soils cause most of the remainder. The gas is a by-product of the biological denitrification process in anaerobic environments and of the biological nitrification process in aerobic environments. About a third of current N2O emissions are anthropogenic. These anthropogenic emissions stem from agricultural soils, cattle feed lots and the chemical industry.
The atmospheric concentration of nitrous oxide (N2O) has increased by 46 ppb (17%) since 1750 and continues to increase (figure 6). The present N2O concentration has not been exceeded during at least the past thousand years.
Future nitrous oxide (N2O) emissions are mainly determined by food supply, because these emissions largely stem from soil processes induced by agriculture. Like CH4, N2O emissions are generally highest in the regional wealth scenario (figure 7).
N2O emissions are generally affected by large uncertainties, because these are mainly caused by bacterial soil processes and are therefore difficult to measure. Extensive research into N2O sources is still required.
Other greenhouse gases
CFCs (chlorofuorocarbons) are gaseous compounds that contains molecules with carbon atoms bonded exclusively to fluorine or chlorine. CFCs are applied for example as refrigerants. These compounds have perhaps the greatest global warming potential among trace gases to induce global warming, because they are very persistent. CFCs absorb infrared radiation in the 8-13 µm wavelength. Each CFC-molecule has the potential to cause the amount of global warming normally caused by tens of thousands of CO2 molecules. The chlorofluorocarbons CFCl3 and CF2Cl2 have been released into the atmosphere in large quantities in the past and have long residence times. Application of CFCs is now prohibited in most countries. Many countries have agreed to reduce emissions under the 1987 Montreal protocol on substances that deplete the ozone layer.
The combined effect of methane, nitrous oxide, ozone and CFCs is now almost as large as that of carbon dioxide. Concentrations of these greenhouse gases are usually summarized as an Effective Carbon Dioxide Concentration.
Dlugokencky, E.J., et al., 1998, Continuing decline in the growth rate of the atmospheric methane burden. Nature, 393, 447–450
IPCC climate change info (http://www.ipcc.ch/present/graphics.htm)
Sorrell, S., Emissions Trading After Kyoto. Introduction to Environmental Economics of Science and Technology Policy Research, 2004 (http://www.sussex.ac.uk/Users/prpp4/lec8.ppt)
World Climate Report (http://www.co2andclimate.org/climate/previous_issues/vol3/v3n20/feature1.htm)