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5.1.1.b
Nitrous oxide N2O | Methane (CH4) | Greenhouse Gases | Ozone??
Trends in N2O
Nitrous oxide (also known as ‘laughing gas’) is formed by many microbial reactions in soils and waters, including those processes acting on the increasing amounts of nitrogen-containing fertilizers. It is also released by burning wood and by some synthetic chemical processes. The industrial sources of N2O include nylon production, nitric acid production, fossil fuel fired power plants, and vehicular emissions.
Change in N2O abundance for the last 1,000 years as determined from ice cores, firn, and whole air samples. Radiative forcing, approximated by a linear scale, is plotted on the right axis. Deseasonalised global averages are plotted in the inset. (Source: IPCC 2001 WG1 p. 253.)
ACTIVITY 1
1. How would you describe the shape of the curve in the main graph? What type of relationship is this?
2. Look at the part of the curve in the main graph that remains at a fairly constant concentration. How can you identify this part of the curve? Estimate the average nitrous oxide concentration for this section.
3. When does the curve start to increase sharply? Account for this sudden increase.
4. What is the current nitrous oxide concentration?
5. Look at the graph in the inset. Describe the shape of this curve.
6. What kind of relationship is represented by this graph. What statement can you make about the global nitrous oxide concentration from 1978 to 2000?
7. Radiative forcing is the heat energy re-emitted back to the Earth’s surface by a greenhouse gas, in Watts per square metre. What is the relationship between N2O concentration and radiative forcing from the graph? Hint: Look at the title for the graph.
Trends in Methane (CH4)
Methane (CH4) is a hydrocarbon and a principal component of natural gas. Methane is also a “greenhouse gas,” meaning that its presence in the atmosphere affects the earth’s temperature and climate system. Methane’s radiative activity refers to its ability to trap infrared radiation (IR), or heat, enhancing the greenhouse effect.
About two thirds of the current emissions of methane into the atmosphere result from cattle farming, rice paddies, landfills, coal mining, oil and gas production, and several other human activities. The other third comes from natural sources, particularly wetlands and termites. The total greenhouse effect from methane has increased by about 0.5 watts (0.3%) the energy striking each square meter of the earth’s surface.
Its atmospheric concentration has been measured globally and continuously for only 2 decades, and the majority of the methane molecules are of recent biologic origin.
As you can see in the graph showing the data from 1840-1994, the concentrations of methane increased rather smoothly from 1520 ppbv in 1978 by about 1% per year until about 1990. The rate of increase slowed down to less than that rate during the 1990s and current values are around 1770 ppbv.
Change in CH4 abundance (mole fraction, in ppb = 10-9) determined from ice cores, firn, and whole air samples plotted for the last 1,000 years. (The firn is the upper, porous, layer of incompletely compacted snow making up the top 50 – 115 m of polar ice sheets. (source: IPCC 2001 WG1 p.249)
Methane was trapped long ago in air bubbles preserved in Greenland and Antarctic ice sheets. These ice sheets the remains of the series of ice ages that Earth experienced over the past 400,000 years. Ice cores are used to determine methane concentrations in the atmosphere in the times before direct measurements were possible.
The methane concentrations have varied during this 400,000 year period between 300 ppbv in the coldest times of the ice ages and 700 ppbv in the warmest times. Methane is more abundant in Earth’s atmosphere now than at any time during the past 400,000 years.
The scale on the right of the graph represents Radiative Forcing. This is the amount of the sun’s energy that is reflected back to the Earth’s surface as heat related to a particular concentration of a greenhouse gas. It is measured in Watts per square metre or Wm-2.
ACTIVITY 2
How long ago did the methane concentration begin to rise steadily?
How much greater is the radiative forcing of current levels of methane than the pre-industrial levels?
Greenhouse Gases
Atmospheric gases that absorb and re-emit infrared (heat) energy are the ‘greenhouse gases’. Most common are carbon dioxide (CO2) and water vapour (H2O). Other greenhouse gases found in lesser amounts are methane (CH4), nitrous oxide (N2O), and ozone (O3). (see Trends in Carbon Dioxide, Trends in Nitrous Oxide, Ozone Depletion, Ozone and Health).
Moreover, there are a number of entirely human-made greenhouse gases in the atmosphere, such as the halocarbons and other chlorine and bromine containing substances, dealt with under the Montreal Protocol.
So-called ‘new’ greenhouse gases, Hydrofluorocarbons, (HFCs), Perfluorocarbons (PFCs), and sulphur hexafluoride (SF6), (See CFCs, Hydrofluorocarbons and other Fluorinated Compounds) are being considered by policymakers with more attention, since they are part of the greenhouse gas commitments of the Kyoto Protocol to the United Nations’ Framework Convention on Climate Change (UNFCCC).
There are some natural processes that produce greenhouse gases such as transpiration, and forest fires started by lighting. Most of the processes are a result of human activities and many of these began with the Industrial Revolution of the 1800’s.
The following diagram shows the major sources of the greenhouse gases involved in commitments in the Kyoto Protocol:
Comparing Ozone in the Atmosphere
These total ozone maps are based on ground-based measurements available from the World Ozone and Ultraviolet Radiation Data Centre. Total ozone values are given in Dobson Units. The numbers represent observations taken from ground stations situated at the bottom left corner of the number.
ACTIVITY 3
1. How many years have elapsed between the data represented on the two maps?
2. What is the highest value for total ozone in 1980? In 2003?
3. In general, where is ozone highest during March in both maps?
4. What has happened to the ozone levels from 1980 to 2003? Where has the decrease been the greatest?
5. Access the following website: http://exp-studies.tor.ec.gc.ca/cgi-bin/selectMap You can request maps for specific years, time intervals and hemispheres. You will now be investigating changes in ozone levels or ‘deviations’, instead of total ozone as in the above maps. You can request two maps at a time.
6. Make the following comparisons in the table and describe trends in ozone concentration.
Ozone Deviations
CHART HERE –Three columns, 6 rows