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Paleo-proxy Data and Trends in other Greenhouse Gases


4.1.c  Paleo-proxy Data and Trends in other Greenhouse Gases

Paleoproxy Data and Trends in Other Greenhouse Gases


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 Climate Change 2001 WG1 p.155)











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.


  1. How long ago did the methane concentration begin to rise steadily?
  2. How much greater is the radiative forcing of current levels of methane than the pre-industrial levels?


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.)



  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.