Introduction

In 1776 the Italian physicist Alessandro Volta noticed bubbles rising from the bottom of a pond. He collected some of these bubbles and found that the collected air was inflammable. Volta had discovered the powerful greenhouse gas (GHG) methane (CH4). Today CH4 is used throughout the world both as an industrial and a domestic fuel source, but in the past few decades there has been increasing concern about the effect CH4 may be having on our climate.

As with the other two main anthropogenic GHGs, carbon dioxide (CO2) and nitrous oxide (N2O), the concentration of CH4 in the atmosphere has been increasing rapidly in the last century or so. We now have records of CH4 concentrations in our atmosphere going back more than 650,000 years. These show that, though we have had many peaks and troughs in that time, levels of CH4 have never been as high as they are today (Cicerone and Oremland, 1988). Peak concentrations were previously ~700 parts per billion (ppb), but since the beginning of the industrial era levels have more than doubled to their current high of about 1800 ppb. Although such concentrations are much lower than those of CO2 (currently about 380 ppm), CH4 molecules are much more effective at trapping the infrared radia tion (heat) reflected from the earth's surface. Indeed, the global warming potential (GWP) of CH4 on a mass basis is 23 times that of CO2 over a 100-year time horizon. CH4 concentrations are rising at around 0.6%/ year. Consequently, CH4 is now a key target in many GHG reduction strategies (Hogan et al., 1991).

The GWP compares the potential warming effects of different trace gases relative to that of CO2. It is also adjusted for any production of secondary GHGs formed when the primary gas is destroyed. The GWP combines the capacity of a gas to absorb infrared radiation, its lifetime in the atmosphere and the length of time over which its effects on the earth's climate need to be quantified (the time horizon). As it has an atmospheric lifetime of only about 10 years, CH4 is 62 times more effective than CO2 on a 20-year time horizon and drops to being 23 times more effective on a 100-year time horizon.

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