In this chapter, I have only sought to explain the physical properties of greenhouse gases, and what are the factors that determine their effectiveness as pollutant gases that can cause global warming. I have not attempted to describe the natural or anthropogenic sources of these greenhouse gases, and why their concentrations have increased since the pre-Industrial era; this will be covered by other chapters in this book.

CO2 and CH4 currently contribute ca. 81% of the total radiative forcing of long-lived greenhouse gases (Table 2), but it is too simplistic to say that control of CO2 levels will be the complete solution, as is often implied by politicians and the media. It is certainly true that concentration levels of CO2 in the earth's atmosphere are a very serious cause for concern, and many countries are now putting in place targets and policies to reduce them. It is my personal belief that CO2 levels in the atmosphere correlate strongly with lifestyle of many of the population, and with serious effort, especially in the developed world, huge reductions are possible. The challenge will be to effect policies to reduce significantly the concentration of CO2 without seriously decreasing the standard of living of the population and negating all the benefits that technology has brought us in the last 50 100 a. I give two examples for possible policy change. First, I query whether the huge expansion in air travel within any one country at the expense of slower methods of transports (e.g., trains) is really worth all the social and economic benefits that are claimed. The price to be paid, of course, is hugely enhanced CO2 emissions. Second, I query whether the benefits of 24 h shopping 7 days a week are really worth the extra CO2 emissions that result from keeping shops open continuously. Would our standard of living drop significantly if shops opened for much fewer hours? Most of Switzerland closes at 4.00 p.m. on a Saturday for the rest of the weekend, yet this country is very close to the top of all international league tables for wealth creation, standard of living and levels of well-being/happiness.

CH4 levels, however, in my opinion pose just as serious a threat to our planet as CO2 simply because they will be much harder to reduce. Whilst it is surprising and remains unclear why the total radiative forcing of methane, 0.48 W m 2, has remained unchanged over the last decade [2], a major component of methane emissions correlates strongly with the number of animal livestock which itself is dependent on the population of the planet. Controlling, let alone reducing world-wide population levels over the short period of time that is apparently available to 'save the planet' (ca. 20 40 a) [19] is a major task. Surely, this could and should be the major policy directive of the United Nations over the next few decades.

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