The Greenhhouse Effect

Many people regard global warming or the greenhouse effect as the most serious environmental threat to our present way of life on earth. Put simply, the greenhouse effect is the natural "trapping in" of heat by some of the gases in the atmosphere. Figure 1 shows the key points of the greenhouse effect.

 
EXAM HINT - Candidates often confuse the greenhouse effect with ozone depletion. Although they have some common causes, their mechanisms and effects are very different.
 

Some of the greenhouse gases, for example, carbon dioxide, occur naturally in the atmosphere, whilst others such as the chlorofluorocarbons (CFCs) are entirely man-made. Regardless of their origin, such gases warm the lower atmosphere by trapping outgoing longwave radiation in a manner similar to that of glass in a greenhouse, hence the term greenhouse effect. It is important to realise that the greenhouse effect is a natural, essential process. Without it, the average temperature on earth would be about -170C and life would be impossible.

However, over the last hundred years human activities have resulted in rising concentrations of all the greenhouse gases. This has led to an increased or "enhanced" greenhouse effect and this in turn seems to have led to an increase in average global temperature.

It is the possible implications of this extra heating effect that are causing so much concern. We simply do not know how much the temperature will continue to rise and what effect any changes will have on local, regional and world climate. Which areas will benefit and which will suffer, and how, is still largely unknown. The temperature will not rise everywhere equally; in fact some areas will probably become cooler (because of this, most of the scientific literature now uses the term "global climate change" rather than global warming). The effects of possible changes will be discussed shortly, but first we need to look at the greenhouse gases in a little more detail.

Why is carbon dioxide so important?

In discussions of the greenhouse effect, attention is almost invariably focused on rising CO2 levels. This is not because it is an unusually powerful greenhouse gas - indeed, gases such as methane are thirty times more powerful weight for weight - but because of its sheer abundance. As a consequence, its effects outweigh those of all the other greenhouse gases combined. Historically, high carbon dioxide concentrations have always coincided with inter-glacial periods and low concentrations with ice ages.



What Can Be Done?

Strategies to reduce the emissions or levels of the greenhouse gases have mainly targeted carbon dioxide. These include:

1. Reducing the consumption of fossil fuels by increasing the fuel efficiency of buildings and vehicles. Improvements to the latter would also reduce N2O  emissions.

2. Switching fuel from coal to oil and gas which release less carbon dioxide upon consumption.

3. Switching from fossil fuels to renewable energy sources such as solar, geothermal, wind, wave, tidal and hydro-electric.

4. Preventing destruction of the tropical rainforests, simultaneously removing a problem and providing a solution.

However, as mentioned earlier, because the other greenhouse gases absorb different infrared wavelengths to carbon dioxide, reductions in this gas alone are unlikely to be sufficient. CFC emissions have already been reduced as a result of the Montreal Protocol in 1989 and subsequent amendments. Alternatives to CFCs are now widely available. Any measures which reduced vehicle use or pollution would simultaneously help to

reduce emissions of carbon dioxide, nitrous oxide and levels of tropospheric ozone. Worldwide however, the trend is in the wrong direction, with one new car joining the roads every second. In Britain fo example, road transport  is responsible for 18% of carbon dioxide emissions,45% of nitrous oxide emissions  and 30% of all hydrocarbon emissions.