The Greenhouse Effect and Global Warming

Extensive measurements have shown that the concentrations of carbon dioxide, methane and some other gases in the atmosphere are steadily increasing. In the 1780s the level of atmospheric carbon dioxide was about 280 parts per million, as it had been for the last six thousand years. Industrialisation increased the level to 315 ppm by the 1930s, 330 ppm by the mid-1970s and 360 ppm by the mid-1990s. In the last ten years the level has risen by a further 20 ppm. By the middle of the present century it could rise to 500 ppm. The annual increase of carbon dioxide is now 0.4%, that of methane 1.2%, of nitrous oxide 0.3%, of the chlorofluorocarbons 6% and of ozone about 0.25%. In the European Union, fossil fuels are the main source: oil 50%, natural gas 20% and coal 28%. Of this, electricity generation accounts for 37%, transport 28%, industry 16%, households 14% and the service sector for 5%. These are established facts, and in addition there is a strong correlation between carbon dioxide concentrations and temperature changes, as shown in Figure 7.2. It is then suggested that these increased concentrations are responsible for global warming and that global warming is responsible for other climate changes and predicted effects such as a worldwide rise in the sea level. The evidence for anthropogenic climate change has increased in recent years and its reality is now generally accepted.

The mechanism of global warming is called the greenhouse effect, though strictly speaking this is a misnomer. The sun's rays pass through the atmosphere and warm the earth. This energy is re-emitted at a longer wavelength that is scattered by the 'greenhouse' molecules in the atmosphere and some of this radiation is scattered back and warms the earth so that its average temperature is around 14°C, whereas without the greenhouse effect it would be around -18°C. If the concentration of the greenhouse gases increases, the earth becomes hotter. The 'greenhouse' molecules are primarily carbon dioxide, methane and the chlorofluorocarbons. In a greenhouse the glass transmits the incoming solar radiation and it warms the earth. The earth radiates energy but the radiation has a wavelength that cannot escape, and so the incoming heat is trapped, thus warming the greenhouse.

o 320

1 300

1000 1200 1400 1600 1800 2000 Year

1000 1200 1400 1600 1800 2000 Year

Figure 7.2. The Average Atmospheric Carbon Dioxide Concentration over the Last Millenium Compared with the Average Temperature Changes in the Northern Hemisphere (Nuclear Issues 23, May 2001).

The greenhouse gases differ greatly in their contribution to the greenhouse effect by their concentration, their scattering efficiency and the time they remain in the atmosphere. The concentration of carbon dioxide is relatively high and it remains for a long time and so it is the most important contributor to global warming. Methane has about sixty times the scattering power but has low concentration and a relatively short life of about twelve years, so it is not so important. This could change if the emission of methane increases due to global warming. The chlorofluorocarbons are extremely potent greenhouse gases and so their emission is now prohibited by international agreement. They are all due to anthropogenic causes and so all machines, such as refrigerators, that emit this gas have been re-designed to use other gases, thus eliminating this source of global warming.

The effect of methane on global warming will become more important due to melting of the permafrost in northern countries such as Alaska and

1000 1200 1400 1600 1800 2000 Year

1000 1200 1400 1600 1800 2000 Year

Figure 7.2. The Average Atmospheric Carbon Dioxide Concentration over the Last Millenium Compared with the Average Temperature Changes in the Northern Hemisphere (Nuclear Issues 23, May 2001).

Siberia. Normally the ground is frozen to a depth of many metres, but an increase in the summer temperature causes the top layer to melt and emit methane as well as carbon dioxide. These gases enhance global warming, leading to further melting and emissions, in a positive feedback process. The melting of the permafrost will also cause mud slides and avalanches that could threaten houses, roads and pipelines (Maslin 2004).

An additional and potentially even more serious danger is the breakdown of gas hydrates. At very low temperatures and high pressures water molecules can form a solid consisting of cages holding molecules of methane and other gases. These are gas hydrates, and are found in deep oceans and beneath the permafrost. It is estimated that there are more than 10,000 gigatonnes of gas hydrates whereas there are just 180 giga-tonnes of carbon dioxide in the atmosphere. If only some of this were released due to global warming it would add to the methane and carbon dioxide in the atmosphere. This would intensify the global warming, causing further releases and so on in another positive feedback effect. There is some evidence that such a runaway greenhouse effect took place 55 million years ago. About 1200 tonnes of gas hydrates were released, and this raised the earth's temperature by 5°C (Maslin 2004).

Another source of global warming is the variation in the brightness of the sun, already observed over several centuries. The sunlight intensity fell by 4 to 6% from the 1950s to the 1980s. Now it is rising again and there has been a 4% rise since 1990; it is estimated that this could account for a rise in temperature of 0.4°C by 2100. There are also daily variations of around 0.2%, and these could produce significant changes in the climate. In addition, there is a correlation between the temperature and the length of the sunspot cycle. The physical basis for this is suggested by another correlation, namely that between the cosmic ray intensity and the low cloud cover. The sunspot cycle is a measure of the solar activity, and this in turn affects the cosmic ray intensity. The cosmic rays produce ions in the atmosphere, and these can form condensation nuclei for clouds, which have a strong influence on the earth's temperature. Detailed studies suggest that solar effects may be responsible for 30 to 57% of the observed global warming. Since this varies with the sunspot cycle, it sometimes reinforces and sometimes weakens the effects of global warming. Some fluctuations have been observed and these could be due to varying amounts of aerosols in the atmosphere (Lomborg, op. cit. pp. 276-278). Recent work by Sloan and Wolfendale has however shown that there is no correlation between the cosmic ray intensity and cloud formation.

Was this article helpful?

0 0
Guide to Alternative Fuels

Guide to Alternative Fuels

Your Alternative Fuel Solution for Saving Money, Reducing Oil Dependency, and Helping the Planet. Ethanol is an alternative to gasoline. The use of ethanol has been demonstrated to reduce greenhouse emissions slightly as compared to gasoline. Through this ebook, you are going to learn what you will need to know why choosing an alternative fuel may benefit you and your future.

Get My Free Ebook


Post a comment