Like a glasshouse around the globe
Given the distance between the Earth and the Sun, our planet should be icy cold (-18°C), but the average temperature at the Earth’s surface is actually +15°C. Why? Because the atmosphere around the Earth acts like a greenhouse, letting the Sun’s rays in to warm us up and preventing the heat from escaping again. This phenomenon is called the “greenhouse effect”.
Gases in tiny amounts but playing a vital role
The water vapour present in the air accounts for two thirds of the greenhouse effect. But there are gases, sometimes present in very small amounts, that also contribute to it. The main greenhouse gases, or GHG, are: carbon dioxide (CO2), methane, nitrogen protoxide, ozone (a sort of super-oxygen found in the lower atmosphere) and compounds called CFCs.
An age-old heating system
Right from the time our solar system was formed, 4.6 billion years ago, the greenhouse effect has regulated the Earth’s climate. But the atmosphere has evolved quite a lot since its original and primitive composition. One marked change was that photosynthesis, starting with the earliest plants, gradually lowered the proportion of CO2 in the atmosphere, helping to maintain a life-friendly temperature on Earth.
A phenomenon amplified by human activity
In its original state, the greenhouse effect was a completely natural phenomenon and invaluable to the Earth’s equilibrium. But the effect can be amplified if the amounts of GHG present are allowed to increase. And this is now happening as a result of the pollution generated by human activities. This is raising concerns about possible climate problems in future.
The main factors behind the greenhouse effect are:
- Water vapour (H20), which is produced by evaporation of the water in plants, soil, watercourses and the ocean. A small amount is also emitted by volcanoes (this is how the planet’s original, primitive atmosphere was formed).
- Carbon dioxide (CO2), which is a natural gas, is generated by: the breathing process of living beings (while the photosynthesis of green plants consumes CO2); combustion (forest fires); and volcanic eruptions. The amount of CO2 in the atmosphere is regulated by the ocean, which can absorb large quantities of the gas.
- Methane (CH4) is also a natural gas. It is given off by the decomposition of organic matter where no oxygen is present (rotting in marshes, mangroves, thawed ground in the Arctic and of course digestion in animal stomachs). It is also given off when volcanoes erupt.
- Ozone (O3), present in the lower atmosphere (troposphere), is a natural gas too. It probably descends from the stratosphere via “pockets” punched by the upper jet stream. Ozone is like oxygen except that is has three oxygen atoms instead of two. The gas is a veritable poison to plants as well as human skin and our respiratory system (asthma attacks). But at altitude, say 20 or 30 km up, ozone is a very useful gas, as it protects us against ultraviolet solar radiation, which is dangerous to most life forms. This is the gas referred to when ecologists talk of the seasonal hole in the ozone layer, first discovered overhead the Antarctic and now reckoned to be diminishing – a phenomenon that worries scientists because of the potential dangers to human beings.
- Nitrogen protoxide is one of the substances produced when nitrates in the soil are broken down by bacteria. It then escapes into the atmosphere and reacts to produce nitrogen oxide NO).
- CFCs or chloro-fluoro-carbons, are produced as a result of human activities. They are mentioned here so as to be compared with other GHGs.
The importance of the various GHGs:
* ppm = parts per million, i.e. 1cm3 per m3 of air
||Greenhouse effect efficiency
||Contribution to the greenhouse effect (W/m2)
|H2O (water vapour)