by Imogen French

Created on: February 25, 2009   Last Updated: March 11, 2009

The greenhouse effect and greenhouse gases are terms that are frequently used in the media in relation to global warming, but what do they really mean, how does it all work, and what can we do about it?

The "greenhouse effect" is the term coined by scientists for the effect of gases in the Earth's atmosphere that cause the planet to warm up. This warming occurs naturally as solar radiation falling on the Earth's surface is trapped in the atmosphere by certain gases that absorb or reflect radiation of different wavelengths, so preventing some of the sun's heat from escaping back out into space. These particular gases in the atmosphere are, in effect, acting like the roof of a greenhouse that lets the light through while keeping the heat in, and so an increase in the proportion of these gases in the atmosphere will contribute to global warming.

The "greenhouse gases" that have this effect are, most notably, carbon dioxide (CO2), methane (CH4), water vapour (H2O), nitrous oxide (NO), ozone (O3) and chlorofluorocarbons. The majority of these gases occur naturally in our atmosphere - people and animals breathe out carbon dioxide, methane is produced by the decay of organic matter, and water vapour comes and goes in the form of clouds as part of the natural water cycle. Without greenhouse gases our world would be much colder, and difficult to survive in. When the system is in balance the overall temperature of the Earth remains, on average, at a constant - in a steady state. The global mean surface temperature (GMST) fluctuates naturally, not only with the seasons but also with much longer term solar cycles that operate over tens of thousands of years causing ice-ages and milder inter-glacial periods. So, how do we know that the current global warming is due to greenhouse gases and not part of some larger natural cycle? Well, these natural cycles are certainly a partial cause of the current situation, but the rate of the rise in GMST is much higher than predicted, and has been shown to directly correlate with the increase in levels of CO2 in the atmosphere.

Human-related CO2 is released into the atmosphere primarily by the burning of fossil fuels, use of which has soared since the industrial revolution at the end of the 19th century, to power our industry, our homes and our transport system. A continuous record of atmospheric CO2 has been kept since 1958 at the Mauna Loa observatory in Hawaii, and records show that levels have increased from 316 parts per million in

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