The Earth is like a greenhouse, which lets in lots of energy from the sun but prevents much of it from leaving again. Rather than glass the planet depends on greenhouse gases such as water vapour, carbon dioxide (CO2), methane and a few others to trap the sun's energy. Historically these gases have trapped just enough heat to make this planet a pleasant and hospitable environment. This delicate balance has recently been disrupted by the release of an ever increasing volume of greenhouse gases, especially CO2, because man has been burning ever greater quantities of coal, oil and gas.
This shift has already resulted in a small increase in the average world temperature, but the fear is that things could change much more dramatically in the future if current trends are not reversed. The exact details are still fairly debateable as even the most comprehensive climate models find it difficult to model all the interactions of Nature, but it is better to be roughly right than precisely wrong, and the general consensus is that temperatures can and will increase significantly.
The best estimates suggest an average increase in world temperatures of between 1.8 and 5.3 degrees Celsius. Such a change would be devastating to already hot areas of the world, but would also feed through into rising sea levels and more variable conditions worldwide, affecting most nations of the globe. It is also possible to construct several disaster scenarios in which a small change in temperature can be amplified by feedback effects, causing irreversible step changes in conditions. Such predictions are at once both necessary and dangerous, reminding us of the worst case scenarios but also promoting an unhealthy fear, disbelief or in some ways even an over-reaction.
Regardless of the exact specification of outcomes it is fair to assume that considerable changes in climate will occur, and potentially devastating ones if nothing is done to combat the trend in rising CO2 emissions.
To deal with the almost inevitable changes which will occur because greenhouse gas emissions cannot be dramatically reduced immediately, it will be necessary to help those who are most affected to deal with the changes. The world's poorest nations will almost certainly be the hardest hit, due to their general geographic location, their reliance on agriculture and their difficulties finding clean and abundant water supplies.
To reduce the likelihood of the severest changes in climate it will, however, be necessary to reduce the volume of greenhouse gas emissions before it is too late. There are many promising ways by which this can be done, including improved efficiency and technology, combined with the tools of taxation and emissions trading, but a political will must make them happen, as idleness will simply make things worse.