In this context global warming can be defined as the increase in the average measured temperature of the Earth since the mid-twentieth century.

The primary reason for global warming is release of greenhouse gases. Greenhouse gases are those which allow solar energy to pass through them, but trap the same energy after it has interacted with the Earth's surface. The Earth would be a cold lump without greenhouse gases, but the equilibrium has been disturbed dramatically by humans since the industrial revolution.

It is not important to understand the exact physics of global warming, but it is important to realise which gases are greenhouse gases the effect that they are having on the Earth and the way in which individuals can work towards slowing this down.

The greenhouse gases in decreasing order of significance are: water vapour, carbon dioxide, methane, nitrous oxide and ozone.

The global concentration of water vapour is not directly affected by human activity, although because warm air can hold more of it, it does result in a positive feedback loop

"A positive feedback loop is, in the case of global warming, a distinctly negative effect. Simply put, it describes a mechanism which magnifies a change in a particular direction; in this case, a rise in global temperature. There are numerous examples of this including, the aforementioned water vapour issue, the albino effect of melting ice caps reflecting less solar energy, and the melting of permafrost across Siberia releasing methane."

The second most important greenhouse gas is carbon dioxide, and the reason behind the term carbon footprint. The major human impact on atmospheric carbon dioxide is through the burning of fossil fuels such as natural gas, coal and oil. This is something that humanity needs to decrease, and decrease rapidly.

The problem is that it is a global problem that can only be dealt with through a coordinated response. In 1997 the Kyoto Treaty represented the first stage towards that, but has faltered due to opposing national interests. Within the UK, the government has been quite slow to realise not only the dangers of allowing global warming to escalate out of control, but also how strongly green issues are affecting the nations electoral decisions. A cynic might argue that the latter realisation has been the most influential. Other recent significant events helping to shape world/UK policy are: The Stern Report, Hurricane Katrina (extreme weather cannot be attributed directly to global warming, but increased average sea temperatures give extra energy to hurricanes, and global warming can result in greater extremes of weather) and Al Gore's An Inconvenient Truth.

The most important publications on global warming are found within peer reviewed scientific journals such as Science and Nature. These can often be quite hard to read, but more accessible and still accurate reports can also be found within The New Scientist and Scientific America. Within these publications, and therefore the scientific community there is no doubt that global warming exists, or that humanity is increasing the average measured temperature. The only controversy is how serious the damage to the earth is, what the long term consequences will be, and how much do we need to reduce emissions to prevent the worst of the suffering.

The last major global decision making event was Copenhagen. Despite new administration in the USA and a strong European treaty already in place there was no legall binding decision.  Until then, there is still responsibility of everyone to decrease their carbon footprint wherever possible.  Further information on carbon footprints can be found here.