Why

However, by burning fossil fuels humans pump billions of tonnes of carbon dioxide (CO2 - the most important greenhouse gas emitted by human activities) and other greenhouse gases into the atmosphere.

These gases create a 'greenhouse effect', thickening the natural canopy of gases in the atmosphere and causing more heat to become trapped. As a result, the global temperature is increasing, throwing the world's climate out of its natural balance and into chaos.

The main source of these human-produced greenhouse gases is burning large amounts of fossil fuels for energy production and transport. Changes in land use and deforestation also release more CO2 into the environment. Trees, for example, are natural 'carbon sinks'-they absorb CO2-and when they are destroyed, CO2 is released into the atmosphere.

While many greenhouse gases occur naturally, the rate humans are adding them to the atmosphere is far from natural. It is estimated that concentrations of CO2 are 30 percent higher than before the industrial revolution, when the wide scale burning of fossil fuels started. Humans are also creating new greenhouse gases such as hydrofluorocarbons (HFCs) from industrial activities.

Even if all greenhouse gas emissions were stopped today, the effects from past activities will persist for many centuries, due to the long life of greenhouse gases in the atmosphere and the long time required for transfer of heat from the atmosphere to the deep oceans.