The greenhouse effect determines the global temperature. Without greenhouse gases - and therefore without the greenhouse effect - the average worldwide temperature would not be 15°C, but about -18°C. In this case, the Earth would be no less than 33 degrees colder - making it quite hostile to life. Somewhat more than 20°C of the entire greenhouse effect are caused by atmospheric water vapour. The remaining 13 degrees are due to a mix of other greenhouse gases: some of these (such as CO2) occur naturally in the atmosphere, but others (such as CFCs) are purely synthetic (artificial) products.

The countries that have ratified the Kyoto Protocol commit to reduce various gases of this type. The greenhouse effect is partially weakened but also intensified at the same time by aerosols (very small solid and liquid particles in the atmosphere, originating from volcanic eruptions or industry, for example). It is relatively difficult to determine the total contribution of these aerosols to the greenhouse effect; its assessment involves major uncertainties.