Since the end of the 19th century, we have known that the Earth’s climate is sensitive to greenhouse gases in the atmosphere. At that time, the Swedish chemist Svante Arrhenius showed that an increase in CO2 concentrations would lead to a warmer climate. However, Arrhenius harbored little hope that this would happen. Consequently, the Swedes would have to continue to suffer in a cold and miserable climate. Since then, much has changed. Annual CO2 emissions have now reached a level that is about 20 times higher than that of 1896. This has caused concern worldwide.
More CO2 in the atmosphere leads undoubtedly to a warming of the earth surface. However, the extent and speed of this warming are still uncertain, because we cannot yet separate well enough the greenhouse effect from other climate influences. Although the radiative forcing by greenhouse gases (including methane, nitrogen oxides and fluorocarbons) has increased by 2.5 watts per square meter since the mid-19th century, observations show only a moderate warming of 0.8 degrees Celsius. Thus, the warming is significantly smaller than predicted by most climate models. In addition, the warming in the last century was not uniform. Phases of manifest warming were followed by periods with no warming at all or even cooling.