Since 1900 the global temperature has risen on average by 0.8°C (Figure 2). Most of this increase took place in the past 30 years. From 1975 onwards, global temperature increased by 0.5°C. Based on comparison of model calculations and observations, it can be concluded that in the past 30 years the temperature increase was mainly anthropogenic in nature. Before 1975 the temperature rise was caused mainly by natural processes. The temperature has not increased evenly over the earth. The increase was largest above the continents in the Northern Hemisphere.

In the Netherlands the temperature has risen, on average, by 1.2°C since 1900 (Figure 3). This increase resulted in a decrease of the number of frost days and an increase of the number of summer days (Figure 4). The ten warmest years since 1900 consist entirely of years after 1988. During the past 20 years, the months February and March have seen the largest increases in temperature. Apart from global warming, this was also due to an increase of the number of days with southwesterly winds. In the Netherlands the dominant wind direction has a clear effect on the temperatures our country experiences. During winter, westerly winds, which convey air from the sea, give rise to milder temperatures. Easterly winds (from the continent) in summer cause temperatures to be higher. It is not yet clear whether the observed increase of 'warm' winds in February and March is connected with the human influence on the climate, or if it is only the product of natural fluctuations.