Global warming: What does the data tell us?
  We analyze global surface temperature data obtained at 13472 weather stations from the year 1702 to 1990. The mean annual temperature of a station fluctuates from year to year by typically +-0.6oC (one standard deviation). Superimposed on this fluctuation is a linear increase of the temperature by typically 0.40oC per century ever since reliable data is available, i.e. since 1702. The world population has doubled from 1952 to 1990, yet we see no statistically significant acceleration of global warming in this period. We conclude that the effect of humankind on global warming up to 1990 is 0.0 +- 0.1oC. 
E. X. Alban and B. Hoeneisen