Every time I read about Global Warming I hear that the earth's average temperature is rising. How do they calculate the Earth's average temperature?

I'd imagine that they do it like we do at school (shudder),
Gather up all the temperatures they have around the world, add them up and when they reach a total, divide by however many temperatures they had in the first place.

ex. Say on my report card I got 95, 93, 95 in English, 88, 78, 83 in French and 96, 83, 88, 86, 94 in Math. Add them together (that gives you 979), now you divide 979 by 11 so your average in those subjects together is 89%.

Matigua is correct. There are thousands of weather stations across the world, both on land and sea. The high and low temperature are recorded each day, added together, and divided by 2 to get the mean temperature for that day at that location, then an average for the month by adding up the daily means and dividing by the # days in the month. Interpolations are made for locales with no reporting station. The average temperature has increased about 1 degree F in the past 100 years. Doesn't sound like much, but small temperature increases can have drastic effects.

http://www.heartland.org/studies/gwscience.htm

http://www.cis.org.au/Policy/autumn98/aut9805.htm

Here are a couple of web-sites that discuss Global Warming and the temperature changes.