The average surface temperature on the earth is 15, maybe 16 degrees Celsius (60 degrees Fahrenheit)," said Alan Robock, a meteorologist and climate researcher from Rutgers University. 
Meteorologists find it through measuring the heat in the oceans, atmosphere, as well as different places on the land. They use primarily satellites today.
- The average temperature of the Earth has varied within a range of about 3°C during the past 3,000 years. It is currently increasing as the Earth recovers from a period that is known as the Little Ice Age.
- ↑ http://www.livescience.com/environment/060925_temperatures_high.html
- ↑ Environmental Effects of Increased Atmospheric Carbon Dioxide