IE- How Does Scientist Message Global Warming

I am learning about global warming and I have found out how global warming is measured. I have been talking to a man called Tim Barnett he is a scientist. He said that scientist uses many instruments to measure the temperature which gives them an understanding of global warming. They use thermometers on ships, land stations and balloons that measure air temperature. All the measurements from different places and temperature get sent back to a there office and there it gets averages. Averaging is done by adding up all the data from the different measuring instruments and dividing that by the number of instruments. They use the heat of different dates and to work out the changes. I think that it is cool that they do average and I think it is quite funny because I do averaging things in my AWS book. But their averaging is a lot bigger and more import.