The problem is that the earth is very large and the temperature is always changing, everywhere, so how can we possibly tell if overall it is a mere half-degree warmer today than it was around 1902? The origin of the theory of global warming (for it is just a theory) lies in taking the thermometer readings we happen to have from the last 100 years and massaging them in various ways. Some places have clearly warmed, others have clearly cooled. Many have gone up and down but with little apparent trend. To get a global result requires a lot of statistical manipulation. […] even if the earth has warmed a little bit there is still the very real possibility that this warming is natural, that the models are no good, that warming is beneficial, etc.