It would appear that the real cause of global warming is the US Post Office! Yes the graph below-click to make larger-which comes from the JoNova blog, show the growth in global temperature and growth in US postal charges. The rise in global temperatures since 1880 closely correlates with increases in postal charges, sparking alarm that CO2 has been usurped as the main driver of climate change. Stamp inflation drives temperature inflation.
Update: In the comments section Brad Taylor alerts me to a rival theory for global warming. The lack of pirates theory. And yes there is a graph. It shows the approximate number of pirates versus the average global temperature over the last 200 years. As can be seen, there is a statistically significant inverse relationship between pirates and global temperature.