One statistic that is used to understand climate is the annual globally averaged temperature anomaly. It is not the only measure of global warming; there are a great many others, but it is one that the media tend to focus on because it is a convenient way of explaining what is happening to surface temperatures as a function of time.
The data here are taken from Global Land-Ocean Temperature Index (C) (Anomaly with Base: 1951-1980), which includes data from 1882-2007. There are updated numbers available for more recent years, but I am using these data to respond to an argument made by a friend.