Most times the temperature data that is used to show Global Warming is only anomaly data. That is the difference from the average temperature. Anomaly data is useful for many reasons because it allows different sources to combine in an easy way. Anomaly data is useful for many reasons. Here is a normal example of the anomaly data that is used to show warming.
There is one important limitation of data presented in this manner. It fails to show the type of variation in temperature that happens on a yearly basis, much less the variation that takes place over time. Few people realize that the global temperature varies between 12-16 °C every year. Dec-February being the coldest months and June-August being the hottest months. The average temperature of the Earth over the course of a year looks like this (Jones 99). The reason why the global average changes is because the Northern Hemisphere is a strong driver of global temperature as discussed here.
From the anomaly and the average temperatures it is possible to reconstruct the actual global temperatures from data that is easily available. Looking at the actual global temperatures for the past 40 years provides an entirely different perspective. One that shows the changes in temperature, but more in line with the natural cycles that the Earth is always going through.
This was created using the NCDC average temperature for each month from 1901-2000 and the blended data (NCDC, CRU, UAH, RSS) that I have been using. What it shows is interesting. The biggest thing that stands out from this perspective is that the Dec-Feb period of the year has been slightly warmer over the past ten years. Other than that not much stands out from a global perspective.
So while anomaly data is useful, it is not everything. Sometimes a bigger picture is needed. If an actual global temperature similar to what is shown above was used to convince people that global warming was happening, how effective would it be? In my opinion it would not be very effective. Perhaps that is another reason that it has not been used.