The graph shown below was downloaded from the U. S. National Oceanic and Atmospheric Administration's National Climatic Data Center website. It includes a summary of temperature records for weather stations in the lower 48 states over the span of time that reliable records have been kept (1895 - present).
The dense black horizontal line (A) represents the 116-year average temperature for the lower 48 states. The gray line (B) represents the temperature trend over the same period. It reveals an overall warming of 1.392 degrees F.
Be cautious about the "average temperature line." It serves merely for reference as the 116-year average. As the average changes with each year's additional data, this line will go up or down, depending on the new average. If I were printing the average line, I would have it be a dotted line. The "trend line," on the other hand, is a smoothed plot of the 116--year net change.
To project this trend forward to 2050 (just 37 years from now) would be to predict a mid-century U. S. average warming of 1.86 degrees F (from 1895 until 2050). If we consider the positive feedback loops that we are discussing (see January 4, 2012 post), there is good reason that the warming will be greater.
If you visit the NCDC website, you can generate 116-year temperature (and precipitation) plots for any state, region, or individual weather station (usually cities, towns, and government facilities). This could be the basis of a school (or university) term paper or science project. The availability of these data enable everyone to study climate change and monitor global warming.
Graph showing annual average temperature over time in the contiguous U.S., 1895-2011. Generated at NCDC website, accessed January 2nd, 2013.