The record reveals temperatures increasing through the 1920s and 1930s, peaking around 1940, and then gradually decreasing through the early 1970s. Over North America, Karl et al. Increases in T max were smaller, for the most part, with T max actually decreasing somewhat in the desert SouthWest. The analysis of U.S. Historical Climatology Network data by Karl et al. The trend magnitude for each climatic division is reflected by the diameter of the circle.
Solid circles represent increases, and open circles decreases.