Figure 23.19 Global warming over time. Based on thousands of measurements around the world, scientists have observed a 0.8°C increase in the average temperature of our planet. Because of year-to-year variation in temperatures, the pattern of increasing temperatures is clearer when we examine the 5-year-running mean temperatures. Temperature anomaly is a comparison of each year’s temperature compared to the mean temperature observed from 1951 to 1980.

Data from http://data.giss.nasa.gov/gistemp/graphs_v3/.