GISS measures the change in global surface temperatures relative to average temperatures from 1951 to 1980. GISS data show global average temperatures in 2017 rose 1.62 degrees Fahrenheit (0.9 degrees Celsius) above the 1951-1980 mean. According to GISS, the global mean surface air temperature for that period was estimated to be 57 F (14 C). That would put the planet’s average surface temperature in 2017 at 58.62 F (14.9 C).