Zero represents the 30-year mean of the years 1951-1980, i.e. some representation of a recent baseline climatology. Choosing 1880, or any other year, might ske

Zero represents the 30-year mean of the years 1951-1980, i.e. some representation of a recent baseline climatology. Choosing 1880, or any other year, might skew perception positively or negatively. From the Earth Observatory [0]: > The period of 1951-1980 was chosen largely because the U.S. National Weather Service uses a three-decade period to define “normal” or average temperature. The GISS temperature analysis effort began around 1980, so the most recent 30 years was 1951-1980. It is also a period when many of today’s adults grew up, so it is a common reference that many people can remember. [0] http://earthobservatory.nasa.gov/Features/WorldOfChange/decadaltemp.php http://earthobservatory.nasa.gov/Features/WorldOfChange/deca...