This report examines the repeated claim that the climate of the 20th century was unusual compared with those of the last 1,000 years. The claim takes several forms – e.g., that the 20th century has been warmer than any other century, that the 1990s were the warmest decade of the millennium, or that 1998 was the warmest year of the millennium.
These claims imply that the temperature of the past 1,000 years is known well enough to allow an accurate comparison of the 20th century with the previous centuries, decades and individual years. This is not the case. A set of direct temperature measurements is only available since 1861 and there are reasons to question whether these data are sufficiently accurate to compute global average temperatures.