According to the IPCC the 20th century climate has warmed 0.8±0.1 C, at a rate and magnitude that is unprecedented over at least the last 500 years. The high confidence placed in this centennial temperature change depends upon the ±0.1 C uncertainty limit. But is that small uncertainty bound justified? There are very few field calibrations of surface temperature sensors. Those that do exist reveal large systematic measurement errors that should be propagated into the temperature record. However, these errors have been ignored. The neglected systematic measurement errors produced by land and sea surface temperature sensors will be described. The surface air temperature record will be corrected to include systematic sensor measurement error. This error constitutes a lower limit uncertainty bound. A conservative estimate of systematic sensor measurement error shows that, at the 95% confidence interval, the 20th century change in surface air temperature is not knowable to better than ±1.2 C. The 20th century global air temperature change is thus indistinguishable from zero.