claim: [NASA] has been adjusting temperatures from the past[...] downward, while adjusting current-day temperatures upward, and those changes are responsible for most of the claimed global warming during that time.
evidence: There are two specific changes to the U.S. temperature observation network over the last century that have resulted in systemic cooling biases: time of observation changes at most of the stations from late afternoon to early morning, and a change in most of the instruments from liquid in glass thermometers to MMTS electronic instruments. Back in July I posted a general introduction to U.S. temperature adjustments that looked at the relative effect of each adjustment. Here I will focus in detail on the Time of Observation adjustment, which is responsible for the majority of the change in U.S. temperatures vis-à-vis raw data. In a future post I will address the pairwise homogenization algorithm, which attempts to correct for factors like the MMTS transition and the impact of urbanization on temperature trends. [...] The net effect of adjustments to correct for time of observation changes is shown in Figue 2. TOBs adjustments increase the amount of warming in the historical temperature record by just under 0.3 C, with similar adjustments to both minimum and maximum temperatures. This adjustment alone is responsible for the majority of the difference between raw and adjusted temperatures reported by NCDC.