Tom's Trick Works and Things Are Worse Than They Seem

Some time ago Eli referred to the US Climate Reference Network as Tom Karl's Trick in that it offered an elegant way of evaluating the data and homogenization of the US Historical Climate Network and by extension the global networks of climate stations

 Its primary goal is to provide future long-term homogeneous observations of temperature and precipitation that can be coupled to long-term historical observations for the detection and attribution of present and future climate change. Data from the USCRN will be used in operational climate monitoring activities and for placing current climate anomalies into an historical perspective. The USCRN will also provide the United States with a reference network that meets the requirements of the Global Climate Observing System (GCOS).

The Bunny's take on this was here is a sensible way of checking the accuracy of older climate networks in the past and calibrating them in the future.  But there was a point that was missed, perhaps even by the designers.  They sensible paired USCRN stations with USHCN ones for evaluation.  Because of this it was implicit in the design that significantly less than the generic 30 climate data evaluation period would be needed to draw conclusions, and that those conclusions would extend back over the entire USHCN period.

Zeke Hausfather, Kevin Cowtan, Matthew J. Menne, and Claude N. Williams, Jr published a short paper on this, Evaluating the impact of U.S. Historical ClimatologyNetwork homogenization using the U.S. Climate Reference Network (open version) where they showed that the USHCN, the historical network, slightly UNDERESTIMATES the warming trend as measured by the USCRN, the optimal network, especially for maximum daily temperatures

Their conclusion was understated, as one would expect

During the period of overlap between the USHCN and USCRN networks, we can confidently conclude that the adjustments to the USHCN station records made them more similar to proximate homogenous USCRN station records, both in terms of trends and anomalies. There are no systematic trend biases introduced by adjustments during this period; if anything adjusted USHCN stations still underestimate maximum (and mean) temperature trends relative to USCRN stations. This residual maximum temperature bias warrants additional research to determine the exact cause.

 The paper also shows that homogenization narrows the distribution of raw data from USHCN stations while leaving the means unchanged which pretty much kills the loud claims of fake data from the peanuts' gallery, but the systematic difference between the USCRN and USHCN warrants further study and a bit more worry.