I read Heller's blog. I've also done a bit of double checking of his work because indeed, what he keeps uncovering is extremely interesting and should I feel really have been discussed in popular journalism about climate before now.

I want to check more of his claims in future. But for now, I only checked how many data points in the NOAA US temperature dataset are now the output of model simulations and not actual measurements:

https://realclimatescience.com/61-fake-data/

I verified the "Percent of USHCN Monthly Temperature Data which is fabricated" graph by downloading the source dataset and writing some scripts to process it. I was able to replicate his numbers. It's alarming that over half of reported temperature readings in the USA are actually not readings at all but rather estimates by a computer program.

I also spent some time reading the source code of the programs doing the temperature simulation but wasn't impressed. It's very old FORTRAN that has no unit tests of any sort. There are code comments giving the basic gist of what it's doing, but it's clear that the code has been grown and patched ad-hoc since the 1980s. There's nothing resembling real software engineering.

However there are also reasons to not be alarmed.

Some years ago climate skeptic bloggers started pointing out extensive problems with the network of weather stations used to calculate US temperatures. It had degraded over time as measuring stations dropped out, moved, had things built next to them etc. This in turn led to climatologists making extensive adjustments to the raw data, which is a very sketchy thing for scientists to be doing.

Congress agreed there was a problem and released funding to build a pristine new temperature network, which started operating some years ago. The skeptic bloggers agreed that the design of this new network was excellent, and the good news is the output of it matches the adjusted output of the old network (in fact, the adjusted old network measurements are still being used as canonical, which is a bit weird). So it seems that the current set of adjustments is not a problem even though it looks alarming.

The bad news is that this is partly because the adjustments made to modern measurements are quite small compared to the extent to which historical temperatures have been adjusted. Heller has been investigating the TOB adjustment in particular, which is near non-existent now but massively alters past readings. It's based on the belief that for most of the 20th century weather station operators didn't know how to properly use a min/max thermometer and that this methodological failure was never documented in primary sources, but has to be inferred from the recorded data.

I haven't made up my mind about these adjustments yet and remain in the neutral "they're probably OK" position. TOB is one but there are many others. The issue for Heller is not only methodological but also "where there's smoke there's fire", that is, the adjustments might be correct but everything around them is extremely suspicious, starting with the fact that - as this thread shows - climatologists keep adjusting even very recent data. How hard can it possibly be to read a thermometer and write down the numbers? Apparently, very hard.