Yes, the procedure is called "calibration." The two principle calibrations are time of day and homogenization. The argument is that weather stations have historically not recorded temperature at consistent times. And that furthermore, temperature stations have been shut down, and new temperature stations have been created. Therefore, there is not consistent temperature data over long (multi-decade) time periods.
That is a very reasonable point. These changes will introduce random variation that could bias the temperature readout at any station.
But... central limit theorem. You have a set of uncorrelated biases. With thousands of temperature stations over decades of sampling, you should expect the cumulative impact of those variations to be distributed normally with a zero mean.
So what is the impact? Well, there are two major climatic records. The NASA set produced by Hansen, and the University of East Anglia's produced by their Climate Research Unit (CRU). When you look at the NASA data set only 5% of the warming in the calibrated data is observable in the raw data. 95% of the "observed" warming is introduced by the calibration procedure. We don't know what the impact on the CRU data because they've always refused to release their raw data. Now they claim the original raw data was lost.
And yes, a 20:1 ratio between pre-and-post calibration observations is pretty remarkable. It's not impossible, but it is really unlikely. Unlikely enough that there should be a big debate over the validity of those calibration schemes, but I've never found a proper validation study in my lit reviews. If you're interested in looking at the procedures, they're implemented in Fortran and they were available from NASA's website. Not sure if they still are, I did this review about ~7 years ago.
Here's where it gets really juicy though. When you look at the observed levels of CO2 in the atmosphere, and you do the energy balance calculation to predict the temperature impact of that increased CO2... you find out that the CO2 increase only explains a fraction of the claimed temperature increase (about 5%-10%).
So what's the explanation for the other 95% "observed" warming? Apparently it's feedback effects. The climate models assume certain positive feedback loops which amplify the increase in temperature. Again, this is possible, but really, really unlikely. Natural systems often have feedback loops, but they're usually negative feedback. Positive feedback systems are unstable. The kind of feedback required to gently bump up the temperature.... again it is possible, but it's extremely unlikely. You require very specific non-linear feedback to get the outcomes that are predicted.
So yeah, it's bullshit all the way down. The earth is warming, and humans are contributing to that. But the extent of the warming is way overstated.