This is just the tip of the iceberg of problems with their data. About the only really solid data they have is from satellites and that's maybe 50 years worth.

Before that they have ground temp stations that are poorly located, uncalibrated and barely maintained. Worse, the data only really covers Part of North America and Europe with any granularity. Go back far enough (150-200 years) and the data is just some guy at a lighthouse randomly taking readings on a thermometer he may have built himself along the coast.

Then they use this crap data to extrapolate globally, and then use that limited inaccurate data set over 200 years to extrapolate earlier from tree rings and ice cores.

If it were actually good data, it still would have margins of error orders of magnitude greater than the changes they forecast. As it is, it's total bullshit.

They claim they can "fix" some of these problems with data massaging. That's not how data works - worshippers of "Science" should know this.