They adjust older data due to problems with both precision and accuracy with 19th century equipment and methods. They adjust newer readings due to changes in land use and population growth around the monitoring stations. They adjust data between surface readings, airborne readings, and space sensors.
I'm sure I've forgotten several others. But you get the idea. Now here's the thing they don't share... All those adjustments to the data are larger than the trends their models predict. That's right, they try to whip us into a panic to "do something" because of a projected 0.6 rise over the next hundred years. But they've adjusted data up and down several degrees.
I used to be a Climate Change Denier. I still think man-made warming (AGW) is a bunch of Red inspired hooey - but...
There are photo of a breeding pair of beavers living North of the Brooks Range here in Alaska. That and where my house sits today used to be a mile-thick layer of ice.
So, yeah, the climate is changing. Since I am sick to death of shoveling snow, I look forward to more ‘warming’....