They are deceptions created and manipulated by technocrats. The objective is to sensationalize the story, by using a period of record that provides the desired result. It is in the practice of modern politics defined by H. L. Mencken a few years ago.
"The whole aim of practical politics is to keep the populace alarmed, and hence clamorous to be led to safety, by menacing it with an endless series of hobgoblins, all of them imaginary."
A simple trick with a climate hobgoblin is to pick a period in which your claim is valid.
The first major and classic example occurred in Chapter 8 of the Intergovernmental Panel on Climate Change (IPCC) 1995 Report. They showed as evidence of human-caused warming a graph of the "Upper Atmosphere Temperature."