varies in the length of time quoted, but the essential fallacy is the
same. You can cherry pick any particular year you wish to get a trend
in the direction you wish. But of course this only works if you
ignore all context or considerations of relevance.
It is quite true that 20K years ago the temperature was some 8 to 100C colder than it is today, but it is highly arbitrary and dubious to simply draw a line from that point to today and say
"Look! 20,000 years of Global Warming!"
If you have a look at the graph of temperature, starting at a point when we were finishing the climb out of deep glaciation, you can clearly see that rapid warming ceased around 10,000 years ago (rapid relative to natural fluctuations, but not compared to warming today). After a final little lift at 8000 years the temperature trended generally downward for the entire period of the Holocene. So the post industrial revolution warming is really the reversal of a many thousand year trend.
A closer view of today's trend with the context of the last 1000 and 2000 years makes it even clearer that today's trend is striking and opposite to what one might expect without an anthropogenic disturbance.