This is just one of dozens of responses to common climate change denial arguments, which can all be found at How to Talk to a Climate Sceptic.
Objection:
Global Warming has been going on for the last 20,000 years.
Answer:
It is quite true that 20Kyr ago the temperature was some 8 to 10oC 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! 20K years of Global Warming!". If you have look at this nice 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 the warming today which is an order of magnitude faster). After a final little lift at 8000 years ago, 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 attached makes it even clearer that today’s trend is striking and opposite to what one might expect without an anthropogenic disturbance.
(Courtesy of Global Warming Art. See source for details on the various reconstructions tangled above.)
If you really wanted to play this "Global warming started X years ago" game you could talk about how we are reversing a five million year cooling trend, or go crazy and track global temperatures right back to the origins of the planet! Not that I can see the point in that…
This is just one of dozens of responses to common climate change denial arguments, which can all be found at How to Talk to a Climate Sceptic.
“Global Warming Is Nothing New” was first published here, where you can still find the original comment thread. This updated version is also posted on the Grist website, where additional comments can be found, though the author, Coby Beck, does not monitor or respond there.