First lets look at a paper that shaped the modern view of past co2 levels.
180 Years of atmospheric CO2 Gas Analysis by Chemical Methods, published in the prestigous Journal "Energy and Environment" by Dr Ernst-Georg Beck.
The paper is also covered in the media:
Below I have reproduced some of the data from this study for the purpose of scientific analysis. This data shows that co2 levels were much higher than today in 1940 and 1827:
There is no question over the accuracy of the earlier measurements.
CO2 levels are caused by temperature changes
This is to be expected because ice core measurements show that co2 levels rise happens 800 years after temperature levels rise. So temperature rise causes co2 rise, not the otherway round. http://joannenova.com.au/global-warming/ice-core-graph/
This is because the oceans release more co2 when they get warmer, and less when they get cooler. http://wattsupwiththat.com/2009/03/21/recent-ocean-heat-and-mlo-co2-trends/
Lets look at the individual periods to demonstrate this:
The 19th century decline in CO2
The drop in co2 in the early 19th century to the start of the 20th century was most likely caused by cooling from the little ice age.
The rise and peak of co2 around 1940
This correlates with the warmest year on record being in 1934: http://www.dailytech.com/Blogger+finds+Y2K+bug+in+NASA+Climate+Data/article8383.htm
Rise in co2 since 1960
While the earlier measurements are beyond questioning, the measurements since 1960 are deeply uncertain. This modern measurement period began warmists started measuring co2 at Mauna Loa in Hawaii. This Mauna Loa sensor reports the gradual co2 rise you see in the graph.
There is just one problem with that:
Mauna Loa is on a Volcano!
That's right, they could have picked anywhere in the world but warmists chose to put the co2 sensor on top of one of the biggest volcanoes in the world! No wonder it shows a rise in co2!
The Medieval Warm Period
Now lets look at a period not covered by the graph, but a period we know was a lot warmer than today. The medieval warm period occured some time in the past around medieval times. Ie 1200AD.
The scientific consensus is that in 1200AD the climate was excruciatingly hot. Plagues and disease swamped the world and people were dying of heat exhaustion. The situation got so bad that the Vikings moved to a large snow covered island north of Iceland to escape the heat. The ice on this island subsequently melted and so the Vikings called it Greenland. Greenland soon became the wine and farming capital of the world.
Didn't last though. Eventually the climate shifted back again and the Vikings were wiped out by ice.
Putting It All Together
Given we know warming causes co2 levels to rise and we know it was very much warmer during the medieval warm period than today, it would be no suprise to find that co2 levels were far higher during the medieval warm period. We can estimate how much higher they were by projecting the co2 fall from the 19th to early 20th century back in time:
The answer is that based on the best science available, co2 levels may have been over 2000ppm in 1200AD! Is it coincidence that 1200AD was 800 years ago? Recall the fact mentioned earlier: "ice core measurements show that co2 levels rise happens 800 years after temperature levels rise". Everything is fitting into place and this last piece seems to complete the puzzle.
It has been shown that co2 levels could have been over 5 times higher just 800 years ago. If this is the case a doubling of co2 is proven to not cause any significant warming and any ideas for more taxes should be immediately abandoned.