[mpitt]

I think it refers to the background radiation increase due to atomic bombs, see also https://en.wikipedia.org/wiki/Low-background_steel

Edit: a nice plot https://commons.wikimedia.org/wiki/File:Radiocarbon_bomb_spi...

[ascorbic]

When archaeologists give a date in years Before Present (BP), they measure from 1st January 1950. I guess that makes today -66BP.

https://en.wikipedia.org/wiki/Before_Present

[whamlastxmas]

I wouldn't have guess such small amounts of radiation in the atmosphere would interfere with carbon dating, but I know literally nothing about how it works. Seems like solar radiation and such would be a bigger contributor?