FAQ: What does cal BP mean?

I know what BP means, but what in the world does "cal" mean?

100 Year Old Beech Tree.
Photo Credit: Astrid Geck

Excellent question, Carol! "cal BP" stands for "calibrated years before the present" and what that means essentially is that archaeologists have discovered wiggles in radiocarbon dating. Basically, the amount of carbon in the atmosphere---which we use to build radiocarbon dates---hasn't been consistent over time, and our mathematical models simply can't be complex enough to resolve that. Luckily, we can correct the regular radiocarbon dates by matching the wiggles with tree ring dates. Tree ring dates are very precise, and continuing research has developed matched tree rings to several thousands of years into the past in many regions of the world. So in archaeological science, you have to list a regular radiocarbon date, and then a corrected one. Here's a longer explanation of the problem, with links to the latest tree ring database.

To be quite frank, I'm having a hard time using cal BP in articles without confusing people like crazy. I try to link to the definition each time, but I know that really isn't optimal. But, because the wiggle really makes a difference---especially when we're talking about the colonization of the Americas when a few hundred years off makes a huge difference theoretically---you gotta do something to express the wigglies.

Thanks for this great question!

• RCYBP and other dating mysteries
• Tree Rings and Culture
• Domestication of Maize
• American Colonization
• More Frequently Asked Questions