Replies

the tree rings are almost exactly the same thickness in the same years. In other words, after crosscutting a tree that has been dead for 400 years, but was 500 years old, you can line up the thickness of the individual rings to match the rings on a tree that died 700 years ago, and was 300 years old in a different country because the climate that created the thickness of the individual ring also has a match in the other tree in the other country. The tree rings tie together an endless chronological map of what the world was doing at any given time. There are some variances, depending where you're looking at the rings, but there are telltale rings that are mutually consistent no matter where you are looking at them from that allow the aligning of the rings within the different trees. The same hold true for ice cores. They're done the same way. for instance, if a meteor struck in one certain year, you could align the ice cores to that strike and go from there. Pollen differences one year compared to another, etc.