Droughts and other variations in the climate make the tree grow slower or faster than normal, which shows up in the widths of the tree rings.
These tree ring variations will appear in all trees growing in a certain region, so scientists can match up the growth rings of living and dead trees.
In regions outside the tropics, trees grow more quickly during the warm summer months than during the cooler winter.
This pattern of growth results in alternating bands of light-colored, low density "early wood" and dark, high density "late wood".
Other critics, perhaps more familiar with the data, question certain aspects of the quality of the fossil record and of its dating.