You learn in grade school that each ring in a tree represents a year. But there's more information hidden away in these concentric rings. The thickness of a ring shows how much the tree grew during that year. If scientists know what temperature and moisture level make the tree happy, they can use the thickness to gauge what the climate was like that year.
Paleoclimatology labs around the country, like the one at the National Oceanic and Atmospheric Administration
, are therefore busily building databases of tree ring data.