lobal warming is a reality. The year 2004 was the fourth-hottest on record; NASA predicts that 2005 will be even worse. But how rapidly is the climate changing? And what does the future look like? To answer those questions, scientists need information about long periods of time over widely spread geographic regions. They use a range of tools to gather the data they need. Here are six:

Most of us know that by counting the rings on a cross-section of a tree trunk we can tell its age. But tree rings can tell much more. Each year's growth is made up of two rings: the first, a ring of a lighter shade of wood composed of large cells that develop during the spring growing season; the second, a dark ring of smaller cells that develop throughout the rest of the year, when the tree receives fewer nutrients. Since scientists know that a tree's growth is limited by the resource that is in shortest supply -- the growth of desert trees, for instance, depends most on the availability of water -- they can tell by comparing the widths of the tree rings what the climate was like during each spring growing season.

Scientists called dendrochronologists (dendron is Greek for tree, and chronos means time) use hollow drills the diameter of a soda straw to extract cores from tree trunks throughout a forest. By comparing the ring widths they can develop a forest's climatic history. And by lining up the rings of dead trees with those of living ones scientists can extend that history back to the forest's beginnings, thereby learning what the climate was like when the forest began to grow and seeing the patterns of drought and temperature change. Climate scientists now have tree-ring data going back as far as 9,000 years.