An Archive in Wood
The annual growth rings produced by some trees are an important clue to past climate. The rings of a single tree can reveal environmental stresses during the tree's lifetime—drought or fire, for instance. Combining ring data from trees whose life spans overlap creates an extended climate record. Like other paleoclimate evidence, tree rings tell us not only what happened in the past—but what may happen in the future.
The Carbon Clock
Scientists use radiocarbon dating to determine the ages of tree rings. Trees absorb carbon from the atmosphere. About 1 percent of it is radioactive carbon, or carbon-14, which is formed in the atmosphere by the interaction of radiation from the Sun with nitrogen-14; the rest is carbon-12. When an organism dies and stops taking in atmospheric carbon, the remaining carbon-14 gradually turns into carbon-12. Carbon dating measures the ratio of carbon-14 to carbon-12, and reveals how many years have passed since the tree died. The technique can be applied to specimens several tens of thousands of years old.
Record of the Rings
This cross section came from a roof beam in the ancient site of Pueblo Bonito, in New Mexico. Builders crafted the beam from a tree cut in AD 1070. Scientists studying this cross section see the thin rings that mark drought in 991, 1005 and 1041.
Layers of information
Scientists can get a picture of ancient climate from rings in corals and layers in ice cores, as well from tree rings. Still other sources include:
Lakes in glacial valleys produce seasonal sediment layers called varves. The thickness of the dark, organic layers in tells us how productive the lake was in the summer. The light layers of sand and silt indicate how much glacial runoff and rain entered the lake in the spring.
Stalactites and stalagmites—speleothems—form in layers as water drips from the roofs of caves. Very thin layers in a core (left) mean less water was available and may be a clue to ancient drought. The chemical composition of the speleothem—and thus, of the water from which it was deposited—also reflects conditions outside the cave.
Clues to Climate
These cores are from two northern red oaks (Quercus rubra); they span 159 and 171 years, respectively. Data from these cores helped scientists identify climatic events that occurred during the past two centuries in an area along New York's Hudson River. Taking cores like these doesn't damage a tree.