Using Ancient Trees to Reconstruct Climate History
Tree rings can tell us how well a tree has grown each year of its life. A cross section of a tree reveals a bull's eye pattern of rings of wood, one ring for each year of growth. If the tree had a good year — usually that means plenty of water — that year’s ring will be a thicker layer of wood; if the conditions were poor for the year's growing season, the growth ring will be thinner. Since trees can live for many hundreds of years, we can count back from the present time to see how climate varied from year to year long before there was an instrumental record of weather.
By sampling tree rings from different regions scientists can ask many questions about historical climate patterns: How long did ancient droughts last? How large an area did they cover? How frequent were El Niños in the past? How severe were they? How did they affect global climate? How did ancient droughts affect human history?
Despite a wave of deforestation that accompanied human development, there are still ancient trees living in many parts of the world. They need attention and protection not only because they are part of our natural heritage, but also because they are a unique source of historical informa- tion. We spoke with Professor David Stahle about how tree rings have extended our knowledge of climate and climate's effects on human history.