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Tree rings are a good place to start thinking about how climate researchers get information about past climates.  In certain cases, trees can live for many hundreds of years and in an extraordinary case, like the bristlecone pine, thousands of years!  Each year trees add growth rings, which can indicate what sort of growing season the tree experienced.  Interestingly these rings are more than a temperature indicator, they also tell the researcher about moisture and cloudiness as well.  

Dendrochronology is the study of climate change as recorded by tree growth rings.  Each year, trees add a layer of growth between the older wood and the bark.  This layer, or ring as seen in cross section, can be wide, recording a wet season, or narrow, recording a dry growing season.  Because the rings are basically recording a good growing season or a bad growing season, they are indirectly recording more than just moisture.  They also document temperature and cloud cover as they impact tree growth as well.  This record of annual summer information is very important when you consider that certain types of trees grow slowly over hundreds and hundreds of years, and therefore contain a record of as many years of climate and climate change.

There are limitations to this research though.  Trees in the temperate zone only record the growing season, so the winter season, no matter how dramatic, will not be seen in the ring record.  Interestingly, trees in tropical regions grow year round and therefore show no real obvious annual growth rings.  Therefore climate data from equatorial areas is difficult to piece out and use. The record is limited geographically in another way too.  Trees do not grow in all places on Earth, therefore we don’t have a tree ring record of climate change for each region and ecologic niche globally.   (No trees in polar regions, high in the mountains, in the ocean!!!)
Image courtesy of Dr. Glenn Juday, Dr. John D. Fox, Jr., Dr. Elena Sparrow, Ms. Valerie Barber, and Ms. Valerie Hendrickson, all at University of Alaska Fairbanks (Click for more information)

In order to know more about climate over an even longer period of time, in some cases thousands of years, it is possible to look at dead trees of an unknown age that are still well preserved.  One can correlate their rings to the rings of a living tree (whose age you know), and get a longer record of climate through time.  An amazing example of this is the tree ring chronologies established by looking at bristlecone pines through time.  Not only are bristlecone pines the longest lived trees on earth, they also live in a place where, even when they do die, they are well preserved over hundreds or thousands of years.  We can therefore look at a living bristlecone, take its ring record , then look at a dead tree and see where the rings match up.  In this way, scientists have established a ring record that records climate signals for over 9,000 years into the past.

  With permission of Leonard Miller ©1995-2005 - All rights reserved.  
Dendrochronology is currently still in its scientific infancy – there are many problems in the use of tree rings, particularly because the growth of tree rings can be impacted by many issues - not just rainfall amount, temperature, and cloud cover – but also by wind, soil properties, disease, or even pollution.  These issues can certainly impact tree ring growth and cloud the scientific record.  Fortunately, scientists are gaining new insight in the reading and use of tree rings, and hope that they can be used to help understand whether global warming has any precedent in the ring record of the past 1,000 years.
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