The science of dendrochronology observes changes in the pattern of tree rings in an attempt to create a time line and infer climate changes. Using tree ring data, it is possible to link the past 11,000 years in parts of Germany & Northern Ireland, but only the past few centuries in other parts of the world.
Let's see how tree ring samples are obtained and dated, and learn how the rings correlate with a change in climate, including one drastic climate change that rings in the Medieval Dark Ages.
Top image is a construction using an image from Howard Pyle's 1903 book The Story of King Arthur and His Knights and a photograph of tree rings by alpe1990.
Tree Ring Growth
Growth in tree rings result from changes in climate throughout the year due to the change in seasons. The portion of the ring acquired by rapid growth in the spring and early summer is called "early wood" - this area is less dense and lighter in color. The tree still grows in the Fall and Winter, but slowly, leading to a denser and visible darker area of growth. The light to dark growth make up a continuous band, with this band called the "annual ring" as it typically accounts for a year of growth."Wet" years with better than average growth feature thicker bands, while off years or years with large atmospheric disturbances (volcanoes) leave behind thinner bands.
Crossdating and establishing a chronology
Matching changes in multiple tree rings allows for crossdating – a joining of tree ring puzzle pieces between living and long dead trees through distinct characterics can extend the chronology of an area back several millennial. In proper data acquisition, samples are taken from several trees in an area - often a minimum of 20 trees per site.
The tree rings are used not just to date trees, but they are also used to date structures in nearby areas. Local buildings containing wood are dated by matching specimens of dead wood used to create the structure to known patterns of growth for trees in the area.
A small portion from the core of the tree is removed using a hollow drill bit. The wooden cylinders removed are typically between 15 and 30 cm in length and around a centimeter in diameter - enough to obtain a reasonable measurement and prevent permanent damage to the tree.
Determining the beginning of the Dark Ages
Tree ring data supports a dramatic worldwide climate event between 535 and 542 AD, with samples from California, Finland, Ireland, Sweden, and Siberia all exhibiting extremely narrow tree rings. The worldwide proliferation of poor rings is likely due to "short summers" and a generally negative change climate. This change in growth is of particular interest to one dendrochronologist, Dr. Mike Baillie at Queen's University of Belfast in Northern Ireland, who links this period of poor growth to the beginning of the "Dark Ages", claiming the downturn is the result of an increase in comet and asteroid activity during the 6th century.
Baille notes climate downturns in several other periods due to a severe drop off in tree ring quality, including the years around 2354 BC, 1628 BC, 1159 BC, and 208 BC. Historic climate records from these periods are not readily available, making it difficult to match these years up with a specific event. The downturn in 1159 BC could correlate with the Greek Dark Ages and the fall of the Mycenaeans if a significant climate event occurred, but this is merely my conjecture.
Intellectual property dispute
Dr. Baille fought a legal battle to keep his data private, holding it as his personal intellectual property. His position as a professor at the government funded Queen's University of Belfast, however, trumped his personal claim, and forced Baille to open up his records to the public.
If you are tempted to take a look at a bevy of tree ring data, several open access databases currently exist. One of the largest is the International Tree-Ring Data Bank, containing trees from 2000 sites on six continents. The database is maintained by the United States' NOAA Paleoclimatology Program and the World Data Center for Paleoclimatology.
Images courtesy of the International Tree-Ring Data Bank, the University of Arizona, and Middlebury College. Sources linked within the article. For an excellent summary of the growth of dendrochronology as a science during the 20th Century, check out Tim De Chant's article at ars technica.