Climate secrets from the past

The magic of reading tree rings.

Trees contain some of nature’s most accurate evidence of the past climatic conditions.

Andrew Ellicott Douglass, a former professor at the University of Arizona teaching astronomy, discovered that trees are sensitive to local climate conditions, such as rain and temperature.

Andrew made this discovery in his quest to further his research about the relation between sunspot cycles and climate. Instead, however, he discovered dendrochronology.

Dendrochronology is the science or technique of dating events, environmental change, and archaeological artefacts by using the characteristic patterns of annual growth rings in timber and tree trunks. It is the study of the relationship between climate and tree growth in an effort to reconstruct past climates. A tree’s trunk is made up of layers and each layer looks like a ring. These ring-like layers are referred to as “tree rings”.

In his research, Andrew discovered that these tree rings not only indicate a tree’s age but also record evidence of rainfall, floods, droughts, insect attacks, diseases, fires, lightning strikes, earthquakes that occurred during the lifespan of the tree and even the kind of particles that were in the air at different seasons.

As a result of such sensitivity, the nature of growth a tree ring assumes depends on the local environmental conditions at a given time. For instance, when the water supply to the tree is adequate, the tree ring will be thicker, however, where the water supply is limited, the tree ring will be thinner. Weather variations can also be detected in the rings in terms of length. For example, longer winter or drought seasons will cause the tree to develop thinner rings and warmer conditions like summer or spring will enable for thicker tree rings for a given year. The tree ring sizes, therefore, change accordingly depending on the weather in a given year.

Thick tree rings show that a tree grew a lot that year and this is attributed to good weather. Thin rings, on the other hand, show that the tree didn’t grow very much because the weather was either too hot, too cold or there wasn’t enough rain. Perhaps there was a drought or a severe winter. Trees with consistent tree ring size mean that the climate was the same year after year. This occurs mainly in the tropics where trees grow year-round.

Further tree ring assessment would show you that tree rings have varying colours. The rings usually have a light colour and dark colour pattern. Light coloured tree rings show that the wood grew in spring and early summer, while the dark tree rings indicate that the tree grew in late summer and fall. A light ring and dark ring together represent one year of growth. The two shades of tree rings help identify what season of growth the tree underwent in a particular year.

With these discoveries, Andrew established the major principles of tree-ring dating. His discoveries led to the founding of the Laboratory of Tree-Ring Research currently found at the University of Arizona. Here, thousands of samples of trees were studied to piece together such accurate pictures of past climates.

A single tree could preserve climate data up to 9000years. For instance, in the White Mountains of California, a bristlecone pine tree called the Methuselah discovered through the tree ring research is believed to have been growing since 2832BC. It is even older than the Pyramids of Egypt. As of 2013, it was the oldest “discovered” living tree in the world. The Methuselah is, therefore, one of those trees that have contributed to giving local climate data regarding climate change and changing weather conditions.

Dendroclimatology has thus proved to be one of the most reliable climate data types. Although it may record short time spans for areas without proper tree conservation cultures as compared to the alternatives, these climate records can cover a timeframe longer than the life of a single tree. For instance, the data from a living 200-year old tree could be combined with data from wood from a tree that was felled 150 years ago to produce a composite dataset covering hundreds of years.

Technology advancement has also advanced the developments in dendroclimatology. To it comes to collect tree ring data for the study, it is not necessary to cut down a whole tree. Instead, scientists use an instrument called the increment borer to extract core samples. The borer is screwed into the tree and pulled out, bringing with it a thin strip about the size of a straw or about 4 millimetres in diameter. The hole in the tree is then sealed up to prevent disease. This strict use of the increment borer emanated in 1964 when a student, Donald R Curry, doing his research on how climate change affects receding glaciers cut down Prometheus, a 4,900-year-old tree. The age was only discovered after the tree had been felled. This discovery prompted heightened security to conserve these very old trees.

The study of tree rings, therefore, clarifies to humans the importance of preserving trees in our communities for purposes of reconstructing regional patterns of climate and reading tree age. These facts do not only show how the Earth’s climate has changed, they have helped scientists understand why these changes came about. Knowing how the Earth’s climate has changed over time helps scientists determine whether the changes that are occurring now are part of the Earth’s natural pattern or caused by human activities. With the current changing climate, tree ring study enables scientists to relate the current changes to past climate occurrences.

Agodo Shabella Patience

Agodo Shabella Patience is a Ugandan Lawyer with keen interest on the Environment and Climate change. She is the founder and Executive Director of Green Teso Initiative a climate change Advocacy NGO based in Eastern Uganda.
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