How to Use a Tree to Determine the Climate History

A cross-section of a tree reveals all the details of its rings.

The story of a tree's life and the changing global climate are told in its rings. Each summer or growing season, a tree expands outward a little bit, forming a growth ring. During winter, a harder layer forms on top of the growth ring. A pattern of concentric rings builds up over time. Included in this story are traumatic and sudden events like fire and lightning strikes. Climate information can be inferred from the relative sizes of the rings. Tree rings can't record precise details like temperature, but they can provide a broad sketch of climate patterns from one year to the next.

Instructions

1. Obtain a cross section of a tree's trunk. A lumber yard is one possible source. Cutting down a living tree is not advisable.

2. Count each ring from the center outward. Each ring indicates one year of growth. If you know when the tree stopped growing, then you can back-date the rings to determine when the tree first sprouted.

3. Compare the relative widths of the rings. Wider rings indicate better growing conditions -- more moisture, more sunlight or both. Narrower rings show that conditions were harsher.

4. Look for evidence of fire or other damage in the tree's past. Fire typically appears as a blackened area along a ring.

5. Make a timeline of the tree's life history. Indicate periods of slow growth, healthy growth and any calamities that the tree survived, like fire.

6. Expand the sample size. Look at the rings from other trees in the area and see that they reinforce the original findings. Trees from the same region with different ring patterns indicate that local factors influenced those trees' growth more than global climate.

Tips & Warnings

• Trees growing in tropical climates do not have growth rings because they grow all year long. This means they can't be used to get the same information as a tree growing in the middle latitudes.

• Tree growth is dependent on local factors like competition and geography just as much as global climate. In other words, don't read too much into the story of a single tree's rings.

• Don't use power tools and saws without the proper training and personal protective equipment. Consider hiring a professional tree-cutter to perform the hard, dangerous work of getting a tree's cross section.

References

• Global Change Project: Studying Climate Change; Tree Rings
• Cornell University Tree-Ring Laboratory: Laboratory Procedures

Resources

• National Oceanic and Atmospheric Administration Paleoclimatology: Tree Ring