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Rock doc: Reading the record of the tree rings

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From page A4 | April 18, 2011 |

E. Kirsten Peters mug

Peters

Scientists have studied natural climate change for quite a while. Part of what we have learned about past climates comes from tree rings, and thereon hangs an interesting tale going back more than a century.

Flagstaff, Ariz., was a pretty small burg in the 1890s, without the street lamps of big cities Back East. It also has an elevation of 7,000 feet, making it well over a mile above sea level.

It was those two conditions that brought a young astronomer named A. E. Douglass to the area in 1894. He was commissioned to set up a new telescope by Percival Lowell.

Lowell was an amateur astronomer, fascinated by telescopic images of Mars that included long lines. The linear features seemed to run from the poles toward the middle of the Red Planet. Lowell speculated the features were Martian canals, dug by an advanced civilization to bring water from the poles to lands were it was becoming more scarce.

Unlike most of us, Lowell had the money to bankroll the investigation of his ideas. (In Lowell’s family, when one generation died, it handed over real wealth to the next round or two of Lowells. When my grandmother died, I inherited a sweater. But I digress.)

Once Douglass got Lowell’s telescope set up, he studied Mars alongside Lowell. But Douglass came to doubt Lowell’s interpretation of what could be seen of Mars. In time, Lowell fired Douglass.

Douglass made a living in Flagstaff for a while as a Justice of the Peace and also by teaching. But he was a true scientist, and he kept his research interests alive as best he could. One of his interests was Our Mr. Sun.

One thing astronomers had noted about the sun were the dark patches, or sunspots, that sometimes could be seen on its face. The sunspots varied a lot in number over substantial periods of time, and they also appeared to go through a much faster 11-year cycle of smaller ups and downs.

Some people wondered if the sunspots created climate change on Earth. It seemed important to understand them better if they determined whether farmers would soon have good or bad yields. But studying the sunspot cycles was limited to the time people had been looking at the sun’s face and keeping records about it.

Here’s where some creative thinking comes into the story.

Douglass couldn’t help but see the lumber industry working around Flagstaff, cutting down the big ponderosa pines of the area. He noticed that the width of the tree rings in the trees varied quite a bit. He wondered if the 11-year cycle of the sun had influenced climate in Arizona in a way that was recorded in the growth of the ponderosas.

Douglass went to work looking at the freshly cut trees. He measured the width of tree rings, the thick ones and the thin ones. He established clear patterns in the ponderosas of the area, with some distinctive thick-thin sequences in the rings. Of course, it was easy to count the rings back through time to learn in what specific year the trees had done well versus when they hadn’t.

Then Douglas started to look at dead wood of the area. The outermost rings of some specimens of fallen trees sometimes matched up with the distinctive sequence of the living trees. When that happened he could count farther back in time with the older wood, and extend his thick-thin record keeping.

Next Douglass started to use old wood in the archeological sites of the Southwest, wood from Navaho hogans and even older structures. Eventually he and his colleagues who had taken up his methods had a good record of the thick-thin rings in the Southwest going back to the days of the most ancient ruins of the region.

What Douglass discovered in the tree rings were clear patterns showing how much climate could vary. There were years and decades of miserable tree growth, then long stretches of time in which the trees had flourished. Most interesting of all, it started to seem likely that climate change was a factor in what had brought early civilizations in the Southwest to their knees.

Tree rings are still being studied around the world. They give us one picture of how climate has varied — and it’s not a comforting bedtime tale about a kindly Mother Nature.

Dr. E. Kirsten Peters, a native of the rural Northwest, was trained as a geologist at Princeton and Harvard. Follow her on the web at rockdoc.wsu.edu and on Twitter@RockDocWSU. This column is a service of the College of Agricultural, Human, and Natural Resource Sciences at Washington State University.

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