Initial Publication Date: April 28, 2017

Part 4: Creating Long Tree-Ring Records to Identify Extreme Climate Events

The forests that you have explored so far consist largely of old-growth trees that can live 300-600 years or more but tree-ring scientists have been able to create paleoclimate records that are much longer--over 1000 years old. They are able to do this because occasionally they discover trees that died long ago that were preserved due to the harsh conditions of the area. Scientists refer to this dead wood as "relict wood".

At Ondur Zuun Nuru, in Mongolia, trees that that die and fall to the ground high up on a rocky mountain slope don't decay because they aren't in direct contact with soil moisture (see image above). So a tree that falls to the ground in a mountainous region can lie there for 500 to a 1000 years or more, decaying very slowly. This type of wood is very valuable to scientists. Once relict wood is found, scientist work very carefully to combine tree-ring data from living trees with the data from the relict wood samples. If there is enough overlap in time between data from the living and relict wood (about 100 years), then scientists can determine the exact calendar dates of each ring from the relict samples. This combination of living and relict wood results in very long tree-ring based reconstructions of temperature in these regions.

At Mendenhall Glacier, in Juneau Alaska, scientists discovered logs being left at the front of the glacier as it slowly receded. Trees, like the ones pictured above, are overrun by glaciers long ago, during a much cooler period when they were advancing. Then, as the glaciers receded as temperatures increased in the region, these overrun trees are exposed. Scientists take cross-sections of these logs and carefully crossdate them with living trees, as described above, to create extremely long tree-ring records of temperature. Through this method, scientists have not only been able to create long records of climate, but also have produced detailed histories that capture periods of glacial advancement and recession.

In Wrangell Saint Elias National Park in southeast Alaska, a large local volcanic eruption covered the forest in a thick ash layer, preserving stumps for more than 1000 years. These logs were only recently exposed as the river eroded the bank. Data from these stumps were used to extend the tree-ring records much further back in time.

Investigate a Long-term Northern Hemisphere Temperature Reconstruction

The interactive time-series graph below shows a very long tree-ring reconstruction for the Northern Hemisphere developed collaboratively by Dr. Rob Wilson and other international scientists, from 54 tree-ring sites across the Northern Hemisphere. This reconstruction spans about 1200 years and was developed in part by combining living and relict wood. You might have already explored three of the sites used to create this reconstruction if you did Lab 5, Parts 2 & 3.

Look carefully at the interactive graph below. It shows several different types of data plotted to produce a thousand-year tree-ring based temperature reconstruction. The orange line marks the data acquired by the combination of tree-ring data from relict wood and living trees. The red line is produced by averaging these data over 10-year periods. This calculation smooths the data to give you a better sense of the decadal-scale patterns of temperature change. The blue line show you historically recorded temperature data from meteorological stations. Notice that it starts around 1850 when weather data initially began to be systematically catalogued. The horizontal green line shows you the long-term average of all the tree-ring data.

1. Begin by focusing on the recorded meteorological data in blue. Note: You can use the sliders at the bottom of the interactive graph to explore that recorded period in closer detail. Take notes on any year-to-year or longer trends you see in the data.

2. Now turn your attention to the red line--the 10-year average of the tree-ring data. Look for long-term patterns in the tree-ring based reconstructions. Take notes on patterns that you see that encompass decades or more.

3. Use the sliders at the bottom of the interactive to find periods 5-10 years in duration that were unusually warm or cold using the orange line data. Write down the five most extreme warm and cold periods you can find.

4. Next, identify the individual years that are the most extremely cold. Example: Roll your pointer over the line that appears to be at year 1600. When the pointer is directly on top of the line, the year--1601 in this case--is shown on the x-axis. Record the coldest 10 years you can find between 1000-2004.

Stop and Think

4.1 What trends did you see in the recorded meteorological temperature data (blue line)?

4.2 Describe the general long-term patterns you discovered in the tree-ring based reconstruction.

4.3 List five periods of 5-10 years in duration that were unusually warm or cold over the past 1000 years.

4.4 List the ten most extreme cold years that you identified.

Investigating Extreme Events

One of the coldest years on record was 1601. What possible geological event could have produced such a dramatic decrease in world-wide temperature? To find out, open a web browser such as Google and type in "1601 climate". Answer the questions below.

Stop and Think

4.5 What happened during 1601 that influenced climate on a global scale? Briefly describe the event and its impacts on people.

4.6 How many of the other nine extremely cold years you identified were also related to similar events? You may have to search the year before the extreme year that you identified in the reconstruction above since these type of events can impact tree growth the following year. This site may be helpful:

4.7 Go back to Part 3 of this lab and evaluate the tree-ring cores from the three northern latitude sites. Can you identify very narrow tree rings (cold years) during known volcanic years such as 1601, and any others that you identified?

4.8 Why do large-scale volcanic events cause global climate to cool so dramatically? Research this questions online and summarize your answer here.