What’s Going On in This Graph? | Tree Rings and Climate (2024)

On Wednesday, Sept. 21, we will moderate your responses live online. By Friday morning, Sept. 23, we will provide the “Reveal” — the graph’s free online link, additional questions, shout outs for student headlines and Stat Nuggets.

1. After looking closely at the graph above (or at this full-size image), answer these four questions:

• What do you notice?

• What do you wonder?

• How does this relate to you and your community?

• What’s going on in this graph? Create a catchy headline that captures the graph’s main idea.

The questions are intended to build on one another, so try to answer them in order.

2. Next, join the conversation online by clicking on the comment button and posting in the box. (Teachers of students younger than 13 are welcome to post their students’ responses.)

3. Below the response box, there is an option to click on “Email me when my comment is published.” This sends the link to your response which you can share with your teacher.

4. After you have posted, read what others have said, then respond to someone else by posting a comment. Use the “Reply” button to address that student directly.

On Wednesday, Sept. 21, teachers from our collaborator, the American Statistical Association, will facilitate this discussion from 9 a.m. to 2 p.m. Eastern time.

5. By Friday morning, Sept. 23, we will reveal more information about the graph, including a free link to the article that includes this graph, at the bottom of this post. We encourage you to post additional comments based on the article, possibly using statistical terms defined in the Stat Nuggets.

Reveal

On July 20, 2022, The New York Times published the above graph in the article “This 500-Year-Old Tree in California Has a Story to Tell.” We have also shown a snapshot of some of the tree rings for the years 1538 to 2021 that were used to calculate the statistics in this graph. The photo came from a slice of an ancient big-cone Douglas fir tree from California’s Mount Pinos grove. The slice was removed without harming the tree by dendrochronologists — scientists who date events, environmental change and archaeological artifacts by examining the patterns of tree rings in timber and tree trunks.

How do tree rings show climate change? Every year, trees grow a new ring under the bark, initially with cells that are light in color and ending with cells that are darker. The width of the ring and the patterns of the cells within the ring indicate the precipitation and temperature levels that year and even when they occurred. The soil moisture content is determined from this data. Then, this data is assigned to percentile categories. (See Stat Nuggets below.)

What is the story that these tree rings tell? The story is that Southwest North America (SWNA), an area that includes the six states of California, Arizona, New Mexico, Colorado, Utah and Nevada, is experiencing the most severe megadrought in 500 years. (A megadrought is a severe drought lasting for two or more decades.) The current SWNA megadrought has broken previous records for the driest 22-year period since the year 800 C.E., as reported in Nature Climate Change.

The questions that scientists and others are trying to answer are: What are the effects of this megadrought? When will it end? Is the harm that has been done to our environment reversible or permanent?

Here are some of the student headlines that capture the main stories of this graph. Four from New Jersey: “Drip, Drip, Drop: A Look at the Southwest’s Lack of Water Since the Mid 1500s” by Katie and “Daunting Droughts in the Southwest” by Sophia, both from Academy of Saint Elizabeth in Morristown, and “Left High and Dry, Megadrought Plagues the Southwest” by Jayna and “A Climate Disaster: Southwest US Megadrought Crisis” by Jesenia, both from Perth Amboy High School. In addition, catchy headlines included “Throwing Our Climate Through the Ringer — The Effects of Climate Change Shown Through Tree Trunk Rings” by Issy Senior of Andover High School in Massachusetts and “Southwest Heading Toward Driest Drought since 1572” by Jessica in New York. Thank you to all students who responded.

You may want to think about these additional questions:

The article “This 500-year-Old Tree in California Has a Story to Tell” includes a visual presentation that shows the Douglas fir tree rings for the time periods 1538 – 1586 and 2002 – 2021. Note the cells are beige at the beginning of the growing season (shown further from the outside bark) and are darker toward the end of the season (shown closer to the center of the tree). Take a look below at the tree rings from 2008 – 2020.

• What do you notice? Relate these tree rings to the above stacked area time series graph at the top of this post.

• What do you wonder?

• Create a headline that captures the main idea of this photograph.

Keep noticing and wondering. We continue to welcome your online responses. We’ll host live-moderation for our next graph on U.S. population trends on Wednesday, Sept. 28. By subscribing to the Learning Network newsletter here, you can receive notices of the “What’s Going On In This Graph?” releases on Fridays preceding Wednesday’s moderation.

Stat Nuggets for “This 500-Year-Old Tree in California Has a Story to Tell”

Below, we define mathematical and statistical terms and how they relate to this graph. To see the archives of all Stat Nuggets with links to their graphs, go to this index.

STACKED AREA TIME SERIES GRAPH

Stacked area time series graphs show trends over time by comparing the relative sizes of subgroups of a whole. The horizontal x-axis is time. The vertical y-axis shows a comparison of subgroups absolute numbers or percentages.

In the Tree Ring and Climate graph, the time period is 1520 - 2020. For each year, the amount of soil moisture for land regions in the six-state SWNA was calculated based on the most recent 22 years running total values. This value was assigned its corresponding historical soil moisture percentile. The percentage of the SWNA land that is in each of the six soil moisture percentile categories is shown in the graph using the color coding given in the graph. As seen in the scale, the soil moisture categories are colored from green to brown, with green representing the greatest moisture content and brown the least. The darker the color, the more extreme is the moisture content. The accompanying map and the graph includes orange, which is not in the color coding, but designates the land which is the driest soil since 1520. The soil moisture content categories appear in order with the categories corresponding to the lowest moisture content at the bottom of the graph.

PERCENTILE

Percentiles are numbers that divide ordered, quantitative datasets into one-hundredths. After putting data in order from smallest to largest, you can find the percentiles. For example, for the 90th percentile, 10% of the data are greater than this value and 90% of the data is less than this value.

For the Tree Ring and Climate graph, one way that the soil moisture percentiles might have been determined is using the data from 1520 - 2020. All soil moisture levels were ordered and divided into one-hundredths. For each year, the graph displays the percentage of the SWNA land that falls into each of the six percentile categories.

RUNNING TOTALS

A running total is a total that is continually adjusted to take account of new data. Running totals for a period, rather than individual values for the period, smooth out short-term variations and make it easier to see trends.

In the Tree Ring and Climate graph, climate data is calculated for each year by incorporating data from the most recent 22 years. This running total data was input into models that calculate the percentage of land that is in each soil moisture category.

The graph for “What’s Going On in This Graph?” is selected in partnership with Sharon Hessney. Ms. Hessney wrote the “reveal” and Stat Nuggets with Roxy Peck, professor emerita, California Polytechnic State University San Luis Obispo, and moderates online with Chris Adkisson, a math and statistics teacher at Pulaski County High School in Somerset, Kentucky.

More?

• See all graphs in this series or collections of 60 of our favorite graphs, 28 graphs that teach about inequality and 24 graphs about climate change.

• View our archives that link to all past releases, organized by topic, graph type and Stat Nugget.

• Learn more about the notice and wonder teaching strategy from this 5-minute video and how and why other teachers are using this strategy from our on-demand webinar.

• Sign up for our free weekly Learning Network newsletter so you never miss a graph. Graphs are always released by the Friday before the Wednesday live-moderation to give teachers time to plan ahead.

• Go to the American Statistical Association K-12 website, which includes teacher statistics resources, Census in the Schools student-generated data, professional development opportunities, and more.

Students 13 and older in the United States and the Britain, and 16 and older elsewhere, are invited to comment. All comments are moderated by the Learning Network staff, but please keep in mind that once your comment is accepted, it will be made public.