Human Energy Systems | Activity 1.4

Return the graphs of arctic sea ice from Activity 1.3 to each pair of students. Ask them to look at their graph for two minutes and describe what information the graph conveys in words to each other. Tell them that “overall” is the key word here: instead of looking at each individual point on the graph, they should look for the general “trend” that is emerging when all of the data points are put together.

• Pass out one copy of 1.4 Drawing a Trend Line Worksheet to each student. Have them write a short description of the trend that they see in their arctic sea ice graph on the first part of the worksheet.
• Ask students to share their initial descriptions of the overall trend of the arctic ice graph. At this point, listen to their ideas, but do not correct their ideas.
• Ask questions that will help reveal how the students came to their conclusions like: Can you tell me more about how you decided that? Or Who can add to that? Or Who had a similar or different description? Or How is your description similar or different?

Open 1.4 Drawing a Trend Line PPT. Use slides 3-4 to show students the graph of land and ocean temperatures during the winter of 2013-2014. Point out that during this winter, the Great Lakes region of North America had temperatures that were below average. However, if we relied only on the data from this region, we might have missed the overall global pattern in temperature for that winter.

• Use slide 5 to have students discuss with each other what the global temperature trend was during the winter of 2013-2014. First, have them turn and talk and describe the global pattern. Then, have a few pairs share their description with the class.
• Use slide 6 to construct a class explanation of the global pattern. Write their ideas on slide 6. At this point, it is ok to evaluate and correct student thinking. If they are having trouble identifying that 2013-2014 winter was warmer than average, ask them for more details about their reasoning. Help them move towards a class description that identifies the global trend of higher temperatures, despite the lower temperatures in a few regions.
• Lead a short discussion about Generalizability. How well does this image of temperature represent global patterns? What about their arctic sea ice graph? Does that tell us anything about what is happening globally, or just in one region? Pose these questions to the class, but do not expect the students to have developed ideas at this point. They will discuss this further in Lesson 2.

Tell students that this first example represented temperature data using colors and a map, and that in the next example we will practice finding a trend (a pattern of change over time) in a line graph (or scatter plot). When trying to identify trends on a scatter plot, we are looking for three options: a negative trend (where the data go downward from left to right), and positive trend (where the data go upward from left to right) and no change (where the data generally move in a flat line from left to right).

• Use slide 7 to demonstrate what a negative and positive trend look like on a scatter plot.
• Use slide 8 to show that there are different ways of calculating a trend line. The graph on the top has a straight line because it was calculated using a mathematical formula that considers all data points in the graph. The bottom graph has a curvy line because it was calculated using 5-year averages, which only considers data points in 5-year periods.
• Tell students that sometimes data sets on a scatter plot are messy, and that it’s not always possible to see the trend with the naked eye. This is why drawing a line (like the lines on slide 7) are helpful for identifying trends that are difficult to see.

Use slide 9 of the presentation to introduce students to a line graph of maximum ice cover on Lake Superior. This graph shows the maximum ice cover from 1973-2013 for each year. Have students look at the graph without a trend line and ask them: Do you see any trend here? Point out that it is difficult to identify trends in messy data sets like this one without drawing a trend line.

• Ask students why a scientist might want to make a trend line. Listen to their ideas. If they do not make the suggestion, add that scientists make graphs because it can help them see the data in ways that they can’t see from looking at just the pictures or just the numbers. It helps them look past short-term variability and to see long term trends. Point out that trend lines help see what is happening when the data points are messy and noisy due to short-term variability.
• Ask students about the short-term variability they see in this graph. Is it predictable?
• Ask students to predict, based on the graph, what the arctic sea ice extent might be in a year from the time the graph ends. They should have difficulty making this prediction! It is difficult to make a prediction due to the stochastic variation in the graph (meaning that the extent of the sea ice changes from year to year based on many random factors, and there is no clear short-term pattern).

Ask: If you wanted to make a trend line, how would you do it? How would you “smooth” out the data? Listen to ideas from the students about how they might look for a trend in the data. Ideas may include:

1. Draw a straight line that goes in the general direction of the trend.
2. Draw a straight line that touches as many data points as possible.
3. Draw an oval around all the points and then draw a line through the oval.
4. Find averages for every few years and make a data point and then connect those points.

Turn students’ attention to #2 of their worksheets. Overview the five steps involved in drawing a trend line for Lake Superior Ice:

• Step 1: Connect the dots (this is already done on the Lake Superior ice graph).
• Step 2: Divide the picture into sections by drawing vertical lines on the graph every five years.
• Step 3: Calculate the average of the data points within those five-year time periods.
• Step 4: Place one star in each section that shows the average for that time period.
• Step 5: Draw a line to connect the new stars.

Have students complete #3 on their worksheet, which asks them to write a description of their trend and to label it positive, negative, or no change.

Divide students into pairs to compare trend lines for Lake Superior ice cover. In what ways are they the same and/or different? Have students work out any discrepancies they may see. When students feel comfortable with the process, move on to the next activity.