Visualizing Arctic Sea Ice Extent Trends

Reader GH sent me an e-mail asking about a previous Arctic sea ice extent trend post (click). GH asked ….

“Why is there such a difference between this type of representation and the chart at link ? What you’ve written above seems to imply that the definitions of extent are the same.   Just looking at 2002 – present,  I’m not clear why the JAXA chart doesn’t appear to demonstrate the same clear trend. ..”

This is a great question and goes to the heart of effective data visualization of climate data. Edward Tufte, the leading data visualization expert, advises…

“To be truthful and revealing, data graphics must bear on the question at the heart of quantitative thinking: “Compared to what?” Source: The Visual Display of Quantitative Information

When making or looking at a data chart, we need to ask the question, “compared to what” to make sure we are clear about what the chart is comparing.

Arctic Sea Ice Extent (ASIE) Data Comparisons

Let’s apply Tufte’s “compared to what” question to 2 Arctic Sea ice Extent charts:

• JAXA’s 2002 – 2010 Daily Arctic Sea Ice Extent chart
• My March Arctic Sea Ice Extent Trend Chart, 1979-2010

Here’s JAXA’s chart:

Here’s my R script based March Arctic Sea Ice Extent Trend Chart:

Both charts are displaying Arctic Sea Ice Extent, the JAXA chart shows daily values using JAXA satellite imagery, my chart shows March average sea ice extent using the NSDIC data. While the JAXA and NSIDC values are different because of  different data sources and interpretation methodologies, the seasonal and long term trends are comparable.

Applying “Compared to What?” To These Charts

• JaXA Chart
  Daily ASIE compared to day of year by year for period 2002 to 2010
• My Chart
  Average monthly March ASIE compared to year for period 1979 to 2010

The JAXA chart shows daily ASIE values compared to day of year. My chart shows  March monthly average ASIE values compared to year. The JAXA chart demonstrates the annual cycle well, it does not clearly show the year to year trend. My chart shows the year to year trend in March ASIE well, it does not show the annual cycle.

It should also be noted that the JAXA data set extends from 2002 to 2010,while the NSDIC data set extends from 1979 to 2010. In addition to its comparison design, the JAXA  chart’s short time period (9 years) and jumble of colors needed to identify individual years seriously limits its value in interpreting the long term trend in ASIE.

The March ASIE values trend chart is more effective than the JAXA chart for showing long term trends because it includes a longer data period (32 versus 9 years) and it provides direct comparison of March ASIE values year to year.

Conclusion

Each chart is designed to address a specific question. Both charts are helpful to understand the Arctic Sea Ice Extent situation. Trying to interpret long term trends from the JAXA chart is unwise. Trying to discern annual cycle information or the trends for a different month from my chart is unwise.

If you find yourself trying to “interpret” a chart, be sure to ask the “compared to what?” question to make sure you are clear about what is being compared.

If you find yourself asking “what about … or what happens when…”, it’s probably time to make another chart that directly addresses your new  “compared to what” question. Don’t expect one chart to answer multiple questions.

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