Climate assessment color gradients

After looking through the 1500 pages of the US 2017/2018 Fourth National Climate Assessment, I was struck by the variety of color gradients used throughout. As someone who writes software that includes making color gradient legends, I was interesting in surveying current scientific practice, so I started clipping out gradients from the report. Here is a collage I made of the color gradients in Volume I: Climate Science Special Report:

Single image showing about 25 different color gradient legends of different colors and styles.

It’s interesting to consider all the ways they differ.

  • Choice of colors, even with differences within the same families of colors
  • Continuous or discrete (and how many discrete levels)
  • Horizontal or vertical
  • Diverging, sequential or rainbow ordering
  • One gradient or separated solid color boxes/dots
  • Labeling the boundaries or centers
  • Squared or triangular end-caps
  • Use of “<” and “>” in labels
  • Use of tick marks for the labels
  • Divider lines between colors: black, gray or none at all
  • Font size for labels
  • Thickness
  • Background color

And that’s not even considering content-based differences like Celsius vs. Fahrenheit degrees. I also made a collage for Volume II: Impacts, Risks, and Adaptation in the United States, which was twice as along:

Single image showing about 35 different color gradient legends of different colors and styles.

Even more variations appeared such as rotated label text and variable-width discrete gradients.

I’m not saying all the gradients should be the same. Many of the differences are driven by the data, such as using a sequential or a diverging order and using semantic colors (blues for water data and reds for heat data). But many others are purely stylistic, such as the frame boxes, tick styles, label locations, and which blue-yellow-red colors to use.

When I first posted these to social media, some responses pointed out the multiple authors involved, which is true. The reports are quite clear that each chapter is authored by a different team of scientists. However, somehow all the chapters use the same fonts, header formatting, bibliography style, sidebar coloring, etc. That is, the authors are all using the same style sheet for the text — why not for the graphs, too?

I know such a mechanism doesn’t exist, at least across multiple graphics tools, but I think a little effort could be made for more consistency. Can’t wait to see if there’s any improvement in the next report currently scheduled for 2023.

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