An open letter to the climate science community
This is a heartfelt plea.
A plea to you all to help rid climate science of colour scales that can distort, mislead and confuse. Colour scales that are often illegible to those who are colour blind.
The main culprit is, of course, the ‘rainbow’:
We have all likely used it, and we have all certainly seen it – presentations, posters, papers, blogs and news articles full of figures with similar colour scales.
However, the most commonly used rainbow colour scales can distort perceptions of data and alter meaning by creating false boundaries between values. There are numerous blogs and published papers from visualisation experts illustrating these issues. In one example, changing to a non-rainbow scale even improved accuracy of heart disease diagnoses.
This is not the first such plea.
A decade ago an article appeared in EOS, demonstrating that contrasting red with green can render a figure illegible to the 8% of the male and 0.4% of the female population who are colour blind. The EOS article suggested that journals should do more to improve the colour accessibility of figures.
But, the problem is now worse than a decade ago. Most issues of every major climate journal have figures which are potentially misleading and colour inaccessible. Maps, line graphs and histograms can all have confusing colour combinations.
Journals, rightly, do not tolerate poor grammar, incorrect spelling, or muddy descriptions of scientific methods. It should be no different for visual communication. We should be equally intolerant to poor use of the grammar of graphics as we are to its written equivalent.
It is not just the journals who need to act. As scientists increase their efforts to make their work accessible to the public through the media, blogs and social media, there are more opportunities to show poor figures.
What are the possible solutions?
We need to be more willing to discuss and criticise the visualisation of the science as well as the science itself.
Authors should be responsible about the colour choices they make. Journals might add colour accessibility to their existing guidelines for acceptable figure types. Reviewers could recommend revision if such colour scales are used. Editors should not accept papers which use inaccessible and potentially misleading colour scales. And, the media might reconsider using such figures from published work.
We know ‘rainbow’ is the default colour scale in many commonly used programming languages, but that doesn’t make it the best. Resources are easily available to change colour scales for R, IDL (& here), MATLAB, Python (& here) & Ferret.
There are numerous websites and online tools giving advice and recommending safe and better colour scales (such as Color Brewer or HCL Wizard). You can even test online or use an app to see how your figures might appear to those who are colour blind.
Choosing a good colour scale is not difficult – it just takes awareness and a few moments of effort. The best choice will probably depend on the situation, so ask yourself why you have chosen that particular colour scale. Adding different shape markers in line graphs might also aid interpretation.
We take heart from some recent progress.
The journal BAMS recently took a step forward by publishing an article pointing out the flaws with rainbow colour scales. MATLAB have just announced that they are changing the default rainbow colour scale, giving a comprehensive explanation considering colour accessibility and perception issues.
All of us could do more in improving the clarity of our figures, the authors of this open letter included. More needs to be done. And, it needs all of us to do more.
So, we undertake this pledge – to never again be an author on a paper which uses a rainbow colour scale.
If you agree to also make this pledge (or disagree), please comment below this post. Or email us. And tell your colleagues.
We hope that you will join us.
UPDATE: Nature have published a small version of this letter in their correspondence pages
Extra links have been added since the letter was originally published. The Climate Explorer has now updated its colour scales due to this campaign and several journals are discussing the issues raised.
Other climate-related case studies:
Example of simulating colour blindness with different colour scales & MATLAB software: Which colour scale is best for you?
Considering different colour scales for sea-level change maps: Better palettes
And, it is not just climate. One of the iconic images in astronomy – the Cosmic Microwave Background (CMB) – is normally in rainbow
Many thanks to all those who have patiently commented on these issues.