Writing Alt Text for Complex Scientific Figures: A Report from the National Climate Assessment

The Fifth National Climate Assessment (NCA5) contains nearly 400 photos, infographics, and charts. For each of these figures, the NOAA Technical Support Unit (TSU) wrote alternative (alt) text—descriptive text that conveys the meaning of the image.

Writing alt text for scientific figures provides a crucial resource for people who use screen readers. It’s also the law: Section 508 of the Workforce Rehabilitation Act of 1973 requires federal agencies to make information “accessible to ... individuals with disabilities.” That means, among other things, that visual elements of federal reports must be accessible to those with visual challenges.

Although federal law requires alt text, it leaves wide latitude as to what shape it should take. On the federal website section508.gov, the first two items of advice are (1) “Alt text should be short and to the point” and (2) “Alt text should communicate the same information as the visual content.” Those two guidelines are often in conflict: when writing alt text for complex figures, generally there is no way to be both brief and thorough.

For NCA5, the NOAA TSU took a maximalist approach, on the theory that alt text should convey as much of the information contained in the figure as possible—that is, it should present information in a way that’s equivalent to the visual experience of the image. We wrote a total of about 83,000 words of alt text, an average of more than 200 words per figure. We think alt text is important, we spend many hours writing and revising it, and we take pride in it. We even added a simple way to copy and share a figure’s alt text on the report website. We know relatively little, however, about whether this work is effective, because we get little feedback on it from those who use screen readers.

This presentation is a summary of practices and a request for input. We will describe NOAA TSU’s approach to writing and implementing alt text, with detailed examples. We will summarize available guidance from the federal government and organizations such as WebAIM and W3C. And we will solicit advice as to how we can improve our work as we begin work on the next edition of the National Climate Assessment. Toward this end, the presentation will address—and seek feedback on—questions including the following: How are people using the alt text in NCA5 and other scientific reports? Is lengthy alt text useful or burdensome? How should alt text for data figures be handled: through text descriptions, through the same data presented as screen-readable tables, or both? How will artificial intelligence reshape the world of alt text? Our goal is both to improve our own work and to help others engaged in writing alt text for complex figures, as we advance the shared goal of making scientific reports accessible to all.