28 February 2011

Graphic analysis: “Climate data challenges”

Posted by Callan Bentley

ResearchBlogging.orgThis image, in the February 11 issue of Science, accompanies an article by Overpeck, et al., in which they talk about climate data challenges. I would argue that this graphic offers some challenges of its own. Though the data are sound and the paper addresses a critical issue in a sober and thoughtful way, I think the design of this graphic could use some work.

My chief critique is that this graphic tries to do too much. It simultaneously attempts to:

  • list sources of information which lead to climate predictions,
  • list natural and human realms that will be affected by climate change,
  • show the past temperature changes (as anomalies),
  • show predictions for future temperature changes (as anomalies),
  • show the global distribution of past temperature changes (as anomalies), and
  • show predictions for the global distribution of future temperature changes (as anomalies).

My critiques are:

  • The color scheme for the past versus the color scheme for the future is odd: the indigo and white of the “past” evokes cold without being quantitative about it — a psychological association which is nonscientific. The background color scheme for the future has less of a color gradient (medium blue to lighter blue), and does not bring any clear psychological associations up. It is less clearly “evocative” of any particular temperature mindset, and so it meets more with my approval than the left side of the background color. However, it is inconsistent to color the graphic in this fashion — why not just a uniform color scheme?
  • Observed “temperature” change (and future “temperature” change) would be a more accurate descriptor of the data being plotted. Climate change includes temperature change, but it also includes variables such as precipitation change, albedo change, storm intensity change, and others. The only variable shown in this image is temperature anomaly, so let’s call a spade a spade.
  • From +2.5°C to +8.0°C, the color changes in “redness” are too subtle to be immediately discernible (and therefore meaningful).
  • The contrast of the red on the scale bar at left with the background is different from the contrast of the red on the globe at right with the (different) background — this makes it more difficult to read the graphic.
  • The arrow with its cavalcade of associated words is clunky and bizarre. It feels very Pravda to me, though I doubtless have Edward Tufte to thank for that association. Is the arrow simply leading from the current state of the Earth to the predicted (high end) future state of the Earth? If so, why is the arrow inclined in slope? Perhaps it has something to do with the fact that …
  • … the position of the “global” maps are located at different vertical positions on either side of the past/future divide.  The bottom of each globe icon sits at the temperature anomaly for the present (+1°C) and the future (year 2100, high-end prediction, +4.2°C), This strikes me as gimmicky at best, redundant certainly (since the graph at bottom already conveys the same average global temperature anomaly information), and deliberately provocative at worst. I think this is the fatal flaw of this graphic — in spite of representing valid and important scientific information, this shifting upward of the future globe by the same vertical distance as 3.2 degrees of temperature anomaly magnitude is hokey and propangandist, and undermines the credibility of the work.

What are your thoughts?

I’m grateful to Lee Allison for posting a note about this study (and including the graphic) on his blog.

Overpeck, J., Meehl, G., Bony, S., & Easterling, D. (2011). Climate Data Challenges in the 21st Century Science, 331 (6018), 700-702 DOI: 10.1126/science.1197869