Building a better slide

I thought it might be interesting to work through the evolution of one of the slides that I'll use in next month's presentation. I started off faced with a common problem - determining the best way to describe a key part of my technical approach to an audience of non-specialists. The work I'll be describing next month falls under the general field of paleoclimatology. Paleoclimatology (literally 'old climate science') uses information stored in natural archives (things like tree rings, lake sediments and many, many others) to describe how the Earth's climate behaved in the past, before people began making measurements with thermometers, rain gauges and satellites.

I needed a graphic to help me describe how paleoclimatologists:

1. relate their natural record to a climate variable like growing-season temperature or winter snow depth;
2. develop a mathematical model that mimics that relationship, and;
3. use the model to estimate climate conditions during the past several hundred or thousand years.

Cramming all those ideas into a single graphic could easily lead to a dense, complex slide – exactly what I didn’t want to show this audience. Spending a lot of time describing my methods would also take time away from more important points. I needed something simple.

My first idea was to borrow an image from William Ruddiman’s excellent book, Earth's Climate: Past and Future. This is really a great textbook for undergraduate and graduate-level courses on the Climate System, but it also stands out because of its visual design. In collaboration with a professional illustrator, Dr. Ruddiman produced a set of graphics that are technical accurate and aesthetically pleasing. One of the figures in the book outlines how tree rings are used to develop estimates of past temperatures. It wasn’t exactly what I needed – I’ll be talking about river flow, not temperature – but it was pretty darn close. And because Dr. Ruddiman has made all of the books graphics available free online, I wouldn’t have needed to do any work other than downloading the file.

Close, but not quite right (Ruddiman, 2000).

Using someone else’s graphic probably would have worked OK, but as I thought about it, I saw a few problems for using this particular diagram for my talk. Ruddiman’s diagram represents tree-ring records and temperature data as two time series – a pair of coloured, squiggly lines. Squiggly lines are basically the hard currency of most climate science. I worried that the audience would be sick and tired of them by the time I spoke. The diagram also has a fair amount of embedded text describing the steps in the methods. It also has five arrows that go off in four different directions. I realized that the diagram was just too complex – great for a printed figure, not so great in a presentation.

I decided to make my own graphic. I wanted something that would highlight my basic point – how information from trees is converted into a quantitative estimate of river discharge. Trees and numbers and water. Trees and numbers that describe water.

My inspiration.

I had just watched Scott Elias’ excellent ‘Taking Your Slide Deck to the Next Level’, and liked the way he used a simple black-and-white scheme in some of his slides. I adapted his design to include an image of a tree on one side, and a stand-in for water and numbers on the other. I briefly considered using a picture of a real tree but decided a vector graphic of a tree silhouette would be more effective in representing trees in general.

Figuring out how to describe numbers and water was harder. I went through a few ideas – wavy lines representing the river, a line plot showing changes in river flow over time – but most of them were too complicated or just confusing and weird. Finally, I decided to strip the idea down as far as I could, and use a set of numbers paired with hydrological units. I added a fade-effect to the numbers, which I thought gave the impression of a continuous series of estimates stretching far back in time.

My last change added a bit of color. I liked the starkness created by the black and white color scheme, but it was maybe a little too harsh. I placed an arrow to connect the tree to the set of numbers, and coloured it to match the other slides in my presentation.

Trees become numbers that describe rivers in the past

I’m very happy with the final slide. It illustrates the main components of my idea, and doesn’t have any extraneous details that could distract me or my audience. It also fits exactly with my point of discussion, which means that I won’t waste time by explaining the differences between my ideas and my illustrations.