Scale on "Global Warming" graphs

Since graphs are visual, we tend to measure them visually at first glance. At first glance, "steep" means visually steep. Consider these two graphs of company profits. Visually, one appears to show stagnating profits, while the other appears to show growth. Yet, the facts depicted are absolutely identical. The only difference is the y-axis scale. This is one of the oldest tricks in the book. Many company annual reports have stuff like this.

When Al Gore present his "global warming" graph, it looks something like this. the upward slope is visually steep. That's the message Gore wants his audience to take home. However. we aren't illiterate, so we take a second look. What's the scale? Is it appropriate? There appears to be an upward trend, but how do we judge if it is significant?

An expert may have an involved answer, but how do I -- a lay-person -- go from first glance to second-glance?

Here is how I approach the notion of an appropriate scale as a layperson: I try to relate the information to some concrete of the same type, which is familiar. If I have a familiar concrete of the same type, and overlay this information over that, I can better relate the visual to reality. So, here is the lay-person attempt:

1. This is a graph of temperature, so let me consider some familiar climate/weather temperature. Let me choose a city in the middle of the U.S. Let's say, I choose temperature variations in Topeka, Kansas.
1a. Google finds information on Topeka's averages for each month.
1b. I convert to Centigrade (because that's what the "global warming" graph uses)
1c. I express the mean of all months as "0 C", and express others as Centigrade above or below. (This does not change the scale, simply the naming of the Y-axis, to make it similar to Gore-style charts.)


2. That was a single "average" year. Next, I ask myself what Topeka's temperature would look like if it had the same "average-year" type of temperature year after year. So, instead of 12 months on the x-axis, I pack in about 30 years. Since I did this manually, it's not exactly even. Still, I would expect the actual year-after-year variations to be far more uneven. So, this is a pretty good depiction of what Topeka's temperatures would look like over 30 years is it is are basically unchanging.

Eyeing the tops and the bottoms, we can see what an almost zero-trend would look like.




3. Now, use the "zero-trend" as a background and overlay the "global warming" graph onto it, bringing the scale of that graph down to match the scale of this one. Here is what we get:

Are you scared now?

Compared to our flat background, the "global warming" graph does show an upward trend, but imagine Gore presenting this graph to his audience. Would it have the same impact?

It is the same data, after all.

I do concede that I'm presenting this as a lay-person. Maybe an expert will explain why this graph that looks insignificant from a lay-person's viewpoint is really significant. That's fine. However, to present this as if it is obviously a steep and significant slope is to play the charlatan.

Look at this correctly scaled graph and add in the fact that some experts question the data and say that it really isn't even as "steep" as depicted. Pardon me if I'm skeptical!