Evidence, Skepticism, and the Scientific Method

Judith Luber-Narod, a high-school science teacher at the Abby Kelley Foster Charter Public School in Worcester, Mass., has incorporated climate change into her environmental studies classes, even though she teaches in a somewhat conservative area.

“I hesitated a little bit talking about something controversial,” she said. “But then I thought, how can you teach the environment without talking about it?”

Her students, on the other hand, love topics some deem controversial, she said. She devised an experiment in which she set up two terrariums with thermometers and then increased the level of carbon dioxide, the main greenhouse gas, in one of them.

The students watched as that terrarium got several degrees hotter than the other.

“I say to them, ‘I’m here to show you the evidence,’ ” she said. “ ‘If you want to believe the evidence when we’re done, that’s up to you.’ ”

I’m still working on that Dark Energy post, but it is proving to be ‘interesting’ to write. In the meantime, I wanted to talk a little about the role of experiment and skepticism in Science. The quote above comes from a New York Times science article New Guidelines Call for Broad Changes in Science Education. I don’t mean to be hard on the teacher. I do mean to be a little hard on the author and editors. But mostly, I’d like to use this as a cautionary tale showing why Good Science is not easy to do.

So, what’s wrong with the little experiment designed to show students how the greenhouse gas carbon dioxide raises Earth’s temperature? Almost everything. In particular, it is a great example of how a little knowledge is a dangerous thing, and how the role of experiment is often misunderstood.

First, the greenhouse effect is not really how a greenhouse warms. The glass of a greenhouse will indeed absorb infrared radiation, reradiating some heat–which would otherwise escape to the outside–back into the greenhouse. But this effect is quite minor, and real greenhouses warm because the glass enclosure blocks convection, preventing hot air from rising and being replaced by cooler air flowing in to take its place. It is (relatively) easy to demonstrate this by replacing the glass panes of a greenhouse with panes made of rock salt. The rock salt is transparent to infrared radiation, and so does not stop radiative cooling. The salt panes do block the formation of convective air currents just as well as glass. A greenhouse with rock salt panes will warm like a glass greenhouse, so the real warming mechanism is the elimination of convective flow and not the reduction in radiative cooling.

Similarly, in almost all terrarium experiments like the ones mentioned above, the real warming mechanism at work is not the carbon dioxide keeping the infrared radiation from carrying off heat energy, but the carbon dioxide inhibiting the formation of convective currents of air. The carbon dioxide, being heavier than air, stays within the open top terrarium. It doesn’t get hot enough to rise over the rim of the terrarium and allow cooler outside air to flow in. This is (relatively) easy to demonstrate by using argon gas instead of carbon dioxide. Argon is heavier than air, and argon is transparent to infrared radiation (like the rock salt). A terrarium filled with argon gas will heat just as well as one filled with carbon dioxide. Ergo, the warming effect has very little to do with the carbon dioxide reducing radiative cooling of the objects in the terrarium.

So, what about “I’m here to show you the evidence. If you want to believe the evidence when we’re done, that’s up to you” that the teacher claims? The problem is the experimental result (the terrarium warming) has more than one explanation, and the experiment isn’t designed to eliminate effects other than greenhouse gas style radiative warming. Good science is really hard, because even if you see a predicted effect, it is necessary to rule out alternative explanations for the observed evidence. If your hypothesis predicts A, but evidence shows B, the hypothesis is wrong. But if the hypothesis predicts A and the evidence shows A, this doesn’t necessarily show the hypothesis is correct. Experiments must be designed to test all other explanations for A and rule them out before the evidence shows the hypothesis is correct.

Science requires skepticism. Science requires more than even a theory agreeing with the evidence. Sometimes, what you see isn’t quite what you (or your teacher) think it is. Don’t be hasty to agree with authority. Be skeptical.

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1 Comment

  1. Fred

     /  April 26, 2013

    The NYT just came out with a nice blog post on this very topic.

    http://opinionator.blogs.nytimes.com/2013/04/25/what-do-scientific-studies-show/

    Reply

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