A Global Warming Worksheet

One of the problems we run into is deciding which experts to trust.

Generally, whatever the topic at hand, you can rely on finding an expert who will say whatever you want to hear about it.

Global warming is one topic where you can build a collection of experts you agree with. Accordingly, Holman W. Jenkins, Jr. offers his "global warming worksheet" in the Wall Street Journal. (2/1/06)

As used by the media, "global warming" refers to the theory not only that the earth is warming, but doing so because of human industrial activity. How can a reasonably diligent citizen assess this claim? Measuring average global temperature is not an easy matter.

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But even if a change is measured, how do we know it's manmade?

The answer is, really, we don't. But how can we evaluate the various claims?

Well, [we] could begin by evaluating the claim that carbon dioxide in the atmosphere has increased from 0.028% to 0.036%...

This finding is so straightforward, it's reasonable to assume it would have been widely debunked if unreliable. Next, the claim that this should lead to higher temperatures because of the heat-absorbing qualities of the CO₂ molecule. A reasonable person might be tempted to take this finding on faith, too, for a different reason: because even ardent believers in global warming accept that this fact alone wouldn't justify belief in manmade global warming.

That's because all things are not equal: The climate is a vast, complex and poorly understood system. Scientists must resort to elaborate computer models to address a multiplicity of variables and feedbacks before they can plausibly suggest (choice of verb is deliberate here) that the net effect of increased carbon dioxide is the observed increase in temperature. By now, a diligent layperson is equipped to doubt any confident assertion that manmade warming is taking place. Models are not the climate, and may not accurately reflect the workings of the climate, especially when claiming to detect changes that are small and hard to differentiate from natural changes.
Note this doesn't make our conscientious citizen a global warming "denier." It makes him a person who recognizes that the case isn't proved and probably can't be proved with current knowledge. He's also entitled to turn his attention now to the nonscientific factors affecting public professions of certainty about manmade global warming.

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...The problems associated with climate change (whether manmade or natural) are the same old problems of poverty, disease, and natural hazards like floods, storms, and droughts. Money spent directly on these problems is a much surer bet than money spent trying to control a climate change process that we don't understand.

Not really a worksheet, but a list of things to consider. In order to evaluate the question of manmade global warming, we have to evaluate the following chain of inferences:

1) Humans have contributed a significant amount of CO₂ to
the environment, increasing levels above where they would otherwise be.

2) The increase in CO₂ is increasing the ability of the planet to trap and retain heat, driving the average temperatures above where they would otherwise be.

3) The increase in temperature, and/or the increase in CO₂ levels, cause, on the margin, more harm than good.

Each of these inferences has a number of assumptions buried in them, and a number of ways they could go wrong. As long as the chain of reasoning has these possible gaps in it, is it worth spending trillions of dollars on the chance it might be true?