Holman Jenkins, Jr. in a Wall Street Journal editorial yesterday pointed out the differences between models and reality, facts and theory, and the sorts of things its possible to do something about and those which it is not.

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. It’s a big planet, with lots of ways and places to take its temperature. Scientists, naturally, have to rely on record keepers in decades past, using different instruments, to produce what has become the conventionally accepted estimate of a one-degree rise over the past century.

But even if a change is measured, how do we know it’s manmade? Giant, mile-thick sheaths of ice have come and gone from North America in recent millennia. In our unstable and evolving planet, temperature is often either rising or falling. Who knows whether a trend is the product of human activity or natural?

The answer is nobody. All we have is hypothesis. Let’s be honest: A diligent and engaged citizen judges these matters based on the perceived credibility of public figures who affiliate themselves with one view or another. Less engaged citizens, whose views are reflected in polls showing a growing public concern about global warming, are simply registering the prevalence of media mentions of global warming.

In both cases, it may be rational to assume there wouldn’t be so much noise about global warming unless responsible individuals had validated the scientific claims. This is a rational assumption, but not necessarily a reliable one. Politicians adopt views that are popular in order to be popular. Scientists subscribe to theories that later are proved to be wrong. There are “belief” processes at work even in the community of climate researchers.

So how else might an intelligent layperson judge the matter?

Well, he could begin by evaluating the claim that carbon dioxide in the atmosphere has increased from 0.028% to 0.036% without necessarily taking the measurements himself. 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.

Nobody doubts, for instance, that when Bill Clinton asserts global warming is the greatest threat to mankind, he’s consulting not the science but a purported “consensus” of scientists. A layman asks himself: What can “consensus” mean if it asserts a judgment nobody is equipped to confidently make?

Likewise, a study that made news worldwide last month purported to show the death of frogs from warming. It did not show the death of frogs from manmade warming — the study contributed zero evidence one way or another on a human role in climate change. You would have thought otherwise from the media reports. Ditto Al Gore, who offers a traveling slide show (now a movie) in which he catalogs possible dire consequences of global warming in non sequitur fashion to persuade audiences that climate change is caused by human activity and would yield to human action.

Myanna Lahsen, an anthropologist who spent several years observing and interviewing staff at the National Center for Atmospheric Research, shows in a new paper that even climate modelers themselves, who appreciate better than anyone the limits of their work, nonetheless slip into unwarranted certainty in public. She quotes one: “It is easy to get caught up in it; you start to believe that what happens in your model must be what happens in the real world. And often that is not true.”

All this explains why, inevitably and unfortunately, today’s debate over global warming revolves almost exclusively around the status and motives of spokesmen for opposing viewpoints, rather than the science and its limits. Yet this is a story of progress.

Tony Blair, whose government has been a steady sounder of climate warnings, now says he recognizes the improbability of nations sacrificing their economic growth based on uncertain climate science.

He and many others also recognize that 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.