As the discussion of global warming progresses, it would be valuable if distinctions could be maintained between what is known, what is reasonably believed and what is mostly being guessed at.

The first of three new reports by the Intergovernmental Panel on Climate Change, which has the best claim to represent a consensus view, covered what was known and what is reasonably believed.

What is known is that global temperatures have been rising. What is reasonably believed is that human greenhouse gas emissions are an important cause and that, unless they are stabilized or reduced, even higher temperatures are likely.

The first IPCC report put the range of the likely increase at 1.8 to 4 degrees Celsius over the remainder of this century.

The next two reports, more recently released, moved very deeply into the mostly-being-guessed-at category.

The second report – dealing with climate change “impacts, adaptation and vulnerability” – was the subject of much doomsday media reporting of a large number of species going extinct, droughts, famine, disease and millions living in coastal areas being wiped out.

The report itself is more circumspect and does not make specific predictions. All of the IPCC reports are expressed in terms of ranges and probabilities.

As the third report, on mitigation, put it: “Because the future is inherently uncertain, scenarios . . . of different futures – not predictions of the future – have been used extensively in this report.”

Many of the doomsday effects spotlighted in media accounts are from the higher end of the temperature change range and with a midrange probability.

This whole report has greater uncertainty since it discusses specific regional effects and the IPCC has consistently said that natural climate variability plays a greater role on a regional scale.

Moreover, what happens as a result of global warming is inherently more speculative than the likelihood of warming itself.

The third report, on possible mitigations, is more speculative still.

Much was made, for example, of the finding that stabilizing greenhouse emissions would cost no more than 3 percent of global Gross Domestic Product in 2030.

Here econometric models are being layered on top of climate models in a gross excess of academic confidence.

Such a projection depends on knowing precisely what global economic output will be 23 years from now and precisely how millions of producers and billions of consumers will respond to a wide array of mitigation regulations and costs.

A number picked out of a hat has as much chance of being accurate.

None of this is a brief for doing nothing about global warming. I’ve previously advocated a carbon tax, and there is some support for that approach in the third report.

Since all participating governments must sign off on these reports, they are in some respects political documents as well. So, the mitigation report makes nice toward all possible remedies.

However, the report does indicate that market approaches (carbon taxes and trading) are likely to be more effective and efficient than a regulation and subsidy regimen.

I suppose that the theory behind blurring the distinction between knowledge and speculation about global warming is that the more serious and certain the perceived problem, the more likely the political will to do something about it will be mustered.

That, however, may be a miscalculation. It might actually be easier to get action based upon what is known, rather than requiring skeptics and the politically reluctant to buy into what is speculated.

Regardless, global warming is a serious issue deserving of a serious discussion. And serious discussions respect important and valuable distinctions.

Read another column by Robert Robb.

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