By MATT RIDLEY

Forget the Doha climate jamboree that ended earlier this month. The theological discussions in Qatar of the arcana of climate treaties are irrelevant. By far the most important debate about climate change is taking place among scientists, on the issue of climate sensitivity: How much warming will a doubling of atmospheric carbon dioxide actually produce? The Intergovernmental Panel on Climate Change has to pronounce its answer to this question in its Fifth Assessment Report next year.

The general public is not privy to the IPCC debate. But I have been speaking to somebody who understands the issues: Nic Lewis. A semiretired successful financier from Bath, England, with a strong mathematics and physics background, Mr. Lewis has made significant contributions to the subject of climate change.

He first collaborated with others to expose major statistical errors in a 2009 study of Antarctic temperatures. In 2011 he discovered that the IPCC had, by an unjustified statistical manipulation, altered the results of a key 2006 paper by Piers Forster of Reading University and Jonathan Gregory of the Met Office (the United Kingdom’s national weather service), to vastly increase the small risk that the paper showed of climate sensitivity being high. Mr. Lewis also found that the IPCC had misreported the results of another study, leading to the IPCC issuing an Erratum in 2011.

Mr. Lewis tells me that the latest observational estimates of the effect of aerosols (such as sulfurous particles from coal smoke) find that they have much less cooling effect than thought when the last IPCC report was written. The rate at which the ocean is absorbing greenhouse-gas-induced warming is also now known to be fairly modest. In other words, the two excuses used to explain away the slow, mild warming we have actually experienced—culminating in a standstill in which global temperatures are no higher than they were 16 years ago—no longer work.

In short: We can now estimate, based on observations, how sensitive the temperature is to carbon dioxide. We do not need to rely heavily on unproven models. Comparing the trend in global temperature over the past 100-150 years with the change in “radiative forcing” (heating or cooling power) from carbon dioxide, aerosols and other sources, minus ocean heat uptake, can now give a good estimate of climate sensitivity.

The conclusion—taking the best observational estimates of the change in decadal-average global temperature between 1871-80 and 2002-11, and of the corresponding changes in forcing and ocean heat uptake—is this: A doubling of CO2 will lead to a warming of 1.6°-1.7°C (2.9°-3.1°F).

This is much lower than the IPCC’s current best estimate, 3°C (5.4°F).

Mr. Lewis is an expert reviewer of the recently leaked draft of the IPCC’s WG1 Scientific Report. The IPCC forbids him to quote from it, but he is privy to all the observational best estimates and uncertainty ranges the draft report gives. What he has told me is dynamite.

Given what we know now, there is almost no way that the feared large temperature rise is going to happen. Mr. Lewis comments: “Taking the IPCC scenario that assumes a doubling of CO2, plus the equivalent of another 30% rise from other greenhouse gases by 2100, we are likely to experience a further rise of no more than 1°C.”

A cumulative change of less than 2°C by the end of this century will do no net harm. It will actually do net good—that much the IPCC scientists have already agreed upon in the last IPCC report. Rainfall will increase slightly, growing seasons will lengthen, Greenland’s ice cap will melt only very slowly, and so on.

Some of the best recent observationally based research also points to climate sensitivity being about 1.6°C for a doubling of CO2. An impressive study published this year by Magne Aldrin of the Norwegian Computing Center and colleagues gives a most-likely estimate of 1.6°C. Michael Ring and Michael Schlesinger of the University of Illinois, using the most trustworthy temperature record, also estimate 1.6°C.

The big question is this: Will the lead authors of the relevant chapter of the forthcoming IPCC scientific report acknowledge that the best observational evidence no longer supports the IPCC’s existing 2°-4.5°C “likely” range for climate sensitivity? Unfortunately, this seems unlikely—given the organization’s record of replacing evidence-based policy-making with policy-based evidence-making, as well as the reluctance of academic scientists to accept that what they have been maintaining for many years is wrong.

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How can there be such disagreement about climate sensitivity if the greenhouse properties of CO2 are well established? Most people assume that the theory of dangerous global warming is built entirely on carbon dioxide. It is not.

There is little dispute among scientists about how much warming CO2 alone can produce, all other things being equal: about 1.1°-1.2°C for a doubling from preindustrial levels. The way warming from CO2 becomes really dangerous is through amplification by positive feedbacks—principally from water vapor and the clouds this vapor produces.

It goes like this: A little warming (from whatever cause) heats up the sea, which makes the air more humid—and water vapor itself is a greenhouse gas. The resulting model-simulated changes in clouds generally increase warming further, so the warming is doubled, trebled or more.

That assumption lies at the heart of every model used by the IPCC, but not even the most zealous climate scientist would claim that this trebling is an established fact. For a start, water vapor may not be increasing. A recent paper from Colorado State University concluded that “we can neither prove nor disprove a robust trend in the global water vapor data.” And then, as one Nobel Prize-winning physicist with a senior role in combating climate change admitted to me the other day: “We don’t even know the sign” of water vapor’s effect—in other words, whether it speeds up or slows down a warming of the atmosphere.

Climate models are known to poorly simulate clouds, and given clouds’ very strong effect on the climate system—some types cooling the Earth either by shading it or by transporting heat up and cold down in thunderstorms, and others warming the Earth by blocking outgoing radiation—it remains highly plausible that there is no net positive feedback from water vapor.

If this is indeed the case, then we would have seen about 0.6°C of warming so far, and our observational data would be pointing at about 1.2°C of warming for the end of the century. And this is, to repeat, roughly where we are.

The scientists at the IPCC next year have to choose whether they will admit—contrary to what complex, unverifiable computer models indicate—that the observational evidence now points toward lukewarm temperature change with no net harm. On behalf of all those poor people whose lives are being ruined by high food and energy prices caused by the diversion of corn to biofuel and the subsidizing of renewable energy driven by carboncrats and their crony-capitalist friends, one can only hope the scientists will do so.

Mr. Ridley writes the Mind and Matter column in The Wall Street Journal and has written on climate issues for various publications for 25 years. His family leases land for coal mining in northern England, on a project that will cease in five years.

A version of this article appeared Dec. 19, 2012, on page A19 in some U.S. editions of The Wall Street Journal, with the headline: Cooling Down the Fears of Climate Change.