You're no climate expert. Do photos of the ice caps also lie? We know the greenhouse effect exists. Does this mean we can pollute at will? Let's seek some clarity in all the confusion.

The problem in a debate that involves both science and politics is that both sides tend to pick holes in each other's arguments, but rarely stop to make a clear statement of what they do and don't believe.

This is an attempt to do so. Let me start by responding to an assortment of specific questions that have been raised, in various forums, by my own columns on this subject. Each response is fairly short, but as they add up in length, it is my hope that they also add up to the conclusion: a statement of where we're at.

You're no expert

The first claim: "You are no climate expert, and you could be wrong." True, and true. I am a journalist and a columnist. As a journalist, I seek out experts and quote them. As a columnist, I distil (secondary) research and (expert) opinion into a coherent argument of my own. Unlike the person who challenged me, who admits to having done no research on the subject at all, I am reasonably familiar with scientific subjects, and have read with interest a great deal on the subject of climate change. Beyond that, I claim no particular expertise, nor qualifications to challenge professional scientists on matters of high science.

However, when scientific findings impact on public policy, one attaches greater importance to the degree of trust we place in them. After all, the proverbial man in the street is being asked to accede to policies that will have clear negative effects on individual liberty, prosperity and economic progress. It is right and proper for the media to query the degree of confidence we can place in the basis for such policies, and the trust we can place in climate science.

Recently, a leak of data and correspondence from the University of East Anglia's Climatic Research Unit has confirmed several long-held suspicions about and allegations against those scientists who claim to form part of a "consensus" about man-made (anthropogenic) global warming, or AGW. Previous revelations exist, such as the refusal to disclose even such elementary information as the location of stations that form part of the climate measurement network. Further revelations have since emerged. The AGW gang are alleged to have cherry-picked data in Russia that confirmed their theories, and ignored data that didn't. A member of the AGW gang, Green Party activist William Connolley, took it upon himself to systematically sanitise large swathes of Wikipedia articles on the subject of climate change.

One does not need to understand higher-order differential equations to find such revelations deeply troubling. It stands to reason that public trust in climate science has been shaken.

Cosy consensus

All this means, of course, that there is no longer a legitimate claim to "scientific consensus".

A recent US Senate report quotes 700 dissenting scientists, up from 400 just two years ago, and including some who contributed to, or withdrew from, the UN IPCC reports. For comparison, and though the UN often claims the support of 2 500 scientists, its 2007 IPCC Report for Policymakers was authored by just 52 of them. Several have complained in public that their views were misrepresented, or amended without their approval by politicians and bureaucrats. So, as a mere matter of fact, the claim to consensus is bogus.

The term itself has always been a nonsense, of course. There once was consensus that medical ailments were caused by an imbalance in the four humours. This consensus held from ancient times, in many cultures, until at least the 19th century, and in some circles well into the 20th century. It was wrong, even though supporters of the consensus could point to countless papers and studies and experts on the subject, while opponents were initially thinly represented.

The notion that consensus has any logical value in determining scientific truth is, quite simply, false. Anyone who relies on claims to consensus, or points simply to a count of papers for and against their views, should be treated with the suspicion that their claims lack objective support. They do not respect the scientific method, which relies for its progress not so much on positive confirmation as on the potential for disprovability and falsification.

If a theory is not falsifiable, it has little scientific merit. If it is, and it stands up to repeated attempts at falsification, it becomes stronger. That's how science works. Evidence that data and methods were withheld from other scientists, on the grounds that they'd only find fault with them, is evidence that the scientific method was repudiated by the very scientists who hide behind the claim of consensus.

Logical fallacies

Here's another one: "Believing that greenhouse gases do not cause climate change is like believing that HIV is not the cause of AIDS." First, the two are unrelated, so any position taken on one issue is wholly independent of a position taken on another. Second, a simple counter-claim could be "Believing that greenhouse gases do cause climate change is like believing humours cause diseases". Of course, the counter-allegation is as invalid as the original, but it shows the logical fallacy at work.

A similar fallacy is the comparison between oil-company-driven climate scepticism and the tobacco lobby's denial that smoking causes cancer. They are entirely distinct, and one can accept or reject each claim independently of the other.

Equally fallacious is the appeal to motive. Even if it were true that sceptics are oil-company funded (I wish!), or are merely arguing their own interests, that alone does not invalidate their arguments. Besides, if they want to compare "vested interests", a far larger amount of money flows to climate research by the alarmist lobby than climate sceptics receive. This would diminish if there wasn't a climate crisis. Thousands of newly minted "climate scientists" would be out of work if there was little to fear, and no basis for political action. Moreover, investments worth billions in expensive "green" technologies, such as those of Al Gore's Generation Investment Management, would be at risk if there was no reason to appeal to governments to force us to give up more cost-effective alternatives. These observations alone do not invalidate the AGW camp's scientific claims, but they do invalidate the hackneyed claim that sceptics are just shills for Big Oil.

This argument also perpetuates the notion that "the climate debate has been hijacked by two extremist camps". While it might appear true, it is not a very useful position to take. That a logical proposition is either true or false does not make claims either way "extremist". And when it is unclear whether something is right or wrong, or how much we do or don't know, we do not seek some comfortable, hazy middle ground. We seek to determine truth, conclusively, one way or the other. That's not extremism. That's debate. That's science.

What is disproven?

Does mere lack of consensus, or a few discredited climate scientists, or evidence of shockingly ill-managed data sets, mean all of climate science is now invalid? Of course not. But it does show that there is reason to distrust some historical reconstructions and that recent records may well have exaggerated warming.

In particular, the revelations once again show the flaws in the Mann "hockey stick" chart that has for almost 15 years been the bedrock of climate policy advocacy. It uses historic tree-ring data grafted onto modern instrument data. Problem is, the two series contradict each other, so they had to "hide the decline" shown by tree rings for the period when it didn't match instrumental data. This alone makes them unlikely matches for a single reconstruction.

However, another problem with concatenating two different data series is the problem of scale. If you can't index one series to another, how do you know how they relate? One can easily compress the amplitude (the height of peaks and depth of valleys) of one series, and it will still appear to match the other.

Analyses by people skilled in statistics suggest that's exactly what happened with the "hockey stick" temperature reconstruction, to make the modern rise seem so remarkably steep. Many of the leaked e-mails point to deliberate attempts to smooth over features of the historical record that would make today's temperatures look ordinary. The Little Ice Age, which is well-documented in science, history and literature, and would make modern warming an artifact of comparing it with a historic minimum, disappears. So does the so-called Medieval Warm Period, which is likewise strongly supported by both scientific data and historical documents, which many sceptics believe may have been even warmer than today, and which resulted in a great expansion of agriculture and prosperity throughout the world.

Moreover, those maxima and minima in the historic record, which the East Anglia bunch were so keen to contain, are important data in support of promising alternative theories about climate change, such as the impact of solar intensity and cosmic rays. They had a clear competitive reason to seek to make them go away.

So the scandal's implication is that while modern temperatures may indeed be relatively warm, historically, they are not extraordinarily so. The only reason why this result would be a problem is that it makes it harder to justify extraordinary measures on the part of governments and harder to justify research grants into this extraordinary situation.

Does NASA doctor pictures too?

What about the ice caps? Surely, we can see that they're melting? Putting aside the inconvenient facts that the Antarctic is, for the most part, neither warming nor melting, and that Arctic ice cover has been recovering somewhat in the last couple of years, the so-called "long-term trend" goes back only as far as 1979. Before that, we have no record at all of Arctic sea ice extent. Since two thirds of this short period of observation coincides with a period in which we know temperatures have been rising (albeit probably by less than the AGW alarmists claim), this is hardly indicative of anything. It is a very small amount of point data in a very large and complex system. It is certainly not a strong basis for dire predictions of a one-way ticket to an ice-free hell. And by the way, the polar bears aren't drowning. Their population numbers are, at worst, stable.

But don't we know carbon dioxide is a greenhouse gas, and that the greenhouse effect is real? Of course, we do. But we know the greenhouse effect of a single gas is not in linear relationship with its concentration. It tapers off at higher concentrations. Since a gas absorbs or reflects only particular frequencies of radiation, and there's only so much of that radiation, further additions to gas concentrations have a diminishing cumulative effect.

We know that long-term data obtained from ice cores shows that temperature changes consistently precede atmospheric carbon dioxide concentrations, rather than the other way around. It's hard to claim that B causes A when A happened first.

We know that carbon dioxide concentrations have been as high, and perhaps higher, in the past, without leading to the runaway feedback effects that were programmed into today's computer models that predict catastrophic warming.

Even worse, the evidence now shows not only a concerted attempt by an influential group of scientists to promote only their carbon-driven view, but also clear admissions that their knowledge about the total energy balance of the atmosphere (let alone the planet as a whole) is so tenuous that they couldn't even tell whether or not specific interventions had any effect.

All this suggests that the importance of carbon dioxide in climate change has been significantly overstated, and that it is only one element of a far more complex system. The notion that rising carbon dioxide levels will lead to a "tipping point" which might trigger runaway global warming appears to be very speculative indeed.

Peer review

What about peer review? Don't we trust it? Sadly, this is the area where the most damage has been done. Peer review is an important element in science. When someone makes a particular scientific claim, it is usually only their peers, with equal knowledge of the subject, who can judge whether the research appears valid and the claims well-supported. A lay person, even with the benefit of a good grasp of science and a lot of reading on the matter, is seldom qualified to challenge complex research.

It now emerges that the peer review mechanism has, at least to some extent, been subverted. The East Anglia scandal shows concerted efforts to boycott publications that dared publish papers that might contradict the work of the AGW camp, and even to oust editors of journals. Appealing to peer review, while actively corrupting that very system, casts a grim shadow over the claims of the AGW camp.

Another common response is to point to articles in "respected publications" which aim to minimise the impact of the scandal at East Anglia. Some are as obvious as RealClimate.org, which was set up by the very scientists implicated in the scandal to make their case and fight the sceptics. It is hardly an impartial source. In other cases, such as Scientific American, it is worth keeping in mind that many of those publications have long discarded any pretence at impartiality in this debate. They've trumpeted fear of AGW as if it were established fact. Few people, the editors of "respected publications" included, are very graceful about humiliating climb-downs.

As noted above, motive is not a conclusive determinant of truth, but a defence of AGW alarmism by publications that long ago hitched themselves to the bandwagon should be taken with a pinch of salt. It took Newsweek 30 years to apologise for promoting global cooling hysteria on its cover in 1975. It took a moon landing before the New York Times could bring itself to retract a snide front-page dismissal in 1920 of the possibility of space flight, on grounds of "knowledge ladled out daily in high schools".

Don't expect to be flooded with admissions from journalists and editors that they turned out to be credulous fools, manipulated by a political green lobby based on dubious science.

Show me the data

Another claim is that climate sceptics often do not present data to make their case, while climate alarmists do. This is only partially true.

Many dissenting papers have been published, despite the best efforts of the AGW gang. Some theorise that the oceans, which are the biggest energy stores on the planet, and in which algea constitutes the earth's single biggest carbon sink, are the main drivers of atmospheric carbon dioxide concentrations, temperature changes, or both. Others theorise that climate cycles are predominantly caused by clouds, solar intensity, the effect of the solar wind on cosmic rays, or a combination of all three. There is plenty data, including in peer-reviewed papers, that contradicts the theory that today's climate change is extraordinary, or that human activity the biggest cause of it.

However, there isn't enough. That is exactly the point the sceptics make: we know too little about the climate system to conclude much about any specific factor in the hugely complex system. We certainly know too little to propose trying to affect it, one way or the other.

Sceptics argue (and the evidence of the leaked e-mails now confirms) that the data on which AGW alarmists base their views are deeply flawed. This claim does not require demonstrating better data. The entire point of the argument is that better data does not exist.

So, where are we at?

Does this mean the climate isn't changing? Of course not. Climate always changes. Does this mean there hasn't been warming in recent history? Of course not. But it has been exaggerated, appears to have tapered off for now, and in any case may not be so unprecedented as the AGW alarmists would have you believe.

Does this mean humans have no impact on the climate? I'd be extremely surprised if they did not. As surprised, in fact, as I would be to discover that their effect is catastrophic.

Does this mean we can just pollute at will? Of course not. Pollution should be minimised on its own merits, after case-by-case evaluation of the costs and benefits of the productive activity that causes it. But more importantly, pollution has nothing to do with climate change. Carbon dioxide is not a pollutant.

Arguing that AGW alarmism is a good thing, lest we become indolent and careless about the environment is misdirected. If we are indeed lazy or careless (a disputable proposition in itself), this should be addressed truthfully on its own merits, and not by inventing some bogeyman or Gaia that'll punish us for our sins.

So, here's what we now know, and don't know:

Climate has always changed, and will continue to do so.

Evidence that current changes are extraordinary is shaky, as is evidence for claimed extraordinary effects such as stronger hurricanes or rising sea levels.

The claim that "runaway warming" is a threat is extremely speculative, and probably false, as is the theory that carbon dioxide is its main cause.

There are alternative theories to explain climate observations, and some of them are at least as strong, if not stronger, than the theory of man-made global warming.

Human activity surely has some impact on the climate, but it is unlikely that this impact is the main reason for changes we see in the climate.

It is possible that climate is a precarious system that can easily be unbalanced, but it seems considerably more likely that climate, like the environment, is a lot more resilient and robust than many "concerned scientists" would have you believe. Once disturbed, any number of natural mechanisms can serve to return it to a stable equilibrium.

Climate remains a field in which a great deal of research is needed. We have hardly scratched the surface in terms of understanding it. Ideally, the entire body of climate research should be reviewed from the ground up, with verifiable data and methods that are open to scrutiny by other scientists and the public at large. Scientists working on the taxpayer's dime have no right to claim secrecy or proprietary interest in their work, especially not when they turn out to have been less than honest with the public.

But above all, what we know is that we haven't even begun to justify massively costly government action that promises only restrict liberty and hobble economic progress in pursuit of some half-seen mirage on the horizon.

And with this mammoth tome – my longest column ever – I'll wish you all a prosperous and happy 2010. May it be free of needless fear and anxiety, and untrammelled by spurious cap-and-trade regulation.