What is causing our climate to change?

Ward said he believed Durkin's claims were so flawed they would unravel if debated in public. The Scotsman invited them to discuss the issues in an e-mail exchange, edited highlights of which are published below:

WARD: The signatories of the letter simply seek for Martin to correct the major misrepresentations contained in his programme before it is distributed on DVD. Seven of these in summary are:

1. It misrepresented a graph of global average temperature published in 1995 and failed to acknowledge the most up-to-date analysis that shows none of the large-scale surface temperature reconstructions indicate medieval temperatures were as warm as in the last few decades.

2. It failed to mention the effect of aerosols in causing a slight cooling of the average temperature in the northern hemisphere between 1940 and 1976.

3. It wrongly claimed that climate models are inconsistent with the data for trends in global average temperature in the lower atmosphere.

4. It wrongly claimed that volcanoes produce far more carbon dioxide than human activities.

5. It wrongly claimed that a paper by Caillon and co-authors suggests that the recent rise in carbon dioxide concentrations must have followed the recent rise in global average temperature, when the authors' paper states the opposite.

6. It wrongly presented Carl Wunsch's views to be that he believes carbon dioxide emissions from humans cannot be responsible for the recent rise in global average temperature, when in fact he is on record as stating that he thinks the opposite.

7. It wrongly claimed that solar activity explains the recent rise in temperature, when the up-to-date published scientific literature suggests that it doesn't.

Perhaps Martin could go through each of these misrepresentations and justify his apparent refusal to correct them?

DURKIN: 1. Bob doesn't like me using a graph depicting the temperature record of the past 1,000 years, which, he says has been "superseded". The problem with my graph (which was published by the IPCC and used to be regarded as the standard account) is that it clearly shows a very warm period (what climatologists call the Medieval Warm Period) followed by a relatively cold period (what they call the Little Ice Age), from which, it appears, we have for the past two or three hundred years, been making a slow, welcome recovery. All in all it's not very alarming.

So the global warming fraternity replaced this inconvenient graph by another - the famous "Hockey Stick" (called so because it looks like one). Far more dramatic. But then two researchers (McIntyre and McKitrick - look it up on the net) examined the computer algorithm used to produce the famous Hockey Stick, and discovered it was very good indeed at producing Hockey Stick shapes.

Their critique was subsequently confirmed in the independent Wegman Report which again I urge readers to look up for themselves on the net. Wegman also criticised the way a small group of Hockey Stick researchers were swapping the same methodological techniques and datasets to come up with numerous studies which were essentially the same one, but purported to be supporting one another.

2. Every record of 20th-century temperature change presents Bob Ward with the same problem. The temperature went up radically from around 1905 to 1940, it fell from the 1940s to the early 70s, it rose in the 80s and 90s and it's done nothing spectacular either way for a decade.

The post-war cooling is especially embarrassing. The post-war economic boom was a big deal - lots of . So why did temperatures go down? The global warmers do a little shrug of the shoulders and suggest that maybe it was (pollution from factories). All reliable accounts of have levels steadily increasing, from the late 19th century till at least 1990. We had dirty industrial production before the Second World War and dirty industrial production after it. Why did the temperature go up in phase one and down in phase two? Why did the temperature go up in the 1980s? China is now the world's biggest producer of . The amount produced by China has increased 27 per cent since 2000. Why aren't we freezing cold?

Here's a thought. Perhaps the temperature change in the 20th century has nothing to do with and . Perhaps it is connected with the fact that solar activity increased from the beginning of the 20th century until the 1940s, fell back till the 1970s and then rose again.

3. He says the temperature rise in the troposphere is consistent with surface temperatures. He quotes as his source Professor John Christy, who is one of the leading scientists in the world on this topic. All I can say is, Prof Christy had a very different story to tell when we interviewed him in his labs in Alabama. According to classic global warming theory, the rate of temperature rise should be greater in the troposphere than at the surface. The observations we have from satellites and weather balloons consistently indicate the opposite.

4. Hurray, Bob's got one right. I wrongly said that volcanoes emit more than humans. He'll be pleased to see I've corrected that in the DVD version. But I find people are still surprised when I tell them that oceans, for example, produce around 80 gigatons of a year, compared to around 7 gigatons from humans. The point here is nothing more than to emphasise that is natural (people often refer to as if is produced only by humans and is a pollutant).

5. The ice core data is frequently cited by global warmers as proof that their theory is true. In Earth's climate record, and temperature seem to move together. What they conveniently fail to mention is that the order is the wrong way round. The temperature goes up, then a few hundred years or more later, so does . The reason is the oceans both emit and suck in , emit more when it's warmer, and suck in more when it's cooler, but it takes centuries to warm up and cool down the oceans.

6. Carl Wunsch was not invited to be interviewed for his views on anthropogenic , but on oceanography, an area in which he is qualified to speak. And everything he said can be found in any textbook on oceanography. His views were most certainly not distorted.

7. Bob Wade's deep attachment to global warming theory means he has to argue, absurdly, that variations in solar activity have little or nothing to do with climate change on earth. This is a sad day for reason. Bob Wade and others have staked their reputations on man made global warming being true. Some have built whole careers on it. I feel very sorry for them.

WARD: I am grateful to Martin for attempting to justify the misrepresentations in his programme. Unfortunately he still has not acknowledged all of the mistakes.

1. The graph attributed to the Intergovernmental Panel on Climate Change (IPCC) in his programme appears to have been published in 1990. But the programme wrongly stated that it showed global average temperatures from AD 900 up to today, rather than up to 1975 as the IPCC's graph stated. Claiming that the graph included temperatures for the last 32 years was a major misrepresentation.

Following controversy over one of these Hockey Stick graphs, the US National Academy of Sciences carried out an authoritative and detailed review of all the work.

The review concluded that "none of the large-scale surface temperature reconstructions show medieval temperatures as warm as the last few decades of the 20th century". The programme ignored this review and all of the evidence that was considered by it.

2. The first broadcast of the programme presented a graph of "world temperatures" erroneously attributed to NASA. In subsequent broadcasts, the attribution was removed and the graph purportedly showed world temperatures between 1880 and about 1990. It showed a marked drop in temperature between 1940 and 1976. But no graph of global average temperature from a reputable source shows this sort of drop in temperature over this period.

The programme's graph may have shown temperatures from the northern hemisphere or more likely from North America, which show a cooling over that 35-year period due to the effect of industrial aerosols. This can be seen on the graphs produced by NASA at http://data.giss.nasa.gov/gistemp/graphs/. Industrial aerosols have the same impact. I do not know where Martin is getting his information from, but the published record of sulphate aerosols show that they increased sharply between 1945 and about 1989, after which they declined rapidly. Global emissions of sulphate aerosols are much less today than they were in the 1970s. Meanwhile greenhouse gas concentrations have continued to mount, which answers his question about "why aren't we freezing cold?".

3. The programme claimed that the record of temperature rise in the lowermost atmosphere (troposphere) is inconsistent with climate models showing the impact of rising greenhouse gas emissions. But this misrepresents the most up-to-date review of the evidence by the US Climate Change Science Programme last year. This review, which was co-authored by John Christy (who appeared on the programme), concluded that "given the range of model results and the overlap between them and the available observations, there is no conflict between observed changes and the results of climate models". The programme failed to present the most up-to-date evidence.

4. How gracious of Martin to admit that the programme was completely wrong about how much carbon dioxide is emitted by volcanoes. But now he's promoting another misrepresentation of the science, with inaccurate figures for the role of the oceans. The scientific evidence shows that the oceans release about 367 gigatonnes of carbon dioxide each year, but absorb about 374 gigatonnes. Therefore, the oceans remove about 7 gigatonnes of carbon dioxide from the atmosphere each year, while human activities add about 22 gigatonnes.

5. The programme showed a graph that was attributed to a scientific paper by Nicolas Caillon and co-authors, showing how carbon dioxide levels in the atmosphere had risen after the initiation of the Termination III deglaciation event about 240,000 years ago. Carbon dioxide levels did not start to rise until about 800 years after deglaciation began, and similar time lags have been recorded for other deglaciation events. But Caillon did not conclude, as the programme wrongly claims, that this proves the recent rise in carbon dioxide must be the result of, rather than the cause of, the recent warming. His paper actually states that "the situation at Termination III differs from the recent anthropogenic increase", noting that "the recent increase has clearly been imposed first". And it points out that the release of carbon dioxide at the beginning of deglaciation events amplified the initial warming caused by fluctuations in the Earth's orbit around the Sun.

Caillon's conclusion is not surprising. The last deglaciation occurred about 12,000 years ago, whereas carbon dioxide levels only started to rise in the 18th century, coincidentally when the start of industrialisation led to widespread burning of coal. On this issue as well, the programme misrepresented the science.

6. In an article shortly after the programme was first broadcast, Carl Wunsch wrote: "In the part of The Great Climate Change Swindle [sic] where I am describing the fact that the ocean tends to expel carbon dioxide where it is warm, and to absorb it where it is cold, my intent was to explain warming the ocean could be dangerous - because it is such a gigantic reservoir of carbon. By its placement in the film, it appears that I am saying that since carbon dioxide exists in the ocean in such large quantities, human influence must not be very important - diametrically opposite to the point I was making - which is that global warming is both real and threatening."

7. The programme showed a graph from a paper by Professor Henrick Svensmark and a co-author that was published in 1997. The graph purported to show a close match between the length of sunspot cycles and temperature since 1860. But the graph in the programme, as it does in the paper, has no record for the length of sunspot cycles after about 1976. If the programme had included more up-to-date evidence, it would have shown that sunspot cycle length has not really changed since 1976 while temperature has continued to climb. Thus the apparent correspondence between sunspot length and temperature does not occur over the past 30 years.

In any case, sunspot cycle length is not a particularly good measure of the amount of heat energy reaching the Earth, which is what influences global temperature. This has been measured directly by satellites since 1978, and the record shows that variations in the Sun's energy have been too small to have contributed appreciably to the accelerated warming over the past 30 years.

And I am afraid that this last point rather means that the central premise of the programme, that solar activity rather than greenhouse gas emissions are responsible for the recent warming, is like a house of cards that completely collapses when the errors in the science are removed.