The House of Lords Economic Affairs Committee has an impressive line-up. It is chaired by former Conservative Energy Minister Lord Wakeham, and includes two former Tory Chancellors, Lord Lawson of Blaby and Lord Lamont of Lerwick.
The inquiry set out to examine the ways in which the estimates of damage arising from climate change have been assessed, and the role played by the IPCC. However, it is already clear from questions that most of the Committee's members - particularly Lord Lawson - are at best sceptical that climate change is a significant or urgent threat.
Another pointer to the Committee's likely conclusions is that it has appointed the influential environmental economist Professor David Pearce as special adviser. In the 1990s, Professor Pearce ran into a storm of controversy for his work on valuing the impacts of climate change - particularly his proposal that the loss of a human life in the developing world should be valued at one-fifteenth that of a life in wealthy nations.
The Committee heard combative evidence from leading climate sceptic Richard Lindzen of Massachusetts Institute of Technology. "I think the public is being misled as to the nature of the controversy and the science," he said. The IPCC "is a very distorting process", he said, particularly the translation of the detailed reports into a summary for policy-makers.
Professor Lindzen claimed that "the one thing that points to high sensitivity [of the climate] is models" - but that these models predict an increase in average temperatures today of 3ºC above pre-industrial levels, and we do not see that". He dismissed the historic temperature record, which the IPCC says shows relatively stable temperatures for most of the last millennium, as being "based on a couple of handfuls of tree rings which only observe growth in the summer."
Professor Lindzen also challenged evidence from Sir John Houghton, who was IPCC chairman for 12 years and oversaw the preparation of the Panel's first three assessment reports. Sir John told the Committee that the 0.6ºC increase in global average temperatures in the 20th century "is phenomenal compared with any variation over the whole millennium."
Professor Lindzen's main scientific dispute with the IPCC is his view that water vapour has a negative feedback on global warming. Sir John claimed that the professor is "the only credible scientist in the world who thinks that" - and "nobody has been able to substantiate the essentially hand-waving arguments that he uses."
Indeed, Sir John pointed out that Professor Lindzen was on the writing team for one of the chapters in the IPCC's 2001 report - but lost the argument because "he is not a man who does his homework; he does not read the rest of the literature; he quotes his own papers." Moreover, Professor Lindzen "does not know anything" about the impacts of climate change.
Richard Tol, an economist at Hamburg University, also launched a bitter attack on the IPCC which, he said, "has become more and more politicised and less pleasant". He accepted that IPCC reports are "a reasonable reflection of the literature", but claimed that "what is then filtered into summaries is a completely different story."
Professor Tol acted as a lead author in the IPCC's second and third assessment reports - but claimed that he is not involved in the current, fourth assessment because the German government has only nominated "people with close connections to the Green Party, and that excludes me immediately".
IPCC chairman Rajendra Pachauri offered a stout defence of the Panel's work and its methods. About 500 authors will be working on the fourth assessment, due to be completed in 2007, and will rely "entirely on peer-reviewed published literature."
"Non-believers" in global warming are involved in the IPCC's work, Dr Pachauri said, "provided they have the research record, publications to their credit and have done scientific work on the subjects for which they are being considered...We certainly do not include armchair critics."
Lord Elder suggested that scientists who support the IPCC consensus are more likely to win funding from official sources. Dr Pachauri said that "any such allegation would be a slur on the dignity and the integrity of the scientists" - and claimed that "it is probably easier to get funding for so-called sceptics these days."
"Every single word" of the IPCC's policy-makers' summary is approved by a plenary of the 192 countries that are members of the IPCC, Dr Pachauri said. "There is hardly any likelihood of something that is indefensible going into the reports."
The debate on climate change has moved a long way over the past five years - to the extent that Professor Lindzen was the only witness to cast serious doubt on the underlying science. Critics of the IPCC and the Kyoto process have now moved on to the detail of the IPCC's emissions scenarios and the costs and benefits of reducing greenhouse gas emissions.
Bjorn Lomborg, author of The Skeptical Environmentalist, was perhaps the highest-profile witness to appear before the Committee. Professor Lomborg said that "global warming is happening and it will have a serious impact... Unfortunately, the problem is it is going to be fairly costly to do fairly little for people far into the future if we decide to go for mitigation, whereas that money could otherwise be spent on many other things that would probably do more good."
Professor Lomborg went on to describe his recent work on the so-called Copenhagen consensus, in which he brought together a group of economists to rank a series of global threats. "They found that the top outcomes were dealing with HIV/AIDS, with malnutrition, with free trade and malaria," he said. "At the bottom came proposals like Kyoto and proposals that would go even further."
He acknowledged the growing concern over major potential impacts such as the melting of the Antarctic ice sheets, collapse of the Gulf Stream or disruption to the Asian Monsoon - but said that to reduce such risks would require "very, very strong changes in your emissions" (see pp 17-21 ).
Professor Lomborg argued that, "certainly when we are talking about realistic cuts in carbon emissions over the coming decade, I would say we are only talking about postponement." He claimed that if emissions are kept at the same level agreed under the Kyoto Protocol until 2100 "it will postpone global warming for about six years."
Professor Richard Tol also warned of "a risk of overspending" in tackling climate change. "If people come up with arguments that climate change will increase malaria in Africa and therefore we should reduce our greenhouse gas emissions, I think that is hogwash because if you are really interested in malaria then you should go after malaria."
Professor Tol claimed that the marginal damage cost of climate change is around $7 per tonne of carbon - much lower than the Government's current yardstick of £35-140 per tonne. "Kyoto does not stand the cost-benefit test," he said, and "the real solution to the climate change problem is not mandatory caps on emissions but developing alternative technologies for the market to take up."
However, Professor Tol admitted, "as a human being I am worried about the fact that climate change does not stop at 2100...If we are still on an upward trend in 2100 then at some point we must run into real trouble."
In contrast, Sir John Houghton maintained that what "reliable economists are saying about the costs of mitigating action is that the cost is very modest." He pointed to Treasury projections which suggest that cutting the UK's CO2 emissions by 60% by 2050 "would cost no more than six months' loss of GDP growth over that period" - and this could be brought down by innovation.
However, Colin Robinson, an economist at the University of Surrey and editorial director of "free-market thinktank" the Institute of Economic Affairs, claimed that the Government had adopted "exaggerated, quite costly and over-hasty action" in its 2003 energy White Paper. "If you become too alarmist," he said, "you can become precipitated into this very costly centralised action to solve problems which would actually have been solved anyway through the normal processes of human ingenuity."
Professor Robinson maintained that "the amount of warming does not appear to be very closely correlated with the science - a claim strongly refuted by Sir John Houghton - and cast doubt on whether the IPCC's emissions scenarios to 2100 have "any value at all" given the uncertainties involved.
The IPCC has projected that global temperatures are set to rise by 1.5-5.8ºC by the end of the century, the wide range reflecting different scenarios of economic growth, population growth and technological development.
Professor Lomborg criticised the IPCC for "declining to say that some scenarios are more likely than others." Instead, he said, "we simply have a vast range of temperature outcomes that could go from only 'slightly troublesome' to 'dramatically problematic'."
Several witnesses dwelt on a long-running argument over the treatment of developing countries' economic growth in the IPCC's emissions scenarios.
Professor David Henderson of Westminster Business School - a former head of economics at the OECD - argued that the scenarios "exaggerated the likely emissions projections" because of unrealistic assumptions of convergence in wealth between developing and industrialised nations.
The IPCC has described his claims as "totally unfounded" and as "disinformation".
Other witnesses suggested that in practice the dispute may make relatively little difference to the level of projected emissions.