By Roger Harrabin
Environment Analyst, BBC News
The UK's plans to cut emissions by 80% by 2050 are fundamentally flawed and almost certain to fail, according to a US academic.
Roger Pielke Jr, a science policy expert, said the UK government had underestimated the magnitude of the task to curb greenhouse gas emissions.
He added that it would be more effective to "decarbonise" economic growth rather than focus on targets.
Professor Pielke made his comments during a speech at Aston University.
Professor Pielke said that a country's greenhouse gas trajectory was determined by three factors: economic growth; population growth; and changes in technology.
This meant, the academic from the University of Colorado suggested, that if people migrate to the UK and the economy boomed, it would be harder for politicians to achieve emissions cuts based on historic levels.
He calculated that the combined effects of possible population growth and economic growth could oblige the UK to increase energy efficiency and reduce carbon intensity of energy at an unprecedented annual rate of 5.4%.
Conversely, if migrants left the UK and the economy slumped, there would be a downturn in emissions, for which politicians would claim unearned credit.
Professor Pielke suggested that a more effective measure would be to track the emissions produced for every unit of wealth generated by individuals. In other words: CO2 per capita GNP.
How to curb climate change will be the subject of heated debates in 2009
This would focus efforts on delivering the technological change needed to reduce emissions, he believed.
However, Professor Pielke's approach also raises a number of questions.
First, there is no guarantee that a change in measurement will provoke the scale of change the author believes is required.
Moreover, his alternative system would reward governments that shifted to service-based economies and moved their emissions "offshore", creating an illusionary cut in emissions.
This difficulty could be overcome with a more complex measure based on CO2 per capita GNP and would include imported "embedded" emissions.
But that has problems too: in modern supply chains: a computer may contain parts from 20 different countries and manufacturers regularly change suppliers, so it will often be impossible to keep an accurate tally of embedded carbon.
It could also be too complex for many people to grasp easily.
Professor Pielke's position is strongly supported by Gwyn Prins, director of the Mackinder Centre at the London School of Economics.
Professor Prins told BBC News: "Professor Pielke is far from being a so-called 'sceptic' on reducing CO2, so this makes his analysis all the more telling.
"To begin to meet the legal targets of the Climate Change Act, the UK will have to achieve and maintain decarbonisation at (unprecedented) rates," he added.
"The Climate Change Act will have to be revisited by Parliament or simply ignored by policymakers. What are the costs in terms of public cynicism about legislators and the legislative process, of passing aspirational rather than codifying laws?"
Colin Challen MP, chairman of the All Party Parliamentary Climate Change Group, said: "This raises questions which I do not think have been factored into the thinking behind the Climate Change Act.
"The task (of cutting emissions by 80% from 1990 levels by 2050) is already staggeringly huge and, as we have seen, well beyond our current political capacity to deliver.
"Heathrow is a prime example of ducking the responsibility," the Labour MP for Morley and Rothwell told BBC News.
"It is hard to see any tough choices being made in the current climate. A greater population implies more embedded CO2 emissions in imported goods, but the climate change committee is only empowered to consider domestic emissions."
Professor Pielke's intervention was rejected by economist Terry Barker, a lead author for the Intergovernmental Panel on Climate Change (IPCC).
"Pielke's analysis does not tell us how fast an economy can de-carbonise, just how much it has done so in the past when there has been a weak carbon price," he said.
"[His] proposals are diversionary; they fail to emphasise the scale of the no-regrets options available to reduce emissions at net benefit and they do not include potential changes in regulations on vehicles and power stations that could lead to rapid de-carbonisation."
Professor Tom Burke from Imperial College London added: "These conclusions are a very marginal addition to our knowledge.
"The argument in his paper amounts to saying that getting 80% will be difficult. This is hardly news.
"There is nothing that supports the contention that the Climate Change Act will fail or that there are flaws in its basic conception or that there is an alternative approach which is better.
"No-one has said this would be easy."
Debates like this will run throughout the year whilst the world staggers towards a climate change agreement to replace the Kyoto Protocol.
The existing EU policy model of capping emissions and allowing firms to trade in carbon permits is criticised for enriching businesses while failing to deliver emissions cuts or setting a long-term carbon price.
Arguments will continue over whether this model can be improved or if any alternative policy structure will be more certain to deliver the emissions cuts the scientific establishment so urgently demands.