Britain’s most prominent climate change sceptic, Lord Lawson of Blaby, has been encouraging audiences over the past few weeks to adopt a recklessly complacent approach to the risks posed by rising atmospheric concentrations of greenhouse gases.

The former Chancellor, who launched the Global Warming Policy Foundation in November 2009, has continued to invite the public to employ the same dubious logic that he set out in his book, ‘An Appeal to Reason: A cool look at global warming’.

On 4 April, Lord Lawson appeared at the Lyttelton Theatre in London as part of the National Theatre’s series of ‘Greenland’ events, and repeated the central claim made in his book that the potential economic impacts of unabated greenhouse gas emissions are comparatively small.

He made exactly the same argument during a debate organised by The Spectatormagazine on 29 March, whilst speaking in favour of the motion: ‘The global warming concern is over. Time for a return to sanity‘.

Lord Lawson’s main claim has been that, even for the ‘worst case’ scenario projected by the Intergovernmental Panel on Climate Change (IPCC), the costs of unmitigated warming by the end of the century would mean that people in developing countries would only be 8.5 richer than now compared with 9.5 times richer in the absence of climate change.

He has been trotting out this line ever since the publication of his book, but it is based on a sleight of hand and faulty logic.

Lord Lawson’s argument, as set out in his book, begins with a reference to temperature projections in the IPCC Fourth Assessment Report: “The report’s best estimates of the likely warming of the planet over the next hundred years range from a rise of 1.8ºC/3.2ºF to one of 4ºC/7.2ºF above the estimated 1980-1999 average temperature, depending on the emissions scenario (or ‘story line’) chosen”.

But this statement does not accurately convey the true range of temperatures projected by the IPCC, which is based on six potential scenarios that depend on factors such as the rate of growth of emissions over the coming century. In the ‘A1FI’ scenario with the highest emissions, as Lord Lawson acknowledges, the ‘best estimate’ is a warming of 4ºC by the end of this century.

However, this ‘best estimate’ also means a 50 per cent chance that global average temperature would exceed 4ºC under this scenario, and the IPCC concludes that the “likely” (i.e. greater than 66 per cent chance) range is 2.4 to 6.4ºC.

Annual global emissions of carbon dioxide were tracking slightly above the A1FI trend before the global economic downturn. Recent research suggests that annual emissions have rebounded after the downturn and could soon return to a path that matches or exceeds A1FI.

Yet Lord Lawson’s approach depends on completely ignoring the very substantial chances of a rise in temperature of more than 4ºC by the end of the century, and he even tries to convince readers of his book that the IPCC similarly neglects such potential warming when estimating the likely costs: “The report then takes the upper end of the range – a 4ºC/7.2ºF warming – and claims that overall, this would mean a loss, by the end of the 21st century, of anything between 1% and 5% of global gross domestic product”.

This cost estimate does appear in the IPCC report and is mainly based on a figure summarising some of the findings of the Stern Review (See Figure 2.3(b)).

However, the IPCC report, like the Stern Review, warns that these figures understate the effects of a warming of 4ºC: “It is very likely [i.e. greater than 90% chance] that globally aggregated figures underestimate the damage costs because they cannot include many non-quantifiable impacts”.

The IPCC report also shows what the costs could be of a warming of 6.4ºC, the top of the likely range for A1FI. It shows that a temperature rise of 6.9ºC above pre-industrial levels (i.e. including the 0.5ºC that occurred before the baseline period) would cost, at a minimum, about 4 to 10 per cent of global GDP.

The economic impacts of climate change are subject to the same degree of uncertainty as the projections of temperature, and the Stern Review showed that a warming of 6.9ºC above pre-industrial levels could actually cost up to 30 per cent of global GDP (see figure 6.5).

Indeed, since the Stern Review was published three years ago, it has become apparent that the true potential costs, when account is taken of disruptive effects such as the migration of large populations away from the worst-affected areas of the world, could be much higher.

But Lord Lawson turns a blind eye to all this and focuses instead on his cherry-picked figures, so that he can maintain that the worst that could happen from unabated greenhouse gas emissions is a warming of 4ºC by 2100, causing a reduction in global GDP of no more than 5 per cent. He goes on to assume in his book that, in the period to 2100, “living standards (measured in terms of gross domestic product per person) would rise, in the absence of global warming, by 1 per cent a year in the developed world, and by 2.3 per cent a year in the developing world”.

He notes “the IPCC’s very proper warning that the loss will be greater than 5 per cent for the developing countries (and thus less than 5 per cent for the developed world)”, and he proceeds with “calculations on the assumptions of a 10 per cent loss of GDP in the developing world and a 3 per cent loss in the developed world”.

After taking into account these losses in GDP, Lord Lawson concludes that “the disaster facing the planet is that our great-grandchildren in the developed world would, in a hundred years time, be only 2.6 times as well off as we are today, instead of 2.7 times, and that their contemporaries in the developing world would be ‘only’ 8.5 times as well off as people in the developing world are today, instead of 9.5 times as well off”.

This all sounds very reassuring, but the deceptive assertion that everybody will be many times richer by the end of the century regardless overlooks the risk of warming of much more than 4ºC, with impacts that could fundamentally cripple economic development and growth. That would mean future generations might not be very much richer, and could be poorer.

Lord Lawson’s book has already been criticised by Sir John Beddington, the Government’s chief scientific adviser, for including “a number of points related to the underlying science of climate change that are incorrect or presented in a misleading way”. And Sir John Houghton, the former chair of IPCC’s working group on the science of climate change, considered that “his book is largely one of misleading messages”.

Yet Lord Lawson continues to rely on the same flawed numbers and arguments to justify his appeal to abandon all reason by recklessly ignoring the very substantial risks posed by unchecked climate change.

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