AFP

THE Intergovernmental Panel on Climate Change (IPCC), set up under the auspices of the United Nations to establish a consensus on global warming and what to do about it, has now completed its fourth assessment report. The first two parts, published earlier this year, about the science and the impacts of climate change, were designed to spread gloom. Change was happening, they said; it was mankind's fault; and it was going to be damaging. The third part, released on Friday May 4th in Bangkok, is about mitigating climate change, and is designed to spread hope. Just as mankind caused the problem, it says, so mankind can stop it—and at a reasonable cost.

In some areas of economic activity, emissions could be cut with no cost to consumers or taxpayers. The heating and lighting systems of many buildings, for instance, are startlingly inefficient. Improving this would cut both emissions and bills. Economists are troubled by this, for it implies that people and businesses are not maximising their economic self-interest; yet the low take-up of energy-efficient lightbulbs suggests this is indeed the case. Governments are therefore beginning to tighten regulations on the energy efficiency of buildings, and to talk about, for instance, banning incandescent lightbulbs. The IPCC reckons that such measures could cut 30% of projected emissions from this sector at no extra cost.

Transport is trickier, because car ownership is rocketing and the demand for fuel is fairly inelastic. If people want to drive they are going to drive, unless governments jack up petrol prices to levels that are politically unacceptable. So for emissions to fall in the transport sector, new technologies, such as more efficient biofuels or electric cars, are needed. Given a big R&D effort in this sector, there is a good chance that those will be forthcoming.

Similarly, in power generation, there is scope for cutting emissions. The cost of renewable energy, such as wind and solar, has been falling. Nuclear generating technology has improved. Carbon capture and storage, which involves taking the carbon dioxide (or C02) out of power station flue gases and injecting it back into the earth, is also a possibility, though that technology is at an early stage.

Technological solutions to climate change, then, are available. But most of those on offer in the power and transport sectors cost more than fossil-fuel generated energy. Fortunately, economics comes to the rescue. Burning fossil fuels imposes a cost to society that is not reflected in their price. Economics says that it should be; and if it were, the price of using fossil fuels would rise in relation to the price of using renewable energy.

Unfortunately, the social cost of carbon is hard to calculate. Plenty of economists have tried, with unconvincing results. It requires estimating the impact of climate change on economic growth, which involves too many unknowns. So the IPCC report starts from the other end. Rather than trying to work out the social cost of carbon, and letting it feed through to reduce greenhouse-gas concentrations in the atmosphere, it starts from a manageable greenhouse-gas concentration and works backwards to a carbon price. Conveniently, it says the “social cost of carbon is at least comparable to, and possibly higher than carbon prices for even the most stringent scenarios assessed by the IPCC”.

And what is the right price? The report says that to stabilise greenhouse-gas concentrations at 550 parts per million (a level most scientists think safeish) would require a price of $20-50 per tonne of carbon by 2020-30. That is along the lines of the carbon price established the European Emissions-Trading Scheme, which varied between $6 and $40 in 2005-06. It has not bankrupted the European economy so far. The IPCC's economic models reckon, on average, that if the world adopted such a price the global economy would be 1.3% smaller than it otherwise would have been by 2050; or, put another way, global economic growth would be 0.1% a year lower than it otherwise would have been.

The world would barely notice such figures; so one might think that climate change can be easily sorted. The problem, of course, is that the numbers work only if they are applied globally. If a few countries—even a few big countries—adopt a carbon price, it will make little difference. All the world's big emitters need to do it. Which brings the world straight back to the problem that sank Kyoto. No country alone can make a difference, and it is in every country's interest to ensure that everybody else bears the burden. As the IPCC report convincingly argues, the technology and the economics of this problem are easily soluble. It is the politics that is so difficult.