To achieve a two-in-three chance of keeping temperature increases below 2 degrees, humanity can only emit about 750 billion tonnes of CO2 between now and 2050, he said. (I assume that after 2050 Schellnhuber believes that net emissions must fall to zero or below.) 750 billion tonnes spread over today's world population of 6.7bn means about 110 tonnes per person between now and mid-century.

It sounds a lot. But European countries today emit about 11 tonnes annually for each inhabitant. It would be more if we counted the embedded emissions in goods we buy from China and elsewhere. Using Schellnhuber's rough numbers, Europeans will exhaust their allowance within ten years if emissions do not fall.

Put another way, to meet the target Europe's countries need to reduce per person emissions by about half a tonne a year every year for the next twenty years. Achieving this would require a near 5% cut next year, rising to 10% a year by 2020 and 20% annual reduction by 2025. As Stern has pointed out (Stern Review, p. 231) the only time even a 5% emissions reduction has ever been achieved over a large country for several years was the 1989 to 1998 period in the former Soviet Union, when emissions fell 5.2% per year.

According to Yvo de Boer, executive secretary of the UNFCCC, the four essentials needing an international agreement in Copenhagen are:

1 How much are industrialised countries willing to reduce their emissions of greenhouse gases?

2 How much are major developing countries such as China and India willing to do to limit the growth of their emissions?

3 How is the help needed by developing countries to engage in reducing their emissions and adapting to the impacts of climate change going to be financed?

4 How is that money going to be managed?

Sweden has had such a tax in place since 1991, when the government imposed a tax equivalent to €28, or $41, for each ton of CO2 emitted. The Swedes currently levy a tax of €128 for each ton of CO2, although industries that are exposed to international competition are permitted to pay the tax at a lower rate.

Emissions in Sweden would be 20 percent higher without the tax, yet the economy has still grown by 44 percent since it was put in place, said Susanne Akerfeldt, a senior adviser on tax issues at the Ministry of Finance in Stockholm.

“We have found that our carbon tax is entirely compatible with economic growth,” she said.

Denmark instituted its carbon tax a year after Sweden, while Finland, Norway, Switzerland and parts of Canada use similar systems, she said.