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In the column entitled "Climate conference: in Lima we need to go further", published in Le Monde on 10 December, Christian Gollier, Pierre André Jouvet, Christian de Perthuis and Jean Tirole give economic legitimacy to an obvious truth: only by applying an annual global emissions cap will the imperative recognized by the whole international community of limiting to 2° an increase in the average temperature by 2100 be met. The rest is just words.

The work of Giraud and Joncovici has shown that the link between economic growth and the consumption of fossil energy has never been broken. With our current economic tools, we know that the famous "decoupling" is a myth. Although the energy efficiency of the European economy has increased, it is above all because Europe imports those goods whose production is the most energy hungry and which today account for a third of our total energy consumption.

Moreover how else could the decoupling have taken place? Through the scarcity of the fossil energy supply and an increase in its price? This increase is always first reflected in the consumer countries becoming indebted, then by a slowdown in their growth and the example of shale gas shows the efforts made to increase supply and kick-start the economic machine. Through a tax on energy? Not such a good idea, as the two failed attempts in France by both right and left has shown. Not such a good idea for three reasons: taxation which is not worldwide would result first and foremost by even more relocation of industries; for it to have an effect on consumption decisions, the levels of taxation needed would be politically untenable; finally, and more importantly it would be a retrograde tax, hitting hardest those households already energy-poor because the poorer the households, the higher the proportion of their income spent on energy.

In truth, it is absurd to talk about decoupling for as long as we use the same currency to pay for what we need to stimulate the economy, human work, creativity and exchange, and what we need to use less of, fossil energy. It is as if we wanted to drive a car in which the brake and accelerator were one and the same pedal! The best way to meet with disaster.

If we want to limit global warming to 2° by the end of the century this involves determining the total quantity of CO2 emitted by 2100 and dividing these emissions between the ninety remaining years, with a trajectory ending in 2100 with zero emissions. Which will determine, year by year, maximum emissions and therefore the quotas to be allocated between countries, between territories and between individuals.

This allocation must be equal for everyone. For three reasons. The first is that the climate is a global common resource; the second that it is only fair that the thriftiest countries, in general simply the poorest, can take advantage of their sparing use; the third is that without carbon sinks, in essence, the oceans, which absorb three-quarters of emissions, our planet would have already become, to use the expression of Michel Rocard, a frying pan. However, the fact is that in the current economic system, the major energy consumers appropriate these sinks.

Allocated quotas are the energy currency. It follows that any purchase must be made in two currencies (a method that has become easy with electronic money and that everyone uses every day through loyalty points): the energy currency, where the total quantity of energy incorporated into the good or service purchased is deducted from the quota and the conventional currency for the rest. Just as VAT has had the effect of aggregating the value-added throughout the production process, the energy currency will instantly create mechanisms for aggregating the fossil energy used throughout the production process.

Unlike taxation, a system of tradable quotas is socially progressive, allowing the poorest countries, territories and households to sell to the rich at high prices the quotas they have not themselves used. Which solves, incidentally, all the more so if it is combined with a global tax at source on fossil energy, the recurring issues of how the rich countries can finance the poor countries in their efforts to adapt to climate change and how they can finance technology transfers. Organising the allocation of quotas nationally, territorially and individually will in particular lead to a strengthening of the role of regions and cities in the transition to sustainable societies.

This completely new way of addressing the issue needs to be accompanied by new measures in the legal system. Currently, the climate does not exist legally. There is no governance of its evolution. It is the result only of obscure negotiations between world powers. The recent bilateral agreement between China and the USA, according to methods which owe more to the nineteenth than to the twenty-first century, is a perfect illustration of this. We need to make climate a common world resource, whose governance must be defined and responsibilities shared. This governance will also be that of a central bank issuing the energy currency -the quotas- and allocating them.