Jean JouzelCNRS Photothèque / Christophe

France is set to become the first major European economy to implement a carbon tax — a levy on activities that emit substantial amounts of carbon dioxide.

Nicolas Sarkozy, the French president, announced on 10 September that the tax would come into effect at the start of 2010. The tax draws largely on recommendations made on 28 July by an expert panel commissioned by the government, and chaired by the former Socialist prime minister, Michel Rocard.

Nature discussed the tax with Jean Jouzel, a member of the Rocard panel, former director of the Pierre-Simon Laplace Institute in the Paris area, and a member of the bureau of the International Panel on Climate Change.

The French carbon tax will be levied at a rate of €17 (US$25) per tonne of CO₂ — the current market price. Is that enough to change people's carbon-emitting habits?

What's most important is that a carbon tax of some sort is going to be introduced. Starting at €32 per tonne, as our report recommended, would have been more courageous. The economists on the panel considered that €40 was the minimum for the carbon tax to be effective in changing consumer behaviour, so €32 was itself already a compromise. It's true that the plan is to phase in higher carbon prices over time, but Sarkozy failed to give further details. In the longer term, by around 2020, we need to reach a price of €100–€200 per tonne.

Read more about the climate at the Road to Copenhagen special.

The Rocard panel called for electricity to be taxed, along with transportation fuel, coal and gas. Sarkozy has left electricity out of his plan, on the grounds that most of France's electricity comes from nuclear power and hydroelectric sources. Is that a problem?

It's true that most electricity in France comes from carbon-free generation. But 10% still comes from fossil-fuel plants, which are also called upon to alleviate peak loads. The panel argued that electricity should be included because the question goes beyond carbon — it's also about encouraging a culture of lower consumption of energy, to spur greater energy efficiency and renewable energy technologies.

The tax saw strong public support during a national consultation on the environment in 2007. Why has there been such a negative reaction to the tax over the past few weeks?

The government moved too slowly. If the tax had been implemented immediately after the consultation, before the financial crisis, it would have sailed through. This slowness — typically French — damaged the momentum.

There were also many errors in communication this summer. The tax was always intended to be fiscally neutral, so that all the tax collected would be repaid to taxpayers via reductions in income tax, or in the form of a 'green cheque'. But it was widely perceived as being a new tax burden, because it's fiscal neutrality had not been well explained.

Sarkozy has also taken on board our recommendations that the amount of offsets to people should be adjusted to take into account the size of households, and whether people live in urban areas, or in rural areas, where they have to use a car more often.

How will the carbon tax affect companies?

Heavy industry, which is already part of the EU's cap-and-trade emissions scheme, will not be affected. Small- and medium-sized companies will pay the tax, and unlike the public they will not be reimbursed, as the government considers that recent cuts in business taxes already provide a sufficient offset.

The tax will eventually need to be modulated to take into account sectors that depend heavily on energy such as fishing, agriculture and transport. The question will be in striking a balance between not unfairly damaging the competitiveness of certain sectors, and yet still providing incentives for them to modify their behaviour. Many companies I've spoken with are very well aware of green opportunities, and their potential for creating jobs.

Will this mean that certain sectors are at a disadvantage compared with competitors in other countries? Although Sarkozy stopped short of announcing firm measures, he suggested that a carbon tax could also be levied on imports.

Many French companies may find themselves at a disadvantage in certain sectors. But taxing carbon imports is not easy because of World Trade Organization rules. Ideally, Europe as a whole would take forward a carbon tax, but not all countries are agreed on this, and Germany and the United Kingdom are completely opposed. A carbon tax at the European level would be useful and totally justified. Europe needs to get its act together on this.

The EU aims to reduce its carbon emissions by 20% from 1990 levels by 2020, and France by 75% by 2050. How realistic are these targets?

The targets set for 2020 are completely attainable. The extra effort needed to reach the 2050 targets will be much more difficult. The warming of the planet is inevitable, and it's now we have to act. We need to stabilise greenhouse-gas emissions between now and 2020, so the next couple of years are critical at the international level. Let's see what happens at the United Nations Climate Change Conference in Copenhagen in December.