In December talks in Paris involving more than 200 countries may result in a new agreement aimed at reducing carbon emissions. In the months leading up to the conference, The Economist will be publishing guest columns by experts on the economic issues involved. Here, Vincent Réquillart of Toulouse School of Economics and the Institut National de la Recherche Agronomique argues that an initial focus on setting clear principles for fair costsharing will clear the way to a broader climate deal.