Article 6 of the Paris climate agreement deals with the trading of emissions reductions credits that might arise from a country beating its own pledges or from a public or private initiative that cut emissions, such as a renewable energy plant or the restoration of a forest. Here in Madrid, as last year in Katowice, countries are struggling to agree the rules of how these markets would work. A number of countries including Brazil want to carry over credits that were created under previous versions of this scheme. The worry is that many of these historical credits are not real reductions. If they are used by countries to meet part or all of their pledges they simply dilute real efforts to cut carbon.