Ln four weeks’ time, the world will awake to news that a marathon round of talks in Paris has produced a new global climate change accord. Or collapsed in failure. If a deal is struck, it will be the third UN climate agreement in 23 years. The first two — a 1992 accord sealed in Rio de Janeiro and the 1997 Kyoto protocol — both failed to meet their chief objective: preventing a rise in the carbon dioxide pollution from burning fossil fuels that scientists say is warming the atmosphere to risky levels. The question is whether a new pact will achieve more than the first two did to shape the fate of the earth’s climate, and an estimated $90tn worth of investment. That is the sum experts think will be spent over the next 15 years on infrastructure for the world’s energy systems, cities and farm sectors. If the Paris meeting delivers a global agreement robust enough to persuade investors they will make more money from backing, say, a wind farm instead of a coal power plant, or green bonds instead of BP shares, it is possible to imagine emissions falling. But even this close to the two-week Paris meeting, which starts on November 30, it is almost easier to say what an eventual agreement will not contain, rather than what it will have in it: