Officials at this year's UN climate talks are hoping to achieve a breakthrough in long-running efforts to agree on rules for a global carbon trading system.
Economists say establishing an international market for carbon could unlock billions of dollars' worth of investments as countries and companies seek the most cost-effective way to cut emissions.
But the issue, known in the parlance of climate diplomacy as 'Article 6,' has defied agreement for years.
With pressure growing from governments to resolve the deadlock, environmental campaigners say the two-week meeting in Madrid might finally result in a deal, after agreements were reached last year on almost all other parts of the rulebook governing the 2015 Paris climate accord.
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