Rio Tinto estimates it would be paying $US600 million ($940 million) per year in carbon penalties by 2040 if it did nothing to reduce emissions, bolstering the business case for spending on decarbonisation.

Rio has vowed to halve emissions by spending up to $US6 billion in the nine years to 2030 and now expects many of its decarbonisation projects to generate returns of between 12 per cent and 15 per cent.