The global market for voluntary carbon offsets – the removal of emissions of carbon dioxide or other greenhouse gases in order to compensate for emissions made elsewhere – has grown rapidly in recent years.

Countries established the REDD+ framework to protect forests as part of the 2015 Paris Agreement. REDD stands for ‘reducing emissions from deforestation and forest degradation in developing countries’. Many of these schemes generate carbon credits by investing in the protection of parts of the world’s most important forests – from the Congo to the Amazon.

But an analysis of 18 major carbon offset projects matched with sites that offer a real-world benchmark for deforestation levels found that more than 60 million carbon credits came from projects that barely reduced deforestation, if at all. Of a potential 89 million credits from these offset schemes, only 5.4 million (6 per cent) were linked to additional carbon reductions through preserved forest...