I was about to do a post debunking the clearly inaccurate claim that More than 20 percent of the world oxygen is produced in the Amazon Rainforest, but those nice people at National Geographic have saved me the bother:
“The net [oxygen] effect of the Amazon, or really any other biome, is around zero,” he explains.
No point me summarising such a neat article, you might as well read it in full, if you're interested.
Debate Magazine
