RIO DE JANEIRO — In the depths of the Amazon, Brazil is building an otherworldly structure — a complex of towers arrayed in six rings, poised to spray mists of carbon dioxide into the rainforest. But the reason is utterly terrestrial: to understand how the world’s largest tropical forest responds to climate change. Dubbed AmazonFACE, the project will probe the forest’s remarkable ability to sequester carbon dioxide — an essential piece in the puzzle of world climate change. This will help scientists understand whether the region has a tipping point that could throw it into a state of irreversible decline. Such a feared event, also known as the Amazon forest dieback, would transform the world’s most biodiverse forest into a drier savannah-like landscape. FACE stands for Free Air CO2 Enrichment. …The construction of the initial two rings is underway and they are expected to be operational by early August.