An international team of researchers is about to launch a project that could help answer whether the mighty photosynthetic engine of the Amazon rainforest will consume enough atmospheric carbon dioxide (CO2) to slow global warming. Called AmazonFACE (Free-Air Carbon dioxide Enrichment), it’s based on a simple idea: For 12 years, researchers will spray pure CO2 into instrumented plots in the rainforest northwest of Manaus, Brazil, raising ambient concentrations to 600 parts per million—a level the world could reach as early as 2050—all the while taking meticulous measurements to determine how the gas affects the growth of plants. That’s a key question because through photosynthesis, land plants currently take up about a quarter of the CO2 humans add to the atmosphere each year, sequestering it as wood and as soil carbon. This natural uptake slows the buildup of CO2 in the air, moderating global warming. FACE experiments have been conducted at dozens of sites in more than a dozen countries, but never before in the Amazon. Researchers must surmount a host of logistical challenges both before and after construction starts next year.