The Innovate to Mitigate project adapts crowdsourcing to support project-based STEM education, posing design challenges for secondary-school students to design feasible innovative strategies to mitigate CO2 emissions and thus global warming. The paper presents evidence that the web-mediated communities of practice support learning of STEM concepts and practices and provides an account of how student and scientist discourse support that learning. Productive talk deployed by community members elicited greater levels of reasoning than was originally manifest in student work and resulted in improved student products.