In this study, we explore the potential of data from large-scale assessments to provide insights into how students' environmental knowledge could address the global challenge of environmental threats to humanity and the transition to sustainable development. We analyze data from the 2015 PISA survey to understand the extent to which 15-year old students in 54 countries are aware of these challenges. We find that students' science activities, self-efficacy and environmental knowledge are positively associated with their awareness about environmental challenges. Students' environmental awareness, in turn, is associated with environmental pessimism, or their outlook on the future of environmental issues. Students who are more engaged with environmental science are more aware about environmental issues and feel less optimistic that environmental issues will improve in the future. Such pessimistic attitudes about the future may be a precursor to pro-environmental behavior. Our results provide a cross-national picture of students' engagement with environmental issues and insight into the potential of large-scale assessment data to inform environmental education policies promoted by individual countries and international organizations.