Two excellent choices. Nordhaus for environmental economics and Paul Romer for economic growth. I did a video for MRUniversity that goes over Paul Romer’s contributions including not only economic growth and charter cities but also his entrepreneurship in developing tools to teach economics! The video was done when MRUniversity was in the early years so it doesn’t have a lot of bells and whistles. On the other hand, Romer told me he really liked this video so this year I don’t think I need to write more!