The second ARCSS Synthesis Retreat was held 8-14 August 2004 at Lake Tahoe, California. The week-long science retreat began by focusing on the question: "How realistic is a conceptual model of a two-state (modern and future seasonally ice free) arctic system, and how well do we understand the processes that may lead to a state change?" Thirty-one participants attended the retreat, including scientists and graduate students representing a wide variety of scientific disciplines.