Problem-based learning (PBL) is an instructional design approach for promoting student learning, in context-rich settings. GlobalEd 2 (GE2) is PBL intervention that combines face-to-face and online environments into a 12-week simulation of international negotiations of science advisors on global water resource issues. The GE2 environment is described examining the impact it has had on middle school students' written scientific argumentation during the intervention. Analyses using HLM on treatment and comparison groups demonstrated a significant positive impact on the written scientific argumentation scores of 1818 middle-grade students from two states with an effect size of 0.257 (p<0.001). [For full proceedings, see ED571332.]