Guillaume Falmagne joined Princeton’s High Meadows Environmental Institute (HMEI) in September 2022. Before that, he worked at LLR (École Polytechnique, Palaiseau/Paris) where he received a Ph.D. in Physics. He investigated properties of the quark-gluon plasma using data from heavy ion collisions in the CMS detector at LHC (the 27-km particle accelerator at CERN, Geneva). In particular, he led the first observation of the Bc particle in that peculiar state of matter. He is now interested in collective phenomena in large groups through the lens of critical transitions and big data analysis. One project, together with postdoc Annie Stephenson, is based on data from a 10-million-user collaborative game on Reddit. He used machine learning techniques to build performant early warning signals for local transitions in the game. The collaborative nature of the game also allows to gain understanding of the organizational structures that favor cooperation at large scales, and of the dynamics of conflicts and group formation. In another project, he determines how the structure of the moisture network affects cascading transitions in the Amazon rainforest. He is also interested in complex systems that show feedback between their social and ecological components.