When it comes to Earth’s climate system, water is often at the center of the story — whether it’s too much, too little or arriving at the wrong time. And while today’s climate models can tell us how much rain might fall or how humid the air might be, they often can’t answer the simpler, and perhaps more important, question: Where did this water come from? A new project led by Rice University and the U.S. National Science Foundation National Center for Atmospheric Research (NSF NCAR) is changing that. Backed by a grant from the National Science Foundation, the initiative — called SCI-SWIM, short for sustainable community infrastructure for stable water isotope modeling — will build a new and improved version of the Community Earth System Model (CESM), which can trace water across the entire planet from the clouds in the sky to the thick ice sheets deep underground.