Soon after, McKibben learned from NASA climatologist James Hansen about new research indicating that the world needed to stabilize the atmospheric concentration of carbon dioxide at 350 parts per million (p.p.m.) to avoid dangerous climate change. (We're already at 392 p.p.m. and counting.) Atmospheric carbon concentration hardly makes for catchy protest slogans, but McKibben saw the number 350 as a clarion call, comprehensible to a global audience without translation. His Internet-savvy friends helped him take the idea worldwide. In October 2009, 350.org organized more than 15,000 rallies in 180 countries. It was likely the biggest mass rally in history.