To help ensure reliably accurate measurements of the big 3 long-lived greenhouse gases, NIST has issued two new Standard Reference Materials. Southern Oceanic Air (SRM 1721) and Northern Continental Air (SRM 1720) contain painstakingly measured concentrations of carbon dioxide, methane, and nitrous oxide. They respond to growing need for greenhouse gas calibration standards that extends beyond organizations participating in the World Meteorological Organization Global Atmosphere Watch Program.