located 254 metres above sea level with gas intakes at 116m and 232m above the streets of San Francisco, the Sutro Tower acts as a climate sentinel, stretching up into the atmosphere to measure CO2 levels. the first US sampling site to be located in an urban centre, it's equipped with automated flask sampling systems that provide daily measurements of a suite of greenhouse gases, carbon isotopes, halocarbons and other compounds. together with other stations, it has witnessed a steady rise in atmospheric CO2 concentrations, with recent figures from the National Oceanic and Atmospheric Administration making for grim reading