as plants and leaves die off and decay, releasing back into the atmosphere. Legacy Global monitoring Due in part to the significance of Keeling's findings, NOAA began monitoring CO2 levels worldwide in the 1970s. Today, atmospheric CO2 levels are monitored at about 100 sites around the globe through the Global Greenhouse Gas Reference Network. Measurements at many other isolated sites have confirmed the long-term trend shown by the Keeling Curve, although no sites have as long a record as Mauna Loa. Ralph Keeling Since Charles David Keeling's death in 2005, responsibility and oversight of the project was transferred to Keeling's son, Ralph Keeling. On the fiftieth anniversary of the beginning of the project, the younger Keeling wrote an article in Science magazine describing his father's life and work, along with how the project has grown and evolved over time. Along with more precise measurement materials and funds for the project of monitoring of the Earth's levels, Keeling wrote about his pride for his father's work and how he has continued it in his memory. Recognition In 2015, the Keeling Curve was designated a National Historic Chemical Landmark by the American Chemical Society. Commemorative plaques were installed at Mauna Loa Observatory and at the Scripps Institution of Oceanography at the University of California, San Diego. Passing 400 ppm in 2013 On May 9, 2013, the daily mean concentration of in the atmosphere measured at Mauna Loa surpassed 400 parts per million (ppmv). Estimates of during previous geologic eras suggest that has not reached this level since the mid-Pliocene, 2 to 4 million years ago. This level of carbon dioxide, causing climate change, suggests a continued worsening in natural and ecological disasters, which increasingly threatens human and animal habitats on Earth, if greenhouse gas emissions are not significantly reduced. See also Greenhouse Gas Monitoring Action for Climate Empowerment (ACE) Paris Agreement References External links Official Keeling Curve website. Scripps Institution of Oceanography, UC San Diego earth: Annually-updated version of the Keeling curve Climate Change Is Clear Atop Mauna Loa, NPR, Day to Day, May 1, 2007 Scripps Institution of Oceanography CO2-Program: Home of the Keeling Curve Atmosphere Historical climatology Carbon dioxide 20th-century neologisms