6. CO2 concentrations in the atmosphere have been measured at an altitude of about 4,000 meters on the peak of Mauna Loa mountain in Hawaii since 1958. The measurements at this location, remote from local sources of pollution, have clearly shown that atmospheric concentrations of CO2 are increasing. The mean concentration of approximately 316 parts per million by volume (ppmv) in 1958 rose to approximately 369 ppmv in 1998. The annual variation is due to CO2 uptake by growing plants. The uptake is highest in the northern hemisphere springtime.