Greenhouse Effect research. A technician checking the trace of an infrared spectrometer to determine the carbon dioxide content of a sample of air. The air sample is taken at the top of a nearby tower, then fed into the laboratory. There, any water vapor is removed before the sample is passed into this spectrometer. Repeated observations over many years have led to the conclusion that the carbon dioxide content of the atmosphere is increasing. CO2 is the main "greenhouse gas", thought to cause global warming. Photographed at the NOAA Climate Monitoring and Diagnostic Laboratory, sited at an altitude of 4170 meters on Mauna Loa, Hawaii.