released again in a cyclic process in the wintertime. NARRATOR: Keeling's sensitive measurements had picked up the Earth's respiration. Every spring, plants bloomed, sucking carbon dioxide out of the air. Every fall, CO2 was released back into the air as the plants decayed. But Keeling noticed that year by year, the total concentration of CO2 increased. Superimposed on the annual cycle was a rising curve. Each year, more CO2 was inhaled and exhaled by the biosphere. Over 40 years, the concentrations rose from 330 parts per million to almost 370 parts per million. The Keeling curve appeared to prove what many had suspected, that humans were changing the composition of the atmosphere. But why should adding an invisible, non-toxic gas be cause for concern? Compared to the major atmospheric gases - oxygen and nitrogen - the amount of CO2 is minuscule, but through an accident of physics - the greenhouse effect -its consequences are significant. Perhaps the most dramatic example of the greenhouse effect is the Earth itself. But for this effect, the Earth would be frigidly cold. Of the energy coming from the sun, about half of the light gets scattered and absorbed before it reaches the surface. The rest is radiated back as heat or infrared radiation. In the absence of a greenhouse effect, this heat energy would escape back into space, leaving the Earth with an average temperature of zero degrees Fahrenheit. All water would freeze. The reason this doesn't happen is that greenhouse substances - gases