Over the past month, several readers have commented that increasing concentrations of CO2 in cannot be the cause of global warming because the concentration of CO2 in the atmosphere today is too low (0.04 percent) compared to nitrogen and oxygen (99 percent).
The warming of the earth, or greenhouse effect, occurs because sunlight, which is composed of shortwave radiation, passes through the atmosphere where it is unaffected by either the major gases or the minor greenhouse gases. The earth absorbs the sunlight, warms the surface and then reradiates heat as infrared or longwave radiation. The outgoing longwave radiation is absorbed by the greenhouse gases in the atmosphere. This heats the atmosphere, which in turn re-radiates longwave radiation in all directions.
The critical fact here is that the nitrogen and oxygen in the atmosphere are essentially transparent to this longwave radiation, so it is only the concentration of the greenhouse gases that is important. Thus, with more carbon dioxide in the atmosphere, we expect to see less infrared radiation escaping to space. The cycle of surface heating and re-radiation then gets repeated over and over again, trapping the heat and further increasing the earth’s temperature.
Since the 1960s, the concentration of CO2 in the atmosphere has increased by 25 percent due to the worldwide burning of fossil fuels such as oil, gas, and coal. An enhanced warming effect from these greenhouse gases such as CO2 and methane has been confirmed by verifiable and repeatable observations by many scientists from all over the world. For instance, satellite measurements of infrared spectra over the past 40 years show decreasing amounts of energy escaping to space at the wavelengths associated with CO2. Over the same time period, surface measurements find increased downward infrared radiation. These observations provide a direct, measurable, causal link between CO2 and global warming. Thus, even though only 0.04 percent of the atmosphere is made up of CO2, its increased concentration over the past 50 years is directly linked to global warming.
Dr. Peter Kroopnick