Long before the term “global warming” became a common household phrase, nineteenth-century Irish physicist John Tyndall said, “Remove for a single summer-night the aqueous vapor from the air which overspreads this country, and you would assuredly destroy every plant capable of being destroyed by a freezing temperature.” This now famous quote reveals the importance of greenhouse gases, like water vapor, in maintaining a balance between the incident solar radiation and the emitted terrestrial radiation. Tyndall understood that without greenhouse gases, water vapor being the most abundant, the earth’s temperature would be markedly cooler. The global average surface temperature is approximately 15°C (59°F) but if the greenhouse gases were removed, the average global temperature would plummet to -18°C (0°F). Remember that these gases make up a small fraction of the composition of the atmosphere! Therefore, adjustments to their concentration will produce dramatic effects.
To understand why these gases are so efficient at keeping the planet warm, let’s examine Figure Atmospheric Transmission . The top panel of this figure shows the normalized intensity of the radiation emitted by both the sun and earth as a function of wavelength. The middle panel shows the total atmospheric absorption spectrum and the bottom panel shows the individual gas absorption spectrum (excluding Nitrogen and Argon). Notice from the top panel that the sun’s peak energy emission falls within the visible portion of the spectrum and suffers very little atmospheric absorption (middle panel). The peak emission wavelength for the earth is in the thermal infrared (IR), and it is effectively absorbed by water vapor (H 2 0), carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N0 2 ). The primary purpose of this figure is to show that the gases in the earth’s atmosphere are transparent to the sun’s peak energy emission (visible light) but not the earth’s peak emission (thermal IR). It is through the absorption of the earth’s outgoing thermal infrared radiation that the global average temperature warms approximately 60°F over what it would be without greenhouse gases.
Are humans altering the natural greenhouse effect? Based upon our assessment so far, this is the final mechanism by which the global climate can be changed. Let’s look into the alteration of the chemistry and composition of the earth’s atmosphere. First are humans increasing the amount of water vapor, the most abundant but also weakest greenhouse gas in the atmosphere? As the air temperature increases, the amount of water vapor the atmosphere can hold also increases. However, a closer investigation of the water cycle is needed to understand what will happen to this increase in water vapor. In this cycle, the amount of evaporation must equal the amount of condensation and thus precipitation on a global scale. This equilibrium must be achieved or else water would end up entirely in its liquid form or in its vapor form. Also due to the speed at which the hydrological cycle operates, a large increase in water vapor would be quickly precipitated out of the atmosphere.