Climate Change Primer
The Greenhouse Effect
Understanding a phenomenon known as the greenhouse effect is a first key step in understanding how climate change is affecting our planet.
Solar radiation interacts with the surface of the earth. This interaction takes several forms: some portion of incoming solar energy is reflected back into space by the earth's atmosphere; another portion is dispersed and scattered by the molecules in the atmosphere; and a large portion penetrates through the earth's atmosphere to reach the planet’s surface. The radiation reaching the earth's surface is largely absorbed, resulting in surface warming (Figure 1).
Much of this absorbed energy is eventually re-radiated in longer infrared wavelengths. As it leaves the earth, it once again interacts with the atmosphere. Some of this re-radiated energy escapes to space, but much of this re-radiated energy is reflected back to the earth's surface by molecules in the earth's atmosphere. This phenomenon is similar to the warming that occurs in an automobile parked outside on a sunny day (Figure 2).
The molecules responsible for trapping re-radiated energy in the earth’s atmosphere are called greenhouse gases because they act like the glass in a greenhouse. The most important greenhouse gases include water (H2O), nitrous oxide (N2O), methane (CH4), and carbon dioxide (CO2). Without these gases, most life on earth would not be possible, as the surface temperature of the earth would likely be about 60°F colder.
Greenhouse gases act like an insulator or blanket above the earth, keeping the heat in. Increasing the concentration of these gases in the atmosphere increases the thickness of this insulator, therefore increasing the atmosphere's ability to block the escape of infrared radiation. Too great a concentration of greenhouse gases can have dramatic effects on climate and significant repercussions for Earth. Too low a concentration can have dramatic effects as well. Climates suitable for human existence are limited above some minimum threshold level of greenhouse gas concentration. In other words, those climates are possible only within a finite window - a limited range of greenhouse gas concentrations make life as we know it possible.