The Earth's physical climate system, which includes the atmosphere, oceans, cryo-sphere, and land surface, is complex and constantly evolving. Nevertheless, the laws of physics, chemistry, and biology ultimately govern the system and can be used to understand how and why climate varies from place to place and over time. For example, the energy balance of the Earth as a whole is determined by the difference between incoming and outgoing energies at the top of the atmosphere. The only significant incoming energy is radiation from the sun, which is concentrated at short wavelengths (visible and ultraviolet light), while the outgoing energy includes both infrared (long-wavelength) radiation emitted by the Earth and the portion of incoming solar radiation (about 30 percent on average) that is reflected back to space by clouds, small particles in the atmosphere, and the Earth's surface. If the outgoing energy is slightly lower than the incoming energy for a period of time, then the climate system as a whole will warm until the outgoing radiation from the Earth balances the incoming radiation from the sun.
The temperature of the Earth's surface and lower atmosphere depends on a broader range of factors, but the transfer of radiation again plays an important role, as does the composition of the atmosphere itself. Nitrogen (N2) and oxygen (O2) make up most of the atmosphere, but these gases have almost no effect on either the incoming radiation from the sun or the outgoing radiation emitted by the Earth's surface. Certain other gases, however, absorb and reemit the infrared radiation emitted by the surface, effectively trapping heat in the lower atmosphere and keeping the Earth's surface much warmer—roughly 59°F (33°C) warmer—than it would be if greenhouse gases were not present.1 This is called the greenhouse effect, and the gases that cause it—including water vapor, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)—are called greenhouse gases (GHGs). GHGs only constitute a small fraction of the Earth's atmosphere, but even relatively small increases in the amount of these gases in the atmosphere can amplify the natural greenhouse effect, warming the Earth's surface (see Figure 2.1).
1 This difference includes the greenhouse effect associated with clouds, which are composed of water droplets, but it assumes that the total reflectivity of the Earth—including the reflection by clouds—does not change.
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