How Is Heat Transferred within the Climate System?

The heat that feeds the climate system comes from two primary sources. The first source is internal to earth, the heat radiating from the Earth itself. This heat is primarily from the decay of radioactive material and residual heat from the formation of earth. This heat is not distributed equally, with more heat escaping in areas where the crust is thinner, such as divergent boundaries. The second, and more significant source, is the heat Earth receives from solar radiation. Again, this heat is not distributed equally across the earth’s surface. The amount of energy received is related to the angle at which the sun’s rays hit the surface. Overall, these angle differences create the large climate differences across Earth. For example, at the equator solar radiation is received perpendicular to the surface which causes more heat to be absorbed per square foot. Conversely, at the poles, solar radiation is received parallel or at an oblique angle, which results in less heat absorption; therefore, the tropics are much warmer than the poles. (Figure 20.2).

Diagram of Earth illustrating how the sun’s rays impact the polar and equatorial regions differently. In the polar region the sun’s rays cover a longer distance and are distributed over a large area. At the equator, the sun’s rays have a shorter distance to travel and are distributed over a smaller area. — **Figure 20.2:** Why are the polar regions colder? Both Poles are cold because they do not receive any direct sunlight. The Sun is always low on the horizon, even in the middle of summer. In winter, the Sun is so far below the horizon that it doesn’t come up at all for months at a time. (CC-BY-SA 2.5; Peter Halas)

The Albedo Effect

While the angle and amount of incoming solar radiation (insolation) is the most important determinant of Earth’s climate, the material on the Earth’s surface is also important, as not all materials will react the same to solar radiation. For example, darker colored materials, like the ocean, absorb and reradiate heat, most of which is retained at the surface of the planet. You are likely familiar with this if you have ever walked barefoot on dark concrete or asphalt in the summer. Lighter colored materials, like ice and snow, reflect solar radiation off the Earth’s surface. This is the reason many skiers and glacial scientists use eye protection to avoid snow blindness.

The proportion of solar radiation that is reflected off the Earth’s surface is called albedo, which varies depending on the type of ground cover (Figure 20.3). Earth’s albedo is higher when it is covered with large expanses of glacial ice and therefore both the amount of sunlight absorbed and the temperature are lower. The distribution of water, ice, snow, vegetation, and other materials on the Earth’s surface control the Earth’s albedo and can change over time.

Sunlight reflecting and being absorbed by sea ice and ocean water. — **Figure 20.3:** Sea ice reflects as much as 85% of solar radiation hitting the surface therefore absorbing only 15%. Ocean water reflects only about 7% of solar radiation and absorbing 93%. (Public Domain; Don Perovich Cold Regions Research and Engineering Laboratory (CRREL)/NOAA; modified by Chloe Branciforte)