Describe the major components of Earth’s annual energy budget.

Globally, Earth’s average temperature remains relatively constant, despite seasonal cold spells and heat waves. This stability indicates that a balance exists between the amount of incoming solar radiation and the amount of radiation emitted back to space; otherwise, Earth would be getting progressively colder or warmer. In addition, the energy exchanged between the Earth’s surface and the atmosphere must also remain stable. This surface-to-atmosphere equilibrium is accomplished through conduction, convection, and the transfer of latent heat as well as by the transmission of longwave radiation between the Earth’s surface and the atmosphere. The annual balance of incoming and outgoing radiation, as well as the energy balance that exists between Earth’s surface and its atmosphere, is generally referred to as Earth’s annual energy budget. Further discussion of Earth’s changing climate can be found in Chapter 14.

Annual Energy Budget

Figure 2.22 illustrates Earth’s annual energy budget. For simplicity, we will use 100 units to represent the solar radiation intercepted at the outer edge of the atmosphere. You have already seen in Figure 2.15 that, of the total radiation that reaches Earth, roughly 30 units (30 percent) are reflected and scattered back to space. The remaining 70 units are absorbed: 20 units within the atmosphere and 50 units by Earth’s surface. How does Earth transfer this energy back to space?

If all the energy absorbed by our planet were radiated directly and immediately back to space, Earth’s heat budget would be simple: 100 units of radiation received and 100 units returned to space. In fact, this does happen over time (minus small quantities of energy that become locked up in biomass that may eventually become fossil fuel). What complicates the heat budget is the behavior of certain greenhouse gases, particularly water vapor and carbon dioxide. As you have learned, these greenhouse gases absorb a large share of outward-directed infrared radiation and radiate much of that energy back to Earth. This “recycled” energy significantly increases the radiation received by Earth’s surface. In addition to the 50 units received directly from the Sun, Earth’s surface receives longwave radiation emitted downward by the atmosphere (94 units).

A balance is maintained because all the energy absorbed by Earth’s surface is returned to the atmosphere and eventually radiated back to space. Earth’s surface loses energy through a variety of processes: the emission of longwave radiation; conduction and convection; and energy loss to Earth’s surface through the process of evaporation—latent heat (Figure 2.22). Most of the longwave radiation emitted skyward is reabsorbed by the atmosphere. Conduction results in the transfer of energy between Earth’s surface to the air directly above, while convection carries the warm air located near the surface upward as thermals (7 units).

Earth’s surface also loses a substantial amount of energy (23 units) through evaporation. This occurs because energy is required for liquid water molecules to leave the surface of a body of water and change to its gaseous form, water vapor. The energy lost by a water body is carried into the atmosphere by molecules of water vapor. Recall that the heat used to evaporate water is referred to as latent heat (hidden heat). If the water vapor condenses to form cloud droplets, the energy released by condensation will be detectable as sensible heat (heat we can feel and measure with a thermometer). Through the process of evaporation, water molecules in gas form carry latent heat into the atmosphere, where it is eventually released.

Latitudinal Energy Budget

Because incoming solar radiation is roughly equal to the amount of outgoing radiation, on average, worldwide temperatures remain nearly constant. However, although there is a balance of incoming and outgoing radiation over the entire planet, it is not maintained at each latitude (Figure 2.23). A rather wide zone that spans the equator receives more solar radiation than is lost to space. The opposite is true for higher latitudes, where more heat is lost through radiation emitted by Earth than is received from the Sun.

We might conclude that the tropics are getting hotter and the poles are getting colder. But that is not the case. Instead, the global wind systems and, to a lesser extent, the oceans act as giant thermal engines, transferring surplus heat from the tropics poleward (Figure 2.24). In effect, the energy imbalance drives the winds and the ocean currents. Stated another way, the transfer of surplus heat between the tropics and the poles drives Earth’s weather system.

Most heat transfer across North America occurs in the middle latitudes—from New Orleans at 30° north latitude, to Winnipeg, Manitoba, at 50° north latitude. Consequently, this is also the zone where the majority of stormy weather occurs. These processes are discussed in more detail in later chapters.

• annual energy budget: The annual balance of incoming and outgoing radiation, as well as the energy balance that exists between Earth’s surface and its atmosphere.

• The annual balance of incoming and outgoing radiation is referred to as Earth’s annual energy budget.

• A rather wide zone around the equator receives more solar radiation than is lost to space. The opposite is true for more poleward locations, where more heat is lost through outgoing longwave terrestrial radiation than is received. It is this energy imbalance between the low and high latitudes that drives the weather system and, in turn, transfers surplus heat from the tropics toward the poles.