7.3: Water and Heat Budgets

Averaged over the entire planet and over time periods of months or years, the amounts of heat and water that enter the atmosphere from all sources must equal the amounts removed from the atmosphere by all routes. If total inputs of heat or water did not equal total outputs, the average atmospheric temperature and/or the amount of water vapor in the atmosphere would progressively increase or decrease from year to year. In fact, the Earth’s climate normally remains essentially unchanged for centuries.

In the Earth’s past, very small imbalances between heat gained and heat lost have slightly changed the total amount of heat in the atmosphere and, thus, the Earth’s climate. Today, anthropogenic releases of greenhouse gases, such as carbon dioxide, may disrupt, or already have disrupted, the balance between heat gained and heat lost that has remained stable over the past several centuries (CC9).

Because the inputs and outputs of heat and water to and from the atmosphere must balance, we can construct global heat and water budgets.

Water Budget

The global water budget is relatively simple (Fig. 7-5). Water evaporates from the oceans, lakes, rivers, soils, and vegetation on land. The amount of water evaporated from land and ocean annually is enough to cover the entire globe with water almost 1 m deep. The evaporated water eventually condenses and falls as rain or snow over both land and ocean. Approximately 93% of the water that evaporates into the atmosphere comes from the oceans. However, only about 71% of the total global precipitation (rain and snow) falls on the oceans. Thus, more water evaporates from the ocean than reenters it as rain, and more rain falls on the land than is evaporated from it. This pattern is fortunate because the excess precipitation on land is the source of the Earth’s freshwater supply. Excess water flows back into the oceans through rivers, streams, and groundwater.

Figure 7-5. The Earth’s water budget. The values represent the estimated volumes of water transferred annually between the various reservoirs by the indicated routes.

Neither the rate of water evaporation nor the amount of rainfall is the same everywhere on the oceans or on land. Clearly, deserts and tropical rainforests receive different amounts of rainfall and sustain different amounts of evaporation. The distribution of evaporation and rainfall is discussed later in this chapter.

Heat Budget 

The Earth’s heat budget (Fig. 7-6) is more complicated than its water budget. The source of heat energy for the atmosphere is the sun. Solar radiation energy is partially absorbed by water vapor, dust, and clouds in the atmosphere and converted to heat energy. Part of the sun’s energy is also reflected back, or backscattered, to space by clouds. Solar energy that reaches the Earth’s surface is partially reflected and partially absorbed and converted to heat energy by the land and ocean waters. The Earth, ocean, atmosphere, and clouds also radiate heat energy. In fact, all bodies radiate heat. The peak wavelength radiated increases as the temperature of the body decreases. Because the Earth is much cooler than the sun, the wavelengths radiated by the Earth, ocean, and clouds (infrared) are much longer than the wavelengths emitted by the sun (visible and ultraviolet). The difference in wavelength and the differential absorption by atmospheric gases at different wavelengths are the basis for the greenhouse effect (CC9).

Figure 7-6. The Earth’s heat budget. Solar radiation, which is concentrated in the visible light wavelengths, is reflected, scattered, and absorbed by the gases and clouds of the atmosphere and by the ocean and land surface. All of the absorbed energy is radiated back to space, primarily as infrared radiation. The diagram shows the estimated percentages of the incoming solar radiation that are absorbed, reflected, and reradiated to space by the various components of the Earth-ocean-atmosphere system. Each of these may change in an unpredictable way as the greenhouse effect is enhanced by anthropogenic inputs to the atmosphere (CC9, CC10, CC11).

About 20 to 25% of the solar radiation reaching the Earth is absorbed by atmospheric gases and clouds and converted to heat energy. Almost 50% of the solar radiation is absorbed by the ocean water and the land, then transferred to the atmosphere by the conduction of sensible heat, as latent heat of vaporization, and by radiation. The net transfer averaged over the global oceans is from ocean to atmosphere. Heat transfer by conduction is relatively small, but the transfer of latent heat from ocean to atmosphere equals about one-quarter of the total solar energy that reaches the Earth. Thus, the oceans effectively capture a major portion of the sun’s radiated energy and transfer much of it to the atmosphere as latent heat of vaporization. This mechanism moderates climate differences between Earth’s tropical and polar regions.

Latitudinal Imbalance in the Earth’s Radiation

At the equator, the sun passes directly overhead at noon on the spring and autumnal equinoxes (Fig. 7-7a). The sun’s radiated energy therefore passes vertically through the atmosphere, and the angle of incidence at the Earth’s surface is 90°. At the poles on the equinoxes, the sun remains on the horizon all day, and its radiation must pass through the atmosphere parallel to the Earth’s surface.

Figure 7-7. (a) The Earth, spinning on its tilted axis, moves around the sun once a year. In northern summer (on the right), the Earth is tilted such that the sun is directly overhead at a latitude north of the equator at a point that moves daily around the Earth. Similarly, in northern winter, this point is south of the equator. At the spring and autumnal equinoxes, the sun is overhead at the equator throughout the day as the Earth spins. (b) The angle of incidence of solar radiation received at the Earth’s surface also changes with the seasons, but generally, the area over which a given amount of the sun’s energy is spread is less near the equator and increases toward the poles. The globe on the left shows the Earth at an equinox. The globe on the right shows the Earth at the winter (northern) solstice.

Because the poles are only about 6000 km farther from the sun than the equator is, and because the mean distance from the sun to the Earth is 148,000,000 km, the intensity of the sun’s radiated energy is only about 0.01% lower at the poles than at the equator. However, solar energy is spread over a progressively larger area with increasing latitude (Fig. 7-7b). Consequently, on the equinoxes, the solar radiation received by an area of the Earth’s surface at 30°N or 30°S is only 86% of that received by an equal area at the equator. The proportion at 60°N or 60°S is only 50%, and at the poles it is zero.

Nearer the poles, solar radiation must also travel a longer path through the atmosphere because of the lower angle to the Earth’s surface. More of the solar radiation is absorbed as it passes through the atmosphere because of the increased path length, but this effect causes a relatively small difference in solar intensity at the Earth’s surface.

The intensity of the Earth’s long-wavelength radiation increases with temperature. However, the temperature difference between equatorial and polar regions produces a latitudinal variation in the Earth’s longwave radiation intensity that is not as great as the latitudinal variation of solar insolation of Earth’s surface (Fig. 7-8).

Figure 7-8. Variation in radiative heat loss and gain at the Earth’s surface with latitude. There is a net gain between the equator and about 35° to 40°, and a net loss at higher latitudes.

The total solar radiation absorbed by the Earth must equal the total long-wavelength radiation lost by the Earth when averaged over the whole Earth, but this balance does not occur at all latitudes. There is a large difference between the rates at which solar heat is absorbed at the poles and at the equator. There is a much smaller difference between the rates at which long-wavelength radiation is lost at the poles and at the equator (Fig. 7-8). Consequently, there must be mechanisms for transferring heat energy from low latitudes to high latitudes. If heat were not transferred, the polar regions would be much colder and the equatorial regions much warmer. Planets and moons that have no (or a limited) mechanism of heat transfer between latitudes have dramatically greater differences in surface temperature between their polar and equatorial regions than does the Earth. Hence, latitudinal heat transfer is one of the critical mechanisms that maintains the habitability of the Earth’s entire surface.

Heat is transported latitudinally in two ways. First, latent heat of vaporization enters the atmosphere as water is evaporated in warm tropical regions and some of the heat is transported to higher latitudes by atmospheric convection cells. At higher latitudes, cooling causes water vapor to condense in the form of rain or snow, thus releasing latent heat to the atmosphere. Second, ocean water warmed by the sun in low latitudes is transported by ocean currents to higher latitudes, where it transfers heat to the atmosphere by conduction and evaporation. Near the equator, ocean currents transport more heat to higher latitudes than does atmospheric transport, whereas near the poles atmospheric transport of heat to higher latitudes is greater than ocean current transport.