Earth is continually bombarded with energy from the Sun, which drives many Earth processes and makes most life possible. Radiant energy exits the Sun and interacts with Earth's atmosphere on its way to the ground or water surface. Solar radiance that makes it through the atmosphere and reaches the planet's surface can be reflected, transmitted, or absorbed and reradiated. This chapter discusses the transmission, reflection, storage, and transport or cycling of solar energy within the Earth system, including an examination of Earth's radiation budget. Also considered are the effects of solar radiation on biospheric processes and how these processes are modified by human activity.
The Sun emits energy in the form of electromagnetic radiation that travels at a speed of about 300,000 kilometers per second (186,000 miles per second), that is, the speed of light. Thus it takes roughly 10 minutes for energy leaving the Sun to reach Earth. Electromagnetic radiation includes many different types of radiation that together compose the electromagnetic spectrum. The spectrum is divided into different classes or types of radiation by the wavelength or frequency (related inversely to wavelength) of the radiation (Figure 2.01). In remote sensing, the usual convention is to categorize radiation by wavelength. Different sources of energy emit radiation of differing spectral (referring to its wavelength) compositions. For example, the Sun's spectral output is composed of approximately 9% ultraviolet (and shorter) wavelengths, 41% visible light, and about 50% infrared radiation. Shorter wavelengths have higher energy content than longer wavelengths.
For the purposes of this discussion, we can reasonably assume that for any given area at the top of the atmosphere, the solar energy reaching that area (for periods of time that the location stays the same distance from the Sun) arrives at a relatively constant rate, referred to as the "solar constant." This rate actually varies seasonally as Earth slowly revolves around the Sun and tilts upon its axis, but it averages about 1400 watts per square meter. Earth life forms exposed to this amount of energy would die!
The spectral composition and amount of solar energy intercepted at Earth's ground and water surfaces are not exactly the same as that arriving at the outer atmospheric edges, because the atmosphere interacts with and modifies the radiation traveling through it. As the solar energy received at the top of the atmosphere continues toward Earth's surface, various wavelengths are selectively transmitted, reflected (scattered), or absorbed by the atmosphere, clouds, and then ground and water surfaces. Figure 2.02 represents a greatly simplified global radiation balance; note that values express the globally averaged effect of the illustrated processes.
Figure 2.03 shows where in the spectrum the atmosphere selectively transmits and absorbs radiation. The graph represents atmospheric transmission as a function of wavelength; where the curve shows peaks (up toward 100 percent transmission) indicates atmospheric windows (wavelengths where radiation travels through the atmosphere) and where the curve shows dips indicates wavelengths where radiation gets absorbed by the atmosphere. Chemical elements and compounds listed on the figure identify the atmospheric absorber.
1.2.1 Absorption and scattering (reflection) by the atmosphere and clouds -- Absorption of solar radiation in the atmosphere occurs as radiant energy interacts with different atmospheric components. On average, about 15% of incoming solar radiation is absorbed by atmospheric molecules such as water vapor, oxygen and small particulates (aerosols). Certain wavelengths of infrared radiation are absorbed by carbon dioxide (CO2) and water vapor (see Figure 2.03). The amount of energy absorbed varies significantly from one geographic location to another. Although the CO2 concentration in the atmosphere is more or less uniform around the globe, atmospheric water vapor content can change dramatically from place to place due to oceanic, meteorological and biospheric effects. Typically, water vapor absorbs the greatest amount of solar radiation passing through the atmosphere, especially in the infrared region of the spectrum.
Scattering of solar radiation within the atmosphere also accounts for a reduction of energy reaching Earth. Atmospheric gas molecules and aerosols deflect solar radiation from its original path, scattering (reflecting) some radiation back into deep space and some toward Earth's surface. Clouds reflect much more incoming solar radiation than they absorb. The high albedo (the ratio of reflected energy to incoming energy) of clouds is a significant factor in the radiation balance, and so the distribution of clouds around the globe can have a large effect on climate. Cloud cover can be highly variable in space and time.
Combining together the percentages of incoming energy absorbed (18%) and scattered (26%) by the atmosphere plus clouds, the overall effect is that nearly half (18% + 26% = 44%) of the energy entering the atmosphere doesn't make it through to Earth's surface.
Of the roughly 56% of the incoming solar radiation making it through the atmosphere to Earth's surface, about 6% gets reflected by the surface and 50% is absorbed at the surface. The fraction of the reflected solar radiation to the incident solar radiation defines "albedo." The larger the albedo, the more bright or reflective a surface or object; the smaller the albedo, the darker or more absorbing a surface. Although surface reflectance typically is small compared to what clouds reflect, its distribution (in both time and geographic location) around the globe affects the distribution of absorbed solar radiation. Surface reflectance of solar energy is also what many remote sensing instruments measure.
Two global views of Earth's albedo (for January 1986) are shown in Figure 2.04. Measurements depicted in the figure were made by a satellite system known as the Earth Radiation Budget Experiment (ERBE). Figure 2.04a shows "total scene" albedo which includes reflection from clouds. In Figure 2.04b, the reflective effect of clouds has been removed (referred to as "clear sky" albedo), revealing the range of albedos for different surface types. Ice and snow in Antarctica and portions of the Arctic have a high albedo, and the Sahara desert in northern Africa has a medium albedo. Forests and oceans have relatively low albedos.
At any given location on Earth, the insolation (incoming solar energy) received on a daily basis depends primarily on 1) the angle of the Sun above the horizon (solar elevation angle, solar incidence angle), 2) the length of time the surface is exposed to the Sun, and 3) atmospheric conditions (discussed above in Section 1.2.1). These factors vary significantly in space and time. As Earth revolves around the Sun over the course of a year, its orbital geometry and tilt cause seasonal and latitudinal variations in insolation. For example, areas in North America receive more insolation in summer months than during winter because 1) the angle of incidence of incoming solar radiation is higher in summer than winter (Figure 2.05), and 2) the day length (hours of sunlight) is longer in summer than in winter. Insolation at any point also continuously changes throughout the day as the Sun rises, arcs up to solar noon, arcs down and then sets.
Generally, equatorial regions experience less fluctuation in daily insolation throughout the year. Further from the equator, seasonal differences are more pronounced. The extreme example is polar regions, experiencing many more hours of sunlight than darkness in their respective summer, and many more hours of darkness than sunlight in their respective winter. On the equator, however, there is a nearly constant 12 hours of sunlight throughout the year. Moreover, the distance light has to pass through the atmosphere near the equator is less than the distance it passes through near the poles (see Figure 2.05). These phenomena combine to create a much greater surface energy insolation in the equatorial regions than in the polar regions. The amount of solar energy then absorbed at any place also depends on the surface albedos of cover types there. For example, polar regions not only receive less insolation, but the high albedos of ice and snow increase reflectance and consequently reduce absorption of sunlight.