To understand most of the processes at work on Earth, it is useful to envisage interactions within the Earth system as a series of interrelated cycles. One of these is the energy cycle, which encompasses the great “engines”—the external and internal energy sources—that drive the Earth system and all its cycles. We can think of Earth’s energy cycle as a “budget”: energy may be added to or subtracted from the budget and may be transferred from one storage place to another, but overall the additions and subtractions and transfers must balance each other. If a balance did not exist, Earth would either heat up or cool down until a balance was reached.

The total amount of energy flowing into Earth’s energy budget is more than 174,000 terawatts (or 174,000 ×10¹² watts). This quantity completely dwarfs the 10 terawatts of energy that humans use per year. There are three main sources from which energy flows into the Earth system.

Incoming short-wavelength solar radiation overwhelmingly dominates the flow of energy in Earth’s energy budget, accounting for about 99.986 percent of the total. An estimated 174,000 terawatts of solar radiation is intercepted by Earth. Some of this vast influx powers the winds, rainfall, ocean currents, waves, and other processes in the hydrologic (or water) cycle. Some is used for photosynthesis and is temporarily stored in the biosphere in the form of plant and animal life. When plants die and are buried, some of the solar energy is stored in rocks, when we burn coal, oil, or natural gas, we release stored solar energy.

The second most powerful source of energy, at 23 terawatts or 0.013 percent of the total, is geothermal energy, Earth’s internal heat energy. Geothermal energy eventually finds its way to Earth’s surface, primarily via volcanic pathways. It drives the rock cycle and is therefore the source of the energy that uplifts mountains, causes earthquakes and volcanic eruptions, and generally shapes the face of the Earth.

The smallest source of energy for Earth is the kinetic (motion) energy of Earth’s rotation. The Moon’s gravitational pull lifts a tidal bulge in the ocean; as Earth spins on its axis, this bulge remains essentially stationary. As Earth rotates, the tidal bulge runs into the coastlines of continents and islands, causing high tides. The force of the tidal bulge *piling up* against land masses acts as a very slow brake, actually causing Earth’s rate of rotation to decrease slightly. The transfer of tidal energy accounts for approximately 3 terawatts, or 0.002 percent of the tidal energy budget.

Earth loses energy from the cycle in two main ways: reflection, and degradation and reradiation. About 40 percent of incoming solar radiation is simply reflected, unchanged, back into space by the clouds, the sea, and other surfaces. For any planetary body, the percentage of incoming radiation that is reflected is called the “albedo.” Each different material has a characteristic reflectivity. For example, ice is more reflectant than rocks or pavement; water is more highly reflectant than vegetation; and forested land reflects light differently than agricultural land. Thus, if large expanses of land are converted from forest to plowed land, or from forest to city, the actual reflectivity of Earth’s surface, and hence its albedo, may be altered. Any change in albedo will, of course, have an effect on Earth’s energy budget.

The portion of incoming solar energy that is not reflected back into space, along with tidal and geothermal energy, is absorbed by materials at Earth’s surface, in particular the atmosphere and hydrosphere. This energy undergoes a series of irreversible degradations in which it is transferred from one reservoir to another and converted from one form to another. The energy that is absorbed, utilized, transferred, and degraded eventually ends up as heat, in which form it is reradiated back into space as long-wavelength (infrared) radiation. Weather patterns are a manifestation of energy transfer and degradation.