Global Energy Transfer

Earth is best thought of as a system with energy inputs and outputs. Energy, mostly heat, comes from the Sun and Earth’s core. As this energy transfers around and through the planet, it drives major processes such as plate tectonics, climate, ocean and air currents, and the distribution of life across the globe.

Earth as a System

Earth is governed by many integrated processes, which together are called a system. The study of global energy (heat) transfer aims to map how these systems interact to move energy around the planet. The transfer of energy, in turn, drives everything from global weather patterns to the formation of mountains to the evolution of plants and animals.

The total amount of energy that moves through the Earth’s system amounts to hundreds of terawatts (1012 watts). For perspective, a typical automobile engine is rated at 50,000 to 100,000 watts, and lightbulbs handle hundreds of watts; Earth deals in hundreds of billions of watts.

The rate at which Earth absorbs and radiates energy is essential to Earth’s capacity to support life. The atmosphere, which slows the release of heat back into the universe, allows the planet to maintain a temperature conducive to liquid water and to the existence of life.

Earth’s Subsystems

Earth’s system can be broken down into four subsystems: atmosphere, lithosphere, hydrosphere, and biosphere. The atmosphere is most notable for the way it acts as a shell around the planet.

The atmosphere bounces back into space much of the incoming radiation that hits Earth, but what the atmosphere does let through, it also often traps on Earth; the atmosphere bounces energy radiating from the Earth’s surface back to the Earth before that energy can leave the system.

The greenhouse gases, such as carbon dioxide (CO₂), get their name because they are especially good at trapping heat in the planetary system like a greenhouse, which traps heat. Earth’s greenhouse gases trap heat by absorbing outgoing infrared radiation and by then reemitting it in all directions. Some radiation then returns toward Earth. The movement of air around the planet, as wind, also transfers some energy. However, the atmosphere is generally unable to hold much energy. Instead, it helps to drive some ocean currents, which move energy around.

The lithosphere is Earth’s landmass. Land is better than the atmosphere at holding heat, but its most important role in global energy transfer comes from its relationship to the planet’s core. In areas of high geothermal activity (volcanoes are perhaps the most dramatic example), energy from the Earth’s core escapes. The gases and chemicals produced by geothermal activity can also influence global energy transfer, such as when ash and gases spewed by volcanic eruptions lead to cooling because they reflect more incoming radiation than does the atmosphere.

The hydrosphere is arguably the most important element of Earth’s energy system. The hydrosphere is the water that covers the planet’s surface, largely the oceans, and the oceans serve as the major energy reservoir for the planet. This is true because of water’s high specific heat (water can store a large amount of energy before its temperature increases). The oceans move this energy around the planet, shaping the climate.

One can consider London’s comparatively mild winters despite its northerly latitude. London benefits from the Gulf Stream, a major warm-water current that carries energy from near the equator up the east coast of North America and then northeast across the Atlantic Ocean to Europe. The interaction of the hydrosphere and atmosphere is largely responsible for Earth’s weather, as heat moves between the oceans and the air above them.

Finally, the biosphere comprises all the living things on the planet, from microbes to plankton to humans to whales. Life uses the energy in Earth’s system to power itself; photosynthetic organisms turn the Sun’s radiated energy into a form (that is, food) usable by other forms of life. Life-forms are most important to global energy transfer because of their roles in how gases and chemicals cycle around the planet. Plants help trap carbon, keeping it from entering the atmosphere as carbon dioxide and acting as a greenhouse gas. The digestion processes in animals (cattle, for instance) can produce greenhouse gases such as methane. The reef-building of corals can help shape coastlines and the flow of energy in ocean currents.

Humans and industry alter the composition of the atmosphere and the way energy moves around the planet at an unprecedented rate. Burning fuel, for example, releases stored energy as heat, and the overall global effects of fuel burning are significant.

Earth’s Energy Sources: Solar Flux and

Across all four subsystems, energy comes from two major sources: the Sun and the Earth’s core. Solar flux is the term for the total amount of incoming solar radiation that enters the Earth’s system. Because of the tilt of the Earth’s axis, sunlight hits different parts of the planet with different intensities depending on the season. This uneven heating creates the three major climate zones: tropical, temperate, and polar. This uneven heating also creates weather systems.

Warmer fluids are less dense than colder fluids; warm air and water in the skies and oceans will be pushed away by heavier, colder air and water. This, coupled with the Earth's rotation, sets the major ocean currents and wind patterns in motion, which are important in moving energy around the planet.

The system’s internal heat source, radiation from Earth’s core, creates currents in the liquid rock that surrounds the core. These currents move the continents around on the Earth's surface. The motion of the crust sliding on the subsurface magma drives plate tectonics, which in turn gives the planet its mountains, basins, and other physical features.

Climate Change

All efforts to mitigate the effects of global climate change are predicated on a thorough grasp of how energy moves around the planet. Global warming, an aspect of global climate change, is, after all, essentially a global energy surplus: More energy remains in Earth’s system than leaves it, thus raising global temperatures.

Using observational satellites, scientists can estimate how much total radiation enters and exits the Earth’s system. However, what happens to this energy once it enters the system remains poorly understood. Scientists know that Earth has a large surplus of unaccounted-for energy that has “disappeared” somewhere on the planet. Earth is not in a state of radiative balance. For this reason, a detailed accounting of Earth’s energy budget and a comprehensive model of the processes that drive global energy transfer are fundamental requirements for success in the face of the challenges presented by global climate change.

Consider once again London’s mild climate. Knowing how ocean and air currents move warm water (energy) around supports predictions of some of the seemingly counterintuitive effects of global warming. For instance, London could experience a temperature drop if the Gulf Stream, a major warm water current, shifts due to climate change. Without the warm water mediating the air temperature around London, the city could see much more severe winters.

Principal Terms

albedo: the amount of radiation a surface reflects; higher albedo reflects more incoming radiation

atmospheric greenhouse effect: the result of greenhouse gases in the atmosphere; trapped energy (heat) in the Earth’s system

electromagnetic radiation: radiation that includes visible light, infrared radiation (heat), radio waves, gamma rays, and X-rays

energy budget: an accounting of all the incoming and outgoing energy for Earth as a system

greenhouse gas: an atmospheric gas that contributes to the greenhouse effect by absorbing infrared radiation and reemitting that radiation

infrared radiation: electromagnetic radiation with wavelengths longer than visible light but shorter than radio waves; the type of electromagnetic radiation perceived as heat

irradiance: the power of electromagnetic radiation over a given unit of area, usually in watts per square meter; used to measure the influx of energy through an area such as the Earth’s surface

radiative equilibrium: a state in which Earth’s incoming radiation and outgoing radiation are equal; it results in a generally stable climate as there is no net gain or loss of energy from the planet’s system

radiative forcing: the total change in irradiance between different layers in the atmosphere; positive radiative forcing indicates a net increase in energy in the system (warming), whereas negative radiative forcing indicates a net release of energy (cooling)

solar flux: the total energy entering Earth’s atmosphere from the Sun

specific heat: the amount of heat (energy) it takes to raise the temperature of the unit mass of a given substance by a given amount, usually one degree; functionally, a substance’s capacity to store heat

Bibliography

“Heat and Energy in the Atmosphere.” In Fundamentals of the Physical Environment. 3d ed., New York: Routledge, 2002.

Kandel, Robert. “Understanding and Measuring Earth’s Energy Budget: From Fourier, Humboldt, and Tyndall to CERES and Beyond.” Surveys in Geophysics, Jan. 2012, pp. 1-12.

Kiehl, J. T., and Kevin E. Trenberth. “Earth’s Annual Global Mean Energy Budget.” Bulletin of the American Meteorological Society, vol. 78, Feb. 1997, pp. 197-208.

Smithson, Peter, Kenneth Addison, and Kenneth Atkinson. “Energy and Earth.” In Fundamentals of the Physical Environment. 3d ed., New York: Routledge, 2002.

“The Transfer of Heat Energy.” National Oceanic and Atmospheric Administration, 2 Jan. 2024, www.noaa.gov/jetstream/atmosphere/transfer-of-heat-energy. Accessed 29 July 2024.

Trenberth, K. E. “An Imperative for Climate Change Planning: Tracking Earth’s Global Energy.” Current Opinion in Environmental Sustainability, vol. 1, 2009, pp. 19-27.

Trenberth, K. E., and John T. Fasullo. “Tracking Earth’s Energy.” Science, vol. 328, Apr. 2010, pp. 316-317.