Global energy balance
The global energy balance refers to the net inflow and outflow of energy to and from Earth, which is crucial for understanding climate dynamics. This balance is influenced by several processes, including atmospheric reflectance, absorption, and the emission of energy back into space. Incoming solar radiation, primarily in the visible spectrum, interacts with Earth's surface, where a portion is reflected, absorbed, or transmitted. About 30% of incoming solar energy is reflected back into space, while 25% is absorbed by the atmosphere, and the remaining 45% is absorbed by the Earth's surface, converting it into thermal energy.
The energy emitted by Earth occurs in longer wavelengths and is partially reabsorbed by greenhouse gases, contributing to warming. Changes to the global energy balance, driven by both natural variations (like Earth's orbital shifts) and human activities (such as greenhouse gas emissions), are key factors in climate change. Historical climate shifts have occurred over millennia due to these natural processes, while anthropogenic effects, particularly from fossil fuel combustion, have significantly increased greenhouse gas concentrations, resulting in a notable rise in global temperatures. Understanding the global energy balance is essential for evaluating the impacts of climate change and exploring mitigation strategies, such as carbon sequestration.
Subject Terms
Global energy balance
Definition
The global is the overall inflow and outflow of energy to and from Earth. The different processes that control this balance are atmospheric reflectance, absorption by the atmosphere and Earth’s surface, and reradiation of energy to space by Earth itself. Electromagnetic energy arrives in the form of solar radiation that passes through the atmosphere and interacts with the surface.
When electromagnetic energy hits an object, such as Earth, three processes can occur. Transmission occurs when the energy passes through an object without affecting it. Reflection occurs when the energy bounces off the object, and absorption occurs when the energy is taken up by the object. Incoming solar radiation exists at all wavelengths, but the dominant intensity is within the visible band (400-700 nanometers), at 550 nanometers. Of this energy, 30 percent is reflected from the atmosphere back into space, 25 percent is absorbed by the atmosphere, and the remaining 45 percent is absorbed by Earth’s surface.
The energy absorbed by Earth’s surface is converted to thermal energy and raises the surface temperature. Because the Earth itself is a warm object, it also emits electromagnetic energy to space. The energy emitted by Earth occurs in longer wavelengths than incoming solar radiation, with the maximum intensity at 10 millimeters. Of this energy, 70 percent passes through the atmosphere and is transmitted to space. However, 30 percent is reabsorbed by atmospheric greenhouse gases (GHGs), including carbon dioxide (CO2), methane, nitrous oxide, and water vapor. This reabsorbed energy contributes to further warming of Earth. The balance between the amount of incoming solar radiation and the amount of reflected, absorbed, and transmitted energy controls global wind patterns and oceanic circulation. Shifts in this balance determine the overall warming and cooling of the planet.
Significance for Climate Change
Changes to the global energy balance, which is affected by both natural and processes, are a fundamental cause of climate change. Processes that affect the global energy balance are often intertwined with one another, resulting in various feedback mechanisms that can enhance or reduce the effects of the original change. Natural changes in the global energy balance can be caused by variations in Earth’s orbit or in the output of solar radiation—both of which affect the amount of energy coming into Earth.
The shape of Earth’s orbit and the tilt of Earth’s axis change slightly over timescales ranging from 11,000 to 100,000 years. These changes can cause the Earth to be somewhat closer to or further away from the Sun, leading to a relative increase or decrease in the amount of solar radiation that reaches Earth. In the geologic past, these types of changes are manifested as climate shifts on timescales of millions of years, known as greenhouse and icehouse conditions. During greenhouse times, Earth is ice-free because of high concentrations of GHGs. During icehouse times, Earth is covered by variable proportions of ice, and the climate shifts between interglacial times (less ice) and glacial times (larger continental ice sheets) on timescales of tens of thousands of years. The most recent peaked at the Last Glacial Maximum, approximately 20,000 years ago. Approximately 850-630 million years ago, Earth may have been completely covered by ice, creating a so-called Snowball Earth. This hypothesis, however, remains highly controversial among scientists. Blocking some proportion of incoming solar radiation remains a goal of certain geoengineering proposals as a means to prevent future global warming.
Changes in GHG concentrations also affect the global energy balance—and therefore, global temperature—because of changes in the amount of reradiated energy trapped on Earth. Volcanoes are natural sources of GHGs, and geologic evidence suggests that there is a correlation between widespread volcanism and greenhouse times in the past. Anthropogenic changes in the global energy balance are thought to be caused by the addition of GHGs to the atmosphere. The most notable of these is the production of CO2 through the burning of fossil fuels, although the production of methane should not be discounted. Increased concentrations of GHGs in the atmosphere lead to increased absorption of energy radiated from Earth, which, in turn, leads to further warming of the surface. The Intergovernmental Panel on Climate Change (IPCC) estimates that anthropogenic changes in atmospheric CO2 concentrations have resulted in a global temperature increase of 0.74° Celsius between 1905 and 2005. By 2024, global temperature had reached a record 1.5° Celsius. Removing CO2 from the atmosphere and sequestering it in reservoirs such as the deep ocean, ocean sediments, or rock formations constitutes a second approach to reduce future global warming.
Bibliography
"2024 Is on Track to Be Hottest Year on Record As Warming Temporarily Hits 1.5°C." World Meteorological Association, 11 Nov. 2024, wmo.int/news/media-centre/2024-track-be-hottest-year-record-warming-temporarily-hits-15degc. Accessed 20 Dec. 2024.
ntergovernmental Panel on Climate Change. Climate Change, 2007—The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Edited by Susan Solomon et al. New York: Cambridge University Press, 2007.
Kunzig, Robert. “A Sunshade for Planet Earth.” Scientific American 299, no. 5 (November 2008): 46-55.
Rapp, Donald. Assessing Climate Change: Temperatures, Solar Radiation, and Heat Balance. New York: Springer, 2008.
Walker, Gabrielle. Snowball Earth: The Story of the Great Global Catastrophe That Spawned Life as We Know It. New York: Crown, 2003.