Forcing mechanisms
Forcing mechanisms refer to the processes that alter the Earth's climate by affecting the balance between incoming solar radiation and radiative energy loss. These mechanisms can be classified into external and internal categories, with impacts that may be anthropogenic (human-caused) or natural. External mechanisms include variations in the Earth's orbit and solar output, as well as extraterrestrial impacts, while internal mechanisms involve geological events and biological changes. Over geological timescales, such as those indicated by the fossil record, significant climate shifts have been observed, often linked to forcings that include catastrophic events like asteroid impacts.
On shorter, human timescales, key forcing factors include changes in atmospheric carbon dioxide levels, volcanic eruptions, and solar cycles. The understanding of these mechanisms is crucial for developing effective climate policies, especially as some may reinforce or mitigate anthropogenic global warming. The Milanković cycles, which describe predictable changes in Earth's orbital parameters, are notable for their long-term climatic effects. Overall, recognizing and analyzing forcing mechanisms is essential for understanding past climate fluctuations and anticipating future changes.
Forcing mechanisms
Forcing mechanisms alter climate by changing the balance between incoming solar radiation and radiative energy loss. They may occur on human or geologic timescales and be anthropogenic or natural. The unpredictability of some large-scale forcing events has been used as an argument against modifying human policies to reduce the greenhouse effect.
Background
Examining fossils, evidence from archaeological sites, and the historical record indicates that climate fluctuates markedly on timescales ranging from decades to tens of millions of years. One age’s tundra may have been another age’s temperate forest. Solar energy drives the Earth’s climate. The amount of solar energy available depends on the balance between the amount reaching the Earth and the amount radiated back into space, both of which are variable. In addition to solar energy, Earth also receives input from gravitation and radioactive decay; these sources are dwarfed by solar input and have been essentially constant throughout the Phanerozoic era.
The Geologic Record: Evidence for Forcing
The geologic record spans of millions of years during which global climate was relatively uniform, divided by much briefer spans characterized by climatic extremes and elevated extinction rates. This record is punctuated by five recognized episodes of catastrophic extinction, believed to correlate with external climatic forcing mechanisms. The best-known of these is the end-Cretaceous event marking the end of the dinosaurs 65 million years ago. Most biologists accept that the precipitating factor was an asteroid collision that (among other effects) abruptly lowered global temperatures by ejecting massive amounts of dust into the atmosphere.
Types of Forcing Mechanisms
Climatic forcing mechanisms can be classified according to whether they are external or internal, whether their effect is to increase or decrease net energy available to the system, and the timescale on which they operate.
EXTERNAL MECHANISMS. External forcing mechanisms include variations in the Earth’s orbit, variation in solar output, impacts by comets and meteorites, and radiation from nearby supernovas. No effect attributable to the last two extremely rare, unpredictable events is evident in the Holocene. The only such event for which there is firm geological evidence is the end-Cretaceous asteroid impact that destroyed the dinosaurs. The predicted climatic effects of an asteroid impact include a short-lived global winter due to obstruction of incoming radiation, followed by a spike in temperatures from released by burning and decaying vegetation. Oceanic currents would also be severely disrupted.
Total solar radiation varies by approximately 3 percent over the course of the eleven-year sunspot cycle. There is evidence for a ninety-year cycle in addition, and longer-term ones probably exist as well. The latter part of the Little corresponds to the Maunder Minimum, a two-hundred-year period of reduced sunspot activity documented by historical records and proxy measurements. Whether similar extended periods of low sunspot activity contributed to at other times is uncertain. The Sun reached a low point in its cycle at the beginning of 2008.
Variations in Earth’s orbit affect both total and its distribution on Earth’s surface. The relevant parameters are degree of eccentricity of the orbit, tilt of the Earth on its axis, and precession of the equinoxes. All of these vary in a predictable way on timescales of thousands of years, with a maximum periodicity of ninety thousand years. The entire orbital pattern is called the Milanković cycles, after its discoverer. These cycles correlate well with Pleistocene episodes, and studies of Triassic-age lake sediments suggest the pattern is ancient. Earth is currently at an intermediate point in this long-term cycle.
INTERNAL FORCING MECHANISMS. Internal mechanisms include geologic events and large-scale biological changes. The timescales of such events vary from weeks, in the case of massive volcanic eruptions, to hundreds of millions of years, in the case of continental drift. The effects of changes in the Earth’s magnetic field on climate are a matter of controversy. This field, generated by currents in the Earth’s molten core, fluctuates on a timescale of 2,300 years and reverses polarity every 780,000 years. When the field is at its weakest, climatic variability is highest. The current weakening of Earth’s and the southward drift of the magnetic pole predict a cooling trend that Earth is not experiencing.
Plate tectonics and continental drift drive volcanic eruptions, which are major determinants of climate. Eruptions spew quantities of ash and sulfur dioxide into the atmosphere, blocking sunlight and reducing ground temperatures. There are several historical examples, most notably the Tambora eruption in 1815. Massive eruptions may also destroy enough vegetation to increase planetary albedo. On very long timescales, continental drift alters the ratio of land to water surface and distribution relative to the equator.
OCEANIC FORCING. Small changes in sea surface temperature create large-scale climatic effects by altering the circulation of oceanic currents. Changes in oceanic currents may be cyclical and relatively predictable, for example those involved in the El Niño-Southern Oscillation, or they may be externally driven, unpredictable, and of longer duration. The cessation of Gulf Stream circulation in the North Atlantic during the Younger Dryas provides an example of the latter category.
BIOLOGICAL FORCING. Changes in the balance between photosynthesis and heterotrophic consumption alter the CO2 content of the atmosphere. The present rise in CO2 levels due to fossil fuel burning by humans mirrors ancient CO2 depletion. Two episodes of massive global glaciation in the Precambrian follow periods of algal reef building and may represent global cooling due to photosynthetic carbon fixation.
Context
On a human timescale of hundreds or thousands of years, the most important forcing mechanisms are changes in atmospheric CO2 content, the solar cycle, and volcanic eruptions. Of these, only the CO2 content can be regulated by human activity. Sunspot cycles, volcanic eruptions, and some of the shorter Milanković cycles have the potential to either reinforce or cancel out present anthropogenic global warming. Understanding and tracking forcing mechanisms is an important step in formulating effective global warming policy.
Key Concepts
- astronomical forcing: climatic change triggered by changes in solar luminosity, variation in the Earth’s orbit, and bolide impact
- Milanković cycles: variations in the eccentricity of the Earth’s orbit, the tilt of the Earth’s axis, and the precession of equinoxes that result in climatic variation on the scale of tens of thousands of years
- planetary albedo: reflectivity of the Earth’s surface to light
Bibliography
Chambers, Frank, and Michael Ogle. Natural Forcing Factors for Climate Change on Time Scales 10-1 to 105 Years. Vol. 2 in Climate Change: Critical Concepts in the Environment. New York: Routledge, 2002.
Cronin, Thomas M. Principles of Paleoclimatology. New York: Columbia University Press, 1999.
Orgel, Isabella Marisol. "What Are Climate Forcings?" The Climate Reality Project, 21 Sept. 2022, www.climaterealityproject.org/blog/what-are-climate-forcings. Accessed 21 Dec. 2024.
Pap, Judit M., and Peter Fox, eds. Solar Variability and Its Effects on Climate. Washington, D.C.: American Geophysical Union, 2004.