By Zack Guido | The University of Arizona | September 14, 2008
At numerous times in the geologic past, the Earth’s astronomical position reduced the amount of solar energy striking the atmosphere, causing massive ice sheets to grow. The cause of these cooling periods, and the subsequent warmer periods that followed, was a change in the amount of solar energy available to power the climate system.
The climate system evolves in part due to changes in external factors that affect the amount of solar radiation available to drive the climate. These factors are called forcings and include solar variations, cycles in the Earth’s position in space, volcanic eruptions, and changes in the atmospheric concentrations of greenhouse gases. All of these phenomena change the radiation balance which has a direct influence on climate. They also cause secondary changes known as feedbacks, such as an increase or decrease in the size of ice sheets and the cloud cover, which in turn magnifies or lessens climate change.
To identify how and to what degree human actions alter the climate, it is important to understand the principal drivers of climate. They include:
Figure 1. Schematic of the Earth's orbital changes (Milankovitch cycles) that moderate insolation and drive the ice age cycles.
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Credit: Intergovernmental Panel on Climate Change, 2007
- Incoming solar radiation
- Variations in the Sun’s solar emissions
- Greenhouse gases
- Volcanic eruptions and aerosols
The amount of solar radiation that strikes the top of the Earth’s atmosphere is not constant. During the course of a year, for example, the Earth’s orbit around the Sun and its tilt alter the amount of incoming solar radiation, or insolation, which cause seasons. Over longer time periods as well, cyclical changes in the Earth’s astronomical position change the amount of insolation, which causes major shifts in the climate. During the past few million years these changes cool the climate, instigating glacial advances, while at other times the climate is warmer and ice sheets and glaciers are smaller.
Three astronomical cycles which impact the amount of insolation are collectively called Milankovitch Cycles (Figure 1), after the scientist who first proposed that the Earth’s ice ages are caused by its planetary motion. These cycles involve changes in the wobble of the Earth (precession), the tilt of the Earth’s axis (obliquity), and the roundness of the Earth’s orbit around the Sun (eccentricity). Each cycle completes a revolution at different rates—the Earth wobbles like a gyroscope around its axis of rotation about every 20,000 years, its tilt varies 2.5 degrees roughly every 41,000 years, and its orbit around the Sun becomes slightly more elliptical on cycles of 100,000 and 400,000 years.1 At times, these three changes combine to reduce insolation, while at other times their combined effect increases insolation.
Changes in the amount of insolation incite feedbacks, or secondary changes in the climate system that either amplify or suppress climate change. The insolation and feedbacks work in concert to control the timing of large-scale climatic swings. In the past two million years, the major ice ages have generally occurred when summer insolation striking the high northern latitudes is low. When summer insolation on the high-latitude northern hemisphere is intense, ice sheets melt.1 The climate response to orbital changes indicates that the Earth will not naturally experience another ice age for at least 30,000 years.1
While the Earth’s astronomical position changes over long time periods, the emission of radiation from the Sun also changes, but on short time cycles lasting 11 years. In 1978 and 1980, satellites were launched in part to measure variations in solar radiation at the top of the atmosphere2. These missions showed that the passage of sunspots—blackened areas on the Sun where temperature is lower—and faculae—bright, hot regions on the Sun—influenced solar energy output. Sunspots and faculae are related so that a high number of sunspots are accompanied by a high number of faculae, and the net effect is greater solar radiation. The satellite measurements, in addition to naked-eye observations that document the number of sunspots as far back as 400 years, suggest that sunspots and faculae wax and wane on an 11-year cycle. At the maximum of this cycle, the amount of solar energy is larger than at the minimum by about 0.1 percent.2 Calculations with three-dimensional models suggest that this variation could cause swings in average global annual temperature around several tenths to a half-degree Fahrenheit.2
The Intergovenmental Panel on Climate Change (IPCC 2007) stated that the changes in solar radiation are not the major cause of the observed temperature changes in the second half of the 20th century unless those changes have caused unknown large feedbacks in the climate system.2 However, the IPCC also stated that more research is needed before the magnitude of solar effects on climate can be stated with certainty.
Greenhouse gases such as carbon dioxide, methane, and water vapor, are naturally part of the Earth’s atmosphere. These gases act as powerful amplifiers of temperature, helping elevate the average temperature in the lower atmosphere to a comfortable 60 degrees Fahrenheit. Without greenhouse gases, the average near-surface conditions would be cold enough to freeze water, at approximately 0 degrees F.
Figure 2. An idealized model of the natural greenhouse gas effect.
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Credit: Intergovernmental Panel on Climate Change, 2001
The greenhouse gas effect functions by trapping in the atmosphere heat from the Sun’s radiation. The greenhouse gases can be thought of as creating a one-way mirror—they allow the short wavelength energy emitted by the Sun to pass through the atmosphere and warm the Earth’s surface, but they do not allow all the longer wavelength energy radiated by the Earth back to space. This trapping effect causes the atmosphere near the Earth’s surface to be warmer than it would be without the gases (Figure 2). Scientists have measured the heat-trapping effect of greenhouse gases since the mid-1800s; as early as the first decade of the 20th century, a few scientists predicted that the average temperature of the lower atmosphere would gradually increase as the greenhouse gas concentration rose—a prediction that is currently being observed.
For the past 650,000 years or so, the atmospheric concentrations of the potent greenhouse gas carbon dioxide (CO2) has wiggled up and down between approximately 180 and 300 parts per million (ppm), with the pre-industrial concentration referenced as 280 ppm. In 2011, CO2 concentrations surpassed 390 ppm.
Global climate models suggest that the GHG effect will help cause the average global temperature to increase between 3–8 degrees F, with the best estimate approximately 5 degrees F, when CO2 concentrations are double what they were prior to the industrial revolution in the mid to late 1700s, or at approximately 560 ppm. At current rates of CO2 increases, 560 ppm will be surpassed in about 75 years. Changes in temperature, however, are slower than increases in CO2 concentrations; if CO2 reached 560 in 75 years, a 5-degree F temperature change would occur in subsequent years.
In June 1991, on an island in the Philippines, Mount Pinatubo erupted and ejected millions of tons of gases, water, and tiny solid particles called aerosols as high as 20 miles into the atmosphere. The eruption killed hundreds of people and altered global climate for the next several years.
The Pinatubo eruption shot material high enough into the atmosphere that trade winds spread the material around the globe. This effectively created a sun shade that lowered global temperatures. A year later, the global temperature was about 1 degree F lower than it otherwise would have been, while the Northern Hemisphere temperature was depressed by about 1.5 degrees F3. Had Pinatubo not been located in the tropical region, the atmospheric circulation patterns would not have spread the sun-blocking material around the globe and temperatures would not have been lowered as much.
Figure 3. Examples of positive and negative feedback cycles.
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Credit: National Academies, 2008
Volcanic eruptions like that of Mount Pinatubo are not unique in geologic history. They are, however, unpredictable. When explosive eruptions occur, the amount of aerosols in the atmosphere increases. Without these additions, the IPCC states, the effect of aerosols from other sources has a negative effect on the energy balance, which slightly suppresses the warming caused by other controls on climate such as greenhouse gases and solar radiation variations4. Sources of aerosols include carbon soot from the burning of fossil fuels, mineral dust from activities like plowing fields and overgrazing, and biomass burning that includes natural fires and human-caused deforestation fires4.
There are many feedback mechanisms in the climate system that moderate the magnitude of climate change. Positive feedbacks incite continued change, while negative feedbacks halt change. With respect to temperature, positive feedbacks tend to amplify warming, while negative feedbacks suppress it. Figure 3 illustrates two important feedback loops that moderate the climate. These feedbacks contribute to uncertainties in future climate change and make pinpointing the exact warming difficult.
General circulation models (GCMs) are used to help understand the effect of the feedbacks on the climate system. Studies that have analyzed GCM results suggest that the combined affect on the climate system of the important positive and negative feedbacks is an amplification of temperature5. However, there is considerable spread in the magnitude of this change5, and this remains an important area of study.
- Solomon, S., et al. 2007. Technical summary. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
- Le Treut, H., et al. 2007. Historical overview of climate change. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
- McCormick, P.M., L.W. Thomason and T.R. Charles. 1995. Atmospheric effects of the Mt Pinatubo eruption. Nature, 373:399-404.
- Forster, P., et al. 2007. Changes in atmospheric constituents and in radiative forcing. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
- Randall, D.A., et al. 2007. Climate models and their evaluation. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.