Orbital Variations and Solar Energy
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Past and predicted variation in Earth’s eccentricity, total solar energy impinging at latitude 65° north during June (insolation), and simulated Northern Hemisphere ice volume over time (thousands of years from the present). These calculations assume no h
Earth Orbit Characteristics
Three characteristics of Earth’s orbit around the sun—obliquity, eccentricity, and precession—change periodically and influence the amount of energy that Earth receives from the sun.
Earth’s daily rotation around its own axis currently has an angle of 23.4° with respect to its orbital plane around the sun. This angle is called the axial tilt or obliquity. The planet, however, wobbles like a spinning top, and the obliquity oscillates between 22.1° and 24.5 every 41,000 years. This means that the seasonal differences in the amount of solar energy reaching Earth varies with a period of 41,000 years. At the extremes, the current Tropic of Cancer (latitude 23.4°N) would receive about a 4.5% change in solar energy from minimum to maximum tilt.
The elliptical shape of Earth’s orbit around the sun is characterized by its eccentricity, a measure of the deviation of an orbit from a perfect circle. Currently, Earth’s orbit has an eccentricity 0.0167, but it oscillates between 0.005 (nearly circular) and 0.0617 (more elliptical) with an average periodicity of about 100,000 years. As the eccentricity of Earth’s orbit increases, the amount of solar energy reaching Earth fluctuates more from summer to winter because of greater seasonal differences both in the distance between Earth and the sun and in the time Earth spends at the aphelion and perihelion. When the eccentricity is minimal (i.e., the orbit is near circular), seasonal variation derives solely from differences in the angle between the axis of Earth’s diurnal rotation and its orbital plane around the sun. [1]
Influence of orbital variations on solar insolation and historical temperature changes (A) Variations in Earth’s solar orbit (obliquity, eccentricity, and precession), total solar energy impinging at latitude 65° north during July (insolation), and changes in benthic (seabed-level) δ18O (inversely related to temperature) over the last 2 million years; a plot of 10,000-year running averages calculated from the orbital data of Laskar et al. (2004) and theoretical insolation data of Berger and Loutre (1991). The δ18O data of Huybers (2006) is the average of twelve sea-bottom core records. Eccentricity (unitless) equals 0 for a perfect circle. (B) Spectral analyses of these data showing the frequencies of the oscillations (like the spectral display on a sound system that shows how much bass or treble is in the music) that indicate that past temperature changes were more sensitive to variation in obliquity and eccentricity.
At present, the summer and winter solstices and aphelion/perihelion (farthest/closest) positions of Earth relative to the sun nearly coincide; the solstices are June 21 and December 21, while the aphelion and perihelion are, respectively, July 4 and January 3. Earth, however, behaves like a wobbling top, and its precession, the alignment of its axis of diurnal rotation with its distance from the sun, oscillates with an average period of about 21,000 years. That is, 10,500 years ago, Earth was closest to the sun on July 4, when the Northern Hemisphere was tilted toward the sun, and farthest from the sun on January 3, when the Northern Hemisphere was tilted away.
By combining the influences of the three orbital factors of obliquity, eccentricity, and precession, one can reconstruct a history of solar insolation, the amount of solar energy reaching Earth per unit area. Solar insolation oscillates with the periods of its components: 41,000 years (obliquity), 100,000 years (eccentricity), and 21,000 years (precession). At the beginning of the twentieth century, estimates for the timing of these periods and of Earth’s glacial cycles became sufficiently accurate to suggest a causal relationship between Earth’s solar radiation balance and climate. Solar insolation in the Northern Hemisphere has a greater effect than that in the Southern Hemisphere because the Northern Hemisphere currently has about 65% of Earth’s land mass, and land absorbs much more solar energy than ocean. Therefore, when the Northern Hemisphere receives more solar energy, so does the planet as a whole. The Serbian astrophysicist Milutin Milankovitch (1879–1958) proposed that when the solar insolation in the Northern Hemisphere was relatively high during December and January, and relatively low during June and July, more snow accumulated during the winter and less melted during the summer, causing glaciers to advance.
Theory VS Climatic Variations
How well does this theory fit the observed climatic variations? The δ18O signature of sea-bottom sediment cores, a temperature proxy, fluctuates at regular intervals. This periodicity reflects changes in obliquity and eccentricity; the influence of precession on δ18O is less evident. More specifically, the Northern Hemisphere experienced a double whammy about 10,500 years ago: As previously mentioned, it was tilted away from and was farther from the sun in the months of December and January and was tilted toward and closer to the sun in the months of June and July. Colder winters and warmer summers in the Northern Hemisphere produced less snowfall in winter and more snowmelt in summer and thereby led to an interglacial period. [2]
A sizeable and growing body of globally distributed, independently dated paleoclimate records collectively identifies a series of naturally occurring contemporaneous abrupt climate change events during the Holocene that are generally cotemporaneous with reductions in solar output. (Buffen, 2008)
Currently, the obliquity of Earth’s orbit is intermediate, its eccentricity is small, and its precessional alignment is such that Earth is farthest from the sun during June and July. These conditions lead to moderate and stable solar insolation in the Northern Hemisphere and explain the long interglacial period that we are now experiencing. If orbital variations were the sole forcing factors, Earth’s climate should remain about the same for the next 40,000 years or so.
References
[1] Arnold J. Bloom (2010) Global Climate Change: Convergence of Disciplines. UCVerse of the University of California.
©2010 Sinauer Associates and UC Regents
[2] Rind, D. (2002) Climatology: The sun's role in climate variations. Science 296:673-677.
[3] Aron Maurice Buffen (2008) Abrupt Holocene Climate Change: Evidence from a New Suite of Ice Cores from Nevado Coropuna, Southwestern Peru and Recently Exposed Vegetation from the Quelccaya Ice Cap, Southeastern Peru. Ohio State University, 2008. 128 pages
