Climate and Earth’s Energy Budget
by Rebecca Lindsey
January 14, 2009
Source: NASA Earth Observatory: http://earthobservatory.nasa.gov/Features/EnergyBalance/

The Earth’s climate is powered by solar energy—it’s powered by the sun. Sunlight drives photosynthesis, fuels evaporation, melts snow and ice, and warms the Earth.


Solar power drives Earth’s climate. Energy from the Sun heats the surface, warms the atmosphere, and powers the ocean currents. (Astronaut photograph ISS015-E-10469, courtesy NASA/JSC Gateway to Astronaut Photography of Earth.)


But the Sun doesn’t heat the Earth evenly. Because the Earth is a sphere, the Sun heats the equator region more than the polar regions. The atmosphere and ocean then work non-stop to even-out solar heating imbalances. They do it through the evaporation of surface water, convection, rainfall, winds, and ocean circulation. Scientists call the circulation of the atmosphere and ocean the Earth’s heat engine because it moves heat around the globe.

The climate’s heat engine must not only redistribute solar heat from the equator toward the poles, but also from the Earth’s surface and lower atmosphere back into space. Otherwise, Earth would endlessly heat up. Earth’s temperature doesn’t infinitely rise because the surface and the atmosphere are simultaneously radiating heat to space. This net flow of energy into and out of the Earth system is Earth’s energy budget.


When the flow of incoming solar energy is balanced by an equal flow of heat to space, Earth is in “radiative equilibrium”, and global temperature is relatively stable. Anything that increases or decreases the amount of incoming or outgoing energy disturbs Earth’s radiative equilibrium; global temperatures rise or fall in response.

Incoming Sunlight
All matter in the universe that has a temperature above absolute zero (the temperature at which all atomic or molecular motion stops) radiates energy across a range of wavelengths.


The surface of the Sun has a temperature of about 5,500 degrees Celsius, or about 10,000 degrees Fahrenheit. At that temperature, most of the energy the Sun radiates is visible and near-infrared light.


At Earth’s average distance from the Sun (about 150 million kilometers), the average intensity of solar energy reaching the top of the atmosphere directly facing the Sun is about 1,360 watts per square meter, according to measurements made by the most recent NASA satellite missions. This amount of power is known as the total solar irradiance.


A watt is measurement of power, or the amount of energy that something generates or uses over time. How much power is 1,360 watts? An incandescent light bulb uses anywhere from 40 to 100 watts. A microwave uses about 1000 watts. If for just one hour, you could capture and re-use all the solar energy arriving over a single square meter at the top of the atmosphere directly facing the Sun—an area no wider than an adult’s outstretched arm span—you would have enough to run a refrigerator all day.


The total solar irradiance is the maximum possible power that the Sun can deliver to a planet at Earth’s average distance from the Sun; basic geometry limits the actual solar energy intercepted by Earth. Only half the Earth is ever lit by the Sun at one time, which halves the total solar irradiance.





Energy from sunlight is not spread evenly over Earth. One hemisphere is always dark, receiving no solar radiation at all. On the daylight side, only the point directly under the Sun receives full-intensity solar radiation. From the equator to the poles, the Sun’ rays meet Earth at smaller and smaller angles, and the light gets spread over larger and larger surface areas (red lines). (NASA illustration by Robert Simmon.)



In addition, the total solar irradiance is the maximum power the Sun can deliver to a surface that is perpendicular to the path of incoming light. Because the Earth is a sphere, only areas near the equator at midday come close to being perpendicular to the path of incoming light. Everywhere else, the light comes in at an angle. The progressive decrease in the angle of solar illumination with increasing latitude reduces the average solar irradiance by an additional one-half.


Averaged over the entire planet, the amount of sunlight arriving at the top of Earth’s atmosphere is only one-fourth of the total solar irradiance, or approximately 340 watts per square meter.


When the flow of incoming solar energy is balanced by an equal flow of heat to space, Earth is in radiative equilibrium, and global temperature is relatively stable. Anything that increases or decreases the amount of incoming or outgoing energy disturbs Earth’s radiative equilibrium; global temperatures must rise or fall in response.



Heating Imbalances
Three hundred forty watts per square meter of incoming solar power is a global average; solar illumination varies in space and time. The annual amount of incoming solar energy varies considerably from tropical latitudes to polar latitudes. At middle and high latitudes, it also varies considerably from season to season.


If the Earth’s axis of rotation were vertical with respect to the path of its orbit around the Sun, the size of the heating imbalance between equator and the poles would be the same year round, and the seasons we experience would not occur. Instead Earth’s axis is tilted off vertical by about 23 degrees. As the Earth orbits the Sun, the tilt causes one hemisphere and then the other to receive more direct sunlight and to have longer days.




The total energy received each day at the top of the atmosphere depends on latitude. The highest daily amounts of incoming energy (pale pink) occur at high latitudes in summer, when days are long, rather than at the equator. In winter, some polar latitudes receive no light at all (black). The Southern Hemisphere receives more energy during December (southern summer) than the Northern Hemisphere does in June (northern summer) because Earth’s orbit is not a perfect circle and Earth is slightly closer to the Sun during that part of its orbit. Total energy received ranges from 0 (during polar winter) to about 50 (during polar summer) megajoules per square meter per day.


In the “summer hemisphere,” the combination of more direct sunlight and longer days means the pole can receive more incoming sunlight than the tropics, but in the winter hemisphere, it gets none. Even though illumination increases at the poles in the summer, bright white snow and sea ice reflect a significant portion of the incoming light, reducing the potential solar heating.



The amount of sunlight the Earth absorbs depends on the reflectivness of the atmosphere and the ground surface. This satellite map shows the amount of solar radiation (watts per square meter) reflected during September 2008. Along the equator, clouds reflected a large proportion of sunlight, while the pale sands of the Sahara caused the high reflectivness in North Africa. Neither pole is receiving much incoming sunlight at this time of year, so they reflect little energy even though both are ice-covered. (NASA map by Robert Simmon, based on CERES data.)


The differences in reflectivness (albedo) and solar illumination at different latitudes lead to net heating imbalances throughout the Earth system. At any place on Earth, the net heating is the difference between the amount of incoming sunlight and the amount heat radiated by the Earth back to space. In the tropics there is a net energy surplus because the amount of sunlight absorbed is larger than the amount of heat radiated. In the polar regions, however, there is an annual energy deficit because the amount of heat radiated to space is larger than the amount of absorbed sunlight.



This map of net radiation (incoming sunlight minus reflected light and outgoing heat) shows global energy imbalances in September 2008, the month of an equinox. Areas around the equator absorbed about 200 watts per square meter more on average (orange and red) than they reflected or radiated. Areas near the poles reflected and/or radiated about 200 more watts per square meter (green and blue) than they absorbed. Mid-latitudes were roughly in balance. (NASA map by Robert Simmon, based on CERES data.)



The net heating imbalance between the equator and poles drives an atmospheric and oceanic circulation that climate scientists describe as a “heat engine.” (In our everyday experience, we associate the word engine with automobiles, but to a scientist, an engine is any device or system that converts energy into motion.) The climate is an engine that uses heat energy to keep the atmosphere and ocean moving. Evaporation, convection, rainfall, winds, and ocean currents are all part of the Earth’s heat engine.


References
Cahalan, R. (n.d.) Solar and Earth Radiation. Accessed December 12, 2008.
Hansen, J., Nazarenko, L., Ruedy, R., Sato, M., Willis, J., Del Genio, A., Koch, D., Lacis, A., Lo, K., Menon, S., Novakov, T., Perlwitz, J., Russell, G., Schmidt, G.A., and Tausnev, N. (2005). Earth’s Energy Imbalance: Confirmation and Implications. Science, (308) 1431-1435.
Kushnir, Y. (2000). Solar Radiation and the Earth’s Energy Balance. Published on The Climate System, complete online course material from the Department of Earth and Environmental Sciences at Columbia University. Accessed December 12, 2008.
Peixoto, J., and Oort, A. (1992). Chapter 6: Radiation balance. In Physics of Climate (pp. 91-130). Woodbury, NY: American Institute of Physics Press.
Peixoto, J., and Oort, A. (1992). Chapter 14: The ocean-atmosphere heat engine. In Physics of Climate (pp. 365-400). Woodbury, NY: American Institute of Physics Press.
Marshall, J., and Plumb, R.A. (2008). Chapter 2: The global energy balance. In Atmosphere, Ocean, and Climate Dynamics: an Introductory Text (pp. 9-22).
Marshall, J., and Plumb, R.A. (2008). Chapter 4: Convection. In Atmosphere, Ocean, and Climate Dynamics: an Introductory Text (pp. 31-60).
Marshall, J., and Plumb, R.A. (2008). Chapter 8: The general circulation of the atmosphere. In Atmosphere, Ocean, and Climate Dynamics: an Introductory Text (pp. 139-161).
Trenberth, K., Fasullo, J., Kiehl, J. (2009). Earth’s global energy budget. Bulletin of the American Meteorological Society.

Earth-Sun relationships and insolation
Published: October 11, 2006
Updated: February 18, 2011, 4:42 pm
Author: Michael Pidwirny



Figure 1: Effect of angle on the area that intercepts an incoming beam of radiation. (Source: PhysicalGeography.net)


Yearly changes in the position of the Earth's axis cause the location of the sun to wander 47° across our skies. Changes in the location of the sun have a direct effect on the intensity of solar radiation. The intensity of solar radiation is largely a function of the angle of incidence (the angle at which the sun's rays strike the Earth's surface). If the sun is positioned directly overhead or 90° from the horizon, the incoming radiation strikes the surface of the Earth at right angles and is most intense. If the sun is 45° above the horizon, the incoming radiation strikes the Earth's surface at an angle. This causes the rays to be spread out over a larger surface area reducing the intensity of the radiation. Figure 1 models the effect of changing the angle of incidence from 90 to 45°. As illustrated, the lower sun angle (45°) causes the radiation to be received over a much larger surface area. This surface area is approximately 40% greater than the area covered by an angle of 90°. The lower angle also reduces the intensity of the incoming rays by 30%.

The yearly changes in the position of the Earth's axis relative to the plane of the ecliptic also causes seasonal variations in day length to all locations outside of the equator. Longest days occur during the summer solstice for locations north of the equator and on the winter solstice for locations in the Southern Hemisphere.

The equator experiences equal day and night on every day of the year. Day and night is also of equal length for all Earth locations on the autumnal and vernal equinoxes. Days are longer than nights in the Northern Hemisphere from the March equinox to the September equinox. Between the September to March equinox days are shorter than nights in the Northern Hemisphere. The opposite is true in the Southern Hemisphere. The seasonal (winter to summer) variation in day length increases with increasing latitude.

The most extreme variations in radiation received in the Northern Hemisphere occur at 90° North. During the summer solstice this location receives more potential incoming solar radiation than any other location. At this time the sun never sets. In fact, it remains at an altitude of 23.5° above the horizon for the whole day. From September 22 (autumnal equinox) to March 21, (vernal equinox) no radiation is received at 90° North. During this period the sun slips below the horizon as the northern axis of the Earth becomes tilted away from the sun.

Earth-Sun geometry
Published: October 11, 2006
Updated: March 25, 2013, 2:20 pm
Author: Michael Pidwirny





Figure 1. Position of the equinoxes, solstices, aphelion, and perihelion relative to the Earth's orbit around the Sun.


Earth Rotation and Revolution
The term 'Earth rotation' refers to the spinning of the Earth on its axis. One rotation takes exactly twenty-four hours and is called a “mean solar day”. If you could look down at the Earth's North Pole from space you would notice that the direction of rotation is counterclockwise. The opposite is true if you viewed the Earth from the South Pole.

The orbit of the Earth around the sun is called Earth revolution. This celestial motion takes 365 1/4 days to complete one cycle. Further, the Earth's orbit around the sun is not circular, but elliptical. An elliptical orbit causes the Earth's distance from the sun to vary annually. However, this phenomenon does not cause the seasons.

It doesn’t cause the seasons, but this annual variation in the distance from the sun does influence the amount of solar radiation intercepted by the Earth by approximately 6%. On January 3rd, perihelion, the Earth is closest to the sun (147.5 million kilometers) (Figure 1). The Earth is farthest from the sun on July 4th, or aphelion (Figure 1). The average distance of the Earth from the sun over a one year period is 150 million kilometers.


Tilt of the Earth's Axis


Figure 2: Annual change in the position of the Earth in its revolution around the sun. (Source: PhysicalGeography.net)


The Earth's axis is not perpendicular to the plane of the ecliptic, but inclined at a fixed angle of 23.5°. Moreover, the northern end of the Earth's axis always points to the same place in space (North Star). Figure 3 shows an animation of the Earth revolving around the sun. In this animation the Earth's axis is colored red. Note that the angle of the Earth's axis in relation to the plane of the ecliptic remains unchanged. However, the relative position of the Earth's axis to the sun does change during this cycle (Figure 2). This circumstance causes the seasons, by controlling the intensity and duration of sunlight received by locations on the Earth. It is also responsible for the annual changes in the height of the sun above the horizon.



Figure 3: Earth revolution animation. (Source: PhysicalGeography.net)


Figure 2 shows the annual change in the position of the Earth in its revolution around the sun. In the graphic, we are viewing the Earth from a position in space that is above the North Pole (yellow dot) at the summer solstice, the winter solstice, and the two equinoxes. Note how the position of the North Pole on the Earth's surface does not change. However, its position relative to the sun does change and this shift is responsible for the seasons. The red circle on each of the Earths represents the Arctic Circle (66.5° N). During the summer solstice, the area above the Arctic Circle is experiencing 24 hours of daylight because the North Pole is tilted 23.5° toward the sun. The Arctic Circle experiences 24 hours of night when the North Pole is tilted 23.5° away from the sun in the winter solstice. During the two equinoxes, the circle of illumination cuts through the polar axis and all locations on the Earth experience 12 hours of day and night.

On June 21 or 22, the summer solstice, the Earth is positioned in its orbit so that the North Pole is leaning 23.5° toward the sun (Figures 2 & 4). During the summer solstice, all locations North of the equator have day lengths greater than twelve hours, while all locations South of the equator have day lengths less than twelve hours. On December 21 or 22, the winter solstice, the Earth is positioned so that the South Pole is leaning 23.5° toward the sun (Figures 2 & 4). During the winter solstice, all locations North of the equator have day lengths less than twelve hours, while all locations South of the equator have day lengths greater than twelve hours.

On September 22 or 23, the autumnal equinox, neither pole is tilted toward the sun (Figures 2, 5). March 20 or 21 marks the arrival of the spring or vernal equinox when once again the poles are not tilted toward the sun. Day lengths on both of these days, regardless of latitude, are exactly 12 hours.



Figure 4: During the summer solstice the Earth's North Pole is tilted 23.5° towards the sun relative to the circle of illumination. This phenomenon keeps all places above a latitude of 66.5° N in 24 hours of sunlight, while locations below a latitude of 66.5° S are in darkness. The North Pole is tilted 23.5° away from the sun relative to the circle of illumination during the winter solstice. On this date, all places above a latitude of 66.5° N are now in darkness, while locations below a latitude of 66.5° S receive 24 hours of daylight. (Source: PhysicalGeography.net)



Figure 5: During the equinoxes, the axis of the Earth is not tilted toward or away from the sun and the circle of illumination cuts through the poles. This situation does not suggest that the 23.5° tilt of the Earth no longer exists. The vantage point of this graphic shows that the Earth's axis is inclined 23.5° toward the viewer for both dates (see Figure 2). The red circles shown in the graphic are the Arctic Circle. (Source: PhysicalGeography.net)


Axis Tilt and Solar Altitude
The annual change in the relative position of the Earth's axis in relationship to the sun causes the height of the sun (solar altitude) to vary in our skies. The total variation in maximum solar altitude for any location on the Earth over a one year period is 47° (2 x 23.5 = 47). For example, at 50° North, maximum solar altitude varies from 63.5° on the summer solstice to 16.5° on the winter solstice (Figure 6). Maximum solar height at the equator goes from 66.5° above the northern end of the horizon during the summer solstice, to directly overhead on the fall equinox, and then down to 66.5° above the southern end of the horizon during the summer solstice (Figure 7).



Figure 6: Variations in solar altitude at solar noon for 50° North during the summer solstice, equinox, and winter solstice. (Source: PhysicalGeography.net)




Figure 7: Variations in solar altitude at solar noon for the equator during the summer solstice, equinox, and winter solstice. (Source: PhysicalGeography.net)


The location on the Earth where the sun is directly overhead at solar noon is known as the subsolar point. The subsolar point occurs on the equator during the equinoxes (Figure 8). During the summer solstice, the subsolar point moves to the Tropic of Cancer because at this time the North Pole is tilted 23.5° toward the sun. The subsolar point is located at the Tropic of Capricorn on the winter solstice. On this date, the South Pole is now tilted toward the sun (Figures 2 and 4).



Figure 8: Relationship of maximum sun height to latitude for the equinox (left) and summer solstice (right). (Source: PhysicalGeography.net)


Figure 8 shows the relationship of maximum sun height to the latitude for the equinox (left) and summer solstice (right). The red values on the right of the globes are maximum solar altitudes at solar noon. Black numbers on the left indicate the location of the equator, Tropic of Cancer (23.5° N), Tropic of Capricorn (23.5° S), Arctic Circle (66.5° N), and the Antarctic Circle (66.5° S). The location of the North and South Poles are also identified. During the equinox, the equator is the location on the Earth with a sun angle of 90° for solar noon. Note how maximum sun height declines with latitude as you move away from the equator. For each degree of latitude traveled, maximum sun height decreases by the same amount. At equinox, you can also calculate the noon angle by subtracting the location's latitude from 90. During the summer solstice, the sun is now directly overhead at the Tropic of Cancer. All locations above this location have maximum sun heights that are 23.5° higher from the equinox situation. Places above the Arctic Circle are in 24 hours of daylight. Below the Tropic of Cancer the noon angle of the sun drops one degree in height for each degree of latitude traveled. At the Antarctic Circle, maximum sun height becomes 0° and locations south of this point on the Earth are in 24 hours of darkness.

Milankovitch cycles

Published: July 7, 2010
Updated: August 1, 2012
Author: Jeffrey Lee

Milankovitch cycles refer to long term variations in the orbit of the Earth that cause changes in climate over periods hundred of thousands of years and are related to ice age cycles. Once Isaac Newton described his laws of motion and of gravity, the orbit of each planet became predictable, not only under the influence the Sun, but the much weaker influences of all the other planets and the Moon as well.

Milutin Milankovitch did not discover the cycles, nor was he the first to calculate their changes. What he did do is improve on the methods of calculating them and relating them to Earth’s climatic variations. Below the graphic illustrating the Milankovitch cycles is a brief description of each:


Milankovitch cycles. Source: UCAR


1. Precession (also called Precession of the Equinoxes)
the gravitational pull of the Sun and Moon on Earth’s equatorial bulge causes the poles to slowly wobble. Over 25,800 years, the polar axis traces out a circle with respect to the stars.

Right now when you look out at the stars at night, the North Pole points to Polaris, and all other stars visible in the Northern Hemisphere appear to rotate around that star in the night sky. But in one-half cycle, or 12,900 years, the North Pole will point to the star Vega, which is forty-seven degrees away from Polaris. And in another 12,900 years, the North Pole will be back to Polaris.

Astronomers in 4,000 BC, for example, noted that the axis pointed to the handle of the Big Dipper (part of the constellation Ursa Major), not Polaris, which is the end of the handle of the Little Dipper (part of the constellation Ursa Minor). An effect of this change is that the time of year that Earth is closest to the Sun, called perihelion, varies through the cycle.

Now perihelion is January 3, so the Northern Hemisphere has slightly milder winters and the Southern Hemisphere has slightly cooler winters. And, conversely, summers are a bit cooler in the North and warmer in the South. In 12,900 years, the North will have colder winters because Earth will be furthest from the Sun (aphelion) in January.

Another aspect of the precession is the length of winter and summer. Because the Sun is not at the center of the orbital ellipse (discussed in the next paragraph), it currently takes seven more days for Earth to travel from the vernal equinox to the autumnal equinox than from the autumnal to the vernal. In other words, the Northern Hemisphere winter now is shorter than the Southern Hemisphere winter. In 12,900 years, the North will have longer winters and shorter summers.

2. Eccentricity of Orbit
Earth travels around the Sun along a flat surface called the plane of the ecliptic, called that because eclipses occur when the Moon intersects this plane. The path taken along this plane is almost a circle, but not quite. It is elliptical, with the Sun just off center as one ‘foci’ of the ellipse. Gravitational pull of other planets causes the path to become slightly more or slightly less elliptical. In other words, it becomes more or less of a flattened circle. Venus, because it is close to Earth, and Jupiter, because it is so massive, have the greatest effect on the eccentricity. There are peaks in eccentricity every 95,000 years, but superimposed on those are larger peaks at 125,000 and 400,000 years. When the orbit is more elliptical, the perihelion is closer to the Sun and the aphelion is farther away than when the orbit is more circular.

3. Axial Tilt (also called Obliquity)
The axis of rotation intersects the plane of the ecliptic at an angle and that angle changes over time. This change is caused by the fact that the Moon’s orbital path is not precisely along Earth’s plane of the ecliptic and so the gravitational attraction of the Moon varies in direction over time. The angle of axial tilt affects the difference between winter and summer in each hemisphere, especially at higher latitudes. Not only does the axial tilt vary over time, but the plane of the ecliptic varies, too. Taking the two into consideration, the obliquity of the axis varies on a 41,000 year cycle and varies from 22.1° to 24.5° from a line perpendicular to the plane of the ecliptic, with the current value at about 23.44°.


Milankovitch cycles over the past 1 000 000 years. Source: Global Warming Art


Soon after the existence of an ice age had been proposed, scientists sought an explanation of their cause. In 1842, Frenchman Joseph Alphonse Adhémar suggested that the varying lengths of winter and summer, an effect of the precession, causes ice to accumulate in the hemisphere with the longer winter. He used the massive ice sheet in Antarctica as evidence, since the Southern Hemisphere currently has longer winter and shorter summer.

Scotsman James Croll combined the eccentricity of the orbit and the precession and in the 1860s and 1870s presented his ideas on the effects of the cycles and how they might influence climate, especially the colder winters when they correspond with the aphelion. In fact, what are typically called ‘Milankovitch Cycles’ are sometimes referred to as ‘Croll-Milankovitch Cycles.’

Milankovitch gets most of the credit for relating the cycles to ice ages because he incorporated all of the pertinent cycles, dealt with them in much greater mathematical precision and showed much more thoroughly how they affect climate. He, at Wladimir Köppen and Alfred Wegener’s suggestion, investigated the role of cooler summers in instigating ice ages. Milankovitch Cycles clearly play an important role in the comings and goings of ice sheets, but the details of just how this happens are far from well understood.

Further Reading
Milankovitch, M. 1941. Canon of Insolation and the Ice-Age Problem. Israel Program for Scientific Translations. Jerusalem (1969).
Muller, Richard A. and MacDonald, Gordon J. 2000. Ice Ages and Astronomical Causes: Data, spectral analysis and mechanisms. Springer. London.
Raymo, Maureen E. and Huybers, Peter. 2008. Unlocking the Mysteries of the Ice Ages. Nature 451: 284-285.

Citation
Lee, J. (2012). Milankovitch cycles. Retrieved from http://www.eoearth.org/view/article/154612

Orbital Variations

Changes in orbital eccentricity affect the Earth-sun distance. Currently, a difference of only 3 percent (5 million kilometers) exists between closest approach (perihelion), which occurs on or about January 3, and furthest departure (aphelion), which occurs on or about July 4. This difference in distance amounts to about a 6 percent increase in incoming solar radiation (insolation) from July to January. The shape of the Earth’s orbit changes from being elliptical (high eccentricity) to being nearly circular (low eccentricity) in a cycle that takes between 90,000 and 100,000 years. When the orbit is highly elliptical, the amount of insolation received at perihelion would be on the order of 20 to 30 percent greater than at aphelion, resulting in a substantially different climate from what we experience today.


The eccentricity of the Earth's orbit changes slowly over time from nearly zero to 0.07. As the orbit gets more eccentric (oval) the difference between the distance from the Sun to the Earth at perihelion (closest approach) and aphelion (furthest away) becomes greater and greater. Note that the Sun is not at the center of the Earth's orbital ellipse, rather it is at one of focal points.
Note: The eccentricty of the orbit shown in the lower image is a highly exaggerated 0.5. Even the maximum eccentricity of the Earth's orbit—0.07—it would be impossible to show at the resolution of a web page. Even so, at the current eccentricity of .017, the Earth is 5 million kilometers closer to Sun at perihelion than at aphelion. (Images by Robert Simmon, NASA GSFC)


Obliquity (change in axial tilt)
As the axial tilt increases, the seasonal contrast increases so that winters are colder and summers are warmer in both hemispheres. Today, the Earth's axis is tilted 23.5 degrees from the plane of its orbit around the sun. But this tilt changes. During a cycle that averages about 40,000 years, the tilt of the axis varies between 22.1 and 24.5 degrees. Because this tilt changes, the seasons as we know them can become exaggerated. More tilt means more severe seasons—warmer summers and colder winters; less tilt means less severe seasons—cooler summers and milder winters. It's the cool summers that are thought to allow snow and ice to last from year-to-year in high latitudes, eventually building up into massive ice sheets. There are positive feedbacks in the climate system as well, because an Earth covered with more snow reflects more of the sun's energy into space, causing additional cooling.


The change in the tilt of the Earth's axis (obliquity) effects the magnitude of seasonal change. At higher tilts the seasons are more extreme, and at lower tilts they are milder. The current axial tilt is 23.5°. Image by Robert Simmon, NASA GSFC)

Precession
Changes in axial precession alter the dates of perihelion and aphelion, and therefore increase the seasonal contrast in one hemisphere and decrease the seasonal contrast in the other hemisphere.


Precession—the change in orientation of the Earth's rotational axis alters the orientation of the Earth with respect to perihelion and aphelion. If a hemisphere is pointed towards the sun at perihelion, that hemisphere will be pointing away at aphelion, and the difference in seasons will be more extreme. This seasonal effect is reversed for the opposite hemisphere. Currently, northern summer occurs near aphelion. (Image by Robert Simmon, NASA GSFC)