Earth:Orbital effects on climate

There are various solar/celestial effects that exist which have an effect on Earth's climate. These effects usually occur in cycles, and primarily include how Earth's obliquity, the eccentricity of Earth's orbit, and the precession of the equinoxes and solstices affect Earth's climate. In addition to these effects, there are also other factors that have an effect on Earth's climate. These other factors include how sun activity affects climate and how celestial phenomena, such as meteors, affect Earth's climate. Some of these factors aren't yet well understood, for instance the ice ages occur on 100,000 year cycles, and it's not completely understood why the various effects with this periodicity have such a strong effect on glaciation (see the 100,000-year problem).

Artist's rendition of geomagnetic storm. Sizes are not to scale.

Overview

Climate in general is an extremely complex system, dealing with temperature and weather conditions over an extended period of time. There are many things which influence Earth's climate. Among these many things, are various solar effects. Earth's climate is affected by a number of factors dealing with the Earth as a whole, in relation to its position in the space relative to the sun. These factors include the angle of Earth's axial tilt (also known as Earth's obliquity), the eccentricity of Earth's orbit (how circular/elliptical Earth's orbit is), and Earth's position in time in the precession of the solstices and equinoxes (with different Earth-Sun distances during any given season).

Although these are the primary three factors in shaping Earths climate, there are other, external, factors that can help shape Earth's climate. These external factors usually affect Earth climate on a very different time scale than the other three, and include factors such as meteors striking Earth and geomagnetic storms. These external forces usually contribute to climate change on a time scale, as meteorites strike the earth, on average, every 50 to 100 million years, where as geomagnetic storms occur periodically through the sun's eleven-year activity cycle. With all of these factors affecting climate in their own way, it becomes easy to see that Earth's climate is in fact, largely dependent on various solar effects/circumstances.

Earth's axial tilt

Perhaps one of the most apparent factors contributing to Earth climate change is the angle at which the earth is tilted. This is the angle at which Earth's axis of rotation is from the vertical, also known as Earth's obliquity. Earth's current tilt angle is approximately 23.5 degrees. The axial tilt angle affects climate largely by determining which parts of the earth get more sunlight during different stages of the year. This is the primary cause for the different seasons Earth experiences throughout the year, as well as the intensity of the seasons for higher latitudes. For example, in the Northern Hemisphere, if there were no axial tilt, i.e. Earth's obliquity would be zero degrees, then there would be no change in the seasons from year to year. This would be because there would be no difference in the amount of solar irradiation received, year-round, anywhere on Earth. On the other hand, if Earth's axial tilt angle was great (45+ degrees), the seasonality of each hemisphere, individually, would be highly exaggerated. Summers would be extremely hot, with substantially more hours of daylight than night each day. Winters would be extremely cold, with substantially more hours of night than daylight each day. This is because, during summer for the northern hemisphere, if the earth is tilted more (pointed towards the sun more), there would be more available hours in which the suns rays can strike any certain place, thereby increasing the number of daylight hours at any given place, with more and more daylight hours at higher latitudes. Also, because the northern hemisphere would be tilted much more towards the sun, it would be physically closer to the sun, thereby increasing the intensity of the sun's rays hitting the northern hemisphere, thereby causing the northern hemisphere to become hotter. Likewise, during winter for the northern hemisphere, there would be fewer hours of daylight because the northern hemisphere would essentially be pointed away from the sun. Fewer daylight hours means less solar radiation hitting the northern hemisphere, especially at higher latitudes, and therefore causing the northern hemisphere to become colder. The same things can also be said about the southern hemisphere, particularly at high latitudes. In either case, the climate around the equator is not affected nearly as much as the higher latitudes, thereby creating a sizable difference in how obliquity affects different latitudes. This is all, of course, dependent on what the actual tilt angle is at any given point in time. The thing is, though, that Earth does in fact change obliquity over time in a cyclic pattern. Earth's obliquity does not change much, though, as obliquity has been determined to cycle between the small range of 22.2 degrees to 24.5 degrees, in a cycle that lasts approximately 41,000 years. Therefore, with the small tilt variation over time, the Earth has always been thought to have had a seasonal climate, at least in the high latitudes due to the solar effect of changing Earth obliquity.

Earth's eccentricity

Earth's orbital eccentricity may play a role in Earth's climate. Eccentricity is defined as the difference in shape between an ellipse and a perfect circle. In a similar fashion to Earth's obliquity, the more uniform Earth's orbit is (more like a perfect circle), the less difference there is in climate change throughout the year. Unlike obliquity, eccentricity affects the entire planet approximately the same, instead of primarily changing polar climate. The base idea with eccentricity is this: “How far away is the earth as a whole from the sun?” If there is no eccentricity to Earth's orbit, then Earth will remain at the same distance from the sun throughout the year, therefore producing no climate change, seeing as how the Earth's orbit would be perfectly circular around the sun. On the other hand, if Earth's orbit has a very high eccentricity, Earth would be very close to the sun (compared to a perfectly circular orbit) during two opposite seasons, and very far away from the sun during the other two opposite seasons. This effect can be seen by analyzing any ellipse, and observing how flat or how skinny the ellipse becomes as its eccentricity increases.

Mathematically, the eccentricity of an ellipse is given by the equation:

[math]\displaystyle{ e = \frac{\sqrt{a^2 - b^2}}{a}, }[/math]

where a is the ellipse's semi-major axis (half of the longest diameter of the ellipse) and b is the ellipse's semi-minor axis (half of the shortest diameter of the ellipse). Thus, for example, if summer and winter are on the part of the orbital ellipse that is furthest away from the sun, then Earth will have the least yearly solar radiation received during those two seasons and the most yearly solar irradiation received during spring and autumn. Currently, Earth's eccentricity is approximately 0.0167, which is closer to the circular side of the eccentricity range Earth cycles through. Earth's eccentricity has always varied over time, and varies within the range of 0.005 to 0.0607 in one eccentricity cycle with a time period of about 100,000 years. There is another eccentricity cycle with a period of approximately 413,000 years, however, it's primarily only noticed as small variations within the 100,000 year eccentricity cycle. When this climate effect is considered with Earth's obliquity, they merge to offset the magnitude fraction of solar irradiation received in a given hemisphere, thus adding to the complexity of the Earth climate system.

Precession of solstices and equinoxes

The precession of solstices and equinoxes is the third factor that plays a role in Earth climate change that deals directly with the earth itself. The basis of the precession of solstices and equinoxes is that it causes the seasons to occur at different times in Earth's revolution around the sun throughout a cycle recurring about every 26,000 years.

In the Northern Hemisphere, when the Earth is at its furthest point from the sun (aphelion) the variation in temperature between winter and summer are less extreme. When the earth is closest to the sun (perihelion), about 5,750 years later, then the variations are at their most extreme. At present the Earth is at its furthest, so the northern hemisphere summers and winters are less extreme and the southern hemisphere climate is more extreme.

This precession movement is the result of two other processes. The first is Earth's slow Axial precession (also known as Earth's wobbling motion), Ruddiman likens it to a top, how the top spins rapidly, revolves around a point on the surface it's spinning on, and also ‘wobbles’ at the top of the top. The second process is referred to as apsidal precession - or the precession of the ellipse. In this second process, the entire ellipse of Earth's orbital path rotates around the sun.

When these two processes are put together, they cause the solstices and equinoxes to effectively move around the earth's orbital path to change how much solar irradiation the hemispheres get during any given season. For details and other subtle effects on the motion, see Precession of the equinoxes#Cause

External/Celestial forces

There are also forces external to Earth itself that affect Earth's climate. Two examples of these external forces are meteors/asteroids becoming meteorites and striking the surface of the earth, and geomagnetic storms from the sun affecting the earth. Asteroids only about two kilometers in diameter, according to Young's paper referencing original author Michael Paine, can cause craters in Earth's surface about 40 kilometers in diameter. Such an impact could throw enormous quantities of dust into the sky, blocking out the sun and causing significant climate changes, somewhat similar in effect to a gigantic volcanic eruption. Among other things, meteorites striking Earth also “affect sea level, rainfall, temperature, ocean currents, and atmospheric circulation”. Asteroids and meteors are not, however, the only external forces to affect Earth climate change. Variations in solar output can also bring about climate change on the Earth. More specifically, varying amounts of sun activity, including sunspots, solar flares, solar wind, and massive solar radiation, can all be grouped together as geomagnetic storms, which together, act to affect Earth's climate.

Our sun has a solar activity cycle of about 11 years. During which time, the sun gets sunspots, shoots off solar flares, and blasts earth with solar wind and geomagnetic activity. Despite the appearance that these events seem random, they do overall follow a fairly steady cycle of approximately 11 years. The difference in average solar irradiation received by the earth due to these ‘random’ events, though, is only about 0.1 percent. Overall, the total irradiation generated by the sun, and received by the earth is a major factor affecting Earth climate. The total solar irradiation received by Earth's surface can be calculated mathematically. This can be done through the following equation:

G_t=G_ND+G_d+G_R

where G_ND is direct irradiation from the sun, G_d is diffuse irradiation from nearby surroundings being heated up by the sun, and G_R is the reflected irradiation from other nearby surfaces. If all that is to be considered is the flat surface of the Earth, then the equation turns into the following:

G_t=G_ND (sin β+C)

where β is the solar altitude angle, which is basically the angle the sun appears to be above the horizon from wherever the solar irradiation is to be calculated, and C is a dimensionless ratio equal to anywhere between 0.103 and 0.138 depending on what day of the year it is where the irradiation is to be calculated. The specific day helps determine for the equation where along Earth's orbit the location being measured is. With this calculation, it is easier to realize just how much the sun's solar radiation contributes to Earth's climate as a whole.

Synopsis

Earth's climate is determined by a compilation of many things and factors. These effects include effects from the primary factors of Earth's axial tilt angle, Earth's orbital eccentricity, and the precession of solstices and equinoxes, as well as some secondary, external effects, such as meteorite/asteroid impacts on the earth's surface and solar activity from the sun, including sunspots, solar flares, and solar winds/geomagnetic storms.

See also

• Milankovitch cycles
• 100,000-year problem
• Precession of the equinoxes