Global Climate Change - Milankovitch Cycles

Although in previous pages I have tried to show numerous other factors which are much more likely to be creating the observed climate changes we are experiencing, there are a couple of other astronomical factors which might have an influence and which are worthy of mention.  It is not currently believed that these factors are likely to be sufficient on their own to create the measured effects, but another thing I think needs to be said is that the Earth's climate is a very complex thing, and it is most likely that the changes are caused by several of the factors working together to create an overall effect.  It's like in most things, there will be times when one or more effects will reinforce each other, and times when they will cancel each other out.

The two additional astronomical factors I want to cover are those caused by the fact that the Earth's orbit is not completely circular - so our distance from the sun varies - and the fact that the axial tilt of the planet can vary - which would of course change the severity of the seasons.

We know that the seasons are caused by the tilt of the Earth's axis of rotation - the 23.4 offset of the axis from a direction perpendicular to the Earth's orbital plane.  The direction of the rotational axis stays nearly fixed in space, even as the Earth revolves around the Sun once each year.  As a result, when the Earth is at a certain place in its orbit, the northern hemisphere is tilted toward the Sun and experiences summer.  Six months later, when the Earth is on the opposite side of the Sun, the northern hemisphere is tilted away from the Sun and experiences winter. The seasons are, of course, reversed for the southern hemisphere.  The solstices mark the two dates during the year on which the Earth's position in its orbit is such that its axis is most directly tilted either toward or away from the Sun.  These are the dates when the days are longest for the hemisphere tilted toward the Sun (where it is summer) and shortest for the opposite hemisphere (where it is winter).

However, as we have said there is a complication.  The Earth's orbit is very close to being a perfect circle, but not quite.  It is slightly elliptical, which means that the distance between the Earth and the Sun varies over the course of the year.  This effect is too weak to cause the seasons, but it might have some influence over their severity.  The Earth reaches perihelion - the point in its orbit closest to the Sun - in early January, only about two weeks after the December solstice.  So winter begins in the northern hemisphere at about the time that the Earth is nearest the Sun.   Now we need to look at something called "precession".  We usually think of the Earth's axis as being fixed in direction - after all, it always seems to point toward Polaris, the North Star.  But the direction is not quite constant: the axis does move, at a rate of a little more than a half-degree per century.  So Polaris has not always been, and will not always be, the pole star.  For example, when the pyramids were built, around 2500 BC, the pole was near the star Thuban (Alpha Draconis).  This gradual change in the direction of the Earth's axis, called precession, is caused by gravitational torques exerted by the Moon and Sun on the spinning, slightly oblate Earth.

Because the direction of the Earth's axis determines when the seasons will occur, precession will cause a particular season (for example, northern hemisphere winter) to occur at a slightly different place in the Earth's orbit from year to year.  At the same time, the orbit itself is subject to small changes, called perturbations.  The Earth's orbit is an ellipse, and there is a slow change in its orientation, which gradually shifts the point of perihelion in space.  The two effects - the precession of the axis and the change in the orbit's orientation - work together to shift the seasons with respect to perihelion.  Thus, since we use a calendar year that is aligned to the occurrence of the seasons, the date of perihelion gradually regresses through the year. It takes 21,000 years to make a complete cycle of dates.

We would not expect the 21,000-year cycle to be very important climatologically because the Earth's orbit is almost circular - the distance to the Sun at perihelion is only about 3% less than its distance at aphelion.  That is, whether perihelion occurs in January or July, it seems unlikely that our seasons would be much affected.  At least, that is the case now; but the eccentricity of the Earth's orbit (how elliptical it is) also changes over very long periods of time, from almost zero (circular orbit) to about three times its current value.  The eccentricity of the orbit varies periodically with a time scale of about 100,000 years, so, it would be reasonable to suppose that if the 21,000-year perihelion shift cycle were to have any effect on climate at all, it would only be during the more widely-spaced epochs when the orbital eccentricity was relatively large.  That is, climatologically, the 100,000-year cycle of eccentricity should modulate the 21,000-year cycle of perihelion.  There is definitely some evidence that this long-term change in the date of perihelion influences the Earth's climate.

These cycles are known as "Milankovitch Cycles" after the Serbian scientist who first discovered them, and they are yet another factor in the complicated series of things potentially influencing our climate.

To close, let's take a look at the oceans, because the influence they have on our global climate are not fully understood and they are often left out of calculations and computer models.

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