It is important that the climate change that occurs as a result of the Milankovitch cycles and the changes of the present day warming due to the enhanced greenhouse effect are not confused. These changes are on completely different time scales with the later happening in decades and the former over many thousands of years.
Here I will mainly focus on the eccentricity factor of the Milankovitch cycle (the fact that the Earth moves in an ellipse and the path of the ellipse changes with time simplified to roughly a 100 thousand year cycle). Often educational material on eccentricity can unwittingly lead to confusion on how it affects the insolation received by the Earth and how it in turn affects other factors in the Milankovitch cycles. When we draw scale diagrams of the solar system we cannot draw the size and distance to the planets to scale on a sheet of paper without the planets become too small to be visible. Likewise to show the effects of eccentricity and to describe mathematical techniques required in handling these changes, the diagram must be exaggerated. A common way is to exaggerate the eccentricity by drawing diagrams like a) or b) below.
Other reasons for exaggerating the Milankovitch cycles:
Contrarians who dislike the idea of climate change today being due to the enhanced greenhouse effect are only too willing to exaggerate the effects of the Milankovitch cycles. It is likely that they will use the exaggerated diagrams of the solar system to leave a false impression that greenhouse gases cannot have any effect. An exaggerated eccentricity also creates a visual impression of massive changes in Sun-Earth distances and would mean precession effects ( discussed briefly below) to be simultaneously exaggerated. This allows disbelief for the need for positive feedback effects and hold on to an whim that there may be some unknown change in the orbits of the Earth that could account for the present warming we see today.
Of course although all the feedback effects are not fully understood the orbital changes of the planets due to these cycles and the causes of these cycles are well understood. Thus they are predicted with confidence for millions of years ahead on well understood physics. Positive feedbacks are needed to explain the glaciation to inter-glaciation cycles and, furthermore, the changing path of the Earth cannot explain the warming in the recent decades today.
There is a fallacy that somehow there might be unknown changes in the orbit of the Earth that could cause the the present warming attributable to greenhouse gas emissions. If these hypothetical changes were fast enough to cause our present warming they would be apparent in changing insolation values or an appropriate and otherwise unaccountable shift in the Earth’s axis. In modern times the insolation changes can be easily measured (as can sunrise or sunset times or position of the stars that would be affected by precessional, obliquity or the Earth’s orbital inclination changes). These insolation changes that we can directly measure today are mainly due to changes in the sun spot cycle that do not explain the current warming either in magnitude or direction. Any theoretical changes in insolation over this short time due to the Milankovitch cycles, known or hypothetical, are much smaller in comparison. Thus the notion that warming today is caused by hypothetical Earth orbit changes are really just a variation of “its the sun” myth.
There is an alternative to the exaggeration of the Milankovitch cycles that is equally flawed. This is the idea that there are changes in these cycles too small to be easily seen but nevertheless these are responsible for the present warming. This would require as yet unidentified positive feedbacks of a magnitude far greater than proposed by current climate science that would mysteriously only apply to these hypothetical changes and not to the changes in solar activity or any other forcing factor. Otherwise this would lead to a climate sensitivity that was very high. It would be hard to explain not only paleoclimate evidence but how the Earth could be stable enough to stay within a habitable zone with such a climate sensitivity.
How does the orbit change with eccentricity?
Is it more like diagram a), b) or c)?
Figure 1. Changes in the elliptical orbit over an approximate 100 000 year cycle: a) major axes unchanged with exaggerated changes in eccentricity b) Major axes increases with increasing exaggerated eccentricity c). Realistic (to scale approximately).
The Milankovitch cycles affect the average isolation that the Earth gets in a yearly cycle and latitudinal variations of insolation that determine the amount of ice at the poles.
So which diagrams are the most helpful?
Of course that depends on what point is being made, although one of these diagrams is very likely just incorrect. Diagrams a) and b) are exaggerated by an enormous amount and c) is an attempt to show the changes to scale
.
If the point that is intended to be delivered is how much the insolation is likely to change compared to “normal” then the diagram to scale would be the most appropriate and the other diagrams would be very misleading. Alternatively if one wanted to explain the mathematics of eccentricity, as is often the case, then the exaggerated diagrams are necessary. It is readily seen from the exaggerated diagrams that the average insolation received by the Earth over the course of a year will change and it is only the eccentricity factor of the various Milankovitch cycles that causes this. The distance from the Earth to the sun changes at different times of the year again due to the eccentricity factor and this will contribute to changes in seasonal variation.
It may be that you thought diagram a) is incorrect because most diagrams on this from educational material are more like b). In fact I will argue that the top diagram a) is more correct apart from the fact of the enormous exaggeration and that b) is incorrect and is further misleading.
Diagram a) indicates that the major axis remains the same and, by Newton’s laws or Kepler’s third law of planetary motion, this would result in the length of year remaining the same. If diagram b) were correct the length of the year would increase and if it were to scale would increase by a factor of about 3! Further diagram a) correctly predicts that average yearly insolation levels would increase with increasing eccentricity, while diagram b) wrongly predicts the reverse.
“However, the semi-major axis of the orbital ellipse remains unchanged; according to perturbation theory, which computes the evolution of the orbit, the semi-major axis is invariant. The orbital period (the length of a sidereal year) is also invariant, because according to Kepler’s third law, it is determined by the semi-major axis.”
A. Berger and M.F. Loutre discuss evidence of insolation cycles that are recorded in proxies from sediments over millions of years that are broadly consistent with the astronomical changes described by Milankovitch here. In calculations of insolation they use the fact that the semi major axis is constant as eccentricity changes.
It is expected that eccentricity values have and will remain within the limits of 0.000055 when it is most circular and 0.0679 at the extreme eccentricity. Using these values it is possible to work out how the yearly average insulation will vary over time and the shortest (perihelion) and longest (aphelion) extremes of the Sun-Earth distance.
Over millions of years the yearly insolation level reaching the whole Earth only varies by somewhat less than 0.3% due to the Milankovitch cycles, which diagram c) could be consistent with. (Eccentricity will have an effect on the differences that each hemisphere or different latitudes will receive over a year since the sun is placed at one of the foci of the ellipse, but very little on the average insolation over the planet over a complete year).
At an eccentricity of 0.067 the minimum perihelion can be readily shown to decrease to a value of about 86.5 million miles from its current 91 million miles (assuming a constant semi major axis). The aphelion would then increase to a maximum of about 99 million miles from its current 94.5 million miles. These changes are of course important in determining the fate of the ice caps.
Eccentricity has not been deemed enough on its own to cause the changes from glaciations to inter-glaciations and indeed Milankovitch came to suspect that it was regional changes in the Northern hemisphere around 65N that would trigger the changes. High summer insolation over the land would not allow the buildup of ice. The obvious changes in obliquity (tilt) of the Earth has been a major factor in many of these transitions since higher tilt will mean greater insolation at summer time in the higher latitudes. The processional cycles whereby the perihelion occurs at different times of the season is another major factor that will affect the insolation regionally. If perihelion occurs at the Northern hemisphere summer then this will add to the effect of high tilt. If the eccentricity is low (zero) then the precession effects will be low (zero). If there is little (no) difference between perihelion and aphelion distances due to low (zero) eccentricity, then which season perihelion occurs will have little (no) effect. (This may play a part in the reasons why the eccentricity cycle has coincided with the glaciations and interglaciations over the last million years). Further discussion on this topic is given in more detail here in this Nature paper.
With a lower than usual maximum for eccentricity this also means that the extremes in perihelion and aphelion are less than normal (whether acting in phase or otherwise). This reduces the magnitude of precessional changes as described above and so a large precessional effect that would now encourage glaciation is not possible.(such as present day aphelion in the north hemisphere summer). Thus the chances of large enough combined cycles encouraging glaciation is minimized. The combination has been favourable and is expected to continue for some time as seen in the figure above for the “daily insolation” which describes the insolation at 65N; more the pity that our actions are now working against an extended stable period.
The Milankovitch cycles and the Holocene.
The Holocene has been a relatively stable period in terms of climate during which (and perhaps the cause whereby) human human civilisation has flourished. There may be many reasons why the climate has been relatively stable over this time (until the recent disruption caused by humans) and the way the milankovitch cycles combined will have played a part in that stability. The different cycles have very different cycle lengths and the way they combine is thus complex and continually changing with no two 100kyr or 400kyr cycle being exactly the same.
This last interglaciation was not unusual in that the cycle of eccentricity peaked and contributions from the relatively shorter cycles of obliquity and precession contributed to seasonal changes encouraging ice melt at the poles. (That is high tilt of the Earth’s axis and perihelion occurring in the northern hemisphere summer producing warmer summers preventing the buildup of ice). However it was unusual in that the peak of eccentricity was lower than has often been the case in the last million years. ( the peak of the 93,000 year cycle coincided approximately with a minimum of the 413,000 year cycle)
With a lower than usual maximum for eccentricity this also means that the extremes in perihelion and aphelion are less than normal (whether acting in phase or otherwise). This reduces the magnitude of precessional changes as described above and so a large precessional effect that would now encourage glaciation is not possible.(such as present day aphelion in the north hemisphere summer). Thus the chances of large enough combined cycles encouraging glaciation is minimized. The combination has been favourable and is expected to continue for some time as seen in the figure above for the “daily insolation” which describes the insolation at 65N; more the pity that our actions are now working against an extended stable period.
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