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Milankovitch Cycles

Posted on 22 July 2011 by Chris Colose

This post is intended to serve as a supplement to SteveBrown’s series on the Last Interglacial, beginning here.

 Changes in the Earth's orbit brought about by astronomical variations have a strong impact on Earth’s climate.  They serve as the pacemaker for the glacial-interglacial cycles over the Quaternary (roughly the last two and a half million years of Earth's history), and provide a strong framework for understanding  the evolution of the climate even over the Holocene (the last 10,000 years, beginiing near the termination of the last glacial period).   Milankovitch cycles are insufficient to explain the full range of Quaternary climate change, which also requires greenhouse gas and albedo variations, but they are a primary forcing that must be accounted for.

Orbital variations are also likely to be a generic feature of other planets, with strong implications for the fate of planetary atmospheres (for example, understanding the potential for habitability on other systems).  This post will serve as a guide to what these so-called Milankovitch cycles are, how they work, and highlight some "to-be-done" work that remains.

Milankovitch cycles are classically divided into the precession, the obliquity, and the eccentricity cycles.  These cycles modulate the solar insolation (i.e., the total energy the planet receives from the sun at the top of the atmosphere) or its geographic distribution.  For example, figure 1 shows the solar insolation change at various latitudes in June over the last one million years.

  

Figure 1: June (daily averaged) insolation (W/m2) over the last 1,000,000 years (0=1950) at (blue= 90 N), (red = 60N), (green =30N), (purple=Equator), (light blue = 30S), (Orange=60S)Data from Berger A. and Loutre M.F., 1991, Insolation values for the climate of the last 10 million years, Quaternary Sciences Review, Vol. 10 No. 4, pp. 297-317

Each of the relevant Milankovitch cycles are described below: 

Eccentricity:  Eccentricity is a measure of how circular a curve is, with e=0 describing a circle, and e=1 describing a parabola. The orbital eccentricity therefore characterizes how circular or egg-shaped a planet’s orbit around the sun is (Fig. 2).  The timescale of Earth’s eccentricity variation is ~400,000 years with a superimposed 100,000 year cycle.  There is also an unimportant 2.1 million year cycle.

Figure 2: Circular and eccentric orbit.  

Because of eccentricity, the distance of the Earth at perihelion (point closest to sun) is slightly different than the distance to aphelion (point farthest from sun). 

Earth’s eccentricity is very moderate, never exceeding approximately 0.07 (almost a perfect circle).  The modern day eccentricity is 0.016, and as a result, the solar insolation that hits Earth varies by ~6.4% over the course of a year.  There are some more extreme examples: Pluto’s eccentricity is about 0.25, higher than any other planet in our solar system. HD 20782b, a newly discovered exo-planet almost 120 light years away has an eccentricity on the extreme end of ~0.97 (similar to Halley's Comet). Eccentricity can introduce very large "distance seasons" on a planet, although this also depends on the thermal inertia, which is large enough on a body with oceans (or a dense atmosphere) to moderate the changes between perihelion and aphelion.  As we will see, Earth's seasonal variations are primarily deterimined by its axial tilt rather than its eccentricity.

Eccentricity is the only Milankovitch cycle that alters the annual-mean global solar insolation (i.e., the total energy the planet receives from the sun at the top of the atmosphere).  For the mathematically inclined, the annually-averaged insolation changes in proportion to 1/(1-e2)0.5, so the solar insolation increases with higher eccentricity. This is a very small effect though, amounting to less than 0.2% change in solar insolation, equivalent to a radiative forcing of ~0.45 W/m2 (assuming present-day albedo).   This is much less than the total anthropogenic forcing over the 20th century.  However, eccentircity does modulate the precessional cycle, as we shall see.

Obliquity:  Obliquity describes how tilted a planet’s axis is (Fig. 3 shows the obliquity of eight planets, plus Pluto which is labeled as a planet).   The tilt of the Earth is ultimately what allows for the existence of seasons, since the Northern Hemisphere is pointed toward the sun in the boreal summer, and away from the sun in the boreal winter.  The tilt of Earth’s axis (in other words, the angle between the spin–axis and a line perpendicular to the orbital plane) varies between about 22 to 25° (currently 23.5°) over a period of nearly 41,000 years, driving changes in the distribution of sunlight between the equator and high latitudes (with more tilt implying more sunlight at high latitudes, and less at lower latitudes; therefore more oblique orbits favor deglaciation on Earth). 

Uranus is at the extreme end with a tilt of ~98 degrees; this would induce a very different structure of solar heating (where at certain times the North or South pole would be receiving most of the sunlight, and allow for a large migration of the solar “hotspot” over the course of one Uranian year); this should drive a different atmospheric circulation than on Earth. For highly oblique planets outside our solar system that have a surface, continents at the Polar Regions would be alternatively cooked and frozen, while the tropical latitudes would have two summers and two winters. At large obliquities (greater than about 54 degrees), the poles receive more annual-mean insolation than the planetary equator (Ward, 1974), and thus the annual-mean energy transport by the circulation would be equatorward.

Figure 3: Obliquity of the Planets, and the direction of spin (note that Venus rotates clockwise, in a retrograde fashion). Taken from www.solarviews.com. Click image to Enlarge


Precession: Precession does not describe how tilted the Earth’s axis is, but rather the direction of its axis.  This changes what star is the “North Star” over time (today it is Polaris, but near the end of the last deglaciation it was Vega), and as described below, governs the timing of the seasons. This is illustrated in Figure 4 (a and b) below.

 

Figure 4: a. Schematic showing the influence of axial precession b. How precession changes the timing of the seasons (4b taken from Ruddiman: Earth's climate Past and Present)

 

There are two key precession effects: axial precession (see an animation here), in which the torque of the other planets exerted on the Earth's equatorial bulge forces the rotational axis to “wobble” like a spinning top; there is also an elliptical precession, in which the ellipse of the Earth itself rotates about one focus. This means that the elliptical shape of Earth's orbit rotates with the long and short axes of the ellipse rotating slowly in space.  This moves the positions of aphelion and perihelion.

Figure 5: Change in the Earth's orbital plane. Even if the spin axis always pointed in the same direction (for example, on a perfectly spherical planet) it it would make a different angle with its orbital plane as the plane moved around

Precession also means that the solstices and equinoxes have changed positions, both with respect to the eccentric orbit, and with respect to the positions of perihelion and aphelion.  Today, the positions of the solstices and aphelion/perihelion line up closely (Fig. 6), and the Northern Hemispheric summer occurs when the Earth is further from the sun, but this is not always the case.

The summer and winter solstices mark the longest and shortest days of the year, and we see the sun moving back and forth throughout the year between the Tropic of Cancer (23.5 N) and Capricorn (23.5 S), which is a reuslt of the revolution around the sun with a 23.5 degree tilt.  Note also that the 90-23.5=66.5 degree mark defines the Arctic and Antarctic circles.  At the shortest winter day (winter solstice), no sunlight reaches latitudes higher than this.  The equinoxes mark the point where the length of day equals the length of night.

Figure 6: Positions of the Equinoxes, Solstices, Perihelion, and Aphelion.  From Ruddimans Earths Climate: Past and Future

 

Today, the Northern Hemisphere winter occurs near Perihelion, and NH summer occurs when the Earth is farthest from the sun.  At present, the Southern Hemisphere has a tendency toward hotter summers , and with a more moderate seasonal cycle in the North, although that simple idea is complicated by differences in land distribution and thermal inertia between hemispheres.  However, about 13,000 years ago, the Northern Hemisphere summer would occur when the Earth is closest to the sun, and NH winter when it is furthest from the sun (Figure 4).  This would enhance the strength of the seasonal cycle.

Precession varies on timescales of 19,000 and 23,000 years, and is thus important even over historical times.  The precessional cycle is the key player behind the Holocene Climate Optimum, a time between ~7,000 and 5,000 years ago of particularly warm Northern Hemispheric  extratropical summers, and colder tropical and extratropical winters.

It is important to note that under the precessional cycle, the change in solar radiation striking the Earth is opposite in each hemisphere, unlike the case of obliquity where a higher tilt will mean more intense radiation at both poles as the planet revolves around the sun (although, obviously at the local summer summer for both poles, and thus at different points in the orbit).  Furthermore, eccentricity modulates the effect of precession.  For zero eccentricity, the precession angle is irrelevant.

What Causes Milankovitch Cycles?

The changes in eccentricity of Earth’s orbit are due to alterations in the gravitational tugs induced by other planets.  Jupiter has a very moderate eccentricity, but if it were larger, it would drive larger changes in Earth’s eccentricity.  It therefore seems likely that exotic cases of highly eccentric orbits may be prominent in other solar systems, where various gaseous planets are known to exhibit large orbital fluctuations.

Obliquity and precession variations arise due to the torque exerted by gravity (i.e., a force that acts perpendicular to he spin axis of the top) which ultimately comes from the pull of the Sun and Moon on Earth’s equatorial bulge.  Precession also varies due to the tilting of the Earth’s orbital plane, as shown above.

The periodicity of Milankovitch cycles is therefore subject to change over geologic time, as the length of day of Earth changes, and the moon becomes further separated from Earth. A shorter Earth-Moon distance would cause the precessional movement to have been larger and the precession and obliquity cycles would have been shorter, as would have occurred in geologically distant paleoclimates.  For example, in the Upper Carboniferous (~300 million years ago), the ~41,000 yr obliquity cycle would have taken about 33,000 years (see e.g., here)

Milankovitch Cycles Beyond Earth

Milankovitch cycles are not unique to Earth, nor are the solar system’s orbital characteristics fixed in time. Even our own solar system may be unstable on timescales comparable to its age. In the inner Solar system, the planets' eccentricities exhibit chaos on billion-year timescales (Fig. 7).  The lighter planets (Mercury and Mars) have the potential for large variations and in fact, it has been calculated that Mercury has ~1% chance of colliding with Venus or the Sun (or being ejected from the solar system) within the next five billion years (Laskar, 1994, Laskar & Gastineau, 2009).

Figure 7: Numerical Integration describing orbital paramters (10 Byr backward, note this is older than the age of these planets, and 15 Byr forwards). The larger planets behave more regularly.  Based on J. Laskar, A&A 287, L9 (1994)

Mars has an obliquity that can vary chaotically between ~0-60°, which has severe implications for its climate evolution.  Milankovitch cycles on Mars can actually play a role in redistributing ice on a global scale.  In particular, it is thought that deposits of large amounts of water ice recently found in certain areas of the mid-latitudes of Mars (e.g., Holt et al., 2008) must have formed at a time when the climate was conducive to glaciation at middle latitudes, as there is no precipitation in these regions today. This probably requires a higher obliquity, a greater amount of sunlight at the poles, driving sublimation and vapor transport equatorward, where it can then be deposited at lower latitudes (Forget et al 2006).  Earth can actually attribute its relatively mild variations in tilt to the stabilizing influence of the moon( Laskar and Robutel, 1993). It would also be possible to have higher obliquity variations if Jupiter were closer to Earth, even with a moon. 

Figure 8, below, shows the relatively recent obliquity variations on Mars.

Figure 8: Recent obliquity variation on Mars (-20 Myr to 10 Myr).  See Laskar et al (2004)

We don't need to think inside just this solar system though.  The influence of exotic spin states or eccentricities is a rather hot topic in the planetary climate community right now (e.g., Spiegel et al., 2010), as new plants beyond our solar system continue to be discovered.  Some open questions for example involve the ability to swing in and out of a Snowball planet (or a runaway greenhouse) at highly eccentric orbits.  Can planets that undergo large variations in the axial tilt remain habitable? Can planets in binary (two-star) systems be stable? Some of these issues have been explored briefly, but with  over 500 planets now discovered outside our solar system and many more expected to come, there's a good amount of work that needs to be done here.  Earth, today, is stable in both its modern configuration or in a cold "snowball" configuration (i.e., if Earth were magically ice-covered, it would stay there, even keeping the solar constant and CO2 the same, due to the ice-albedo and water vapor feedback) (Figure 9). 

Fig. 9: Bifurcation diagram of Temperature (purple curve) vs. Solar Insolation (blue curve).  Because of the ice-albedo feedback, the equilibrium is stable at several points.

Figure 9 cuts into the heart of various planetary climate "extreme" problems.  For a relatively circular orbit, the problem of determining where Earth falls into and out of a Snowball is challenging.  What happens though if you make the planet slowly rotating? What if the eccentricity is very high, so it the planet swings in and out of the "habitable zone" over the course of one planetary year? Milankovich cycles have the potential to make this issue a lot more interesting, although it is not a solved problem. 

Some Final Words

There are still a number of unresolved questions that remain in the astronomical theory of climate change, even during the more familiar Quaternary timeframe.  For instance, while we know changes in the orbit pace ice ages, the precise way the three Milankovitch variations conspire to regulate the timing of glacial-interglacial cycles is not well known.

For example, about 800,000 years ago a shift of the dominant periodicity from a 41,000 yr to 100,000 yr signal in glacial oscillations occurred (called the Mid-Pleistocene Transition, see e.g., Clark et al., 2006), and while a lot of ideas exist for why this should be the case, there's no bulletproof answer to this.  Explaining the 100,000 yr recurrence period of ice ages is difficult because although the 100,000 yr cycle dominates the ice-volume record, it is small in the insolation spectrum. Therefore, there's still a lot to be done here.

It seems that the Earth listens to the Northern Hemisphere when deciding to have an ice age.  If the North and South are alternatively near and far from the Sun during summer, why has glaciation been globally synchronous? What connections are there between Northern insolation and Antarctic climate at the obliquity and precession timescales? What are the competitive roles between a further distance from the sun during summer and a longer summer, following Kepler's law? These quesrions are still not resolved (for a flavor of the discussion, see  Huybers, 2009...see also Kawamura et al 2007; Huybers and Denton, 2008; Cheng et al 2009; Denton et al 2010 ).  This problem also involves work at the interface of carbon cycle and ice sheet dynamics, processes that are in their infancy in terms of modeling. 

 

Acknowledgments: I'd like to thank fellow SkS contributor "jg" for terrific work in piecing together various visuals used in this post.

Recommended Reading: I also recommend Tamino's multi-part series on Milankovitch cycles (the rest of the posts are linked at the bottom).  Involves some math, but a good read.

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Comments 51 to 68 out of 68:

  1. Moderator JH, I would like to ask, if the website is named skeptical science and most people in the scientific community agree with global warming, then why are all of the skeptics expected to formulate their questions like a scientist?  Also want to point out post @47 somehow duplicated into @49

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    Moderator Response:

    [JH] Septics of mainstream science are more than welcome to post comments on this website if they are polite and abide by the rules. Skeptics who cannot and/or will not document the source of their assertions are, by and large, wasting everyone else's time. They are effectively functioning as "climate science deniers" who grossly underestimate the climate science knowledge base of the regulars who post and respond to comments. 

    To learn more about why the name Skeptical Science was chosen, click on the "About" button in the blue bar on the top of each page.

  2. Moderator JH I would like to point out again comment @49 was a duplicate that can be deleted as well as this comment.  A user here pointed me to an interesting website that answers most of my questions which is what I was hoping for.  I will not be asking more here

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    Moderator Response:

    [JH] At the end of the day, we all do what we consider to be in our own self interest.

  3. MAP,

    In general, if you think of a question scientists have answered it.

    If not for humans the descent into the next glacial period would have started several thousand years ago.  The hockey stick shows declining temperature until 1850.  The descent would have been faster but human land use slowed it down.

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  4. Responding to comment here.

    Perhaps you need to spell why you dont "beleive" in Milankovich cycles since the cycles themselves are extremely well observed in astronomy and the effect of the cycles on the insolation hitting the earth is readily calculated. Ie this is not some hand-wavy speculation. From memory, Milankovich did the calculations by hand while in prison so not too daunting. The detail of the maths and the results are detailed here (among many other places) - see bottom of the page.

    The match of the variation of insolation at 65N from Milankovich and the glacial cycle as revealed from ice cores and benthic forams is extraordinary. Any competing theory would need to do at least as well.

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  5. mkrichew @32elsewhere,

    The inclination of our slightly-less-than-round Earth doesn't appear to impact the area subject to insolation by very much. The Earth's dimensions are given as a polar minimum radius of about 6,357 km and an equatorial maximum radius of about 6,378 km. If we were to consider the Earth as an elipsoid with these dimensions, its area facing-the-sun with a pole pointing at the sun would be just 0.3% greater than with the tropics-facing-the-sun but that would be assuming the axis is tilting through 90º relative to the sun and staying there throughout the year. Yet the actual change in tilt is nothing like 90º and is only fully acting at the solstices.

    The tilt varies between 22º & 24½º through its 40,000 year cycle, so just a 2½º variation, and that inclination is achieved relative to the sun only at the solstices, twice a year. So the increase in Earthly area facing the sun would vary by perhaps (0.3% x 3% x 70% =) 0.006% or a forcing of  very roughly  0.015Wm^-2. That's only about 4-months-worth of AGW so not exactly significant. And bear in mind the bigger winter/summer temperature range at the two poles resulting from any increase in tilt. That would firstly see more energy leaking away into space (as the energy loss to space is T^4 so a constant temperature is more energy-efficient than hot-summer:cold-winter) and secondly the albedo change from the greater area of winter snow will reduce solar warming. These two cooling effects should well-exceed the warming from the greater earthly area catching the sun from there being a greater axial tilt.

    (Note also the calculated effect of orbital eccentricity in the link @54 is 0.167%, some 30x greater. Even with this larger increase in insolation leads John Baez to the conclusion "if changes in eccentricity are important in glacial cycles, we have some explaining to do.")

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  6. April Milankovitch thoughts

    1. What is the area of the elipse vs. the area of the circle that the earth's orbit makes?

    2. What different areas does the earth's elipsoidal shape present to the sun or hemispherical shape?

    3. Somewhere I read that previous to our current hundred thousand year trend for ice age cycles ( matching the eccentricity cycle ), there was a 41 thousand year cycle of ice ages ( matching the obliquity cycle ) but I did not see a reference ( which does not matter as I cannot access a library due to the pandemic ).

    So, why am I asking these questions? As you may have guessed, I am not a great fan of the Milankovitch cycle theory of ice age cycles. I have my own theory which asks "Where does all this CO2 come from in the past cycles?" The Mike Krichew Theory says that it comes from the oceans. This means the oceans must have warmed. What would cause them to warm? I suggested the tail of a comet might provide the increasing insolation to slightly warm the ocean and increase the atmospheric concentration of CO2 near sea level as warming sea water gives off CO2. This increased CO2 level would warm the atmosphere slightly which would slightly warm the ocean resulting in the release of more CO2 causing more atmospheric warming etc. This might explain the lag between warming and CO2 levels that opponents to worrying about greenhouses gases are quick to point out. The rest of my theory states that at some point the CO2 levels in the upper atmosphere reach a point where they capture the suns rays up there and radiate the heat out into space. This assumes the excess energy of a CO2 molecule is disapated by electron cascading rather than molecular collisions. Having said all this, then why am I asking the questions about orbital elipse size and earth cross sectional area at various points in the spin cycle of the earth.? If I were a rich scientist, I should be releasing weather weather balloons at statistically significant points in the Pacific ocean and measuring the CO2 levels at different altitudes. Unfortunately, I lack the funding. Maybe someone has already done this? The reason I am asking the questions is that in looking at the Vostok ice core graphs or the benthic graphs, it is hard to imagine coming up with modelling equations that would match the steep slope at the start of the warming cycle. The same can be said of Milankovitch cycles. I looked into magnetic pole flips and crons were just not in the picture.

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    Moderator Response:

    [PS] This is repetition. You have not addressed MA Rodgers above. Your objections to Milankovich do not appear to be a problem with the calculations. Increasing temperature (from albedo change and feedbacks) did indeed warm ocean and cause more CO2 (which then warmed earth more including southern hemisphere ). Why go for the wierd and wonderful and ignore the observable milankovitch cycles with their easily calculatable effect?


    Hansen and Sato reproduce the ice core data rather well with conventional physics. Real science doesnt have your problem.

     Balloon measurement of gases (including CO2) at various levels in atmosphere have been done for ages. eg here. Just google for them.

  7. To MA Rogers @55: Thank you for your kind response.

    Also thank you to the moderator for the graphs of CO2 concentration at different altitudes.

    I apologize that my submission came out in orange and is almost illegible. It was not when I submitted.

    Concerning Milankovitch cycles, I still do not think they provide the insolation forcing that is found in past cycles at the start of warming. This was mentioned by at least one other in the comments section.

    Getting back to CO2 concentrations at altitude, I would have expected them to be higher as I thought earth was entering into another glacial period. Back in 1955 I thought I was taught that most glaciers were advancing. Also, I believe there was a brave scientist who obtained ice core samples from high in the Andes mountains. I wonder what CO2 levels he found there and how they compared to other core samples taken at lower altitudes? Once again, thank you for your patience in looking at my comments. I also thank scaddenp @54, I hope I answered his query and yes I was aware where Milankovitch did his work. He must have been very clever.

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  8. Mkrichew @57 , to give you a very brief reply :-

    Your final paragraph ~ what reason would you have, to think that a higher CO2 concentration means Earth entering another glacial period?

    ~ most glaciers were advancing during the last approx 5000 years (as Earth surface temperature gradually reduced . . . until the rapid warming of the last 150 years.  (Up until 1955 date you mentioned, the available evidence was not as strong as today.)

    May I point out that, so far, you have not provided any evidence to demonstrate any error in the mainstream climate science.

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  9. Eclectic @58

    Thank you for your question concerning higher CO2 levels at the start of glacial periods. From the ice core curves it appears that the CO2 concentration is highest just prior to the start of the downward negative slope that signals the start of a cooling glacial period.

    Further, your comment about not providing any evidence for my theory mentioned above in 56 and elsewhere concerning CO2 concentrations reaching high enough levels in the upper atmosphere to trap and dissipate the sun's radiation before it can warm the lower atmosphere is correct.

    I thank the moderator and others for providing some evidence, even if not always supportive of my theory.

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    Moderator Response:

    [PS] it would perhaps improve discussion if you could clarify whether you contest that:

    1/ Milankovich cycles exist (this would be very hard to dispute)

    2/ that the calculations for variations in radiation at TOA at 65N due to cycle are correct.

    3/ that the variation in radiation (+/- 50W/m2) is insufficient to cause the change observed

    or

    4/ that ice age cycles do not closely correlate the cycles.

    Thank you.

  10.  

    To the moderator:

    Thank you once again for providing the graphs of CO2 concentration verses altitude.

    1. In answer to your implied query in 1/ above concerning the existance of the Milankovitch cycles. I believe they exist, although I have not read his paper. I am taking your word and explanation as well as the one in Wikipedia.

    What I was questioning and am continuing to look into was the forcing at the onset of deglaciation. John Cook's article in this URL "Why does CO2 lag Temperature?" has given me food for thought in the area of the speed of onset and fast temperature rise.

    2. I have not looked into the math done to get the curves shown concerning your comment 2/.

    3. Your comment 3/. I have read 5 W/m2 and 50 W/m2 as the intensity changes. I will see if I get interested enough to do the math.

    4. Your comment 4/. I had read somewhere that the cycles timing did not match physical evidence of timing of ice ages. However, this may have been in reference to the obliquity cycle and not the eccentricity cycle. I can't remember, I think it was ten years ago when I read this.

    Thanks once again.

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  11. mkrichew @60,

    Briefly as we are off-topic, regarding CO2 at altitude, it is effecively well mixed up to 50,000km, the scatter measured at low altitude being simply local influence and more generally the annual CO2 cycle.

    CO2 with altitude

    What I would add is that the ability of CO2 to "capture the suns rays," something you suggest is significant @56, is very small. Of the absorption bands of CO2, only the 2.9 micron band operates within the wavelength of solar radiation and that at the very tag end of the insolation's frequency distribution.

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  12. There is a useful little summary of the Milankovic cycles in Physics Today, including the critical feedbacks

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  13. What causes more climate change then, Milankovitch cycles or anthropogenic causes?

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  14. Kylesa @63 , your question is a bit off the bulls-eye.

    The climate change caused by the Milankovitch cycle during the past 1 million years, has occurred in cycles of approx  100,000 years.  It is much more correct to say that those climate cycles have been triggered by the Milankovitch orbital alterations ~ because the Milankovitch changes in solar heating of the Northern Hemisphere are very slight (purely in themselves much too weak to make a difference in global climate).  However, these slight changes are then greatly magnified by the consequent change in atmospheric CO2, as the atmospheric CO2 leaves or enters the planetary oceans.

    Basically, I think of the recent glaciation/de-glaciation cycles as being caused 10% by the Milankovitch changes (which are the trigger) and 90% by the CO2 rise/fall (the CO2 being the main charge of gunpowder moving the bullet).

    More than 1 million years ago, the Milankovitch cycles were still in operation, but were having near-zero effect on climate because the atmospheric CO2 level was so high it swamped the tiny Milankovitch effect.

    The anthropogenic causes (mostly the fast-rising CO2) have been so rapid and powerful in causing GW, that it's fair to say that the weak and ultra-slow Milankovitch effects are tiny/negligible ~ like comparing a cockroach to an elephant.

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  15. Hmm I think Hansen & Sato calculated this and reported in AR4. More like 4% for global solar change, 21% for albedo change and remainder from GHG (CO2 + CH4). However, locally (65N) the milankovich forcing is very high, enough to determine whether snow melts out in summer or not and so trigger the large scale albedo changes.

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  16. My understanding is that the current Milankovich forcing is to make the temperature go down.  So since temperatures are going up it cannot be due to Milankovich forcings.

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  17. Approximate figures (as I recall) quoted are :-

    A gradual global temperature fall of 0.7 degreesC over the past 5,000 years, but a rapid rise of 1.2 degreesC over the past 150 years.

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  18. Scientists have evaluated all natural forcings and factors capable of driving the Earth's climate to change using multiple lines of consilient and converging evidence, including the slow, long-term changes in the Earth’s movement around the Sun (Milankovitch cycles or orbital forcings), and it is only when the anthropogenic forcing is included that the observed and ongoing warming since 1750 can be explained.

    Natural vs Anthropogenic Climate Forcings, per the 4th US National Climate Assessment, Volume 2, in 2018 (orbital forcings shown in the top frame):

    Forcings

    https://nca2018.globalchange.gov/img/figure/figure2_1.png

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