Talk:Milankovitch cycles

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Contents 1 Graph 2 Summary cartoon 3 Explanation vs Forcing 4 Modulation of eccentricity 5 There might not be a 100 ky signal!?! 6 Effect of Eccentricity 7 Commons 8 seasons of peri/aphelion 9 Arctic/Antarctic circles 10 How about some successes? 11 Long-term cycles and data 12 Prefer no merging. 13 Global Warming 14 Pfvlloyd's revisions 15 Maximum and minimum latitudes for the arctic and antarctic circles? 16 Question regarding time-frames 17 Re. "Question Regarding Time Frames" 18 Predicted temperature changes based on change in eccentricity, tilt, and wobble of the earth 19 Wikiproject Earth 20 Axial tilt angle

[edit] Graph

Why would the graph at the top of the page show time flowing from right to left? It is unconventional and might appear misleading to some. GBMorris 13:31, 25 July 2007 (UTC)

[edit] Summary cartoon

Image:Earth_axis_tilt_and_precession.jpg has two apparent errors:
The blue box indicates the Minimum, not Maximum, Axial Tilt.
The duration of 21000 years is half of, not twice, 41000 years;
thus there are about two 21000 year periods in a 41000 year period.
Joe Kress 05:34, Aug 26, 2004 (UTC)

(William M. Connolley 08:52, 26 Aug 2004 (UTC)) I think you are right. I've removed the image for now. BTW, there is no reason for the image to have the large bold text at the bottom - that can be done by caption (which would be fixable if done that way).

Is that image still necessary? Although I think it would be better to use one similar to the top of http://3dparks.wr.usgs.gov/nyc/images/fig142.jpg (SEWilco 20:49, 8 Mar 2005 (UTC))

(William M. Connolley 22:58, 8 Mar 2005 (UTC)) That pic is nice but isn't it missing the 400kyr eccentricity signal? Why only the 95kyr?

First let me commend SEWilco on finding a nice set of PD images. However, one of them, Image:Milankovitch_patterns.jpg I strongly object to. The first and third lines (precession and eccentricity) are misleading because they only show single frequencies and neither of these has a pure spectrum. The fourth line labeled "Relative combined effects of the above cycles on the warming and cooling of the Northern hemisphere" is just the sum of the first three lines in some (apparently) arbitrary weighting, and bears no relationship to actual solar forcing (or anything else as far as I can tell. The last line is some unidentified sediment record (it can't be ice cause only EPICA is long enough and it isn't that), and has some features which I would regard as atypical and hence not a good representative pic. Beside which who came up with "interglacial" 3? While the last line is questionable, it is really lines 1,3 and 4(especially 4)that I regard as too misleading to justify using that pic. Dragons flight 01:07, Mar 9, 2005 (UTC)

Addendum: After looking at it some more, the phasing and weighting of the first three lines is such that line 4 does resemble N. Hemisphere Summer insolation, though it is still not quite accurate. Dragons flight 01:31, Mar 9, 2005 (UTC)

Second Addendum: Given how misleading I though that figure to be, I decided to replace it one I felt was more realistic. Dragons flight 15:21, Mar 9, 2005 (UTC)

You could have overlaid my image file with an equivalent improvement. Yours is nice, but the yellow is hard to see. Maybe purple or brown. Although the red might be a little more appropriate for the forcing compound curve than the other colors (yes, yellow is also appropriate but is hard to see on a white background). (SEWilco 02:52, 10 Mar 2005 (UTC))

[edit] Explanation vs Forcing

I note the orbital forcing content. I thought the explanation of the cycles should be here, as the physical movements are part of this topic while it is possible that the orbital forcing article may be more focused on the effects of movements. Whatever happens in the other article, this seemed like the place to begin explanations. — SEWilco 07:26, 7 Mar 2005 (UTC)

• I realize the material is growing beyond M. Cycles. Let it accumulate here for the moment and it will be sorted out soon. (SEWilco 02:55, 10 Mar 2005 (UTC))

Personally I'm against merging these two articles, although there is a considerable overlap. In favour is the fact that the orbital forcing article is almost exclusively confined to a discussion of precession, so it might mislead the hasty browser into thinking that orbital forcing consists of precession.

On the other hand, orbital forcing is not really synonymous with the Milankovitch theory, which was once a particular theory with an identifiable history. It was not the first theory to posit that the Earth's climate history is related to orbital variations (as the article notes, and see "Ice Ages" by John Imbrie and Katherine Palmer Imbrie). The Milankovitch theory has come to function as "shorthand" for the orbital forcing or pacing of climate change, Milankovitch cycles for any orbital cycle affecting climate (on any planet). The merging of the two articles would suggest this is acceptable and should be encouraged; maybe it should! What do other people think?Orbitalforam 19:25, 25 January 2007 (UTC)

[edit] Modulation of eccentricity

(William M. Connolley 10:04, 7 Mar 2005 (UTC)) I removed this:

A 100,000 year pattern is present in recent glacial events, but Milankovitch calculations of eccentricity should also carry a strong 413,000 year component. It has been pointed out that the 100,000 year pattern behaves as if it carries a frequency modulation (FM) of the 413,000 component. The mechanism of such a modulation is not known, although the oscillation in size of ice sheets resembles an expected pattern and may reflect a relevant resonance in the climate system.

I couldn't make sense of it. The orbital periods are all well known, how could it be otherwise? The 413 kyr period is largest (say Imbrie and Imbrie).

I understand the logic in the paper, but it still seemed odd. I included it because it is "Rial[6]" in Ice_age#Causes_of_ice_ages. I was going to check references to the paper, as I'd expect the 413,000 component to also have a direct effect, and perhaps the FM effect was a poor analogy. (SEWilco 22:06, 7 Mar 2005 (UTC))

Rial has more recent work where he casts more math upon the winds yet no progress on mechanisms. I removed it from Ice age. (SEWilco 04:34, 9 Mar 2005 (UTC))

(William M. Connolley 09:36, 9 Mar 2005 (UTC)) I re-inserted it. I can't see why you removed it.

Because it only seems like mathematical analysis of a False analogy. The 400ky cycle (±0.011 eccentricity or a range of 0.022 change) is supposed to be one which alters the distance to the Sun, which produces an amplitude change. The 100ky cycle (0-0.05 eccentricity) also produces an amplitude change. This should produce 100ky cycles which vary slighly in total amplitude through the 400ky cycle. To FM modulate the 100ky orbital patterns would require something like pushing and pulling on the timing of the 100ky orbits, making the orbits happen sooner or later than calculated with the laws of physics. Since the 400ky cycle is calculated from the orbit based on physics, it seems unlikely to find something unexpected in its behavior. If the 400ky cycle is not present in the orbital pattern, what are we talking about? Are the climatologists using poor calculations by Milankovitch which astronomers have not checked against Earth's orbit? Rial happened to discover an unexplained similarity in two spectrum patterns. Either he detected an odd behavior of the physical world (gravity?), the 400ky cycle effects exist but below our ability to detect it, or the 400ky orbital component has absolutely no effect (perhaps due to being overwhelmed by climate system noise). At the moment it seems to me that the 400ky component is not being detected (one of the latter 2 situations). Rial's math only seems to prove the similarity to an FM pattern, and is at the moment as relevant to these articles as finding Jughead's face in a stellar nebula. I don't think "encyclopedic" has to include every interesting shape of cracks in Antarctic ice, until someone studies a "that's funny..." result and finds there is indeed something interesting. (SEWilco 03:56, 10 Mar 2005 (UTC))

For detection of the 400,000 year cycle in the longer geological record, see (for example) Science 13 April 2001, Vol. 292. no. 5515, p. 191 (a discussion of a paper by Jim Zachos of a 3.4 million year isotopic record from the Oligocene-Miocene). These cycles were also noted in the Oligocene ODP Leg 199 records from Site 1218: Science, 22 December 2006, Vol. 314. no. 5807, pp. 1894 - 1898. The cycles seem to be clearly present in the geological record, and it would be wrong to imply that they haven't been detected.Orbitalforam 17:19, 5 June 2007 (UTC)

[edit] There might not be a 100 ky signal!?!

(William M. Connolley 22:11, 15 Mar 2005 (UTC)) SEW added:

There might not be a 100 ky signal: It has recently been noted that many studies have displayed power frequency information using a linear scale which distorts any 100 ky peak. Records contain only 10-20% of their energy in the obliquity and precession bands. There are indications of the obliquity and precession signals, but the 100 ky energy is indistinguishable from a broadband stochastic process. The main issue is that statistical analysis of only seven Pleistocene ice ages is difficult due to the small number of samples.^[1]

This seems weird to me. The 100 kyr peak is the largest in the spectral record (Imbrie, Science, 1980; many other places too). Records contain only 10-20% of their energy in the obliquity and precession bands. doesn't seem to be too relevant, except to demonstrate that they are small - but since they are not the 100 kyr signal, that doesn't seem to be relevant to whether there is a 100 kyr signal (except to support it).

I looked at the paper. In my opinion, it is basically silly, since much of the interpretation is predicated on the existence of a linear response between climate and orbital variation, and if a response to 100 kyr eccentricity variation exists at all, it pretty obviously can't be a linear response. But, it is published, so I have to put my personal reservations aside. However, I did try to edit the comment to be more directly on point with the 100 kyr cycle. Dragons flight 00:03, Mar 16, 2005 (UTC)

PS. Wunsch agrees that the 100 kyr cycle frequency band is the largest part of the spectrum but basically comes to the conclusion that it is not strong enough to meet his criteria for being significant.

Yes, Wunsch points out that there are many studies which report the 100 kyr peak is the largest in the spectral record. (SEWilco 09:19, 16 Mar 2005 (UTC))

• First issue is whether the obliquity and precessional energy drive the 100 kyr behavior.
  • He dismisses as an extreme hypothesis that there is "a true, linear, resonant response to the small eccentricity forcing at 100ka".
  • He examines whether the 0.01-0.05 C/ka band has variance dominated by astronomical forcing (whether there are astronomical peaks).
    • If that band is dominated by signals arising from the Milankovitch forcing, then there is a case for suspecting their interactions may be significant.
    • If that band is dominated by a continuum, "the Milankovitch rectifier hypothesis is much less compelling". (If there are only random variations, it is unlikely those drive the 100 kyr behavior.)
  • He examines whether the obliquity and precessional bands may have enough power to drive the 100 kyr behavior.
  • The spectral display being used in previous reports is the linear-linear of Figure 4. There obviously is a large peak at 0.01 there. But look at the log-log form in Figure 2. There is a dip at 0.02, but to the left is a broad plateau rather than a peak. To the right, the peaks are much less dramatic than in Fig. 2.
    • The linear-linear graph "has the effect of exaggerating the importance of the peak at the obliquity period."
  • He tries a stochastic pattern with an overall waveform similar to the original.
    • Except for a small obliquity signal, the results are similar.
  • Thus, "in these two Vostok records, the overall variance...is governed by a process indistinguishable from a stochastic one."
  • Similar results with other records; records tuned to orbital frequencies have certain peaks due to tuning.

• He next examines the 100 kyr signal.
  • The SPECMAP stack was used by Ridgwell to examine the last 7 glacial/interglacial events.
    • Those 6 intervals happen to be about 100 kyr each.
    • Ridgwell showed "the apparent coincidence of delgaciations with 4 or 5 precessional cycles".
      • The precessional period is nominally 20 kyr, so of course matches with 5*20=100 are likely.
      • Ridgwell also accepted 4*20 as matching a 100-ish period.
    • The SPECMAP stack has errors of ± 5 kyr.
    • Wunsch considers the evidence for determinism to be shaky.
  • He looks for a 100 kyr sinusoidal signal hidden in the SPECMAP data.
    • No evidence for a dominant signal.

[edit] Effect of Eccentricity

1.) I have been studying a book on the Milankovitch cycles (The Dynamic Earth, OU) which suggests that the main effect of the changing shape of the orbit is to change the amount of solar radiation reaching the Earth as a whole, in addition to a subsidiary effect accentuating the seasonal changes in one hemisphere. I don't see any mention of such a total increase in radiation for highly elliptical orbits here, and I would have thought intuitively that the greater time spent further away from the Sun would balance the time spent in close approach. Does this effect exist? Tonderai 18:27, Apr 25, 2005 (UTC)

For fixed semi-major axis, the total radiation recieved over a year is constant. The changes in eccentricity do have accompanying changes in semi-major axis, but the fractional change is of order eccentricity², or in other words less than 0.1% in the typical case, and consequently not considered climatically important. I would assume that your text is refering to the instantaneous solar radiation which does vary considerably as a function of eccentricity. For example, in its most extreme configuration, at perihelion the Earth as a whole recieves about 25% more radiation then it does as a whole during the aphelion of the same orbit. Dragons flight 19:19, Apr 25, 2005 (UTC)

This is incorrect. The semi-major axis is fixed, period. The total radiation changes by an order eccentricity² amount for a given semi-major axis. It is generally, but not universally, thought implausible that such a small effect could be significant. Eccentricity is more typically thought to affect climate through a rectification of the perihelion cycle Climateguru

Yes it must be referring to differences between aphelion and perihelion, if there is little difference over the course of a whole year. Thanks for clearing that up, my elliptical geometry's not what it might be! Tonderai 08:11, Apr 26, 2005 (UTC)

2.) From the article Due to the eccentricity resembling a circular orbit, there is a 6% difference in the amount of solar radiation during summer in the two hemispheres. (from Present Conditions). I understood that it was the current eccentricity of the orbit that caused the difference in solar radiation between summers in different hemispheres. A circular orbit would mean there was no difference between the hemispheres. Correct? Tonderai 18:27, Apr 25, 2005 (UTC)

I clarified this in the text. What it meant to say is that we are presently in a relatively low eccentricity orbit (nearly circular), so the variation is only 6%. If it were totally circular, the variation would be 0, as you surmised. Dragons flight 19:19, Apr 25, 2005 (UTC)

That reads much more clearly now. Just out of curiosity, how much would the difference be in a highly eccentric orbit? Tonderai 08:11, Apr 26, 2005 (UTC)

For fixed semi-major axis (such as is essentially relevant for the Earth), the flux at perihelion is proportional to 1 / (1 − e)² and at aphelion proportional to 1 / (1 + e)², so obviously the difference is going to become arbitratily large as you consider ever more elongated orbits, i.e. as e -> 1. However, the Earth's orbit only gets as high as 0.058.

If you were to instead fix the perihelia distance (rather than the semi-major axis), then the flux at aphelia would go as (1 − e)² / (1 + e)². This is perhaps easier to understand, since longer orbits then clearly show a decay towards zero flux the farther away they get. Dragons flight 19:41, Apr 26, 2005 (UTC)

Why does the eccentricity change? 84.191.227.100 17:16, 26 January 2006 (UTC)

[edit] Commons

Please copy the images to Wikimedia Commons. --Saperaud 5 July 2005 11:30 (UTC)

I won't. You figure out the copyright issues and do it. I put my images in as generalized a place as I could. There seemed to me to be issues between their copyright and the requirements of Commons. Someone else can move and delete them if that is appropriate. (SEWilco 5 July 2005 18:41 (UTC))

My interest is in providing the best experience in the English Wikipedia that I can. Beyond that you can do what you will provided you respect my rights to the full extent of the GFDL. Don't expect me to help populate Commons however, I'm just not interested in navigating those issues. Dragons flight July 5, 2005 19:22 (UTC)

[edit] seasons of peri/aphelion

Perihelion and aphelion are points (in space or time). They occur during a season. Perihelion may occur during summer, but summer cannot occur during perihelion. The mistake occurs several times in the article.

[Above comment by 24.112.15.149 on 8 October 2005]

Good point. I've rewritten two sentences where this occurs. -- JimR 12:28, 27 October 2005 (UTC)

[edit] Arctic/Antarctic circles

The page needs a bit on how the axial tilt affects the latitude of the Arctic and Antarctic circles, including the distance and degrees latitude they range over. Including the total surface area ranged over would be a good datum for those who don't have the foggiest notion of how to calculate the surface area of a section of a sphere. ;) As the circles move towards the poles, there's less area in 24 hours to six months continous darkness per year, which reduces the net heat radiation of Earth.

The History Channel's recent documentary on the "Little Ice Age (Big Chill)" had nothing on Milankovitch cycles. That was a rather large omission!

[edit] How about some successes?

It would be nice to see some of the successes of this theory, rather than just the problems! As it stands, it looks like there is very little reason to accept that Milankovitch explains anything. I suggest a figure showing the coherency and power spectra of the 65N insolation and some of the untuned ODP cores. Also the new EPICA Dome C records?

I agree, I think the preceding page over-emphasises the problems with the Milankovitch theory. However, problems are interesting and can lead to progress! Most palaeoclimatologists studying the interval 50kyr-30myr BP look to astronomically tuned records to give the most precise absolute or relative timescales for time-series. The approach was vindicated by (for example) Shackleton, Berger and Peltier's bold revision of the age of the Brunhes-Matuyama polarity reversal on the basis of Milankovitch cyclicity in ODP 677 (Trans. Royal Society Edinburgh, 81, 252-261, 1990) referred to here:[2]. The revised age has been validated by subsequent more precise radiometric dating of the event.

As far as ice-core records are concerned, ice-core workers in general tend to avoid astronomic calibration of their timescales as long as possible, perhaps because they regard the use of astrochronology as a circular argument, preferring to use ice-accumulation models. These can work well over geologically short intervals (a few thousand years) but on time-spans of hundreds of thousands of years go badly astray, so that ice core modellers are forced to use a few "astronomically tuned" ages anyway. The argument about the alleged circularity of Milankovitch timescales can get complex but in my view the dialogue between between timescale development and other sources of information about the age of deposits delivers the debate from circularity. Milankovitch-based timescales make "predictions" about accumulation rates, the timing of biological or geophysical events, and relationships between cycles and amplitude modulation that would be extremely hard to make functional if the theory were not valid. The revised ice-core ages have come to look more and more like the astronomically tuned marine records as time has gone by, and data and agemodels have improved. The precise timing of deglaciations, however, may prove an exception to this observation.

The Gradstein et al. Geologic Time Scale 2004 makes extensive use of astronomically tuned timescales for their chronology for the last 30 million years. However, relative chronologies have been constructed using Milankovitch cyclicities for geological sections from most parts of the Phanerozoic, including the Triassic and Cretaceous. These tend to be isolated glimpses of cyclostratigraphic clarity amidst a sea of geological uncertainty, however; and when orbital tuning is done incorrectly, it can go spectacularly awry.Orbitalforam 18:05, 25 January 2007 (UTC)

I've just modified the header of the "problems" section to reflect what I know of why its accepted. And the intro does state, explicitly, that orbital forcing *does* cause the ice ages. But more detail from an expert would be good William M. Connolley 18:59, 25 January 2007 (UTC)

I think that gives a better overall perspective without minimising the problems. The Milankovitch theory is widely regarded as having been validated by its explanatory power in relation to the waxing and waning of ice sheets over the last 800,000 years (the Imbrie, Hays, Shackleton "Pacemaker of the Ice Ages" paper in Science, 1976 [3] got this off to a good start). The irony is that in terms of the long term geological record (as reflected in eg the Mediterranean sapropels studied by Hilgen and others, the Leg 154 Ceara rise record, etc.) the giant ice sheets are actually a kind of "disruptor" to the more straightforward climatic response to orbital forcing over the last 50 million years and more. So the theory was established (though correctly, in my view) on the basis of one of its more "awkward customers"...Orbitalforam 10:09, 2 March 2007 (UTC)

[edit] Long-term cycles and data

I've added some references on orbital forcing climate effects before the Quaternary (the articles of Zachos are the ones usually cited), and links to the simulation data of Berger (1978 and 1992, 1-5 My BP), Laskar et al. (1993 and 2004, up to ~50-65 My BP) and Varadi et al. (2003, up to ~50 My BP). I had quite some trouble finding actual simulation data and accurate information on longer-term (ie. over more than 1-5 My) orbital forcing elsewhere, so this might be useful for others as well. The article by Varadi et al. might be a bit too technical/astrophysical, but has a very nice introduction on the subject of orbital cycles and climate, and good references. Accurate calculation of planetary orbits seems to be restricted to ~50 million years, because chaos causes errors to accumulate over long time periods. Lvzon 10:45, 23 February 2006 (UTC)

The longer term data clarify a number of points. In particular, Milankovitch cycles have been pervasive in the marine sediment record through the Cenozoic. These include "split" eccentricity cycles and show 400 kyr modulation of eccentricity (as well as even longer term modulations), even when tuning is oriented to resolving the tilt and precession frequencies. Cf the composite tuned benthic isotope record from ODP 677 and Leg 138, Jim Zachos' composite record, and the tuned lithological signal from ODP Leg 154 and others.

Nick Shackleton's paper in Science, September 2000, showed how multifaceted the periodicities involved in the Quaternary marine isotope record are. In some ways, the large ice sheets of the Quaternary obscure the response to orbital forcing seen more clearly earlier in the geological record. In contrast, the oxygen isotopic signal at Vostok has a clear relationship to orbital "climatic" precession, can be readily and unambiguously tuned, and reveals the true drivers of Quaternary climate change: a linear response to orbital forcing and a nonlinear response involving carbon dioxide that is particularly pronounced at eccentricity periodicities, and which still needs to be understood. Orbitalforam 24 April 2004

I've made some edits to reflect the fact that the majority of researchers interpret climatic variation at around 100ky periodicities to be linked (nonlinearly in the last million years) to orbital eccentricity. In the longer records (extending over millions of years) there is spectral power and amplitude modulations at these periodicities that appears robustly linked to eccentricity; interpreting the ca. 100kyr variations as inclination would be incompatible with the additional strong presence of tilt and precession periodicities in the records, as well as the amplitude modulation of tilt and eccentricity at longer periodicities. Two sentences - "The cycles are not being detected in climate" and "to date climate records show a single frequency" - were very misleading in this respect and I've therefore changed them. An interesting footnote is that the geological imprint of climate change may be strong enough to place constraints on the Earth's orbital parameters before 30-50 Ma.

Orbitalforam 8 May 2006

[edit] Prefer no merging.

While true there is some overlap, it is not so large as that. The distinction ought to be that orbital forcing is only one of many climate forcings, whereas M. cycles are primarily used in an attempt to explain the paleoclimate record.

134.121.64.253 23:07, 14 February 2007 (UTC)

I'd prefer no merge as well. People will come looking for the article under this name, and will not be able to find it. I was glad to have found it here. Jiminezwaldorf 08:10, 30 August 2007 (UTC)

It seems this merge was suggested long ago, with no proponent arguments surviving on the talk page. Change history indicates previous frustration at the lack of merge discussion, along with previous attempts to remove the merge (29 October 2005). This matter should be considered settled. - Rgrant 00:46, 5 October 2007 (UTC)

It should not be considered settled if there hasn't been any discussion. I can't see any convincing argument that the topics should be separate, and redirects will find it under either name. The way, the truth, and the light 00:51, 5 October 2007 (UTC)

Fair enough. The topics indisputably share orbital body mechanics and solar radiation changes, so it's not controversial whether that should be described clearly in one place - it should. Therefore, let this discussion center on current and historical ties to climate analysis, and indications of similarity in the minds of climatologists who are creating articles suitable for NPOV reference. If Milankovitch cycles imply certain narrow theories of orbital forcing, and certain predictions regarding climate change, then orbital forcing as a separate idea may better capture other more general usage, or usage that is not tied to "old-Milankovitch" climate theories. Meanwhile, separating solar radiation changes from climate predictions, in both topics, may allow readers to find what they need, without tripping on all the differences. - Rgrant 19:00, 9 October 2007 (UTC)

For an initial impression of what's going on in this space, i used three separate Google scholar searches. one on "Milankovitch cycles", one on "orbital forcing", and one on both quoted terms. At present, Google found more than twice as many references to "orbital forcing". Only a fraction of articles crossed memespace and referred to both terms. This technique does not tell us what specific climate theories are being discussed.

I did run across a reference indicating that popular understanding ties Milankovitch cycles to a specific mechanism for climate change.

The theory of Milankovitch cycles states that periodic changes in Earth's orbit cause increased summertime sun radiation in the northern hemisphere, which controls ice size.

However, this study suggests that the pace-keeper of ice sheet growth and retreat lies in the southern hemisphere's spring rather than the northern hemisphere's summer.

– Terradaily staff writers , from "Carbon Dioxide Did Not End The Last Ice Age"[4], Oct 04 2007

Confusingly, this recent research seems to suggest that orbital forcing is occurring, that the physical orbital mechanics first described by Milankovitch cycles are its cause, and that the theorized mechanism of climate change associated with Milankovitch cycles (longer summers in the Northern hemisphere) is causally backwards from the details of these newly observed climate changes. I look forward to seeing this topic space well organized. - Rgrant 19:00, 9 October 2007 (UTC)

• Time's up. I removed the merge tag today to merge to Orbital forcing. There's no active discussion, no consensus, and furthermore the two tags (one on each page) did not even link to the same location for a discussion. Let a proposer put forward, with a link to a well-stated argument to merge on the same talk page for both articles.
  -- Yellowdesk (talk) 00:51, 8 January 2008 (UTC)

[edit] Global Warming

It is implied that humans cause global warming in this article ("anthropogenic effects (global warming)") when that fact is still hotly debated. This asserts a particular point of view that I don't think is neutral. I think it should be changed to conform to a NPOV. --Rcronk 20:01, 27 April 2007 (UTC)

The passing mention of anthropogenic global warming is couched in probabalistic terms ("are likely to..."). I would be surprised if that was not compliant with a NPOV, given the widespread agreement that there is at least some anthropogenic contribution to recent warming. The extrapolation of climate cyclicity into the future seems to require a consideration of the levels of greenhouse gases; Berger and Loutre, and Nick Shackleton, used a combination of orbital forcing and greenhouse gas levels to help account for the non-linear aspects of the climate reponse to Milankovitch forcing over the glacial-interglacial cycles. Orbitalforam 17:01, 1 May 2007 (UTC)

The fact is only 'hotly debated' in the public media, not the scientific community. I can't remember the exact details but a recent study of a representative sample of recent scientific literature found that 99% of it agreed that humans were making some contribution to global warming. The same test run on a sample of popular press articles found that only 50% of them reached the same conclusion. Verisimilus T 18:05, 9 May 2007 (UTC)

The quote then goes farther in saying "anthropogenic effects (global warming) are likely to exert a larger influence, at least over the short term" which doesn't just imply that humans are affecting climate but that they are affecting climate more than the Milankovitch cycles do. There is no citation for that assertion. Remove the assertion or at least cite the evidence supporting it. Fair enough? --Rcronk 17:16, 16 May 2007 (UTC)

Well since the Milankovitch cycles don't do much over the short term, that statement doesn't seem very controversial to me. Eve 20:36, 16 May 2007 (UTC)

This is true, since the most rapid Milankovitch cycle is climatic precession, which operates at frequencies of around 20,000 years (with a probable harmonic at around 10,000 years). Most people would not consider these to be short term variations! Interestingly, some of the "worst case" scenarios for anthropogenic climate change do envisage it exceeding the circa 5 degree centigrade mean change associated with the 100,000 year glacial-interglacial cyclicity. Even if the eventual effect is much less strong, it seems uncontroversial to assert that anthropogenic effects are likely to be stronger than Milankovitch ones over the next 100 years, and the latest IPCC report would support that.Orbitalforam 16:07, 5 June 2007 (UTC)

[edit] Pfvlloyd's revisions

Pfvlloyd please explain the changes you are making. You keep citing Berger and Loutre without identifying which one (they've published together several times). Maybe you know of something new, but I don't know of anything that justifies the large scale changes you are making. Dragons flight 17:10, 5 August 2007 (UTC)

Hi ! - Thanks for your interest. I see a note which says I have not replied to your questions, but since this was only placed a few hours ago, I think it's a bit steep to cancel my edits without even seeing my side of the story. Maybe this is just how you do things here!

Firstly, nearly all my comments arise from a sraightforward application of the celestial mechanics calculations of Earth orbital eccentricity which are universally accepted.

Secondly, Berger and Loutre's refinements of 1991 and 2002 provided very clear data over the past 750,000 years showing the timings and extent of eccentricity cycles. I refer you to 1991 Quaternary Science Reviews, 10: 297-397; and to Science, 23/8/02, pp1287-1288.

If you find that any of my statements are not upheld by this rather basic material, L'ld be very pleased to review and discuss.

But I would have preferred to have had the chance to do so before a summary withdrawal of my suggestions with no notice! Pfvlloyd 22:25, 5 August 2007 (UTC)

I left the note here and at your talk page while you were editing, so you did have a chance to respond. But we can put the changes back if we can agree. However, right now I don't agree. Neither B&L 1991 or 2002 address most of the changes you've made. In particular, the problems section deals with differences between the climate record and the orbital theory. B&L 1991 shows the orbital variations continue over millions of years, but the climate record shows deviations from those expectations (e.g. the transition problem). Similarly the "unsplit peak" and "stage 11 problem" are issues with the climate record that can't be explained with just a study of orbital structure. The 2002 is closer to those topics but focuses on the current interglacial and says little about the timing of the stage-11 deglaciation occurring despite low eccentricity.

You also said orbital insolation is reduced 81% during high eccentricity, but that's simply not right. You are comparing aphelion to perihelion at high eccentricity, but you ought to compare aphelion at high eccentricity to aphelion at low eccentricity. And that's without considering that at high latitudes, precession and obliquity have a larger variation on total insolation during a particular season (e.g. during summer) than the effect of eccentricity does. You also say that eccentricity uniquely varies the total solar heating while precession and obliquity only redistribute it, but that's not entirely true either. It's true that eccentricity is the only term that varies that annual integrated power received at Earth's position, but the absorbed radiation also depends on albedo. Since albedo varies significantly with spatial position on Earth, redistribution also alters the total absorption. So in practice the total energy actually absorbed by the Earth varies with all three components.

Also, you made several minor errors, such as labelling eccentricity of 1.07 as 0.07%, etc. If it is a percent it is 7%, but eccentricity is generally written as a number not a percentage. My biggest problem however, is generally your purely justified removal of large parts of the problems section. In my opinion, those issues are generally still unresolved. Dragons flight 23:13, 5 August 2007 (UTC)

Reply -

I didn't see your note before I logged off - it was getting late here. But it was still a rather brief time allowed before posting that I had not responded to requests for an explanation. However, that's water under the bridge.

Thanks for your comments on my edits. OK, I see where you have problems with them. I think I can offer you reasonable arguments for most of your comments, but some are just plain differences of interpretation which we may never agree on, but that's science!! And I found some of the text very confused, so obviously so that I did not think an explanation for an edit was necessary in those cases.

However, I have deadlines to meet just now and cannot reply properly. Give me a day or two and I will respond properly. I really look forward to discussing these issues with you - an excellent opportunity to improve insight.

Regards.Pfvlloyd 15:22, 6 August 2007 (UTC)

In addition to Dragons flight's questions about interpretation of the sources (which I can't comment on because I haven't read those sources), I found the writing in Pfvlloyd's version harder to read than the pre-Pfvlloyd-edit (or post-Dragons flight-revert) version. In particular, the lead section was bulked-up considerably, with a lot of numbers that might trip up readers looking for a quick overview -- a problem that remains to a degree in the pre-edit, post-revert version. Correctness is more important than readability, but clear writing also matters. (Apologies if this sounds like I'm picking on your writing. It's possible that my impression of unclarity is partly because I looked at the diff more closely than the post-Pfvlloyd-edit version; if that's the case I apologize for mentioning anything more than the bulk in the lead section.) -- Steve Schonberger 16:14, 6 August 2007 (UTC)

[edit] Maximum and minimum latitudes for the arctic and antarctic circles?

The axial tilt changes slightly on a short period, less than a year, which moves the circles toward and away from the poles a bit. What would the maximum and minimum latitudes be over the 41,000 year cycle? Now for the math wizard question. What's the total surface area of the two belts the circles wander over? An important thing to remember as this relates to climate change is the very different topography of the north and south poles. The north is water surrounded mostly by land while the south is land surrounded by an unbroken ring of water. In winter, the area above the circle can get heat only via convection and a small amount that comes from the interior of Earth. The north gets heat both from water and air convection while most of the south gets heat only from air convection. the convection can't keep up with the radiant heat loss during the time the sun doesn't rise above the horizon. As the tilt decreases, there's less surface area with 24 hours or more constant darkness to radiate heat. Anyone who remembers the basic science classes they had in school knows that a planet with less axial tilt has an overall more temperate climate than a planet with a greater axial tilt, yet I've talked with several people who are convinced this should make the poles *cooler*, and thus "proof" of anthropogenic "global warming". —Preceding unsigned comment added by Bizzybody (talk • contribs) 13:30, 20 October 2007 (UTC)

[edit] Question regarding time-frames

I want to understand the basis for these theories. What is the basis being used for the timeframe estimates used in this article? How did they calculate the 41,000 years and the 100,000 years? Are those numbers calculated based on geological observations tied to assumptions about radiometric dating and stratigraphic principles? Thank you. 68.253.24.169 13:53, 29 October 2007 (UTC)

[edit] Re. "Question Regarding Time Frames"

It is inappropriate to insert personal queries into Wikipedia articles to get attention. It is a form of vandalism. I have removed the query from the article.

Re. "What is the basis being used for the timeframe estimates used in this article? How did they calculate the 41,000 years and the 100,000 years? re those numbers calculated based on geological observations tied to assumptions about radiometric dating and stratigraphic principles?":

I am not a geologist or paleoclimatologist, but since nobody with real credentials has answered the query I will say this: Data about climate variations in the range of tens to hundreds of thousands of years ago are available from a rich range of sources that do not depend on radiometric dating. The study of climate over this time scale depends heavily on ice cores and cores of ocean-floor sediments. These deposits record the yearly layering of snow and sediment from the present time back in unbroken series to, in the case of at least one Antarctic ice core, about 800,000 years ago. Each year adds a new, thin layer like the growth of a tree. Although it is my understanding that the precise dating of these layers is a complex professional matter, to a rough approximation, at least, one dates the layers simply by counting them, exactly like tree rings.

See the Wikipedia articles on Paleoclimatology and Ice Cores for more information. —Preceding unsigned comment added by 72.173.17.89 (talk) 18:41, 3 November 2007 (UTC)

[edit] Predicted temperature changes based on change in eccentricity, tilt, and wobble of the earth

In the October issue (2007) of National Geographic there is a poster called Greenhouse Earth. The bottom graphic line shows how the change in eccentricity, tilt, and wobble of the earth should have impacted global climate changed for the last 400,000 years. What is most cool about this is you can see changes in sea level, temperature, and co2 concentrations at the same time. The graph suggests that we should be in a cooling period for the last 100 thousand years....which is a concern because we are in a one of the most warm periods over the last 400,000 years. Is this correct. I didn't find any citations for this. It strange that the dotted line doesn't have a label. How solid is this evidence. Perhaps it belongs on this page. SoilMan2007 (talk) January 1, 2008

Not sure exactly which poster you mean. "the last 100 thousand years" must be wrong. About 12kyr ago we were leaving the last glacial, ie warming. For the last ~10kyr we've been in an interglacial, in which temperatures have been roughly constant. The future (in the absence of human forcing) prediction, is quite complex but probably not a full glacial for 50 kyr. See-also global cooling William M. Connolley (talk) 20:04, 1 January 2008 (UTC)

Hi William. Thanks for your response. I have a feeling that it is likely an error. According to the national geographic poster (October 2007 issue), they show a graph that indicates temperature changes that would be expected from changes in eccentricity, tilt, and wobble. That graph would indicates that about 120,000 years ago or so, there should have been cooling when in fact we didn't have much change in temperature till about 10,000 years ago. This would be very significant if it were true. But I think it has to be wrong. Take a look if you have a chance. Thanks again! SoilMan2007 (talk) (Tom) January 1, 2008. —Preceding comment was added at 23:32, 1 January 2008 (UTC)

Try Image:Vostok-ice-core-petit.png for one version of the temperature record William M. Connolley (talk) 22:08, 2 January 2008 (UTC)

I'm talking about what would be predicted based on those orbital factors rather what actually occurred. —Preceding unsigned comment added by SoilMan2007 (talk • contribs) 12:59, 3 January 2008 (UTC)

I doubt I can make sense of that without seeing the Nat Geog thing you're referring to. But T can certainly be "postdicted" from insolation, if you do enough fitting William M. Connolley (talk) 22:18, 3 January 2008 (UTC)

I think you are being overly generous William. With sufficient free parameters one can curve fit in an ad hoc manner, but such fits have little explanatory power. If instead you start with insolation and try to extract a temperature model, then I've yet to see a model that correctly captures the timing and intensity of the different ice ages without having to put those in by hand. Long time-scale modeling with dynamic ice sheets is still a very young field, and there are many unresolved problems there. I'm sure that whatever model National Geographic showed, it was an illustrative one rather than a definitive study of what solar forcing should produce. Simply put, we don't yet have a sufficient understanding of how solar forcing triggers the growth and decay of continental ice sheets to have a definitive set of predictions. Dragons flight (talk) 23:03, 3 January 2008 (UTC)

Aren't we pretty close with this challenge (and the 8 models?) [5]? --Kim D. Petersen (talk) 23:36, 3 January 2008 (UTC)

Those models are predictions of CO2 from measured temperature data. That is a radically different and much easier problem than predicting temperature from orbital dynamics data. Dragons flight (talk) 23:48, 3 January 2008 (UTC)

Yes - i was aware of that - but at least the Paillard model uses only insulation. It was with this in mind that i asked. --Kim D. Petersen (talk) 06:50, 4 January 2008 (UTC)

I agree that it was likely for illustrative purposes. The reason I was concerned was that the graphic implies that we should be in a cooling cycle for the last 100 thousand years rather than warming (warming seems to be happening). I would think that if we are so sure of the impacts of these factors, that scientists should be able to come up with a model to predict their effect. Anyway, thank you all for your responses. I'm really grateful. SoilMan2007 (talk) —Preceding comment was added at 02:48, 8 January 2008 (UTC)

[edit] Wikiproject Earth

Hello i have recently proposed the Wikiproject Earth. This Wikiproject`s scope includes this article. This wikiproject will overview the continents, oceans, atsmophere and global warming Please Voice your opinion by clicking anywhere on this comment except for my name. --Iwilleditu^{Talk :)Contributions} —Preceding comment was added at 15:35, 30 March 2008 (UTC)

[edit] Axial tilt angle

The opening paragraph of the article states that the axial tilt varies between 21.5° and 24.5°. However, the section "Axial Tilt (Obliquity)" states that it varies between 22.1° and 24.5°. The article Axial tilt seems to agree with the latter range although it mentions the 21.5° low value in section "Long period variations", but also questions it as a possible one-time overshoot. I just happened to read this article and am not an expert on the subject, so I don't feel comfortable fixing this disparity, but most likely in the same article we should either stick to same numbers or explain the differences. --Antti Salonen (talk) 18:35, 4 June 2008 (UTC)