Richard Alley states in 'Two-Mile Time Machine' that the sharp weakening of the Earth's magnetic field evidenced by the spike in increased 10Be in ice and sediment cores is not relevant to the climate story because there is no corresponding climate change. I googled a few exceptions to his apparent claim:

http://gsa.confex.com/gsa/2001SE/fin...tract_4731.htm
http://tinyurl.com/40-000-B-P
http://tinyurl.com/41-000B-P

Was there abrupt climate change or not?

Do any members have a tentative explanation for the reduction in the Earth's magnetic field around 40,000 yrs ago?

I'm surprised Ophiolite doesn't have an answer.

emm I need more proof that earth magnetic field supposevely abruptly changed during those times

Richard Alley is a leading expert in his field and wouldn't have made a mistake regarding the observation of the spike. I'd be pleased if anyone could locate the original beryllium-10 data. I've got it for Antarctica but not for Greenland.

The Earth's magnetic field is variable. It frequently (on a geological time scale) undergoes variation of strength, polarity and overall expression. I see nothing remarkable about an apparent fluctuation at any particular point in the Earth's history. One of the delights of science is that we are steadily filling in the gaps in our picture of 'what happened when'.

Your first link provides evidence for a local decrease in temperatrure around 40,000 years B.P. (41,700 +/-).
The third link provides evidence for a local increase in temperature between 39,000 and 43,500 yr B.P.
The second link does not appear to cover the time period you are interested in. If you are cherry picking you are not doing it terribly well.

What is your point?

The problem is however that interpretation of data is a choice that may be wrong.

Problem also with abstracts only, is that we cannot analyze the exact method. Furthermore carbon dating past about 22,000 years is not robust enough to compare with counted records.

The second link is related to the time period in question, and lends support to the other two links that abrupt climate change did indeed happen around 40,000 B.P. The oxygen-18 isotope data doesn't show a recognisable change, but this would only show climate change of a certain magnitude and duration. The change in the Earth's magnetic field indicated by the spike does therefore appear to corrrespond with a short lived abrupt climate change.

The point is that geomagnetic excursions could well be significant in the climate story, and shouldn't be dismissed so readily. See the Wiki explanation http://en.wikipedia.org/wiki/Geomagnetic_excursion:

"A minority opinion, held by such figures as Richard A. Muller, is that geomagnetic excursions are not a spontaneous processes but rather triggered by external events which directly disrupt the flow in the Earth's core. Such processes may include the arrival of continental slabs carried down into the mantle by the action of plate tectonics at subduction zones, the initiation of new mantle plumes from the core-mantle boundary, and possibly mantle-core shear forces and displacements resulting from very large impact events. Supporters of this theory hold that any of these events lead to a large scale disruption of the dynamo, effectively turning off the geomagnetic field for a period of time necessary for it to recover."

No. You are misreading the text.
1. Quote me where the second link makes reference to climate changes within the time period you are addressing.
2. The other two links do not support an abrupt change and indeed refer to changes that are opposites - one cooling, the other warming.

Your quote about the influence of plate tectonics on the Earth's magnetic field has zero relevance to your claim that the magnetic field could effect the climate. Why are you even quoting it? No one is denying that the magnetic field varies. No one is denying that we are not fully certain of why it varies as it does. The quotation is irrelevant.

I have a working hypothesis that the Earth's inner core is affected by external influences which alters the magnetic field. You may have remembered my idea of 'extra gravitational pull' on the (exotic) inner core from the Astronomy section. You weren't very receptive to a new idea then, and I can sense that your opinions haven't changed. I bet you can even remember my reason for the 40ka reduction in the magnetic field. Time will tell whether I'm right or wrong.

I shouldn't be at all surprised if this was true. I shouldn't be at all surprised if it wasn't.
I would be amazed if you could explain to me why you would be prattiling on about external influences on the magnetic field, in a thread that purports to be about how the magnetic field effects the external. Will you at least try to explain?

I've tried enough. I'm sending my hypothesis to somebody working more closely in the field. No offence, but it would too difficult to explain to members of a forum.

So, based on your logic, if you are a member of a forum you must be a bit thick.
However you are a member of a forum, this one to be precise.

How do you reconcile this logic?

Entropy,
common_sense_seeker's statement - this would be too difficult to explain to members of a forum - is a reflection on his own poor communication skills, not upon the intellect of the forum members. It is ironic that common_sense_seeker does not understand this.

I endeavour, as always, to assist people in constructing logically structured thoughts and sentences.
Even if, as is sadly all too often the case, it is futile.

There was abrupt climate change at 41,000 B.P. It was a date when the North American ice sheet suddenly went into a sharp retreat. There is a very clear graph in J.Imbrie's 'Ice Ages: Solving The Mystery'. The other sharp retreat was at 60,000 B.P and then the Milankovich predicted ending of the ice age , starting around 18,000 B.P and then retreating at a slower rate with warming and cooling cycles.

Magnetic field is a human concept quantitatively describing something that humans don't really understand. Magnetic field doesn't exist outside of (some) human brains

Citation? Hmm, it's pretty much obvious - humans don't understand action at the distance, learned humans use "magnetic", "electric", "gravitational", etc. fields as a bypass around understanding enabling measurements, practical applications and even some theoretical futility.

Read Feynman's lectures on Physics, even better - listen mp3s of the lectures given by Feynman (he did not actually write the books himself). He said something like this, "During the Middle Ages, educated people thought that angels push Earth around Sun by flapping their wings. Today we explain Earth movement by action of the gravitational fields. Both explanations are equivalent, it just that angel's wings have been renamed into gravitational field."

once you get into quantum physics, the "wings" cease to exist