This is one of those science stories that is on one hand fairly simple, and on the other hand fairly complex, where the interface between simplicity and complexity causes little balls of misunderstanding to come flying out of the mix like pieces of raw pizza dough if the guy making the pizza was the Tasmanian Devil from the cartoons.
What is true: A scientist named Ryskin proposes that decadal or century scale minor wiggling in the measured Earth’s magnetic field is influenced by changes in ocean currents. Plausible. Interesting. Could explain some things. Not earthshaking.
What is not true: The earth’s magnetic field is caused by ocean currents. The earth’s magnetic field’s long term variations, like reversals in field orientation, are caused by ocean current changes. The Earth’s magnetic field causes oceanic current changes or the currents are the sole cause of secular variation. The cause of the earth’s magnetic field is not, as previously thought, the molten dynamo thingie inside the earth.
Let me explain.
The Earth has a polarized magnetic field, with one pole (which we call “North”) located …. up north … between Canada and Russia, and another pole (which we call “South”) located down south in Antarctica. The magnetic field is caused by something.
Every now and then, over long term historical time at scales of hundreds of thousands of years or more, the orientation of the Earth’s magnetic field reverses. That reversal may or may not be related to the present article, but if it is, the relationship is not proposed by the author or anyone else. So let’s forget about reversals.
Over shorter time scales, there is this phenomenon called “secular variation.” The word “secular” comes from the same root as the French “siecle” (century, or age or period) and refers to “short term” variation. Centuries are short from the perspective of geologists.
How thick is a layer of sand laid down by the spring floods each year in a delta? How thick is the layer of mud laid down between winter die offs in a cold deep Pleistocene lake? How far does an active fault creep each year over the three or four thousand years during which it moves?
Who cares? We add up all those numbers and average them out, or we don’t even measure them; We simply take the large scale phenomenon (like meters of mud or sand) and that tells us what we really want to know about the geological phenomena under study. Right? Oh, what’s this you say? You are actually interested in the small scale variation for some reason? Well, then, you are studying secular variation.
Archaeologists are interested in secular variation in magnetic fields because, if you know within a couple of hundred thousand years or less what a site dates to, and you have a few or more geomagnetic samples from the layers in which your site exists, you can guess where in time the site dates to by examining the exact orientation — not just ‘reversed’ and ‘unreversed’1 state — of the magnetic field.
Geomagnetic scientists believe that the earth’s magnetic field is caused by a dynamo that functions in the earth’s core, a dynamo consisting of one or more giant masses of iron spinning (as is the whole earth) around. Changes in the earth’s magnetic field, including both secular variation and reversals, have typically been explained in relation to this dynamo. However, it is true that these explanations have been weaker than one would like, and it is true that over the last several centuries of consideration of this matter, no explanation for secular variation has ever stuck as well as one might like.
So, it has previously been assumed that secular variation in the magnetic field, which constitutes a trend of a westward drift of the orientation of the poles, is caused by details of this dynamo, but this idea has not been nailed down. Also, as mentioned by the author, some of our key ideas about how this dynamo works depend on the assumption that secular variation is caused by stuff happening with the dynamo (or its subterranean context) itself.
The new paper, by Ryskin, takes a slightly approach, comes up with an interesting model which may be plausible, demonstrates empirically that there may be something new to the model, then somewhat overstates it’s significance in a way clearly designed to annoy most geomag experts. So yes, a study on perturbation in the Earth’s magnetic field is sure to perturb, academically.
Here is the theory in the author’s own words, excerpted from the abstract, introduction, and conclusions. (Believe me, you don’t want to see most of the middle of this paper. Unless you like watching them make sausage.):
…I propose a different mechanism of secular variation: …the magnetic field induced by the ocean as it flows through the Earth’s main field … The predicted secular variation [based on my model] is in rough agreement with that observed. Additional support is provided by the striking … correlation … between the intensity of the North Atlantic oceanic circulation and the rate of secular variation in Western Europe; this explains… the recently discovered correlation between secular variation and climate. Spatial correlation between ocean currents and secular variation is also strong.
Owing to secular variation, the International Geomagnetic Reference Field (IGRF) must be revised every 5 years … Since secular variation is believed to originate in the Earth’s core… theoretical studies of secular variation have been limited to solving the inverse problem: find the flow at the core-mantle boundary that would produce the observed secular variation … The results of these studies cannot be compared with observations: seismic data indicate that the Earth’s outer core is fluid, but flow in the core cannot be measured or observed.
I propose a different mechanism of secular variation: ocean water being a conductor of electricity, the magnetic field induced by the ocean as it flows through the Earth’s main field may … manifest itself globally as secular variation.
… The ocean-induced magnetic field certainly exists, and may depend on time, including, possibly, on annual and longer timescales. … The crucial question is whether the ocean-induced field may vary on secular variation timescales. The bulk of [this paper] deals with this issue. Fortunately, one is on a firm ground here: the analysis is based on magnetohydrodynamics. The difficulties, though considerable, are largely mathematical; no hypotheses need to be invoked or introduced. The answer is in the affirmative
…the only remaining question is whether the ocean-induced secular variation is a part or the whole of the observed secular variation. … while the comparison [presented here] does suggest that secular variation in its entirety is induced by the ocean flow, this by no means constitutes a proof of the proposal. In fact, a definitive proof may never be possible, but as the accuracy and completeness of the data continue to improve, and further computations are carried out, sufficient clarity on this issue should be achieved …
The results presented suggest that the observed secular variation of the Earth’s magnetic field owes its origin to the ocean flow. … Data analysis exhibits striking temporal correlation between the intensity of the North Atlantic oceanic circulation and secular variation in Western Europe…
There is little doubt that these conclusions will be met with skepticism. And so they should: the results presented by no means constitute a proof. But the possibility of direct connection between the ocean flow and the secular variation of the geomagnetic field is bound to stimulate further research, especially in view of the implications for the question of the origin of the main field.
The current consensus is that the main field is generated by the hydromagnetic dynamo in the Earth’s fluid outer core. Secular variation has been taken as evidence of motion in the core since the time of Halley (1692). Halley thought that secular variation, the westward drift in particular, was caused by differential rotation of a magnetized solid core, separated from the `external parts of the Globe’ (also magnetized) by a `fluid medium’. Contemporary theoretical studies use the westward drift to estimate the characteristic large-scale velocity in the outer core, and to conclude that dynamo action is possible. If secular variation is caused by the ocean flow, the entire concept of the dynamo operating in the Earth’s core is called into question: there exists no other evidence of hydrodynamic flow in the core.
With respect to the relationship between climate variation and magnetic variation, the author offers a weak but intriguing empirical data set. Have a look at this graph:
Figure 1. Does ocean flow cause the geomagnetic jerks? Comparison of oceanographic and geomagnetic data shows that the trend in secular variation is closely correlated with the trend in the ocean-flow intensity. Points–the oceanic transport index, a measure of intensity of the North Atlantic gyre circulation (Curry and McCartney 2001; data absent in some years, especially between 1979 and 1984). Lines–secular variation of the geomagnetic field (differences between successive annual means, east component) at three observatories in Western Europe: solid line–Eskdalemuir, dotted line–Niemegk, dashed line–Chambon la Foret (World Data Centre for Geomagnetism 2007). It is seen that the hitherto unexplained geomagnetic jerks of 1969, 1978, 1991 and 1998 (De Michelis and Tozzi 2005) are correlated with sharp changes in the trend of the ocean-flow intensity. See section 8 for details.
The dots are the climate data, and the squiggles are magnetic variations. We are not impressed. But we also cannot reject this out of hand. What is interesting here is this: Every few years, almost like clockwork, somebody comes up with a correlation between either sunspots and climate change or magnetic variations and climate change. The fact that these two attempts at explanation come up all the time may be little more relevant than as an argument that untrained people should not be let near real data and statistical analysis tools. But it could also be that there are vague enough links among these phenomena that they will continue to peek out from the data, and failure to confirm or replicate preliminary conclusions is due to a lack of proper causal paradigm than anything else. The present paper asserts that the climate change causes the magnetic change, which is the REVERSE of all prior work of which I’m aware. (Here, “climate change” = oceanic circulation.) Having the causal arrows backwards or otherwise mixed up can certainly slow down research.
The other major conclusion, referred to in the extended quote above, is that since the dynamo model is based in part on the old and now in question secular variation model, geomag experts have to go back to square one, or at least, square two or three, and rethink what they’ve got.
At the beginning of this post, I noted that there was a certain amount of ‘getting it all wrong’ out there. The author’s final statement about our need to rethink the dynamo has certainly contributed to this, but inappropriately so. This paper does not suggest that the dynamo is not there or that the dynamo does not create the Earth’s magnetic field.
Nonetheless, the summary overview from the same journal in which the paper is published makes this statement:
400 years of discussion and we’re still not sure what creates the Earth’s magnetic field, and thus the magnetosphere, despite the importance of the latter as the only buffer between us and deadly solar wind of charged particles (made up of electrons and protons). New research raises question marks about the forces behind the magnetic field and the structure of Earth itself.
This statement has all the hallmarks of a typical bad press release: Alarmism, fear, hints that everything we know is wrong, and the suggestion of an impending paradigm shift.
This may be in part what lead the contributor on Slash Dot who tagged this story to totally screw the pooch, in cooperation with the Slash Dot editors:
Ocean Currents Proposed As Cause of Magnetic Field
…a recently published paper proposing that ocean currents could account for Earth’s magnetic field. …
…This reader adds, “The currently predominant theory is that the cause of Earth’s magnetic field is molten iron flowing in the outer core. There is at present no direct evidence for either theory.”…
Uffda. Shame on you Slash Dot.
1We call the unreversed state of the magnetic field “normal.” Typical chrononormative colonialist scientist jargon.
Ryskin, G. (2009). Secular variation of the Earth’s magnetic field: induced by the ocean flow? New Journal of Physics, 11 (6) DOI: 10.1088/1367-2630/11/6/063015