Past Climate Change Cannot Be Tied to Earth Passing Through Galactic Plane
Written by Nancy Atkinson
Earth's climate has changed over time, but the cause for the changes has been hotly debated. One idea (Shaviv and Veizer,2003), suggested that perhaps two-thirds to three-fourths of the variance in Earth's temperature over the past 500 million years may be attributable to when our solar system passes through the spiral arms of the Milky Way galaxy. The evidence seemed to fit: there appears to be a 140 million year cycle of global climate change, and that correlates when our solar system seems to move between spiral arms, too. Or at least it used to. Since 2003 we have revised our map of the galaxy, which changes the estimation of when Earth transits through the spiral arms.
"Although previous work found a correlation between the 140 Myr climate cycle on Earth and the intersection with spiral arms," write researchers Adrian Melott, Andrew Overholt, and Martin Pohl, "with new data on the structure of the galaxy, this correlation disappears."
On Earth, the 140 million year cycle is estimated from the timing of ice ages and abundances of fossils.
The basic idea of the earlier research was that when the solar system journeys through the Milky Way's spiral arms the event rate of cosmic rays in the Earth’s atmosphere greatly increases, since the number of supernovae in spiral arms is clearly much larger than in between the arms. This could affect cloud formation on Earth and therefore strength of greenhouse effect.
But that assumed the Milky Way had four arms, and was less massive than new calculations show. In 2008, new information from the Spitzer Space Telescope helped astronomers conclude that the Milky Way consisted of two spiral arms and a large central bar. Additionally, in 2009 Spitzer data helped scientists conclude that our galaxy is much more massive than originally thought, and is moving faster than originally estimated.
So just when has Earth passed through the galactic arms? With changing estimations of mass and smaller number of arms, no one can be absolutely sure. But Melott and his team have compared the times of transit between regions of the new galactic map with changes in Earth's climate and found that the 140 million year correlations no longer apply.
The team also says the 140 million year cycle cannot be made to match up with any cyclical movement of the solar system through the galaxy.
"The only periodic trend that can be found with the new data is the relative orbital period of our solar system," the team writes in their paper, "relative to the previously assumed pattern speed around the galactic plane, which is slightly larger than 500 Myr. Though one could create varying periodic trends by changing this pattern speed, the orbital period relative to the galactic pattern could never reach the 140 Myr time as this is less than the orbital period itself, meaning the pattern and the Sun would be required to move in opposite directions."
So, the researchers conclude, the solar system passing through the plane of the galactic arms could have no direct tie with past climate change on Earth.
The team's paper can be read here.
Graphic caption: Red vertical lines represent the midpoints of the last seven ice ages, which don't correlate with the passage of the solar system through the galactic plane. Credit Melott, Overholt and Pohl.
Source: arXiv, Technology Review Blog
Filed under: milky way
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June 26th, 2009 at 12:55 pm
The underlying problem for such models seems to be that they for no adequately substantiated reason try to fit contingent processes to some periodic function. That periodicity goes away with better data, as for example when more fossils AFAIU removed the evidence for periodic extinctions seen in earlier records.
I'm reminded of statistician Cosma Shalizi IIRC complaint on the popularity to fit phenomena to power distributions, when AFAIU simply fitting can't distinguish between these and exponential distributions. The eagerness to see patterns _after data analysis_ that simply aren't there is astounding.
June 26th, 2009 at 6:20 pm
I'm confused.
Surely "passing through galactic plane" and "passing between spiral arms" are completely different things, yet this article seems to refer to them interchangeably.
Given that the number of spiral arms is cited as relevant, it appears to me that this article is really about the cycle of "passing between spiral arms", and that the conflation between that and "passing through the plane" is an error.
(Also note that according to Phil Plait in DFTS page 240, the "passing through galactic plane" cycle is 64 million years.)
Could someone clarify these points, please?
June 27th, 2009 at 4:52 am
Someone pls clarify whether the motion of solar system around the AGN is highly non homogeneous; passing above and below the galactic plane, switching between spiral arms and experiencing turbulance and probably all of them happen in a complex and non homogeneous way.
Now the bigger question is how much would it affect our observations on rest of the universe? If it is braking and accelrating, the rest of the universe would appear to red shift of blue shift accordingly.
As shown in the graph, we are close to a transition and will that have an influence?
June 29th, 2009 at 9:07 pm
@ Adrian Morgan : I'm not sure that I understand this well enough myself I'm afraid. But I'll try.
I think the galactic midplane is everything in our Galaxy on a line that extends from the "equator" of the Galactic bulge" and includes both the spiral arms (the dense regions of stars and gas clouds) and the interspiralarm medium where our Sunis now. (Less densely packed, thinner dustand gas clouds, generally older & less massive stars.)
Our Sun "bobs" up & down across the galactic midplane ("Galactic equator") and also encounters the denser spiral arm regions as it orbits the Glactic Centre … But I too am somewhat confusedover exactlywhich phenomena theyare referring to here.
See also on the BA blog post on this.
Anyone else?
June 29th, 2009 at 9:16 pm
@ Essel – the Sun is an older star which is part of the "thick disk population" as opposed to the "thin disk" (staying at the core of the galactic equator, younger stars, some much more massive than our Sun) and "halo" (very old stars generally less massive in highly ecentric orbits taking them foar formteh spiral arms and galactic plane /equator) As I understand it our Sun's orbit does bob up & down and also pass through denser spiral arm regions -but this take sso long and is so slow thast it would have little to no effect on cosmological observations.
In other words yes – but -no – the Sun does move like you suggest but no this is too slow and too small an effect to make any difference.
I think! (Could be wrong.)
June 29th, 2009 at 9:18 pm
"taking them foar formteh spiral arms "
Aaaaaarrgh! That *should* read :
taking them far from the spiral arms.
Sigh. (We can't edit these comments? Drat. That's a pain & out of date.)
July 8th, 2009 at 1:11 am
Would be really surprising, if the cause of ice age phenomena was found within the solar system only, or even just some relationships on Earth.
Why not take an opposite approach, and try to work out, what has happened in the universe at same intervals as the ice ages? As long as no correlation is found, there obviously is some data missing.