The Earth’s climate is believed to be affected, not just by people driving Humvees, but by a series of complex cycles, which can occur over a few years (such as El Niño) or even tens of thousands of years (changes in the Earth’s axis of rotation).
But another cycle which may have an effect lasts tens of millions of years and is literally galactic in origin. The entire solar system is believed to ‘bob’ up-and-down relative to the centre of the galaxy, and a team of scientists from Israel and Canada has conducted a study to see if there is link between these cycles and the Earth’s climate.
So, what’s the point?
The Milky Way is a barred spiral galaxy, with our solar system positioned on the inside of one of the arms, some 27,000 light years from the centre.
As the galaxy rotates, the solar system vertically oscillates relative to the centre, which is densely packed with stars. A full up-and-down oscillation is estimated to take between 60 and 84 million years (so galactic central is crossed every 30-42 million years).
The idea that this oscillation might affect the Earth is not as crazy as it sounds: The authors state that links have previously been made between the Earth’s position, relative to the galactic centre, and its climate.
The precise reason for this is unknown, but they suggest 3 possibilities: The solar system’s position could influence the frequency of comet impacts on Earth, collision with interstellar dust clouds and the radiation received from cosmic rays.
So while the exact mechanism is unclear, it may be possible to use information about how the Earth’s climate has changed over hundreds of millions of years to better understand the solar system’s vertical oscillation.
This could help in a number of areas of astrophysics, including with estimations of the galactic disc and dark matter.
What did they do?
The researchers complied a library of geological data on δ18O (a rare isotope of oxygen). When marine organisms make shells, the ratio of δ18O to common oxygen (16O) that they take up is affected by the surrounding water temperature.
So by looking at loads of shells from the same time period, and correcting for latitude and depth at which the organisms lived, the average temperature of the sea for that time can be estimated. This means that the researchers were able to estimate how the temperature has changed over the past 488 million years (the earliest shellfish are thought to have evolved around 570 million years ago).
In total, the study looked at around 24,000 δ18O values. The researchers fitted this data to a sine curve, to estimate the period of the δ18O, and therefore temperature oscillation over time.
Did they prove anything?
The scientists reckoned that the data best fit to an oscillation with a ‘stable cycle’ of 32 million years. This fits well to previous estimates of the solar system’s vertical oscillation (30-42 million year half-period).
Assuming that this provides an accurate measurement for the half-period of oscillation of the solar system, they were able to estimate the density of the galactic disk (which determines the oscillation characteristics.
Their estimate was about 70% larger than can be explained by known stars alone, and they suggest that the galactic disk may therefore contain a ‘dark matter component’.
So, what does it mean?
While the data do appear to oscillate, there is obviously a significant error in the fitting (which should not be unexpected). This could occur due to local events (e.g. major volcanic eruptions) or processes which occur over different time spans (e.g. ice ages).
The scientists warn that while the cycle time fits well with that estimated for solar system oscillation, ‘terrestrial processes’ such as mantle convection could also cycle at similar timescales, so a definite link between the apparent temperature oscillation and solar system vertical oscillation cannot be confirmed.
Essentially, we still can’t tell for sure, but it is interesting to consider that climate could be affected by factors beyond that of the Earth and Sun.
It also shows that simple long-dead life-forms on this pokey little rock in a galactic suburb might help us to explain some of the mysteries of the galaxy and the universe itself.
Original article in Scientific Reports Aug 2014
All images are open-source/Creative Commons licence.Credit: NASA (via gnews pics) (First); RJHall (Second); A Z Colvin (Third); opencage.info (Fourth); N J Shaviv et al. (Fifth); USGS/Hawaiian Volcano Observatory (Sixth)
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Shaviv, N., Prokoph, A., & Veizer, J. (2014). Is the Solar System’s Galactic Motion Imprinted in the Phanerozoic Climate? Scientific Reports, 4 DOI: 10.1038/srep06150