Earth’s magnetic field helps protect life from energetic particles that might arrive from space. Mars now lacks a strong magnetic field, and conditions on its surface are considered so devastating to life that any microbes that might inhabit the planet are thought to be safely below the surface. On Earth, the magnetic field ensures that life thrives on the surface.
However, this is not always true. The Earth’s magnetic field varies, the poles move and sometimes they alternate places and the field sometimes weakens or effectively fades. However, a look at these events revealed nothing particularly interesting – no clear links with the extinctions, nor major environmental disruptions.
A paper published yesterday in the journal Science provides an impressively accurate history of inverting an earlier magnetic field using rings from trees that have died for tens of thousands of years. It appears that the upturn is linked to changes in climate. But the paper then proceeds to try to relate the face to everything from a minor extinction event to an explosion of cave art by our ancestors. In the end, the work is a mixture of established science, provocative hypotheses, and unfettered speculation.
Old trees, but how old are they?
We will start with the solid science to which it all belongs Kauri trees, One of the distinctive species native to New Zealand. These trees are very large and long-lived, and regularly reach more than 1,000 years of age. The tree’s wood often remains buried in swamps, where some specimens are tens of thousands of years old.
The team behind the new work is based on the discovery of kauri wood dating back to the time of the Laschamps Expedition, a period when magnetic poles briefly swapped places about 40,000 years ago. Old trees tell many tales. The carbon 14 they enter can provide fairly accurate dates to the sample, and then individual tree rings allow inferring conditions present in individual years. Studies of other isotopes found in wood can provide rough estimates for everything from solar activity to precipitation patterns.
The team behind the new work found that the dating put in some of the material they had at the time of the Laschamps Journey. There was an increase in the amount of excess carbon 14 in the deposited tree rings at that time, consistent with more particles getting to the ground due to the lower magnetic field strength. This would usually be enough to quit dating, and it limited our ability to pinpoint accurate Laschamps flight dates with past samples.
But the captured details in the tree rings allowed the research team to correlate their data with data from other sources that have accurate dates associated with them. These include an annual sediment that was made in a cave, which contained carbon-14 records and dates provided by the thorium isotope. Researchers can arrange the data more precisely with basic ice records as well, which also capture information from the time of Laschamps’ flight.
Once these records were combined, it provided accurate timing of magnetic field reversal, as well as information on magnetic field strength during that time. The combined record also provides some information about the prevailing climate and details on things like precipitation and solar activity.
Not stuck in reverse
The record indicates that the magnetic field began to decline at 42,350 years ago and reached its lowest level in 41,800 years, 300 years before the actual reversal of the pole. Hence, the weak magnetic field at that time was more a harbinger of a reversal than an electrode exchange effect. Because of the timing, which was centered around 42 kilometers, the researchers decided to name the Adams Transitional Geomagnetic Event, after the author Douglas Adams.
The data alignment also indicated that Earth wasn’t the only thing doing something unusual at the time. The isotope beryllium-10 is mostly made of cosmic ray particles that affect the atmosphere, so they are an indicator of solar activity. This is because the sun’s magnetic field is related to its level of activity, and this magnetic field could deflect incoming particles that would otherwise travel to the solar system and possibly affect the Earth.
So two independent events have been working to allow more high-energy particles to reach Earth’s atmosphere. Using an atmospheric chemistry model, the researchers found that these particles could generate chemicals that destroy ozone. According to NASA’s Gavin Schmidt, the ozone losses not big Like the one that created the current ozone hole, though it is expected to be distributed somewhat differently, both geographically and seasonally.
Ozone loss leads to a range of relatively subtle climate impacts, altering the Arctic jet stream and Southern Hemisphere precipitation patterns. These are the results of a limited number of cycles from a single chemical-climate model, so the researchers themselves acknowledge that the impact of ozone loss really needs to be studied with additional models to see how strong these effects are.
However, using the carbon-14 signature associated with an Adams event, the researchers identified equivalent time periods in some of the sediment records. Both indicated changes in the atmospheric circulation patterns that occurred during the event, which are consistent with the effect on climate.
Time to speculate
In general, the new accurate timing should be very beneficial for any situations involving a carbon-14 preservation sample and dates from around this period. In this respect, the work provides service to the field. The potential link with climate and the arrival of more high-energy particles is an interesting hypothesis, and it differs from previous attempts to link solar activity with climate change. It’s an idea that seems worth pursuing.
But for most of the remaining leaves, researchers are looking for anything that happened roughly 42,000 years ago and attempting to relate it to a mix of changing environmental conditions that they believe were caused by the Adams event. This includes cooler conditions prevailing in the northern hemisphere, as evidenced by glacial expansions. With the exception of changes in the magnetic field, changes in the magnetic field last only a few hundred years, while a cooler climate lasts for thousands of years. So they had to suggest that Adams’ event pushed the climate beyond a tipping point, allowing him to maintain his altered state in the absence of the original trigger. Plus, there are some climate records that show very little change in the time of an Adams event.
Australia experienced a massive extinction of megafauna that peaked around 42,000 years ago; This indicates a link to the variable precipitation that the Adams event appears to have caused in the Southern Hemisphere. It’s an interesting idea, although extinction events like this usually extend for some time before and after the peak.
Other potential connections become very weak. Modern humans, despite having been in Central Asia for tens of thousands of years, appear to have appeared in Europe around the time of the Adams event, and Neanderthals became extinct shortly thereafter. While it is reasonable to suspect that these last two events are related, it is not clear why any of them are related to magnetic field flipping and any effect on climate.
This period also witnesses a growth in the extent and development of cave art by these modern humans. Once again, researchers are trying to link this growth to the Adams event. More humans must be in the caves to escape the harsh radiation environment! And they were using red ocher as a sunblock because of that, so they had an art substance with them!
The truth is that humans (and Neanderthals) have been using red ocher for technical reasons for tens of thousands of years at that point – and have been inhabiting caves for a long time. There may have been a difference in degree about 42,000 years ago, but it wasn’t immediate.
Both climatologists and anthropologists have expressed many doubts about these claims thus far, although a number of them have found the individual claims interesting and worth pursuing. The real test of some of these insights will come when the researchers use the carbon-14 signature described by the paper to search for other samples that record environmental changes, such as in sediment cores, from the same time period. This will give us a clearer picture of whether the events that occurred around the same time really represent the kinds of global changes that are being proposed.
Other insights likely remain outside our ability to create clean tests. It’s not clear how we ever learned the ratio of sunscreen use to art of red ocher use by ancient inhabitants or whether more people were in the caves because they somehow felt the atmosphere was becoming dangerous. So it appears that the researchers were broadcasting some provocative ideas that would not clearly affect the field.
One obvious way to continue working is to look closely at other magnetic field reflections; The paper specifically mentions one that occurred 35,000 years ago. But when the researchers simulated the reversal of a magnetic field without much of a decrease in solar activity, nothing happened. It really looks like we’re going to need both to see the far-reaching effects that the researchers suggest. Given that the chance of both things happening at the same time seems remote, it is not clear how much the other examples might tell us.