The Earth's magnetic field helps protect life from energetic particles that would otherwise arrive from space. Mars now lacks a strong magnetic field, and the conditions on its surface are considered so damaging to life that any microbes that might inhabit the planet are thought to be safely beneath the surface. On Earth, the magnetic field ensures that life can flourish on the surface.
Except that's not always true. The Earth's magnetic field varies, with the poles moving and sometimes swapping places and the field sometimes weakening or effectively vanishing. Yet a look at these events has revealed nothing especially interesting—no obvious connections to extinctions, no major ecological upsets.
A paper published yesterday in Science provides an impressively precise dating for a past magnetic field flip by using rings of trees that have been dead for tens of thousands of years. And it shows the flip was associated with changes in climate. But the paper then goes on to attempt to tie the flip to everything from a minor extinction event to the explosion of cave art by our ancestors. In the end, the work is a mix of solid science, provocative hypothesizing, and unconstrained speculation.
Old trees, but how old?
We'll start with the solid science, which all comes back to kauri trees, one of the distinctive species native to New Zealand. These trees are quite large and long-lived, regularly reaching over 1,000 years of age. And the wood from the tree often survives being buried in marshes, with some samples being tens of thousands of years old.
The team behind the new work relies on the discovery of kauri wood that dates back to the time of the Laschamps Excursion, a period when the magnetic poles briefly swapped places roughly 40,000 years ago. Old trees tell a lot of tales. The Carbon-14 they incorporate can provide fairly precise dates for the sample, and the individual tree rings then allow the conditions present in individual years to be inferred. Studies of other isotopes found in the wood can provide rough estimates of everything from solar activity to rainfall patterns.
The team behind the new work found that dating placed some of their material at the time of the Laschamps Excursion. And there was a spike of excess carbon-14 in the tree rings deposited at the time, consistent with more particles reaching Earth due to the drop in magnetic field strength. This would normally be enough to throw off the dating, and it has limited our ability to place precise dates on the Laschamps Excursion with prior samples.
But the details that were captured in the tree rings allowed the research team to line its data up with data from other sources that did have precise dates attached to them. These include annual deposits made in a cave, which had both carbon-14 records and dates provided by an isotope of thorium. The researchers could line the data up more precisely with ice core records, too, which also capture information from the time of the Laschamps Excursion.
Once combined, these records provided precise timing on the magnetic field reversal, as well as the information on the magnetic field strength during the time. The combined record also provides some information about the prevailing climate and details on things like rainfall and solar activity.
Not stuck in reverse
The record suggested that the magnetic field began to drop at 42,350 years ago and reached its lowest level 41,800 years ago, which is 300 years prior to the actual pole flip. Thus, the weakened magnetic field of the time was more a precursor to the flip than an impact of the poles swapping places. Because of the timing, centered on 42 kiloyears, the researchers decided to name this the Adams Transitional Geomagnetic Event, after author Douglas Adams.
The alignment of data also indicates that the Earth wasn't the only thing doing something unusual at the time. The isotope beryllium-10 is mostly formed by cosmic ray particles impacting the atmosphere, so it serves as an indication of solar activity. That's because the Sun's magnetic field correlates with its activity level, and that magnetic field can deflect incoming particles that would otherwise travel into the Solar System and potentially impact Earth. Levels of the isotope in ice cores indicate the Sun went into a long quiet period that overlapped with the Adams Event.
So there were two independent events that would both act to let more high-energy particles reach the Earth's atmosphere. Using a model of atmospheric chemistry, the researchers found that these particles would generate chemicals that destroy ozone. According to NASA's Gavin Schmidt, the ozone losses from this aren't as large as the ones that created our current ozone hole, though they would be expected to be distributed somewhat differently, both geographically and seasonally.
The loss of ozone creates a range of relatively subtle climate effects, altering the Arctic jet stream and precipitation patterns in the Southern Hemisphere. These are the results of a limited number of runs from a single coupled chemistry-climate model, so the researchers themselves acknowledge that the impact of ozone loss really needs to be studied with additional models to find out how robust these effects are.
Still, using the carbon-14 signature associated with the Adams Event, the researchers identified the equivalent time periods in some sediment records. Both indicated that there were changes in the atmospheric circulation patterns that occurred during the event, which is consistent with an impact on the climate.
Speculation time
Overall, the new precise timing should be very useful for any research that involves a sample that preserves carbon-14 and dates from around this period. In that regard, the work provides a service to the field. The prospect of a link to climate and the arrival of more high-energy particles is an intriguing hypothesis, and it's one that is distinct from previous attempts to link solar activity to climate change. It's an idea that seems worth following up on.
But for most of the rest of the paper, the researchers search for anything that happened at roughly 42,000 years ago and try to tie it to the mix of altered environmental conditions they think were triggered by the Adams Event. This includes colder conditions prevailing in the Northern Hemisphere, as evidenced by glacial expansions. The changes in the magnetic field, however, only last a few hundred years, while the colder climate persists for thousands of years. So they had to propose that the Adams Event pushed the climate past a tipping point, allowing it to maintain its altered state in the absence of the original trigger. Plus there are some climate records that show very little change at the time of the Adams Event.
Australia saw a major extinction of its megafauna that peaked at roughly 42,000 years ago; this is suggestive of a link to the altered rainfall that the Adams Event seems to have triggered in the Southern Hemisphere. It's an intriguing idea, although extinction events like this typically extend for quite some time before and after the peak.
Other potential connections are extremely tenuous. Modern humans, despite having been in Central Asia for tens of thousands of years, seemed to show up in Europe around the time of the Adams event, and Neanderthals went extinct shortly thereafter. While it's reasonable to suspect that those latter two events are linked, it's not clear why either would be associated with the magnetic field flip and any impact it had on climate.
The period also sees a growth in the extent and sophistication of cave art by those modern humans. Again, the researchers try to tie this growth to the Adams Event. More humans must have been in caves to escape the harsh radiation environment! And they were using red ochre as sunscreen because of it, so they had the material for the art with them!
The reality is that humans and Neanderthals had been using red ochre for artistic reasons for tens of thousands of years by that point—and inhabiting caves for just as long. There may have been a difference in degree around 42,000 years ago, but it wasn't instantaneous.
Both climate scientists and anthropologists have voiced a lot of skepticism about these claims so far, although a number found individual claims intriguing and worth following up on. The real test of some of these ideas will come as researchers use the carbon-14 signature the paper describes to look at other samples that record environmental changes, like in sediment cores, from the same time period. This will give us a clearer picture of whether the events that happened around the same time truly represent the sorts of global changes that are being proposed.
Other ideas are likely to remain beyond our ability to devise clean tests. It's not clear how we'd ever learn the sunscreen-to-art ratio of red ochre use by ancient populations or whether more people were in caves because they somehow sensed the atmosphere was becoming dangerous. So it seems like the researchers were airing out some provocative ideas that won't clearly influence the field.
One obvious way of following up on the work is to look more closely at other magnetic field reversals; the paper specifically mentions one that happened 35,000 years ago. But when the researchers simulated a magnetic field reversal without a big drop in solar activity, nothing much happened. It really looks like we'd need both to see the far-reaching impacts the researchers are proposing. And given that the chance of both happening at the same time seems remote, it's not clear how much other examples might tell us.
Science, 2021. DOI: 10.1126/science.abb8677 (About DOIs).
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