Earth’s magnetic field helps to protect life from energetic particles that would otherwise come out of space. Mars now does not have a strong magnetic field, and the conditions on its surface are considered so life-damaging that any microbes that can inhabit the planet are safely below the surface. On Earth, the magnetic field ensures that life on the surface can flourish.
Except that it’s not always true. The earth’s magnetic field varies, with the poles moving and sometimes changing places, and the field can sometimes weaken or effectively disappear. Yet a glance at these events revealed nothing particularly interesting – no obvious connection with extinctions, no great ecological upheaval.
A paper published in Science yesterday provides an impressively accurate dating of a magnetic field shift in the past through the use of trees that have been dead for tens of thousands of years. And it shows that the flip is associated with climate change. But the newspaper continues to tie the flip to everything, from a minor extermination event to the explosion of cave art by our ancestors. Ultimately, the work is a mixture of solid science, challenging hypothesis and unlimited speculation.
Old trees, but how old?
We will start with the solid science, which all goes back to kauri trees, one of the distinctive species native to New Zealand. These trees are quite large and of long life and often reach more than 1,000 years old. And the wood of the tree often survives buried in swamps, and some specimens are tens of thousands of years old.
The team behind the new work relies on the discovery of kauri wood dating from the time of the Laschamps excursion, a period when the magnetic poles briefly changed places about 40,000 years ago. Old trees tell many stories. The carbon-14 they contain can give fairly accurate dates for the sample, and the individual tree rings then allow the conditions to be deduced in individual years. Studies of other isotopes found in the wood can provide a rough estimate of everything from solar activity to rainfall patterns.
The team behind the new work found that dating of the material they had during the Laschamps excursion had come out. And there was an increase in carbon-14 in the tree rings that were deposited at that time, in line with more particles reaching the earth due to the decrease in the magnetic field strength. That would normally be enough to eliminate the dating, and that limited us to placing exact dates on the Laschamps excursion with prior samples.
But the details captured in the tree rings enabled the research team to compile its data with data from other sources that attached exact dates. These include annual deposits made in a cave, containing both carbon-14 records and dates provided by an isotope of thorium. The researchers can also compile the data more accurately with iron core records, which also record information from the time of the Laschamps excursion.
Once combined, these records gave precise timing for the reversal of the magnetic field, as well as the information on the magnetic field strength over time. The joint record also contains information on the prevailing climate and details on things like rainfall and solar activity.
Not stuck in reverse
According to the record, the magnetic field began to decline 42,350 years ago and reached its lowest level 41,800 years ago, 300 years before the actual pole flip. The weakened magnetic field of the time was thus more of a precursor to the turn than an impact of the poles changing places. Because of the timing, centered on 42 kilograms, the researchers decided to name the Adams Transitional Geomagnetic Event, after the author Douglas Adams.
The alignment of data also indicates that the earth at the time was not the only thing doing something unusual. The isotope beryllium-10 is mostly formed by cosmic ray particles that affect the atmosphere, and it therefore serves as an indication of solar activity. This is because the magnetic field of the sun correlates with its activity level, and that the magnetic field can emit incoming particles that would otherwise move into the solar system and affect the earth.
So there were two independent events that would both work to allow more high-energy particles to 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, ozone losses is not that big than those who created our current ozone hole, although it would be expected to be geographically and seasonally different.
The loss of ozone creates a series of relatively subtle climatic effects, altering the Arctic jet stream and precipitation patterns in the Southern Hemisphere. These are the results of a limited number of runs of 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 strong these effects are.
However, using the carbon-14 signature associated with the Adams event, the researchers identified the equivalent periods in some sediment records. Both indicated that there were changes in the atmospheric circulation patterns that occurred during the event, consistent with the impact on the climate.
Speculation time
In general, the new, precise timing should be very useful for any case involving a sample that stores carbon-14 and dates from this period. In this regard, the work provides a service to the field. The prospect of a link with climate and the arrival of more high-energy particles is an interesting hypothesis, and it is one that differs from previous attempts to link solar activity to climate change. This is an idea worth following up on.
But for most of the rest of the article, the researchers are looking for something that happened about 42,000 years ago and trying to connect it to the mixture of changing environmental conditions that they say was caused by the Adams event. This includes colder conditions in the Northern Hemisphere, as evidenced by ice extensions. Except that the changes in the magnetic field last only a few hundred years, while the colder climate lasts for thousands of years. They therefore had to suggest that the Adams event push the climate past a tipping point so that it could maintain its altered state in the absence of the original trigger. On top of that, there are some climate records that show very little during the Adams event.
Australia has seen a major extinction of its megafauna that reached a peak about 42,000 years ago; it points to a link to the changing rainfall that caused the Adams event in the Southern Hemisphere. This is an interesting idea, although extinction events like these usually take some time before and after the peak.
Other potential connections become extremely small. Despite having been in Central Asia for tens of thousands of years, modern humans in Europe seemed to have appeared around the time of the Adams event, and Neanderthals soon became extinct. Although it is reasonable to suspect that the latter two events are linked, it is not clear why they are related to the magnetic field cut and any impact it had on the climate.
The period also saw an increase in the scope and refinement of cave art by those modern people. Once again, the researchers are trying to link this growth to the Adams Event. More people must have been in caves to escape from the harsh radiation environment! And so they used red ocher as sunscreen, so they had the material for the art with them!
The reality is that people (and Neanderthals) at that time had been using red ocher for artistic reasons for tens of thousands of years – and had lived in caves for just as long. There may have been a difference in grade about 42,000 years ago, but it was not instantaneous.
Climate scientists and anthropologists have so far expressed much skepticism about these claims, although some of them find individual claims interesting and follow-up. The real test of some of these ideas will come when researchers use the carbon-14 signature described in the article to look at other samples that record environmental changes, such as in sediment cores, from the same period. This will give us a clearer picture of whether the events that took place at the same time really represent the kind of global changes.
Other ideas are likely to remain beyond our ability to set up clean tests. It is not clear how we would ever learn the sunscreen-to-art ratio of the use of red ocher by ancient populations or more people were in caves because they somehow felt the atmosphere was becoming dangerous. Thus, it seems that the researchers have broadcast challenging ideas that will not clearly affect the field.
One obvious way to follow up on the work is to take a closer look at other magnetic field conversions; the newspaper specifically mentions one that happened 35,000 years ago. But when the researchers simulated a reversal of the magnetic field without a large drop in solar activity, nothing much happened. It really seems like we both need to see the far-reaching consequences the researchers are proposing. And since the chances of both happening simultaneously seem remote, it’s not clear how many other examples we can tell.
Science, 2021. DOI: 10.1126 / science.abb8677 (On DOIs).