Coupled with a great solar minimum, the last geomagnetic excursion would have brought dramatic environmental changes on Earth. Also, for several centuries, the aurorae flourished all over the globe. The collapse of the ozone layer was also fueling climatic upheavals and a level ofUV unprecedented on the surface. In any case, this is reported by a new study published in the journal Science this February 19.
780,000 years ago the last known reversal of the Earth’s magnetic field occurred. During such an event, the magnetic poles reconfigure in phase or in phase opposition with the geographic poles. Also, during the transition, the global field weakens strongly. This inversion marks the beginning of the current geomagnetic period, called Brunhes period.
However, although the Earth’s magnetic field has not undergone an inversion for 780,000 years, it has not, however, stood still. Indeed, around fifteen excursions have been identified since then. One thus defines abrupt interruption phases but not leading to a permanent inversion of polarity. The time scale specific to these fluctuations extends from a thousand to ten thousand years. The most recent – the Laschamp excursion – took place around 42,000 years ago, at the end of the last Ice Age.
On the link between geomagnetic excursion and climate
If the consequences of disturbances in the magnetic field on life and the global climate are still poorly understood, most climatologists nevertheless believe that they are weak. In a study published in the journal Science, a group of researchers looked into the question. By analyzing the wood ofAgathis australis fossilized for more than 40,000 years, scientists were able to establish a detailed time scale of Laschamp’s excursion. Indeed, dark circles show a very clear carbon-14 peak during this episode. However, taking into account the annual resolution provided by the rings, it was possible to date the event precisely and to link together a whole set of records distributed over the globe.
The image that emerges is quite terrifying. It reveals major environmental changes such as a shift in belts of tropical rains and westerly winds towards the poles, leading to deadly droughts in regions such as Australia and South America. At the same time, the ice cap present over northern America rapidly progressed to the eastern United States and Canada. The authors also note that it was during this period that in Europe, Neanderthals marked the beginning of their extinction. An event that is probably not fortuitous.
Like an allure of the end of time
The researchers therefore wanted to go further. Using a climate model with atmospheric chemistry, they analyzed the effects of a field reduced to just a few percent of its current value at the peak geomagnetic excursion. During the Laschamp event, the solar star also suffered a great minimum. To better assess the state of the global climate during this period, the authors also took it into account. In total, it appears that these two factors have a synergistic effect, bringing a very high level of cosmic radiation into the upper atmosphere.
The results obtained confirm what the environmental archives mentioned above reveal. They also indicate that these climatic disturbances would have been triggered following an ozone layer collapse. During this episode, the Northern Lights were probably blooming all over the globe, making the nights sometimes as bright as the day. Furthermore, the authors argue that “These dramatic changes coupled with unprecedented UV levels prompted the first humans to take refuge in caves, explaining the apparent explosion of rock art across the world 42,000 years ago”.
Let us conclude by pointing out that the conclusions of the present study are still marred by many uncertainties and will need to be confirmed by further work. Some researchers, for example, have questioned the realism of certain hypotheses made by the authors. The fact remains that these intriguing advances question the paradigm of a negligible influence of magnetic changes on life and climate. To be continued !