41,000 years ago, Earth’s shield collapsed

The Earth is bare without its protective barrier. The planet’s magnetic shield surrounds the Earth and protects it from the natural attack of cosmic rays. But sometimes, the shield weakens and wobbles, allowing cosmic rays to hit the atmosphere, creating a shower of particles that scientists think can wreak havoc on the biosphere.

This has happened many times in the history of our planet, including 41,000 years ago in an event called the Laschamps excursion.

Cosmic rays are high-energy particles, usually protons or atomic nuclei, that travel through space at relativistic speeds. Normally, they are deflected into space and away from Earth by the planet’s magnetic shield. But the shield is a natural phenomenon and its strength fluctuates, as does its orientation. When this happens, cosmic rays hit the Earth’s atmosphere.

This creates a shower of secondary particles called cosmogenic radionuclides. These isotopes become embedded in sediments and ice cores and even in the structure of living things like trees. There are different types of these isotopes, including calcium 41 and carbon 14.

Some of the isotopes are stable and some are radioactive. Radioactives have half-lives ranging from just 20 minutes (Carbon 11) to 15.7 million years (Xenon 129).

As the Earth’s shield weakens, more of these isotopes reach the planet’s surface and accumulate in sediments and ice. By studying these cores and sediments, scientists can determine the history of the magnetic shield. Their observations show that Earth experienced a geomagnetic excursion or reversal 41,000 years ago. The Laschamps excursion is named after the Laschamps lava flows in France, where geomagnetic anomalies revealed their occurrence.

Every few hundred thousand years, Earth’s magnetic poles rotate. North becomes south and vice versa. Between these big events are smaller events called excursions. During excursions, the poles are moved for a while without changing their place. Excursions weaken the Earth’s shield and can last from a few thousand to tens of thousands of years. When this happens, more cosmic rays hit the atmosphere, creating more radionuclides that fall to Earth.

Scientists often focus on a particular radioactive isotope in paleomagnetic studies. Beryllium-10 has a relatively long half-life of 1.36 million years and tends to accumulate at the soil surface.

Sanja Panovska is a researcher at GFZ Potsdam, Germany, studying geomagnetism. At the recent General Assembly of the European Geosciences Union (EGU) 2024, Panovska presented new research on the Laschamps excursion. He discovered that during the excursion to Laschamps, the production of Be 10 was twice the normal.

To further understand the Laschamps excursion, Panovska combined cosmogenic and paleomagnetic radionuclide data to reconstruct Earth’s magnetic field at the time. He found that when the field decreased in strength, it also decreased. The transition from the normal field to the inverted field took about 250 years and remained inverted for about 440 years. During the transition, the Earth’s shield weekend up to 5% of its normal strength. When fully inverted, he was at about 25% of his normal strength. This weakening allowed more Be 10 and other cosmogenic radionuclides to reach the Earth’s surface.

These radionuclides do more than collect in sediments and ice. Some of them are radioactive. The weakening of the shield also weakened the ozone layer, allowing more UV radiation to reach the Earth’s surface. The high-altitude atmosphere also cooled, which modified the wind flows. This could have caused drastic changes to the Earth’s surface.

For these reasons, the Laschamps event has been linked to the extinction of the Neanderthals, the extinction of the Australian megafauna and even the emergence of rock art. These links have not stood up to scientific scrutiny, but that doesn’t mean events like the Laschamps event aren’t dangerous. If it happened now, it would wipe out our power grids. Earth’s equatorial region would be lit up with auroras.

“Understanding these extreme events is important for their future occurrence, space weather predictions, and assessment of effects on the environment and the Earth system,” Panovska said.

Scientists are learning that the magnetic shield is not static. There are anomalies. One of them is the South Atlantic Anomaly, a region where the magnetic field is weaker near the Earth. When satellites pass through this region, they are exposed to higher levels of ionizing radiation. The anomaly is likely caused by a deposit of dense rock within the Earth, illustrating the complexity of the magnetic shield.

Scientists do not know what effect cosmic rays have on life when the magnetic shield is weak. It is tempting to correlate extinctions with events like the Laschamps Excursion when they align temporally. But the poles have shifted, weakened and reversed many times and life is still here and still thriving.

If humanity lasts long enough, we will go through one of these reversals. Then we’ll know.