A reversal in Earth’s magnetic field and a decline in solar activity 42,000 years ago caused major climate shifts that led to global environmental change and mass extinctions, according to a new study. The researchers called it the “Adams Event”after sci-fi writer Douglas Adams who declared the number 42 as the ultimate answer.

The Earth has a magnetic field that works as a protective shield against damaging electromagnetic radiation. But when the poles switch, as it has happened many times in the planet’s geological history, the shield weakens significantly and leaves the planet exposed to high-energy particles. The last time this happened was 42,000 years ago and lasted for about 1,000 years, during what scientists refer to as the Laschamp event.

Previous studies couldn’t find much evidence of the impact of the event on the planet, probably because the focus wasn’t on the period in which the poles were actually reversing, Turney and his team argue. Now, they discovered that the pole switch could have been the reason for a wide array of climate and environmental phenomena with severe consequences.

For the study, the researchers carried out radiocarbon analyses of the rings of ancient kauri trees located in northern New Zealand wetlands, some of which were more than 42,000 years old. This allowed tracking over time the increase of carbon-14 levels in the atmosphere, which is generated by the increasing levels of high-energy cosmic radiation reaching the Earth.

“For the first time ever, we have been able to precisely date the timing and environmental impacts of the last magnetic pole switch,” co-author Chris Turney from the University of New South Wales said in a statement. “The findings were made possible with ancient New Zealand kauri trees, which have been preserved in sediments for over 40,000 years.”

The researchers looked at numerous records and materials from all over the world, including from lake and ice cores, and found that a host of major environmental changes happened at the same time as the carbon-14 levels peaked. This included a shift in tropical rain belts in the West Pacific and a large growth of the ice sheet over North America, for example.

They used a model to understand how the chemistry of the atmosphere might change if the Earth’s magnetic field was lost and there was a prolonged period of low solar activity, which would have further reduced Earth’s protection against cosmic radiation. Ice core records suggest such drops in solar activity, known as the “grand solar minima”, coincided with the Laschamps event.

The atmospheric changes would have caused big environmental and climate changes, the researchers believe. This would have accelerated the growth of ice sheets and contributed to the extinction of Australian megafauna. Plus, it could explain the emergence of red ochre handprints, as humans might have used the pigment as a sunscreen to cope with the increased levels of ultraviolet radiation.

But that’s not all. The harsh climate conditions would explain the larger use of caves by our ancestors, as the underground spaces offered shelter. This would have increased competition and could have contributed to the end of the Neanderthals. “It’s the most surprising and important discovery I’ve ever been involved in,” Alan Cooper, lead author of the study, said in a statement.

While the last time the poles switched was 42,000 years ago, the researchers said there’s currently rapid movement of the north magnetic pole across the Northern Hemisphere. This could eventually lead to another reversal. The magnetic field has already weakened by about 9% over the past 170 years.

“Our atmosphere is already filled with carbon at levels never seen by humanity before. A magnetic pole reversal or extreme change in Sun activity would be unprecedented climate change accelerants,” Turney said in a statement. “We urgently need to get carbon emissions down before such a random event happens again.”

The study was published in the journal Science.