Nearly 42,000 years ago, when Earth's magnetic fields reversed, this triggered major environmental changes, extinction events, and long-term changes in human behavior, a new study reports. The findings, made possible by a new radiocarbon record derived from New Zealand's ancient kauri trees, raise important questions about the evolutionary impacts of geomagnetic reversals and excursions throughout the deeper geological record, the authors say. "Before this work," says author Chris Turney in a related video, "we knew there were a lot of things happening around the world at 42,000 years ago, but we didn't know precisely how... For the first time, we've been able to precisely date what happened when Earth's magnetic fields last flipped." Written in the geological record are numerous instances where the planet's magnetic poles flipped. Today, such an event would almost certainly wreak havoc with modern electronic and satellite technologies. However, the potential environmental impacts of such events are virtually unknown. The most recent major magnetic inversion, the Laschamps excursion, a relatively short-lived geomagnetic event that occurred ~41,000 years ago, provides one of the best opportunities to study the potential impacts of extreme changes to Earth's magnetic field. However, despite compelling evidence from several paleoenvironmental records that suggest it coincided with significant environmental and ecological changes , the ability to precisely characterize this event and determine its role - if any - in contemporaneous global changes has been limited by an uncertain radiocarbon calibration for the period. In this study, Turney, Alan Cooper and colleagues present a new, precisely dated atmospheric radiocarbon record derived from the tree rings of ancient kauri trees preserved for millennia in New Zealand wetlands. Like a missing keystone, this new record allowed the authors to better align other global radiocarbon and ice core records with the Laschamps. Cooper et al. identified a significant increase in atmospheric radiocarbon during the period of weakening magnetic field strength that preceded polarity reversal. By modeling the consequences of this increase, they found that the geomagnetic field minimum, when Earth's magnetic field was estimated to be only ~6% of current levels, triggered substantial changes in atmospheric ozone concentration and circulation. These shifts may have caused synchronous global climate and environmental changes observed in other climate records that occurred ~42,000 years ago. The discovery that geomagnetic field fluctuations can affect atmospheric temperature and circulation on a global scale provides a model for understanding anomalous and sudden paleoenvironmental shifts, the authors say.