In searching for planets and studying their stars, I’ve had the privilege to use some of the world’s great telescopes. However, our team has recently turned to an even larger system to study the cosmos: Earth’s forests.
In searching for planets and studying their stars, I’ve had the privilege to use some of the world’s great telescopes. However, our team has recently turned to an even larger system to study the cosmos: Earth’s forests.
We analyzed radioactive signatures left in tree rings around the world to study mysterious “radiation storms” that have swept over Earth half a dozen times in the past 10,000 years or so.
Our results, published today in Proceedings of the Royal Society A, rule out “solar superflares” as the culprit—but the true cause remains unknown.
A history written in tree rings
When high-energy radiation strikes the upper atmosphere it turns nitrogen atoms into radioactive carbon-14, or radiocarbon. The radiocarbon then filters through the air and the oceans, into sediments and bogs, into you and me, into animals and plants—including hardwoods with their yearly tree rings.
To archaeologists, radiocarbon is a godsend. After it is created, carbon-14 slowly and steadily decays back into nitrogen—which means it can be used as a clock to measure the age of organic samples, in what is called radiocarbon dating.
To astronomers, this is equally valuable. Tree rings give a year-by-year record of high-energy particles called “cosmic rays” going back millennia.
The magnetic fields of Earth and the sun shield us from cosmic rays shooting through the Galaxy. More cosmic rays reach Earth when these magnetic fields are weaker, and fewer when the fields are stronger.
This means the rise and fall of carbon-14 levels in tree rings encodes a history of the 11-year cycle of the solar dynamo (which creates the sun’s magnetic field) and the reversals of Earth’s magnetic field.
