A powerful blast of cosmic rays—carrying the energy of 167 megatons of TNT—entered Earth from a star 1,200 years ago and filled the remote reaches of the Earth with beryllium, carbon isotopes and other elements.

The cosmic event left its imprint in tree rings and ice cores, which are natural history books for times before the human era. And using it, scientists have resolved a long-standing argument on how the planet responds to volcanic eruptions.

They have found that almost every major volcanic eruption in the past 2,500 years has rapidly cooled the planet. The study was published yesterday in Nature.

“Volcanic eruptions are one of the major influences on the climate system,” said Kevin Anchukaitis, a geologist at Woods Hole Oceanographic Institution (WHOI) who is not affiliated with the study.

The first inkling scientists had of volcanoes’ impacts was in 1991, when Mount Pinatubo in the Philippines released 20 million metric tons (about the weight of 3 million African elephants) of sulfur aerosols into the atmosphere. The particles reflected sunlight back into space and lowered global temperatures for the next two years.

Scientists who studied Pinatubo theorized that volcanoes—and aerosols, in particular—must cool the planet.

They had to confirm this using historic data, a challenging task since very few detailed records of eruptions exist. Even records of global temperatures stretch back only to the 1800s.

The task was important, since the aerosols spewed by volcanoes cut to the heart of disagreements over climate change. Most scientists agree that human-caused global warming is happening, but they disagree on how quickly the planet is warming. The Intergovernmental Panel on Climate Change (IPCC) says that a doubling of CO2 concentrations warms the planet by anywhere between 2 and 4.5 degrees Celsius. Therefore, understanding aerosols and their cooling effect on the planet would be important for narrowing down this range.

A history unveiled through trees and ice
To stretch history beyond the instrumental record, paleoclimatologists rely on tree trunks and ice cores.

Trees add a new layer of wood to their trunk every year and capture temperature at the time of growth.

Similarly, annual snowfall at the poles traps aerosols and other particles in the atmosphere within ice. So cores drilled out of Greenland and Antarctica contain these records. Spikes of aerosols in a layer of ice can indicate past volcanic eruptions.

Tree ring and ice core scientists independently assembled their timelines of temperature and volcanic eruptions over the past 2,500 years.

They then compared their results.

“They need to match; they need to be synchronized with each other,” said Joseph McConnell, a hydrologist at the Desert Research Institute and a co-author of the study.

But the timelines did not synchronize well. For instance, tree rings showed that global temperatures had plunged in A.D. 536, and scientists theorized that it had been preceded by a volcanic eruption. Historians from the time had written about a mysterious “dust veil,” famines and plague.

But when scientists examined the ice core record, they did not find a major eruption.

That implied one of two possibilities: Volcanic eruptions did not always trigger global cooling. Or the ice core record was slightly off and had to be recalibrated.

‘Now everything makes sense’
Calibration is key to all measurements, as students learn early in chemistry labs.

When it comes to past climate, scientists work with a limited set of calibration tools created by chance. One such tool was created in A.D. 775 when an extraterrestrial cosmic ray event set off by a star, or perhaps a supernova, split atoms in the atmosphere and suffused remote reaches with isotopes such as carbon-14 and beryllium-10.

Cedar trees from Japan contain spikes of the carbon-14 isotope corresponding to A.D. 775. And ice cores from the poles contain spikes of beryllium-10.

Noticing the parallel spikes, McConnell and his colleagues forced a match at this point in history between their ice core and tree ring timelines.

“It gives us a very definite marker so that we can absolutely tie the tree ring chronology to the ice core chronology,” McConnell said.

This brought the two records into full alignment.

The A.D. 536 global cooling event was now preceded by a major eruption.

“So it reconciles everything; now everything makes sense,” McConnell said.

The new, recalibrated ice core record can be used by scientists to refine their understanding of “the earth system and how the ocean and atmosphere respond to perturbations in the amount of energy reaching the surface,” said Anchukaitis of WHOI.

“This is a very elegant approach and an important result,” he said.
