Scientists Shed 'Light' on Arctic Tree Divergence Problem

Changes in tree-ring density in the Arctic may be evidence of changes in light intensity during the trees' growth, according to a new study by San Francisco State University researcher Alexander Stine.

The study sheds light on the tree-ring "divergence problem" - a phenomenon in which Arctic tree-ring densities do not keep pace with increases in temperature.

Tree rings consist of a low density ring and a high density ring, which, respectively, forms early in the growing season and late in the growing season. The dense latewood rings tend to be denser during warm years in cooler climates.

Divergence doesn't appear to be a huge issue, if it weren't for the fact that scientists use tree rings to study past climates. Stine explained in a press release, "It's a problem because if we want to use these tree rings as a proxy for temperatures of the past, we need to make sure that we understand what's happening now."

Changes in light intensity could explain, researchers say, the divergence problem. Since the 1960s, the amount of sunlight reaching the Earth's surface has declined.

In the study - published in the journal Nature Communications - Stine looked at regional variations in cloud cover and light availability throughout the Arctic. He found divergence was most prevalent in the darkest regions, where changes in light should have the largest effect.

He also examined tree-ring density following volcanic eruptions - when sulfur dioxide spews into the air after an eruption its decreases the amount of sunlight that reaches the surface.

Stine concluded that variations in light intensity caused by volcanic eruptions and "global dimming" both affect tree-ring density, and this impact is greatest in the darkest Arctic regions. In the brightest areas, the divergence problem is essentially nonexistent, and tree-ring density is most closely linked to temperature instead.

The bright side is that this research may be able to help scientists better understand climate change.

"We could learn more about past variations in light intensity at the Earth's surface, and we may be able to deepen our understanding of both trees and climate," Stine concluded.