The researchers tested whether global dimming might be responsible for the decline in tree-ring density in the Arctic. This idea had been proposed in the past, but scientists had not been able to test the hypothesis. "It's very hard to distinguish a record that's controlled by temperature from one that's controlled by light, because light and temperature tend to vary together. Sunnier days are usually warmer," Stine added.
To get around this challenge, the researchers took advantage of the fact that there are regional variations in cloud cover and light availability throughout the Arctic, allowing them to compare trees that grew in the brightest and darkest areas but in comparable temperature ranges. They found that divergence was largest in the darkest parts of the Arctic, where changes in light should have the largest effect.
The researchers used changes in tree-ring density following volcanic eruptions to confirm the findings. Major volcanic events such as the 1991 eruption of Mount Pinatubo in the Philippines also have spewed tons of light-scattering sulfur dioxide particles into the atmosphere, decreasing the amount of sunlight reaching the surface.
Their analysis for seven different tree species suggests 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 essentially disappears, and tree-ring density is mos
|Contact: Jonathan Morales|
San Francisco State University