Arctic Study Sheds Light on Odd Tree-Ring Divergence Problem in Tracking Temperatures

Tree rings can tell us a lot about climate. Now, scientists have found that changes in tree-ring density in the Arctic may be evidence of changes in light intensity during the trees' growth. The findings have direct implications for the tree-ring divergence problem.

Tree rings are mostly made up of a low density ring, which forms early in the growing season, and a high density ring, which forms late in the growing season. In colder parts of the world, the dense rings tend to be denser during warm years. This means that, in theory, tree rings can be used to track temperatures. Yet scientists have found that after the 1960s, Arctic tree-ring densities did not keep pace with increases in temperature, an issue called the divergence problem.

Now, scientists may have found the solution to the divergence problem. Scientists set out to understand why tree-ring density was declining in the Arctic. One possible explanation could be changes in light intensity that affected the trees' ability to grow.

In order to test this, the researchers took advantage of the fact that there are regional variations in cloud cover and light availability throughout the Arctic. They compared trees that grew in the brightest and darkest areas, but in comparable temperature ranges. This allowed them to see if light was impacting the way tree rings formed. They then confirmed their findings by examining how trees grew and responded after major volcanic events, obscuring sunlight.

So what did they find? The scientists discovered that variation in light intensity affects tree-ring density, and that this impact is greatest in the darkest Arctic regions. In the brightest areas, though, the divergence problem essentially disappears. Instead, the tree-ring density is most closely related to temperature instead.

The findings could have implications for geoengineering proposals that hope to block sunlight by pumping aerosol particles into the atmosphere. It appears that if these proposals were instituted, Arctic trees might not grow as much and thus not soak up as much carbon; this, in turn, would mean that the geoengineering scheme wouldn't be nearly as effective.

The findings are published in the journal Nature Communications.