Redwood Trees Reveal Climate History of West Coast Rain and Fog

Redwood trees are the tallest living organisms in the entire world. Stretching hundreds of feet high, these trees can live for centuries. For years, scientists have tried to use redwoods to help reconstruct historic climate, yet they've proven to be too erratic in their growth patterns in the past. Now, though, researchers have developed a way to use these trees as a window into ancient coastal conditions.

"This is really the first time that climate reconstruction has ever been done with redwoods," said Jim Johnstone, one of the researchers in a news release. "Redwoods are restricted to a very narrow strip along the coastline. They're tied to the coastline, and they're sensitive to marine conditions, so they actually may tell you more about what's happening over the ocean than they do about what's happening over land."

In order to examine past climate conditions using redwoods, the researchers used cores from Northern California coastal redwoods. Although tree-ring research typically involves a detailed look at a cross-section of a tree trunk, the rings of a redwood are uneven and don't fully encircle the tree. In order to get past that, the scientists approached the tree cores like ice cores; they examined molecules captured in the wood to sample the atmosphere of the past.

Most oxygen in Earth's atmosphere has an atomic mass of 16, which makes it O-16. However, a small percentage of oxygen is the heavier O-18 isotope. When seawater evaporates off of the ocean to form clouds, some drops fall as rain over the ocean and more of the heavier O-18 molecules rain out.  By looking at the proportion of O-16 and O-18 in the wood from each season, the researchers could measure the contribution of fog and rain.

"We actually have two indicators that we can use in combination to determine if a particular summer was foggy with a little rain, foggy with a lot of rain and various combinations of the two," said Johnstone in a news release.

The findings revealed that the amount of West Coast fog is closely tied to the surface temperature of the ocean. This means that redwoods could tell researchers quite a bit about the long-term patterns of ocean change.

The findings are published in the Journal of Geophysical Research-Biogeosciences.