It's been a tough few months for tree-rings, perhaps unfairly. Back in April, we commented on a study [that one of us (Mike) was involved in] that focused on the possibility that there is a threshold on the cooling recorded by tree-ring composites that could limit their ability to capture the short-term cooling signal associated with larger volcanic eruptions. Mostly lost in the discussion, however, was the fact-emphasized in the paper-that the trees appeared to be doing a remarkably good job in capturing the long-term temperature signal-the aspect of greatest relevance in discussions of climate change.
This week there have been two additional studies published raising questions about the interpretation of tree-ring based climate reconstructions.
The first of these by Steinman et al (Mike is again a co-author) appeared in PNAS, and compared evidence of winter precipitation changes in the Pacific Northwest over the past 1500 years using a physical model-based analysis of lake sediment oxygen isotope data to statistical reconstructions of drought based on tree ring data. Steinman et al note that the tree-ring and lake estimates track each other well on multidecadal timescales, but show some divergence in their lower frequency (i.e. centennial and longer timescale) trends. They conclude that this divergence may simply reflect the differing and, in fact, complementary seasonal information reflected by the two proxy records, noting in the abstract:
Differences in seasonal sensitivity between the two proxies allow a more complete understanding of the climate system and likely explain disparities in inferred climate trends over centennial timescales.