Tree Rings and Climate: Some Recent Developments

by Michael E. Mann, Gavin Schmidt, and Eric Steig

Update 7/12/12: Media Matters comments on the latest misrepresentations of the Esper et al study discussed in our article: ‘Surprise: Fox News Fails Paleoclimatology’

Update 7/13/12: Further comment from Bob Ward of the Grantham Institute in Huffington Post UK “The World’s Most Visited Newspaper Website Continues to Regurgitate Nonsense from Climate Change ‘Sceptics’”

Update 7/14/12: Some additional context provided by this LiveScience article

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.

The authors amplified this point in their press release (emphasis added):

Tree ring and oxygen isotope data from the U.S. Pacific Northwest do not provide the same information on past precipitation, but rather than causing a problem, the differing results are a good thing, according to a team of geologists.

Nonetheless, some of the coverage (e.g. “Scientists see ancient climate patterns in lake-bottom ‘muck” by Lauren Morello, E&E/Climatewire, July 3, 2012) emphasized conflict between scientists over the discrepancies, rather than the more positive message about making use of complementary strengths of diverse sources of information about past climate change. In other words the ‘signal’ (moving forward with the science) became buried in the ‘noise’ (scientists on record arguing with each other).

Page 1 of 4 | Next page