Tree Rings and Climate: Some Recent Developments

Jan Esper and colleagues have an article in Nature Climate Change that introduces a new reconstruction (N-Scan) of high-latitude (Fennoscandian) summer temperature changes over the past two millennia based on Maximum Latewood Density (‘MXD’). The most exciting–and in our view important–development is that they seem to have greatly ameliorated the “divergence problem” that has plagued some surface temperature reconstructions based on these types of data; given that the revised MXD data appear to be able to track the most recent warming provides increased confidence in the estimates they provide of past temperature changes.

Another interesting finding is that N-Scan exhibits a substantially larger pre-industrial (pre 1900) millennial cooling trend (around -0.31C/1000yr) than a tree ring width (TRW) based summer temperature reconstruction from the same trees. The authors interpret this finding as indicating that TRW reconstructions may be unable to recover millennial-timescale temperature trends owing to non-biological impacts on growth and limitations of detrending procedures used to separate climatic and non-climatic growth components. This seems a plausible conclusion, arrived at through a thoughtful and elegant case study. Yet the article extrapolates quite a bit, in terms of its conclusions regarding proxy-based temperature reconstructions more generally. The authors make much of the importance of long-term radiative forcing due to the changes in the earth’s orbit for millennial timescale temperature trends. They argue that TRW data which fail to record this forced long-term cooling might therefore underestimate variability on millennial timescales more generally, and potentially underestimate the warmth of past warm periods (e.g. medieval and Roman periods).

Orbital forcing is indeed substantial on the millennial timescale for high-latitudes during the summer season, and the theoretically expected cooling trend is seen in proxy reconstructions of Arctic summer temperature trends (Kaufman et al, 2009). But insolation forcing is near zero at tropical latitudes, and long-term cooling trends are not seen in non-tree ring, tropical terrestrial proxy records such as the Lake Tanganyika (tropical East Africa) record (Tierney et al, 2010) (see below).

Long-term orbital forcing over the past 1-2 millennia is also minimal for annual, global or hemispheric insolation changes, and other natural forcings such as volcanic and solar radiative forcing have been shown to be adequate in explaining past long-term pre-industrial temperature trends in this case (e.g. Hegerl et al, 2007). Esper et al’s speculation that the potential bias they identify with high-latitude, summer-temperature TRW tree-ring data carry over to a bias in hemispheric temperature reconstructions based on multiple types of proxy records spanning tropics and extratropics, ocean and land, and which reflect a range of seasons, not just summer (e.g. Hegerl et al, 2006; Mann et al, 1999;2008) is therefore a stretch.

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