Tree Rings and Climate: Some Recent Developments

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.

Indeed, there are a number of lines of evidence that contradict that more speculative claim. For example, if one eliminates tree-ring data entirely from the Mann et al (2008) “EIV” temperature reconstruction (see below; blue curve corresponds to the case where all tree-ring data have been withheld from the multiproxy network), one finds not only that the resulting reconstruction is broadly similar to that obtained with tree-ring data, but in fact the pre-industrial long-term cooling trend in hemispheric mean temperature is actually lessened when the tree-ring data are eliminated—precisely the opposite of what is predicted by the Esper et al hypothesis.

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The wider hypothesis doesn’t get much support from looking at the pre-industrial millennial-scale temperature trends in published proxy reconstructions of Northern Hemisphere mean temperature, all of which indicate cooling of varying magnitudes. Ordered from smallest to largest cooling trend, we have:

Moberg et al (2006): -0.06 ºC/1000yr (0-1900)

Esper et al (2002): -0.11 ºC/1000yr (831-1900)

Hegerl et al (2007): -0.14 ºC/1000yr (558-1900, 30º-90ºN land, Chblend-dark)

Ljungqvist (2010): -0.18 ºC/1000yr (0-1900, 30º-90ºN)

Mann et al (1999): -0.19 ºC/1000yr (1000-1900)

Mann et al (2008): -0.23 ºC/1000yr (300-1900, nhcru_eiv_composite):

This can be loosely compared to the -0.31 ºC/1000yr estimate derived for N-Scan and trends of -0.10 and -0.19 ºC/1000yr at that latitude in summer seen in two model estimates discussed – though note that the model simulations will have smaller trends for the whole hemisphere and for the annual mean.

There are a few rather interesting observations here. One is that the Moberg et al (2006) reconstruction, which–unlike all of the other reconstructions listed above–uses no tree-ring proxy data at all to estimate centennial and longer-timescale temperature variations, shows the smallest cooling trend of all. That is in contrast to Esper et al’s hypothesis that including tree-ring data leads to reduced long-term cooling trends. Another interesting observation is that trends calculated from Ljungqvist (2010), Mann et al (1999), and Mann et al (2008) are quite similar to the theoretical cooling trends cited by Esper et al (based on forced multi-millenial GCM experiments; in fact the Mann et al 2008 trend is substantially greater than the model estimates). So there is no support, at least with these reconstructions, for any systematic underestimate of forced millennial temperature changes, if the climate models–and forcings used to drive them—are indeed correct.

Finally, as these latter reconstructions target full hemispheric mean temperature, including tropics & extratropics, and annual mean conditions, the impact of orbital forcing is expected to be far smaller than for high-latitude summer reconstructions, such as this new N-Scan reconstruction. So the fact that a larger millennial cooling trend is seen in this latter case is hardly surprising.

In any case, the Esper et al paper represents a valuable contribution, suggesting important steps forward in the science of dendroclimatology. The paper provides a promising approach to at least reducing the vexing “divergence problem”, and it suggests prospects for tree-ring reconstructions of extratropical summer temperature with substantially greater fidelity at millennial timescales.

Only by understanding the relative strengths, weaknesses, and limitations of various sources of proxy evidence can we continue to refine proxy estimates of past climate change, something that is of interest not just to the paleoclimate community, but to the broader climate research community which relies, in part, on paleodata as a benchmark for testing and evaluating our mechanistic understanding of the climate system.

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