There is a new paper on the relationship of tree ring data to near-surface air temperatures. It is
Rosanne D’Arrigo, Rob Wilson, Beate Liepert and Paolo Cherubini, 2007: On the ‘Divergence Problem’ in Northern Forests: A Review of the Tree-Ring Evidence and Possible Causes. Journal of Global and Planetary Change. In press
The abstract reads,
âAn anomalous reduction in forest growth indices and temperature sensitivity has been detected in tree-ring width and density records from many circumpolar northern latitude sites since around the middle 20th century. This phenomenon, also known as the “divergence problem”, is expressed as an offset between warmer instrumental temperatures and their underestimation in reconstruction models based on tree rings. The divergence problem has potentially significant implications for large-scale patterns of forest growth, the development of paleoclimatic reconstructions based on tree-ring records from northern forests, and the global carbon cycle. Herein we review the current literature published on the divergence problem to date, and assess its possible causes and implications. The causes, however, are not well understood and are difficult to test due to the existence of a number of covarying environmental factors that may potentially impact recent tree growth. These possible causes include temperature-induced drought stress, nonlinear thresholds or time-dependent responses to recent warming, delayed snowmelt and related changes in seasonality, and differential growth/climate relationships inferred for maximum, minimum and mean temperatures. Another possible cause of the divergence described briefly herein is ‘global dimming’, a phenomenon that has appeared, in recent decades, to decrease the amount of solar radiation available for photosynthesis and plant growth on a large scale. It is theorized that the dimming phenomenon should have a relatively greater impact on tree growth at higher northern latitudes, consistent with what has been observed from the tree-ring record. Additional potential causes include “end effects” and other methodological issues that can emerge in standardization and chronology development, and biases in instrumental target data and its modeling. Although limited evidence suggests that the divergence may be anthropogenic in nature and restricted to the recent decades of the 20th century, more research is needed to confirm these observations.â?
The paper is an excellent review of this subject. Excerpts read,
âThe divergence problem has important consequences for the utilization of tree ring records from temperature-limited boreal sites in hemispheric-scale proxy temperature reconstructions (Jones et al. 1998, Mann et al. 1999, Briffa 2000, Briffa et al. 2001, Esper et al. 2002, Cook et al. 2004a, Moberg et al. 2005, D’Arrigo et al. 2006, Hegerl et al. 2006). The principal difficulty is that the divergence disallows the direct calibration of tree growth indices with instrumental temperature data over recent decades (the period of greatest warmth over the last 150 years), impeding the use of such data in climatic reconstructions.â?
âA recent analysis by Cook et al. (2004a) suggests that the divergence is restricted to the recent period and is unique over the past thousand years. It is thus likely to be anthropogenic in origin.â?
âWe have discussed a number of factors that can potentially impact climate sensitivity on local to regional scales and cause or simulate divergence effects (e.g., Moisture stress: Jacoby and D’Arrigo 1995, Barber et al 2000, Lloyd and Fastie 2002, BÃ¼ntgen et al. 2006; Complex non-linear or threshold responses: Vaganov et al. 1999, D’Arrigo et al. 2004a; Local pollution: Wilson and Elling 2004, Yonenobu and Eckstein 2006, Differential response to maximum and minimum temperatures: Wilson and Luckman 2002, 2003, Youngblut and Luckman (in press); Detrending end effects: Melvin 2004; see below). The observation that the divergence phenomenon appears confined to recent decades strongly suggests an anthropogenic cause (Cook et al. 2004a). Its widespread nature may also imply a cause that is hemispheric to global in scale,
possibly related to (likely anthropogenic) air pollution effects (Briffa et al. 1998a and b).â?
â? There has been expressed concern that the divergence problem challenges the uniformitarianism assumption in tree rings (e.g., National Research Council 2006). However, if the divergence is in fact anthropogenic in origin then it will only directly impact reconstructions within the past few decades. Some evidence suggests that this is the case, and that the divergence is limited, and unique to this recent period (Briffa et al. 1998a, Cook et al. 2004a). Nevertheless, there are still significant implications for the development of dendroclimatic reconstructions, as we have 1 noted in this paper. For example, reconstructions based on northern tree-ring data impacted by divergence cannot be used to directly compare past natural warm periods (notably, the MWP) with recent 20th century warming, making it more difficult to state unequivocally that the recent warming is unprecedented.â?
âInterestingly, the dimming phenomenon may be a cause of the slower increase in maximum vs. minimum temperatures in recent decades (Dai et al. 1999; Wild et al. 2007, Romanou et al. in press).â?
There is another explanation for the divergence between the proxy determined temperatures and in-situ near-surface observed temperatures in that the later measurement may not be an accurate description of the regional recent changes in temperatures. Indeed, as we have documented in our paper
Pielke Sr., R.A., C. Davey, D. Niyogi, S. Fall, J. Steinweg-Woods, K. Hubbard, X. Lin, M. Cai, Y.-K. Lim, H. Li, J. Nielsen-Gammon, K. Gallo, R. Hale, R. Mahmood, R.T. McNider, and P. Blanken, 2007: Unresolved issues with the assessment of multi-decadal global land surface temperature trends. J. Geophys. Res. in press,
there are significant issues with the use of the land surface temperature record to assess multi-year trends and variability if near-surface air temperatures.
If the in-situ near-surface air temperature warm bias that we have identified is real, the lack of warming seen in the recent proxy tree ring data could be real and would explain the â’divergence problem’ in Northern Forestsâ?. This would mean that the high latitude land areas may not have increased in temperature as much (or at all) as concluded in reports such as the 2007 IPCC Statement for Policymakers.