Southwestern US temperatures ‘have been relatively stable over last 500 years’

Russ Steele

A new paper published in Geophysical Research Letters indicates that “Temperature trends in Southwest US have been relatively stable over last 5 centuries” and that there has been “no sustained monotonic rise in temperature or a step-like increase since the late 19th century.”  This would imply that there has been no significant influence of man-made CO2 on temperatures of the Southwest US. It would also indicate that Dalton Minimum was not a major factor in the regions climate variability.


from Geophysical Research Letters by M. Berkelhammer and L. D. Stott

Key Points

  • Temp. trends in the SW US can be reconstructed using isotopes in tree rings
  • A process model of the proxy can be used to characterize uncertainty in proxy
  • Temperature trends in SW US have been relatively stable over last 5 centuries

Pre-instrumental surface temperature variability in the Southwestern United States has traditionally been reconstructed using variations in the annual ring widths of high altitude trees that live near a growth-limiting isotherm. A number of studies have suggested that the response of some trees to temperature variations is non-stationary, warranting the development of alternative approaches towards reconstructing past regional temperature variability. Here we present a five-century temperature reconstruction for a high-altitude site in the Rocky Mountains derived from the oxygen isotopic composition of cellulose (δ18Oc) from Bristlecone Pine trees. The record is independent of the co-located growth-based reconstruction while providing the same temporal resolution and absolute age constraints. The empirical correlation between δ18Oc and instrumental temperatures is used to produce a temperature transfer function. A forward-model for cellulose isotope variations, driven by meteorological data and output from an isotope-enabled General Circulation Model, is used to evaluate the processes that propagate the temperature signal to the proxy. The cellulose record documents persistent multidecadal variations in δ18Oc that are attributable to temperature shifts on the order of 1°C but no sustained monotonic rise in temperature or a step-like increase since the late 19th century. The isotope-based temperature history is consistent with both regional wood density-based temperature estimates and some sparse early instrumental records.

It should be noted that Bristlecone pines from California were a source of Michael Mann’s hockey stick reconstruction and use of strip back trees were not recommended for temperature reconstructions by the National Academy of Sciences. However, this approach used a different method for determining temperature. 


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