Secular temperature trends for the southern Rocky Mountains over the last five centuries

Abstract

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 betweenδ18Ocand 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 inδ18Octhat 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.