Late summer temperature variability for the Southern Rocky Mountains (USA) since 1735 CE: applying blue light intensity to low-latitude Picea engelmannii Parry ex Engelm

Abstract

Our study examines the application of using blue intensity (BI) methods to develop a late summer maximum temperature (Tmax) reconstruction for the Southern Rocky Mountains—a mid-latitude (i.e., 36° N), arid region in North America. We reconstruct August–September (AS) Tmax for the period 1735–2015 CE using a composite latewood BI (LWB) Engelmann spruce (Picea engelmannii Parry ex Engelm.) chronology from multiple sites across the Sangre de Cristo Mountains in Northern New Mexico, USA. This study presents the first BI-derived temperature reconstruction for the lower mid-latitudes (30–45° N) of North America. We compare the climate response of multiple tree-ring parameters: LWB, earlywood BI (EWB), ∆BI (earlywood BI minus latewood BI), ring width (RW), and maximum latewood density (MXD). Of all parameters, the site-composite LWB and ∆BI chronologies show the strongest correlations with AS Tmax. Reconstructed AS Tmax demonstrates fluctuating warm and cool periods during the latter portion of the Little Ice Age (ca. 1730–1850) and pronounced warming through the early to mid-twentieth century (ca. 1920–1950s). The reconstruction also documents substantial warming over the last decade, the trend of which appears to be anomalous within the context of the past ca. 280 years. We highlight the potential for BI methods to be successfully used at high-elevation, mid-latitude locations where temperature proxy datasets are scarce or non-existent. As many places across the mid-latitudes lack contiguous, temporally resolved, decadal-scale paleotemperature proxies, we suggest here that BI methods can be effective at improving the spatial gaps in the Northern Hemisphere temperature proxy network.