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. Past climate variations may be uncovered via reconstruction methodsthat use proxy data as predictors. Among them, borehole reconstruction is awell-established technique to recover the long-term past surface airtemperature (SAT) evolution. It is based on the assumption that SAT changesare strongly coupled to ground surface temperature (GST) changes andtransferred to the subsurface by thermal conduction. We evaluate the SAT–GSTcoupling during the last millennium (LM) using simulations from the CommunityEarth System Model LM Ensemble (CESM-LME). The validity of such a premise isexplored by analyzing the structure of the SAT–GST covariance during the LMand also by investigating the evolution of the long-term SAT–GSTrelationship. The multiple and single-forcing simulations in the CESM-LME areused to analyze the SAT–GST relationship within different regions and spatialscales and to derive the influence of the different forcing factors on producingfeedback mechanisms that alter the energy balance at the surface. The resultsindicate that SAT–GST coupling is strong at global and above multi-decadaltimescales in CESM-LME, although a relatively small variation in thelong-term SAT–GST relationship is also represented. However, at a global scalesuch variation does not significantly impact the SAT–GST coupling, at localto regional scales this relationship experiences considerable long-termchanges mostly after the end of the 19th century. Land use land cover changesare the main driver for locally and regionally decoupling SAT and GST, asthey modify the land surface properties such as albedo, surface roughness andhydrology, which in turn modifies the energy fluxes at the surface. Snow cover feedbacksdue to the influence of other external forcing are also important forcorrupting the long-term SAT–GST coupling. Our findings suggest that suchlocal and regional SAT–GST decoupling processes may represent a source ofbias for SAT reconstructions from borehole measurement, since the thermalsignature imprinted in the subsurface over the affected regions is not fullyrepresentative of the long-term SAT variations.

Influence of radiative forcing factors on ground–air temperature coupling during the last millennium: implications for borehole climatology