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

Abstract

Abstract. Past climate variations may be uncovered via reconstruction methods that use proxy data as predictors. Among them, borehole reconstruction is a well-established technique to recover the long-term past surface air temperature (SAT) evolution. It is based on the assumption that SAT changes are strongly coupled to ground surface temperature (GST) changes and transferred to the subsurface by thermal conduction. We evaluate the SAT–GST coupling during the last millennium (LM) using simulations from the Community Earth System Model LM Ensemble (CESM-LME). The validity of such a premise is explored by analyzing the structure of the SAT–GST covariance during the LM and also by investigating the evolution of the long-term SAT–GST relationship. The multiple and single-forcing simulations in the CESM-LME are used to analyze the SAT–GST relationship within different regions and spatial scales and to derive the influence of the different forcing factors on producing feedback mechanisms that alter the energy balance at the surface. The results indicate that SAT–GST coupling is strong at global and above multi-decadal timescales in CESM-LME, although a relatively small variation in the long-term SAT–GST relationship is also represented. However, at a global scale such variation does not significantly impact the SAT–GST coupling, at local to regional scales this relationship experiences considerable long-term changes mostly after the end of the 19th century. Land use land cover changes are the main driver for locally and regionally decoupling SAT and GST, as they modify the land surface properties such as albedo, surface roughness and hydrology, which in turn modifies the energy fluxes at the surface. Snow cover feedbacks due to the influence of other external forcing are also important for corrupting the long-term SAT–GST coupling. Our findings suggest that such local and regional SAT–GST decoupling processes may represent a source of bias for SAT reconstructions from borehole measurement, since the thermal signature imprinted in the subsurface over the affected regions is not fully representative of the long-term SAT variations.

Characterization of Air and Ground Temperature Relationships within the CMIP5 Historical and Future Climate Simulations

“…Koven et al () employed the difference between GST and SAT as part of an analysis to evaluate the propagation of heat into the soil and its effect on permafrost soils within the CMIP5 simulations. Melo‐Aguilar et al () most recently used the temperature difference as metric to study the effect of radiative forcings on the coupling between air and ground temperatures within the CESM‐LME ensemble.…”

“…Soil temperature at 10 cm from the CMIP5 simulations are obtained using a linear interpolation between nodes of each soil layer. The difference between GST and SAT has been previously used to track the relationship between air and ground surface temperatures at daily, seasonal, and annual scales (Koven et al, 2013;Melo-Aguilar et al, 2018;Smerdon et al, 2006). Smerdon et al (2006) proposed a method to study and validate the long-term coupling between air and ground surface temperatures, which is the main assumption of paleoclimate reconstructions from geothermal data (e.g., Beltrami, 2002).…”

Extending the climatological concept of ‘Detection and Attribution’ to global change ecology in the Anthropocene

“…A control run produced with constant pre-industrial forcing specifications (piControl; Taylor, Stouffer, & Meehl, 2012) can represent internal climate variability as a reference for non-perturbed climate conditions. If the aim is to consider the relative effects of natural versus anthropogenic forcing within the historical period or the last millennium, groups of natural-only or anthropogenic-only forcing responses can be considered to compare with the evolution of the climate variables during a certain period of time (Duan et al, 2019;Hegerl et al, 2010Masson-Delmotte et al, 2013;Melo-Aguilar, González-Rouco, García-Bustamante, Navarro-Montesinos, & Steinert, 2018;Schurer et al, 2013Schurer et al, , 2014Schurer et al, , 2019Wang et al, 2018); ensembles of simulations incorporating all possible forcing agents or specific simulations addressing a single forcing are also typically considered (Otto-Bliesner et al, 2016).…”

WRF v.3.9 sensitivity to land surface model and horizontal resolution changes over North America

“…The representation of near-surface conditions (e.g. air and soil temperatures, soil moisture...) and energy and water exchanges at the land surface in a climate model depends on the processes simulated by the atmospheric and soil model components, and on the degree of coupling implemented between both model components (Koster et al, 2006;Melo-Aguilar et al, 2018). Different Land Surface Models (LSMs) include varying levels of realism in the representation of soil physics.…”