Ground surface temperature and continental heat gain: Uncertainties from underground

Abstract

Temperature changes at the Earth?s surface propagate and are recorded underground as perturbations to the equilibrium thermal regime associated with the heat flow from the Earth?s interior. Borehole climatology is concerned with the analysis and interpretation of these downward propagating subsurface temperature anomalies in terms of surface climate. Proper determination of the steady-state geothermal regime is therefore crucial because it is the reference against which climate-induced subsurface temperature anomalies are estimated. Here, we examine the effects of data noise on the determination of the steady-state geothermal regime of the subsurface and the subsequent impact on estimates of ground surface temperature (GST) history and heat gain.Wecarry out a series of Monte Carlo experiments using 1000 Gaussian noise realizations and depth sections of 100 and 200mas for steady-state estimates depth intervals, as well as a range of data sampling intervals from 10mto 0.02 m. Results indicate that typical uncertainties for 50 year averages are on the order of ±0.02 Kfor the most recent 100 year period. These uncertainties grow with decreasing sampling intervals, reaching about ±0.1 Kfor a 10msampling interval under identical conditions and target period. Uncertainties increase for progressively older periods, reaching ±0.3 Kat 500 years before present for a 10m sampling interval. The uncertainties in reconstructed GST histories for the Northern Hemisphere for the most recent 50 year period can reach a maximum of?0.5 K in some areas.Wesuggest that continuous logging should be the preferred approach when measuring geothermal data for climate reconstructions, and that for those using the International Heat Flow Commission database for borehole climatology, the steady-state thermal conditions should be estimated from boreholes as deep as possible and using a large fitting depth range (∼100 m).