Jason E. Smerdon

Borehole climate reconstructions: Spatial structure and hemispheric averages

[1] Ground surface temperature (GST) reconstructions determined from temperature profiles measured in terrestrial boreholes, when averaged over the Northern Hemisphere, estimate a surface warming of � 1 K during the interval AD 1500– 2000. Other traditional proxy-based estimates suggest less warming during the same interval. Mann et al. [2003a] have raised two issues with regard to borehole-based reconstructions. The first focuses on the need for spatial gridding and area-weighting of the ensemble of borehole-based GST reconstructions to yield an average hemispheric reconstruction. The second asserts that application of optimal detection techniques show that the GST only weakly displays the spatial structure of the surface air temperature (SAT). We demonstrate the consistency of GST warming estimates by showing that over a wide range of grid element area and occupancy weighting schemes, the five-century GST change falls in the range of 0.89–1.05 K. We examine the subhemispheric spatial correlation of GST and SAT trends at various spatial scales. In the 5-degree grid employed for optimal detection, we find that the majority of grid element means are determined from three or fewer boreholes, a number that is insufficient to suppress site-specific noise via ensemble averaging. Significant spatial correlation between SAT and GST emerges in a 5-degree grid if low-occupancy grid elements are excluded, and also in a 30-degree grid in which grid element means are better determined through higher occupancy. Reconstructions assembled after excluding low-occupancy grid elements show a five-century GST change in the range of 1.02–1.06 K. INDEX TERMS: 1645 Global Change: Solid Earth; 1699 Global Change: General or miscellaneous; 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); 3322 Meteorology and Atmospheric Dynamics: Land/atmosphere interactions; 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; KEYWORDS: boreholes, paleoclimate, spatial analysis, surface temperature

Continental heat gain in the global climate system

[1]xa0Recent estimates have shown the heat gained by the ocean, atmosphere, and cryosphere as 18.2 · 1022 J, 6.6 · 1021 J, and 8.1 · 1021 J, respectively over the past half-century. However, the heat gain of the lithosphere via a heat flux across the solid surface of the continents (29% of the Earths surface) has not been addressed. Here we calculate that component of Earths changing energy budget, using ground-surface temperature reconstructions for the continents. In the last half-century there was an average flux of 39.1 mW m−2 across the land surface into the subsurface, leading to 9.1 · 1021 J absorbed by the ground. The heat inputs during the last half-century into all the major components of the climate system — atmosphere, ocean, cryosphere, lithosphere-reinforce the conclusion that the warming during the interval has been global.

Air‐ground temperature coupling and subsurface propagation of annual temperature signals

[1] Borehole-based reconstructions of ground surface temperature (GST) have been widely used as indicators of paleoclimate. These reconstructions assume that heat transport within the subsurface is conductive. Climatic interpretations of GST reconstructions also assume that GST is strongly coupled to surface air temperature (SAT) on timescales of decades and longer. We examine these two assumptions using records of SAT and subsurface temperature time series from Fargo, North Dakota; Prague, Czech Republic; Cape Henlopen State Park, Delaware; and Cape Hatteras National Seashore, North Carolina. The characteristics of downward propagating annual temperature signals at each site clearly indicate that heat transport can be described as one-dimensional conduction in a homogeneous medium. Extrapolations of subsurface observations to the ground surface yield estimates of annual GST signals and allow comparisons to annual SAT signals. All annual GST signals are modestly attenuated and negligibly phase shifted relative to SAT. The four sites collectively demonstrate that differences between annual GST and SAT signals arise in both summer and winter seasons, in amounts dependent on the climatic setting of each site. INDEX TERMS: 1645 Global Change: Solid Earth; 1875 Hydrology: Unsaturated zone; 3322 Meteorology and Atmospheric Dynamics: Land/atmosphere interactions; 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; 3367 Meteorology and Atmospheric Dynamics: Theoretical modeling; KEYWORDS: heat transport, air-ground termperature coupling, paleoclimate

Conduction‐dominated heat transport of the annual temperature signal in soil

[1] Conductive heat transport of temperature signals into the subsurface is a central assumption of ground surface temperature (GST) reconstructions derived from presentday temperatures in deep boreholes. Here we test this assumption and its implications for annual relationships between GST and surface air temperature (SAT) by analyzing two decades of shallow soil temperature (0.01–11.7 m) and SAT time series measured at Fargo, North Dakota. We spectrally decompose each of these temperature time series to determine the amplitude and phase of the annual signal at each depth. Conductive heat transport of a harmonic temperature signal in a homogeneous medium is characterized theoretically by exponential amplitude attenuation and linear phase shift with depth. We show that transport of the annual signal in the soil at Fargo follows these theoretical characterizations of conduction closely: the depth dependence of both the natural logarithm of the amplitude and the phase shift are highly linear. Interval wave velocities and thermal diffusivities calculated as functions of depth suggest a diffusivity gradient in the upper meter of the soil. We estimate the annual signal at the ground surface by extrapolating amplitude and phase shift regression lines upward to the surface. We compare this estimate of the annual signal at the ground surface to the annual signal contained in the SAT and show the ground surface signal to be attenuated � 20% and negligibly phase shifted relative to the SAT. INDEX TERMS: 1645 Global Change: Solid Earth; 1875 Hydrology: Unsaturated zone; 3322 Meteorology and Atmospheric Dynamics: Land/atmosphere interactions; 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; 3367 Meteorology and Atmospheric Dynamics: Theoretical modeling; KEYWORDS: paleoclimate, soil temperature, conductive heat transport

Variable seasonal coupling between air and ground temperatures: A simple representation in terms of subsurface thermal diffusivity

[1]xa0The utility of subsurface temperatures as indicators of temperature changes at Earths surface rests upon an assumption of strong coupling between surface air temperature (SAT) and ground surface temperature (GST). Here we describe a simple representation of this coupling in terms of a variable thermal diffusivity in the upper meter of the subsurface. The variability is tied to daily SAT, precipitation, and snow cover, but does not incorporate the physical details of these and the many other factors that influence the air-ground interface in many high-fidelity land-surface models. Our simple model reduces the difference between observed and modeled temperatures by a factor of 3 to 4 over a model with uniform diffusivity driven only by SAT. This simple representation of air-ground coupling offers a means of simulating subsurface temperatures using only archived meteorological records and creates the potential for examining the long term character of air-ground temperature coupling.

A model study of the effects of climatic precipitation changes on ground temperatures

[1] Temperature changes at the Earth surface propagate into the subsurface and leave a thermal signature in the underlying soil and rock. Inversions of subsurface temperature measurements yield reconstructions of ground surface temperature (GST) histories that provide estimates of climatic changes. A question remaining in the interpretation of reconstructed GST histories is the extent to which GST changes reflect changes principally in surface air temperature (SAT), or whether other factors may be significant. Here we use a Land Surface Processes (LSP) model to examine the influence of precipitation changes on GSTand subsurface temperature and moisture fields on annual to decadal timescales. We model soil and vegetation conditions representative of a prairie region in the southern Great Plains of North America and force the model with meteorological data synthesized from a typical year in the region. Model responses are observed after changes in the amount of daily precipitation, the intensity and frequency of daily precipitation, and the diurnal and seasonal timing of precipitation. We show that: (1) increasing daily precipitation cools mean annual GST, (2) increasing the intensity and reducing the frequency of daily precipitation, while holding the annual amount of precipitation constant, cools mean annual GST, and (3) shifting maximum precipitation to occur in the warmest months cools mean annual GST. We compare modeled results to observed precipitation changes during the 20th century and conclude that the observed precipitation changes would cause only small changes to GST within the modeled region, on the order of 0.1 K or less. INDEX TERMS: 3322 Meteorology and Atmospheric Dynamics: Land/ atmosphere interactions; 1878 Hydrology: Water/energy interactions; 3354 Meteorology and Atmospheric Dynamics: Precipitation (1854); 1866 Hydrology: Soil moisture; 1875 Hydrology: Unsaturated zone;

Surface temperature trends in Russia over the past five centuries reconstructed from borehole temperatures

[1]xa0We analyze borehole temperature logs from 101 sites in Russia and nearby areas to reconstruct the ground surface temperature history (GSTH) over the past five centuries. The data are drawn principally from three regions: the Urals, southwest Siberia, and northeast Siberia. We derive GSTHs for each region individually, and a composite “all-Russia” GSTH from the full ensemble of sites. The results show that over the past 500 years, the investigated areas have on average warmed ∼1 K, with more than half of the warming occurring in the 20th century alone, and 70–80% in the 19th and 20th centuries taken together. The 16th through 18th centuries in the Urals and southwest Siberia were on average 0.1–0.2 K cooler than at the beginning of the 19th century, but northeast Siberia was more moderate in the 16th through 19th centuries, relative to the present-day, than the Urals or southwest Siberia. A wide variety of instrumental, proxy, and indirect evidence support these geothermal results.

The influence of soil moisture upon the geothermal climate signal

Estimates of regional climate warming over the past few hundred years are being obtained from profiles of borehole temperature versus depth. The assumptions in recovering mean annual Surface Air Temperature (SAT) are that the relationship between the Ground Surface Temperature (GST) and the temperature-depth profile is purely conductive, and that SAT is uniquely coupled to GST. While these assumptions have been demonstrated to be approximately valid, they ignore the role of moisture transport in soil between soil and atmosphere. In this study we examine the influence of climatic changes in precipitation upon mean annual GST with climatic SAT held constant. We use the most recent version of our Prairie SVAT model for a set of 80 years simulations. Our findings are 10 increasing precipitation reduces mean annual GST, 2) phasing maximum precipitation to occur during the warmest months reduces mean annual GST, and 3) increasing the variance of precipitation reduces mean annual GST. The amplitudes of the effects are small but potentially not insignificant fractions of the geothermal climate signal. One of the long-term objectives of this investigation is to use global EOS SAT and remotely sensed soil moisture to link region-specific, geothermal climate signal histories to evolution of regional climate.