Continental heat gain in the global climate system

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95084/1/grl15494.pd

Propagation of linear surface air temperature trends into the terrestrial subsurface

Previous studies have tested the long-term coupling between air and terrestrial subsurface temperatures working under the assumption that linear trends in surface air temperature should be equal to those measured at depth within the subsurface. A one-dimensional model of heat conduction is used to show that surface trends are attenuated as a function of depth within conductive media on time scales of decades to centuries, therefore invalidating the above assumption given practical observational constraints. The model is forced with synthetic linear temperature trends as the time-varying upper boundary condition; synthetic trends are either noise free or include additions of Gaussian noise at the annual time scale. It is shown that over a 1000 year period, propagating surface trends are progressively damped with depth in both noise-free and noise-added scenarios. Over shorter intervals, the relationship between surface and subsurface trends is more variable and is strongly impacted by annual variability (i.e., noise). Using output from the FOR1 millennial simulation of the GKSS ECHO-G General Circulation Model as a more realistic surface forcing function for the conductive model, it is again demonstrated that surface trends are damped as a function of depth within the subsurface. Observational air and subsurface temperature data collected over 100 years in Armagh, Ireland, and 29 years in Fargo, North Dakota, are also analyzed and shown to have subsurface temperature trends that are not equal to the surface trend. While these conductive effects are correctly accounted for in inversions of borehole temperature profiles in paleoclimatic studies, they have not been considered in studies seeking to evaluate the long-term coupling between air and subsurface temperatures by comparing trends in their measured time series. The presented results suggest that these effects must be considered and that a demonstrated trend equivalency in air and subsurface temperatures is inconclusive regarding their long-term tracking

Ground surface temperature and continental heat gain: uncertainties from underground

Temperature changes at the Earthʼs surface propagate and are recorded underground as perturbations to the equilibrium thermal regime associated with the heat flow from the Earthʼs 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. We carry out a series of Monte Carlo experiments using 1000 Gaussian noise realizations and depth sections of 100 and 200 m as for steady-state estimates depth intervals, as well as a range of data sampling intervals from 10 m to 0.02 m. Results indicate that typical uncertainties for 50 year averages are on the order of ±0.02 K for the most recent 100 year period. These uncertainties grow with decreasing sampling intervals, reaching about ±0.1 K for a 10 m sampling interval under identical conditions and target period. Uncertainties increase for progressively older periods, reaching ±0.3 K at 500 years before present for a 10 m 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. We suggest 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)

Impact of borehole depths on reconstructed estimates of ground surface temperature histories and energy storage

Estimates of ground surface temperature changes and continental energy storage from geothermal data have become well-accepted indicators of climatic changes. These estimates are independent contributions to the ensemble of paleoclimatic reconstructions and have been used for the validation of general circulation models, and as a component of the energy budget accounting of the global climate system. Recent global and hemispheric analyses of geothermal data were based on data available in the borehole paleoclimatology database, which contains subsurface temperature profiles from a minimum depth of 200 m to about 600 m. Because of the nature of heat conduction, different depth ranges contain the record of past and persistent changes in the energy balance between the lower atmosphere and the ground for different time periods. Here we examine the dependency of estimated ground surface temperature histories and the magnitude of the subsurface heat content on the depth of borehole temperature profiles. Our results show that uncertainties in the estimates of the long-term surface temperature are in the range of ±0.5K. We conclude that previous estimates of ground surface temperature change remain valid for the period since industrialization, but longer-term estimates are subject to considerable uncertainties. The subsurface heat content shows a larger range of variability arising from differences in depth of the borehole temperature profiles, as well as from differences in the time of data acquisition, spanning four decades. These results indicate that estimates of subsurface heat should be carried out with caution to decrease cumulative errors in any spatial analysis

Characterizing land surface processes: A quantitative analysis using air-ground thermal orbits

A quantitative analysis of thermal orbits is developed and applied to modeled air and ground temperatures. Thermal orbits are phase-space representations of air and ground temperature relationships that are generated by plotting daily or monthly ground temperatures against air temperatures. Thermal orbits are useful descriptive tools that provide straightforward illustrations of air and ground temperature relationships in the presence of land surface processes related to snow cover, soil freezing, and vegetation effects. The utility of thermal orbits has been limited, however, by the lack of quantitative analyses that describe changes in orbits across different environments or in time. This shortcoming is overcome in the present study by developing a linear regression analysis of thermal orbits that allows changes to be tracked in time and space and as a function of depth within the subsurface. The theory that underlies the thermal orbit regression analysis is developed herein, and the utility of the application is demonstrated using controlled model experiments

Simulation of air and ground temperatures in PMIP3/CMIP5 last millennium simulations: implications for climate reconstructions from borehole temperature profiles

For climate models to simulate the continental energy storage of the Earth's energy budget they must capture the processes that partition energy across the land-atmosphere boundary. We evaluate herein the thermal consequences of these processes as simulated by models in the third phase of the paleoclimate modelling intercomparison project and the fifth phase of the coupled model intercomparison project (PMIP3/CMIP5). We examine air and ground temperature tracking at decadal and centennial time-scales within PMIP3 last-millennium simulations concatenated to historical simulations from the CMIP5 archive. We find a strong coupling between air and ground temperatures during the summer from 850 to 2005 CE. During the winter, the insulating effect of snow and latent heat exchanges produce a decoupling between the two temperatures in the northern high latitudes. Additionally, we use the simulated ground surface temperatures as an upper boundary condition to drive a one-dimensional conductive model in order to derive synthetic temperature-depth profiles for each PMIP3/CMIP5 simulation. Inversion of these subsurface profiles yields temperature trends that retain the low-frequency variations in surface air temperatures over the last millennium for all the PMIP3/CMIP5 simulations regardless of the presence of seasonal decoupling in the simulations. These results demonstrate the robustness of surface temperature reconstructions from terrestrial borehole data and their interpretation as indicators of past surface air temperature trends and continental energy storage

Impacts of the Last Glacial Cycle on ground surface temperature reconstructions over the last millennium

Borehole temperature profiles provide robust estimates of past ground surface temperature changes, in agreement with meteorological data. Nevertheless, past climatic changes such as the Last Glacial Cycle (LGC) generated thermal effects in the subsurface that affect estimates of recent climatic change from geothermal data. We use an ensemble of ice sheet simulations spanning the last 120 ka to assess the impact of the Laurentide Ice Sheet on recent ground surface temperature histories reconstructed from borehole temperature profiles over North America. When the thermal remnants of the LGC are removed, we find larger amounts of subsurface heat storage (2.8 times) and an increased warming of the ground surface over North America by 0.75 K, both relative to uncorrected borehole estimates

First assessment of continental energy storage in CMIP5 simulations

Although much of the energy gained by the climate system over the last century has been stored in the oceans, continental energy storage remains important to estimate the Earth's energy imbalance and also because crucial positive climate feedback processes such as soil carbon and permafrost stability depend on continental energy storage. Here for the first time, 32 general circulation model simulations from the fifth phase of the Coupled Model Intercomparison Project (CMIP5) are examined to assess their ability to characterize the continental energy storage. Results display a consistently lower magnitude of continental energy storage in CMIP5 simulations than the estimates from geothermal data. A large range in heat storage is present across the model ensemble, which is largely explained by the substantial differences in the bottom boundary depths used in each land surface component

Effects of bottom boundary placement on subsurface heat storage: Implications for climate model simulations

A one-dimensional soil model is used to estimate the influence of the position of the bottom boundary condition on heat storage calculations in land-surface components of General Circulation Models (GCMs). It is shown that shallow boundary conditions reduce the capacity of the global continental subsurface to store heat by as much as 1.0 x 10²³ Joules during a 110-year simulation with a 10 m bottom boundary. The calculations are relevant for GCM projections that employ land-surface components with shallow bottom boundary conditions, typically ranging between 3 to 10 m. These shallow boundary conditions preclude a large amount of heat from being stored in the terrestrial subsurface, possibly allocating heat to other parts of the simulated climate system. The results show that climate models of any complexity should consider the potential for subsurface heat storage whenever choosing a bottom boundary condition in simulations of future climate change

Long-term tracking of climate change by underground temperatures

Underground temperatures contain a record of past changes in the energy balance at the Earth's surface, such that borehole temperature data can be used to reconstruct long‐term trends of ground surface temperature (GST) changes. In addition to surface air temperature, however, GST is the response of the ground to other near surface processes that govern the surface energy balance. In order to compare GST histories constructed from geothermal data with surface air temperature (SAT) data, it is necessary to ascertain the relationship between these quantities. Here we jointly interpret four borehole temperature logs within a small area and SAT records from a nearby station. The subsurface temperature anomalies are consistent with the SAT data even in the presence of a variable snow regime, and different surface conditions. Our results indicate that borehole records are robust long‐term paleoclimatological indicators