Français

Carolyne Pickler

Doctorate's candidate at UQAM

Supervisors: Dr. Hugo Beltrami, Dr. Laxmi Sushama & Dr. Jean-Claude Mareschal
Enrolled in CREATE program since January 2013

Borehole Climatology : Retrieving Climate from Under Ground

The climate system response to radiative forcing is a crucial area of research because the interpretation of GCMs’ future climate projections depends upon it. Thus, a record of past climatic changes is needed to separate natural climate variability from changes induced by anthropogenic activities. This matter is essential in assessing societal actions to be taken regarding future climate change. In the last few years an important growth in research on climate variability of the last millennium has taken place, fuelled by an increasing concern about rising global temperatures (e.g. Houghton, 2005; Solomon et al., 2007). Paleoclimate research has helped to place the relatively short instrumental record in a broader temporal context (Jones et al., 2001; Mitchell et al., 2001; Jones and Mann, 2004; Mann, 2007), with results coming from proxy climate reconstructions and from paleoclimatic simulations from GCMs. Paleoclimatic reconstructions are built from a variety of proxy indicators (tree-ring variables, ice cores, etc.) which provide information at different time resolutions and sensitivities to a variety of climate parameters (e.g. Jones and Mann, 2004), while climate reconstructions for the last millennium exist at local, regional and global scales (e.g. Moberg et al., 2005).

Data from borehole temperature profiles (BTPs) have been one source of information that has significantly contributed to our understanding of centennial temperature changes (e.g. see Pollack and Huang, 2000; Bodri and Cermak, 2007 for reviews). Climate reconstructions based on BTPs assume that surface air temperature (SAT) changes are coupled to GST changes and propagate to the subsurface by thermal conduction. In other words, if we assume the Earth’s upper crust is at thermal equilibrium, then the temperature distribution in the upper few kilometers is determined by the long-term (>1000 years) surface temperature and the internal heat flow (constant at the 100-105 year scale). Under these conditions, for known thermal properties of the rocks, the temperature increases linearly with depth. If the temperature at the Earth’s surface changes, then a quantity of heat will be gained or lost by the ground; these changes in the energy balance at the surface propagate and are recorded underground as perturbations to the equilibrium thermal regime. The downward propagation of the climate disturbance is a function of the thermal diffusivity of the rock (ca. 10−6 m2s−1) so that the first few hundred meters of the surface crust store the integrated thermal signature of the last millennium.

Due to the nature of heat conduction (Carslaw and Jaeger, 1959) the amplitude of surface disturbances is exponentially attenuated and their phase shifted with depth as a function of their time scales. The process operates as a low pass filter such that the low-frequency variations propagate deeper than those with higher frequency. Typically, perturbations penetrate about 20 m in a year, 150 m in 100 years and about 500 m in a millennium, such that recent energy balance changes at the surface remain recorded in the shallow subsurface. Analysis of these underground perturbations provides the base of the Borehole Climatology method. While the ground integrates the effects of energy exchanging processes at the air-ground interface, continuously recording the energy balance at the surface, other surface factors such as changes in vegetation cover, underground hydrology, topography variations, lateral heat conduction, etc., can all potentially affect the underground thermal regime independently of climate. Before data is analyzed most works on the borehole method screen affected data.

Mathematically, the recovery of the GST history can be described as determining the time dependent surface temperature boundary condition that has given rise to an observed BTP. Several methodologies have been developed for this purpose (See González-Rouco et al.; and references therein; Beck et al., 1992; Shen et al., 1992, 1996). Contrary to other climate reconstruction procedures, temperature inversions obtained from borehole data are not calibrated against the instrumental record and provide an independent measurement of past temperature. Climate reconstructions from borehole temperature profiles have gained increased recognition among climate researchers since the so-called “climategate” incident as Borehole Climatology’s results support the IPCC report, but have remained outside this controversy because they are independent of the meteorological record.

One very important contribution of Borehole Climatology to climate research has been the clarification of the role of heat storage in the shallow crust within the global climate system energy budget (Levitus et al., 2001; Pielke, 2003; Levitus et al., 2005; Hansen et al., 2005) through the calculation of the energy stored in the ground in the latter half of the 20th century. These first estimates of heat absorbed by the Earth’s continents in the last 50 years is of the same order of magnitude as the heat absorbed by the atmosphere (8.0x1021 J) (Beltrami, 2001b; Beltrami et al., 2002). The work on Beltrami (2001b), Beltrami et al., (2002), Beltrami et al. (2006) and Levitus et al. (2001, 2005) comprise Figure 5.4 of the IPCC (2007) showing that the present warming of the planet has a global character. Significant work has been conducted on to understand the long-term SAT and GST coupling within the BC context (e.g. Smerdon and Stieglitz, 2006). During the last few years climate model simulation studies have targeted questions related to Borehole Climatology from a variety of perspectives. As in the case of other reconstruction methodologies, some studies have addressed aspects of the borehole method using model simulations as an artificial reality in which the coupling between SAT and GST, and the uncertainties mentioned above can be addressed (e.g. Mann and Schmidt, 2003; Gonzalez-Rouco et al., 2009, and references therein).

Other studies have compared model simulations and borehole reconstructions serving both the purpose of model validation and also the understanding of mechanisms contributing to climate variability (e.g. Beltrami et al. 2006; Stevens et al. 2008). Further investigations involve the study of the expected change of subsurface energy in climate change scenarios and also whether climate models address the physics of the subsurface to account for subsurface heat content [e. g. MacDougall et al. (2010), Stevens et al. 2007a, Stevens et al. 2008, Risk et al. 2008]. At the moment one of the great problems facing borehole climatology is the lack of data from the southern hemisphere, and lack of a coordinated world-wide data collection effort.

The project will consist of exploring the borehole uncertainities and sources of error. Because the results from BC have now reached the broad community in climate, it is important to continue developing analytical and numerical tools for interpreting the data, because uncertainties on the GST histories and subsurface heat content have been largely overlooked. Data will be analyzed to study the dependency of the GT and HF perturbations on the underlying thermo-physical structure and from the effects of the last glaciation on the thermal regime of the subsurface.

Another important point that will be focused on is the subsurface thermal effects of landuse and air-ground coupling using regional models. In MacDougall et al. (2010), Stevens et al. (2007a), Beltrami et al. (2006), and MacDougall et al. (2008) we showed that a large amount of heat absorbed by the continents is neglected in models. Reconciling the ground heat flux and temperature histories from geothermal data with model simulations as well as with meteorological and proxy records requires also the clarification of the heat transfer regime at the air/ground interface. For example, what is the relation between air and soil temperature at high and low frequencies, if the regime is not purely conductive? Does mass heat transfer dominate over conduction? What are the effects of surface conditions, slope orientation, vegetation cover, land use, soil moisture and ice content? This is a difficult problem, because of the large number of interacting processes involved (Nitoiu and Beltrami, 2005; Smerdon et al., 2009; Lesperance et al., 2010; Bense and Beltrami, 2007; Ferguson et al., 2006; Ferguson and Beltrami, 2006). This is serious problem since an important energy sink is ignored and models could affect the understanding of energy dependent processes in the shallow subsurface (MacDougall et al., 2010). Balancing this is an urgent task for climate modelers (Hansen et al., 2005). The goal of the project is to help in this new science with the use of regional models MM5 and WRF as tools to study.


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