April 20th, 2016

General Circulation Models (GCMs) are complex mathematical tools used to simulate the past, recent and future evolution of the Earth’s climate. Each GCM uses different approaches to solve the mathematical equations that describe the physical phenomena and processes driving our planet’s climate.  They allow us to ask important questions about how the climate system works and how it is expected to change in the future under different greenhouse gas emissions scenarios. In the most recent report by the Intergovernmental Panel on Climate Change (IPCC-AR5), published in September 2014, a large set of GCM simulations were used to evaluate and understand the impacts of climate change, as well as to study climate system dynamics; these experiments were also used to inform the Conference of the Parties (COP21) international climate agreement of December 2015 in Paris.

Each GCM’s performance is evaluated over many time and spatial scales to check the reliability of their projections as well as the reproducibility of known present climatic and paleoclimatic conditions. These controls rely on observational and paleoclimatological data. Paleoclimatic records are mostly a variety of indirect climate indicators – or proxy data, such as tree-ring, corals or oxygen isotope data from ice cores. The IPCC-AR5 report published an ensemble of temperature reconstructions for the last millennium compiled from a variety of records. Temperature reconstructions from geothermal data are included in this ensemble, which suggest a larger warming over the period extending from the sixteenth to the nineteenth centuries, known as the Little Ice Age. Such discrepancies among temperature reconstructions require that all forms of paleoclimatic information be carefully assessed, including those based on geothermal data.

To help interpret the geothermal data, M. Sc. students Almudena García-García and Francisco José Cuesta-Valero (both in StFX’s NSERC CREATE program in Climate Sciences) collaborating with Dr. Hugo Beltrami (St. Francis Xavier University, Canada Research Chair in Climate Dynamics) and Dr. Jason E. Smerdon (Lamont-Doherty Earth Observatory of Columbia University) have used five simulations of the climate of the last millennium from the set of IPCC-AR5 simulations.  These simulations provided the researchers with a synthetic ‘laboratory’ that can be used to examine in detail the relationship between the air and ground temperatures. Geothermal data come from temperature measurements typically made in mining exploration boreholes and they extend from the surface to several hundred meters underground.  These data have been used as part of the paleoclimatological record as they allow for the reconstruction of past variations in ground surface temperature, as well as provide estimates of continental heat storage. Nevertheless, to relate the paleoclimatic information retrieved from borehole temperature data to other above ground indicators of climate change, it is assumed that there exists a stable temporal relationship between the lower atmosphere and subsurface, that is, a coupling between near-surface air temperatures and subsurface temperatures. The stable relationship between air and ground temperatures have been supported by previous studies, but some analyses have shown the existence of a short-term decoupling between air and ground temperatures in winter because of snow cover and ground freezing, which could in principal violate assumptions used to interpret the borehole data. Understanding air-ground coupling and the potential effect of the winter short-term decoupling on the past temperature reconstructions is therefore important for incorporating the reconstruction of past climate from geothermal data in the larger set of paleoclimatological records.

The authors present the results of their study in a recent paper published in Environmental Research Letters [1], in which they evaluate the coupling between air and ground temperatures using the five IPCC last millennium GCM simulations. The authors find a strong simulated coupling between air and ground temperatures in summer from 850 to 2005 CE within all of the simulations. The insulating effect of snow and ground freezing do produce a short-term decoupling between air and ground temperatures in the simulations at northern high latitudes in winter.  For all the simulations, however, the paleoclimatic reconstructions retain the long-term variations of the simulated surface temperatures throughout the last millennium.

These findings provide additional support for the use of underground temperatures for reconstructions of past variations in ground surface temperature, and as indicators of climate change in last millennium. The group’s research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grants and CREATE programs, as well as from the Canada Research Chairs program.