Submitted to: Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/30/2001
Publication Date: N/A
Interpretive Summary: There is considerable public concern and much uncertainty about the possible impact of rising carbon dioxide concentrations on the earth's climate, particularly changes in temperature. Climate models have been used extensively in this area, but shortages of certain types of data have hindered the development and testing of these models. To address this we have analyzed a 37 year record of soil temperatures collected at depths down to 12.8 m at a field site in St. Paul, MN. The depth and temporal extent of this data set are unique. We found that mean ground temperature has increased more than 1 degree C at all depths over this time period. This indicates ground storage of heat at a rate of approximately 1.3 MJ per square meter per year (about 29,000 calories per square foot per year). Current global climate models are incapable of properly simulating ground heat storage and generally ignore it. Our results suggest that this will cause such models to over predict changes in surface and air temperatures. However, the data should be useful in refining models to correct this problem, resulting in more accurate predictions of global climate change.
Technical Abstract: We analyzed a long-term (37 year) record of monthly average below-ground temperatures, at depths ranging from the surface down to 12.8 m, to determine the ground heat flux. Temperatures at all depths have increased over the period, evidence of a non-zero mean ground heat flux. Analysis indicates an average downward flux out of the root zone (below 1.6 m) over the period of approximately 1.3 MJ m**-2 year++-1. The corresponding average flux through the bottom plane of measurement has been approximately 0.22 MJm**-2 year**-1, indicating that 17 percent of the heat storage during the period has been at depths greater than 12.8 m. Current representations of ground heat flux in global climate models are inadequate to realistically simulate these results, perhaps one reason why model estimates of atmospheric warming have often exceeded observations.