A decade of air-ground temperature exchange from Fargo, North Dakota

In borehole paleoclimatology, it is commonly assumed that a direct coupling exists between air and ground temperatures. This assumption is valid only if variables affecting ground-surface temperature exchange have remained constant through time. In an analysis of a 9-year record of air and ground temperature data, we found that several critical variables changed in ways that cause decoupling between air and ground temperatures. Mean-annual ground temperatures in the upper 12 m increased by 0.93 +/- 0.09 degreesC during the study. Air temperatures used as model-forcing signals generated ground temperatures that exhibit no significant increase. The decoupling of winter air and ground temperatures is due to snow cover and latent energy effects. Maximum residual temperatures for freezing, summer and thawing modeling periods averaged +/-0.18 degreesC, +/- 0.30 degreesC, and +/-0.75 degreesC, respectively. Duration of winter snow cover increased during the time of record and correlates with winter air-ground temperature differences (r(2) = 0.71). Annual values of modeled latent energy of ground freezing show a dependence upon total precipitation 60 days prior to ground freezing. (C) 2001 Published by Elsevier Science B.V.