ABSTRACT: Climate change inferred from borehole temperatures: minimal "snow effect" from North America.

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Borehole temperature-depth profiles contain information about surface ground temperature histories over time scales of several centuries and in particular prior to the widespread availability of surface air temperature records [{\it Huang et al, Nature, 2000; Harris and Chapman, GRL, 2001}]. Borehole-based reconstructions on the regional and hemispheric scale yield significantly different magnitudes of warming in the past 500 years when compared to proxy-based reconstructions. Borehole reconstructions suggest that the Northern Hemisphere warming has been about 1.1 C while proxy methods indicate warming closer to 0.7 C [{\it Mann et al, Nature, 1999}]. One suggested reconciliation of borehole and proxy reconstructions is that long-term variations in seasonal snow cover may bias the borehole record. A spurious long-term warming signal relative to SAT trends could be introduced by alteration of the duration or onset of seasonal snow cover over the course of decades or longer. We have developed a "snow effect" model that predicts transient warming or cooling of the surface ground temperature due to changes in the onset, duration, and depth of snow events [{\it Bartlett et al, in review}]. We use our model to compute the response of ground temperatures at the regional scale to seasonal snow cover of the past century in North America. Snow and air temperature data used in the model come from the United States Historical Climatology Network (NOAA-NCDC NDP-070) and the Canadian Daily Climatic Dataset (CDCD). Results indicate that variations in snow onset and duration have had the greatest influence in Central North America, leading to ground warming on the order of 0.1-0.2 C / 100 yrs in this region relative to SAT trends. Other regions within North America have experienced negligible effects over the past century. We conclude that the magnitude of the snow effect in North America is insufficient to reconcile completely regional borehole and proxy reconstructions of climate change. References:\\ Bartlett, M.G., D. S. Chapman, and R. N. Harris, Snow and the ground temperature record of climate change, {\it JGR - Earth Surface}, submitted, 2004.\\ Harris, R. N. and D. S. Chapman, Mid-Latitude (30-60deg N) climatic warming inferred by combining borehole temperatures with surface air temperatures. {\it GRL}, 28, 747-750, 2001.\\ Huang, S., H. N. Pollack, and P. Y. Shen, Temperature trends over the past five centuries reconstructed from borehole temperatures, {\it Nature}, 403, 756-758, 2000.\\ Mann, M. E., R. S. Bradley, and M. K. Hughes, Global-scale temperature patterns and climate forcing over the past six centuries, {\it Nature}, 392, 779-787, 1998.

Bibliography of Canadian Geomorphology