Climatic Warming in North America: Analysis of Borehole Temperatures

The primary database used to assess climatic warming over the last 100 to 150 years is the history of surface air temperatures (SATs) as recorded on a daily basis for weather forecasting. Climatic information missing from the truncated SAT record may be found in borehole temperature profiles. Changes in ground surface temperature (GST) propagate into the subsurface, exponentially decreasing in amplitude with increasing time and depth. Studies to date have shown that changes in SAT tend to be tracted in GST changes and GST is a valid indicator of climate change. Studies of borehole temperatures provide a relatively good constraint on the total magnitude of warming; inferences concerning the date at which the warming trend began and the rate at which it proceeded are much less certain. The available evidence from both GST and SAT studies is sonsistant with a major climatic warming over the North American Continent that likely began near the middle of the 19th century in the east, later in the west. The magnitude of warming in the east estimated from changes in GST significantly exceeds that estimated from SAT. A cause and effect relationship between anthropogenic activities and climatic warming cannot be demonstrated unambiguously at themore » present time. 29 refs., 2 figs.« less