Problems in evaluating regional and local trends in temperature: An example from eastern Colorado, USA

We evaluated long-term trend,, in average maximum and minimum temperatures. threshold temperatures, and growing season in eastern Colorado. USA. to explore the potential shortcomings of many climate-change studies that either: ( 1) generalize regional patterns from single station,,. single seasons. or a few parameters over short duration from averaging dissimilar stations: or (2) generalize an a average regional pattern from coarse-scale general circulation models. Based on I I weather stations, some trends Acre weakly regionally consistent with previous studies of night-time temperature warming. Long-term (80 + year,,) mean minimum temperatures increased significantly ( P < 0.2) in about half the stations in winter. spring. and autumn and six Stations had significant decreases in the number of days per year with temperatures less than or equal to - 17.8 C (less than or equal to0 F), However, spatial and temporal variation in the direction of change as enormous for all the other weather parameters tested. and, in the majority of tests, few stations showed significant trend, (even at P < 0.2). n summer. four stations had significant increases and three stations had significant decreases in minimum temperature,,,. producing a strongly mixed regional signal, Trend,, in maximum temperature varied seasonally and geographically, as did trends in threshold temperature days greater than or equal to32.2 C (greater than or equal to90 F) or days greater than or equal to37.8 C (greater than or equal to100 F). There was evidence of a subregional cooling in autumn's maximum temperature,,, with five stations showing significant decreasing trends. There were many geographic anomalies where neighbouring weather stations differed greatly in the magnitude of change or where they had significant and opposite trends. We conclude that sub-regional spatial and seasonal variation cannot be ignored when evaluating the direction and magnitude of climate change, It is unlikely that one or a few weather stations are representative of regional climate trends. and equally unlikely that regionally projected climate change from coarse-scale general circulation models will accurately portray trends at sub-regional scales. However. the assessment of a group of stations for consistent more qualitative trend, (Such as the number of days less than -17.8 degreesC, such as we found) provides a reasonably robust procedure to evaluate climate trends and variability. Copyright (C) 2002 Royal Meteorological Society.