Is recent climate change across the United States related to rising levels of anthropogenic greenhouse gases?

Global warming as a result of rising concentrations of anthropogenic greenhouse gases is predicted by current climate models. During the period 1948–1987, the concentration of anthropogenic greenhouse gases increased by more than 30%, and the mean annual temperature of the northern hemisphere increased by about 0.15°C. The mean annual temperature of the contiguous United States, however, does not show any significant trend. To gain a better understanding of why the United States' temperature record does not reflect the anticipated greenhouse warming, we studied the inter‐relationships between trends of temperature, cloudiness, sunshine and precipitation. Both the seasonal and annual trends for 23 geographic regions covering the United States were analyzed using Monte Carlo field significance tests. Several seasonal and regional differences were noted. While winters and autumns cooled, springs and summers warmed. Annually, cooling has occurred across the eastern half of the country, while warming dominates in the West. The largest changes in maximum temperature, daily temperature range, cloud amount, percent of possible sunshine and precipitation occur during autumn. Autumn also has the most significant correlations between trends. We found that the recent decrease of the maximum temperature and daily temperature range in autumn is statistically associated with increasing cloud amount and precipitation, and with decreasing sunshine. The widespread reduction in the temperature range is a result of decreased maximum and increased minimum temperatures. Cloud amount increased over most of the country during all seasons except spring. During spring the cloud amount remained fairly constant even though precipitation increased. Interestingly, no significant correlation was found between trends of mean temperature and cloud amount. Although several features of the recent climate change across the United States agree qualitatively with the model‐predicted impact of increasing anthropogenic greenhouse gases, the regional and seasonal distribution of the observed trends do not appear in line with the model results. We conclude that either the recent changes of temperature, cloud amount, sunshine and precipitation over the United States are as yet unrelated to the increasing anthropogenic greenhouse gases, or that the transient response of regional climates to the greenhouse effect is not proportional to the modeled difference between the 1×CO 2 and 2×CO 2 equilibrium climates.