Nature of observed temperature changes across the United States during the 20th century

The annual temperature history of the United States during the 20th century shows 3 distinct periods of change: warming from 1900 until about 1940, cooling from 1940 to 1969, and warming from 1970 to the present. The characteristics of daily temperature change during these 3 periods are very different. The first 2 periods are marked by a tendency toward more temperature extremes -higher extreme maxima during the first period, and lower extreme minima during the second. In contrast, the warming during the most recent period, often used as evidence of humaninduced climate change, is characterized by temperature moderation -the pattern of temperature rise exhibits a strong, preferential warming of the coldest days of the year.