Tropical storm activity in the Atlantic Ocean, the Caribbean, and the Gulf of Mexico has increased during the past 20 years. The occurrence of abnormally high annual precipitation totals (as defined by the National Oceanic and Atmospheric Administration) has also increased. Nationwide, nine of the top 10 years for extreme one-day precipitation events have occurred since 1996. The prevalence of extreme single-day precipitation events remained fairly steady between 1910 and the 1980s but has risen substantially since then. In recent years, a higher percentage of precipitation in the United States has come in the form of intense single-day events.

However, shifting weather patterns have caused certain areas, such as the Southwest, to experience less precipitation than usual. Since 1901, precipitation has increased at an average rate of 0.2 inches per decade over the contiguous 48 states. Total annual precipitation has increased over land areas in the United States and worldwide. The average heat wave season is 49 days longer, and individual heat waves are lasting longer and becoming more intense. Heat waves are occurring three times more often than they did in the 1960s-about six per year compared with two per year. Heat waves are occurring more than they used to in major cities across the United States. Record-setting daily high temperatures have become more common than record lows. Although the United States has experienced many winters with unusually low temperatures, unusually cold winter temperatures have become less common-particularly very cold nights (lows). This trend indicates less “cooling off” at night. Unusually hot summer nights (lows) have become more common at an even faster rate. Since the 1970s, unusually hot summer days (highs) have become more common over the last few decades in the United States. Many extreme temperature conditions are becoming more common. Spring temperatures have increased by about 2☏, while summer and fall temperatures have increased by about 1.5☏. Since 1896, average winter temperatures across the contiguous 48 states have increased by nearly 3☏.

As the Earth warms overall, average temperatures increase throughout the year, but the increases may be larger in certain seasons than in others. Within the United States, temperatures in parts of the North, the West, and Alaska have increased the most. Average global temperatures show a similar trend, and all of the top 10 warmest years on record worldwide have occurred since 2005. Nine of the top 10 warmest years on record have occurred since 1998.

Average temperatures have risen across the contiguous 48 states since 1901, with an increased rate of warming over the past 30 years. While increased precipitation can replenish water supplies and support agriculture, intense storms can damage property, cause loss of life and population displacement, and temporarily disrupt essential services such as transportation, telecommunications, energy, and water supplies. More frequent and intense extreme heat events can increase illnesses and deaths, especially among vulnerable populations, and damage some crops. More extreme variations in weather are also a threat to society. For example, warmer average temperatures could increase air conditioning costs and affect the spread of diseases like Lyme disease, but could also improve conditions for growing some crops. Long-term changes in climate can directly or indirectly affect many aspects of society in potentially disruptive ways. This chapter focuses on observed changes in temperature, precipitation, storms, floods, and droughts. Scientific studies indicate that extreme weather events such as heat waves and large storms are likely to become more frequent or more intense with human-induced climate change. Rising global average temperature is associated with widespread changes in weather patterns.

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