Arizona
Arizona, the sixth-largest U.S. state, encompasses diverse climates and topography. The deserts in the south are some of the hottest and driest areas of the country, while the higher terrain of the Colorado Plateau in the northeast has a cooler climate, with cold winters and mild summers. The mountain ranges that run from the northwest to the southeast experience heavier precipitation and wide temperature variations. Annual average (1991–2020 normals) temperatures range from the 40s (°F) at the highest elevations in the mountains to the mid-70s (°F) in the lower elevations of the south. The southern deserts frequently experience summer temperatures between 105°F and 115°F. Phoenix has the hottest climate of all major U.S. cities. Extreme temperatures in Arizona range from a record high of 128°F at Lake Havasu City (June 29, 1994) to a record low of −40°F at Hawley Lake (January 7, 1971). The hottest year on record was 2017, with a statewide annual average temperature of 63.0°F, which is 3.3°F above the long-term (1895–2020) average.
Figure 1
Temperatures in Arizona have risen about 2.5°F since the beginning of the 20th century. The first 21 years of this century have been the warmest period on record for the state (Figure 1). Since 1995, the number of days with a maximum temperature of 100°F or higher has been near to above average, reaching a record high during the 2015 to 2020 period (Figure 2a).The number of nights with a minimum temperature of 80°F or higher has been trending upward since 1995, also reaching a record high during the 2015 to 2020 period (Figure 3); this increase in high nighttime minimums is observed statewide, but the increase is much larger in the Phoenix metropolitan area. The number of nights with a minimum temperature of 0°F or lower has been below average since 1980 (Figure 4). One notable trend is an increase in both daytime high and nighttime low summer temperatures, which has implications for the intensity of future heat waves in a state that already experiences very hot conditions (Figures 2b and 2c).
Much of Arizona is characterized as arid to semiarid, with annual average precipitation ranging from less than 4 inches in the southwest to around 40 inches in the White Mountains in the east-central region. Precipitation is highly variable from year to year, with statewide total annual precipitation ranging from a low of 6.0 inches in 1956 to a high of 22.8 inches in 1905. The driest multiyear period occurred during the early 1900s and was immediately followed by the wettest multiyear period (Figure 2d). The driest consecutive 5-year interval was 1899–1903, with an annual average of 8.3 inches of precipitation, and the wettest was 1905–1909, with an annual average of 16.5 inches. The years since 1995 have also been relatively dry, with 17 of the last 26 years experiencing below average precipitation. Snowfall is rare in the southern desert region but does occur at the higher elevations, where it can reach depths of more than 100 inches. Snowpack plays a critical role in supplying water for both urban and agricultural areas in the lower Salt River valley and the lower Gila River valley and is vital for forest health and groundwater recharge across the entire state.
An important feature of Arizona’s summer climate is the North American Monsoon, which causes large amounts of rain to fall from late June or early July to mid-September. Precipitation during the monsoon season is highly variable from year to year (Figure 5). Since 2000, monsoon precipitation has been below average, with the exception of the 2010–2014 period, which was above average due to warmer sea surface temperatures off the Pacific coast and a very active hurricane season in 2014. In the southernmost portion of the state, monsoon rainfall accounts for more than half of the annual precipitation and plays an important role in supporting agriculture and ecosystems. The monsoon rains are highly beneficial but can occasionally be destructive. On September 8, 2014, extremely heavy monsoon rain associated with a decaying eastern Pacific hurricane caused significant damage and flooding around the Phoenix area. The record for single-day rainfall was broken, with several stations reporting more than 4 inches. The 2020 monsoon season was the driest on record, with only 1.5 inches of precipitation, well below the previous record low of 2.8 inches in 2009.
The historical record indicates periodic prolonged wet and dry periods (Figure 6). Arizona is currently in a long-term drought that has lasted more than 20 years. Multiyear periods of high and low precipitation can cause significant variations in reservoir supplies. The latest western U.S. drought has resulted in record-low water levels in Lake Mead, which is a critical water resource for Arizona, as well as southern Nevada, southern California, and northern Mexico. Since reaching high levels in the late 1990s, water levels have been falling, reaching historic lows in 2015 and 2016 (Figure 7). Long-term droughts also raise the risk of wildfires, already a concern for this arid state. In 2011, the Wallow Fire consumed more than 500,000 acres in eastern Arizona, making it the state’s largest wildfire on record.
Unlike many areas of the United States, Arizona and other southwestern states have not experienced an upward trend in the frequency of extreme precipitation events. The number of 1-inch extreme precipitation events has been variable throughout the period of record (Figure 2e). Since the 1990s, the number of these events has been near to below normal, with the exception of the 2010–2014 period.
Under a higher emissions pathway, historically unprecedented warming is projected during this century (Figure 1). Even under a lower emissions pathway, annual average temperatures are projected to most likely exceed historical record levels by the middle of the century. However, a large range of temperature increases is projected under both pathways, and under the lower pathway, a few projections are only slightly warmer than historical records. Extreme heat is a particular concern for Phoenix and other urban areas, where the urban heat island effect raises summer nighttime temperatures. Rising temperatures will increase the intensity of future heat waves, which is a concern for a state that already experiences extremely hot conditions.
Although projections of overall annual precipitation are uncertain, there is a risk of decreases in spring precipitation (Figure 8); Arizona is on the northern fringe of an area of projected decreases over Mexico and Central America. Additionally, projected rising temperatures will raise the snow line—the average lowest elevation at which snow falls. This will increase the likelihood that precipitation will fall as rain instead of snow, reducing water storage in the snowpack, particularly at lower mountain elevations that are now on the margins of reliable snowpack accumulation. Higher spring temperatures will also result in earlier melting of the snowpack, further decreasing water resources needed for irrigation during the hot summer months.
Naturally occurring droughts are expected to become more intense during the cool season. As noted above, future projections of overall precipitation are uncertain, including those related to the North American Monsoon. However, even if precipitation does not decrease, higher temperatures will intensify naturally occurring droughts by increasing water evaporation. This will further reduce streamflow, soil moisture, and water supplies. Drought will not only challenge limited agricultural resources but also increase the frequency of dust storms and the frequency of the risk of very large wildfires.