Extreme temperatures can create dangerous conditions for people and may lead to property damage. Summer temperatures in the arid Southwest can reach dangerously high levels, and temperatures around or above 38°C (100°F) are not uncommon. High heat can lead to a series of health complications if not properly dealt with—heat exhaustion, heat stroke, and dehydration can all result from exposure to extreme temperatures. Since the human body can only survive a few days (typically three) in the desert without water, a stranded and unlucky hiker or camper can easily die of dehydration if a suitable water supply cannot be reached in time. Heat waves are periods of excessively hot weather that may also accompany high humidity. Temperatures of just 3°C (6°F) to 6°C (11°F) above normal are enough to reclassify a warm period as a heat wave. Under these conditions, the mechanism of sweating does little to cool people down because the humidity prevents sweat from evaporating and cooling off the skin. Heat waves have different impacts on rural and urban settings. In rural settings, agriculture and livestock can be greatly affected. Heat stress recommendations are issued to help farmers protect their animals, particularly pigs and poultry, which, unlike cattle, do not have sweat glands.

The impacts of heat waves on urban settings include a combination of the natural conditions of excessive heat and the social conditions of living in a densely populated space. Cities contain a considerable amount of pavement, which absorbs and gives off more heat than vegetation-covered land does. Air conditioning units that cool down the inside of buildings produce heat that is released outside. Pollution from cars and industry also serve to elevate the outdoor temperatures in cities. This phenomenon, in which cities experience higher temperatures than surrounding rural communities do, is known as the heat island effect. Other social conditions can increase the hazards associated with heat waves in urban areas. People who are in poor health, live in apartment buildings with no air conditioning, or are unable to leave their houses are at greatest risk of death during heat waves. In the summer of 2015, a recordsetting heat wave occurred across the desert Southwest, scorching Arizona and leaving Phoenix with a new daily record of 46°C (115°F). Temperatures hovered above 38°C (100°F) even later than 10 pm. The heat wave contributed to severe drought, amplified heat-based health emergencies, and caused a heavy spike in electricity usage (related to increased air conditioning use) that generated a record-breaking demand on the power grid. Other times when heat waves have affected the Southwestern states include the heat wave of June-July 2013, which scorched the Southwest and Great Plains, baking Utah and setting records for extreme heat across New Mexico (Figure 9.33); and the heat wave of June-July 2012, which broke previous record highs across Colorado.

Figure 9.33: Air temperatures across the continental US during July 2013.

While high temperatures can be directly dangerous, a larger scale hazard arises when these temperatures are coupled with lack of precipitation in an extended drought period. The Southwest has experienced both short-term and even decade-long periods of drought. Unlike other hazards, drought sets in slowly and takes time to be recognized. Agricultural areas can be seriously affected by a lack of rainfall and insufficient water supplies. Even higher-altitude forests show signs of stress since the combination of heat and long-term lack of precipitation deprives the land of one of its key resources. Lack of precipitation does not simply mean a lack of rain—it also means less seasonal snowfall in the mountains. Relatively little mountain snow in the winter translates into a lack of water for crop irrigation and household use in desert portions of the Southwest. Change in the flow rate of the Colorado River, which originates in the Rocky Mountains, serves as an excellent diagnostic for the effects of drought. This river is crucial for the irrigation of crops, and it feeds manmade reservoirs such as Lake Powell that supply drinking water to much of the region.

Many significant droughts have occurred in the Southwestern states—one notable instance of catastrophic drought in the Southwest was the Dust Bowl of the 1930s. Severe drought led to a drying of much of the topsoil, which was crucial to the agriculture of the area. High winds stripped the land of this topsoil, making crop growth impossible. This, in turn, led to the collapse of the farming industry, which was one of the main factors contributing to the Great Depression. More recently, severe drought conditions in 2002 forced Denver, Colorado to impose mandatory limits regarding water use; in addition, from 2011–2014, New Mexico was struck by its worst drought since the Dust Bowl. As of May 2016, much of the Southwest, especially Arizona, is experiencing conditions of moderate drought or abnormal dryness. Compiled tree-ring records over the past several thousand years shows that there have been past “megadroughts” that have been worse, and lasted longer, than recent ones. Models suggest that the likelihood of such droughts is expected to increase due to the effects and continuing patterns of climate change. Recent research using both models and data suggests that the climate of the Southwestern US has become and will remain drier, as subtropical dry zones move north.

Careful planning for seasonal drought, as well as extended drought, is the most effective way to reduce the chance of storage depletion in the Southwest. Conservation must be implemented as a series of progressive steps to be taken as water becomes scarcer. Out of necessity, the Southwest actually implements some of the most effective water management strategies in the United States. Still, no amount of planning can eliminate the long-term threat of drought, especially in an area dominated by deserts and under threat of the influence of changing climate.