Average annual temperature has increased about 2°F since the beginning of the 20th century. Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century, with associated increases in heat wave intensity and decreases in cold wave intensity.
Nevada is the driest state in the United States, and future projections of average annual precipitation are uncertain. Increases in temperature are projected to lead to reductions in late winter and spring snowpack with potential negative impacts on water supplies.
Drought has been frequent since the beginning of the 21st century. Higher temperatures will increase the rate of loss of soil moisture during dry spells, increasing the intensity of future naturally occurring droughts. The frequency of wildfire occurrence and severity is projected to increase in Nevada and surrounding states.
Nevada is largely a dry state. Due mostly to the large elevation range (from less than 500 feet to more than 13,000 feet), the climate is highly diverse, ranging from scorching lowland desert in the south to cool mountain forests in the north. Las Vegas is one of the hottest cities in the United States with summer high temperatures averaging 102°F and regularly exceeding 110°F (about 8 days per year on average reach 110°F or higher). Much of the state lies within the Great Basin, a region between the Rockies and Sierra Nevada Mountains containing numerous small mountain ranges and high-elevation desert valleys. Due to the climate and rugged mountainous terrain, much of the land is sparsely populated. Nevada lies on the eastern side of the Sierra Nevada mountain range, which blocks moisture from the Pacific Ocean. The majority of the population lives in two concentrated urban areas—Washoe and Clark counties, supported by water from Lake Tahoe and the Colorado River, respectively. Nevada is the driest state in the United States with a statewide average annual precipitation of only 10 inches. Locally, average annual precipitation varies from 4 inches to more than 50 inches on high mountain peaks of the Sierra Nevada Mountains.
Temperatures in Nevada have increased about 2°F since the beginning of the 20th century (Figure 1). In the last 15 years, the annual number of days of extreme heat (above 95°F), averaged over the state, has been above average, with the highest 5-year averages occurring in 2000-2004 and 2005-2009 (Figure 2), partly because of very high values in 2002, 2003, and 2007. In addition to a general daytime warming, Nevada has experienced an above average occurrence of warm nights (minimum temperature >70°F) since 2000 (Figure 3). The state is the most urbanized in the nation with 94% of the population living in areas defined as urban. The urban heat island effect has likely exacerbated these trends in Las Vegas in particular where explosive growth has taken place.
After wet conditions in the late 1990s, the 2000s have been generally drier than normal; however, there is no overall trend in annual precipitation for the 118-year period of record (Figure 4). Seasonal precipitation patterns vary across the state, with most locations experiencing a maximum in precipitation during the winter months. However, eastern and southern areas, including Las Vegas, can experience intense rainfall in the summer from the North American Monsoon system.
Drought is a critical climate threat for this arid state (Figure 5). Since 2000, the Colorado River Basin, the source of water for the southern part of the state, has experienced drought conditions, with impacts on Lake Mead. In addition, precipitation was well below average from the 2011–2012 through the 2014–2015 water years (October–September) and each of those years was abnormally warm. This has led to a strain on water supplies in agricultural areas relying on surface water. A majority of counties have been designated as natural disaster areas due to the extreme drought conditions. Lake Mead, the largest man-made reservoir in the United States, provides water for southern Nevada, as well as Arizona, southern California, and northern Mexico. Water storage in Lake Mead is at 40 percent capacity, and water levels have been dropping since 2000 (Figure 6). Due to aggressive conservation policies, metropolitan areas have been able to manage the reductions in water supplies. Parallel declines in snowpack have been observed over this same time period (Figure 7). Snowpack refills Lake Tahoe every spring and lake levels slowly decrease throughout the year. Warm and/or dry years lead to low snowpack, which negatively affect the lake’s water levels. Since 1900, the lake has fallen below the natural rim 19 times (Figure 8).
Heat waves and wildfires are Nevada’s other most frequently occurring extreme weather events. Since 2004, the state has received multiple federal disaster declarations for wildfire events. Following the national 2012 drought, western wildfires burned an estimated 9 million acres causing $1 to $2 billion in damages, with Nevada being one of eight states affected. Damaging floods can also occasionally occur. Severe flooding along the Truckee River affected Reno in 1997, and again in 2005. Summer monsoon rains frequently lead to disruptive flooding in the Las Vegas Valley.
Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century (Figure 1). Even under a pathway of lower greenhouse gas emissions, average annual temperatures are projected to most likely exceed historical record levels by the middle of the 21st century. However, there is a large range of temperature increases under both pathways, and under the lower pathway, a few projections are only slightly warmer than historical records. The already extreme temperatures in Las Vegas will become even more extreme. Combined with the urban heat island effect, this will likely impact Las Vegas residents and tourists. Poor air quality during heat waves may also worsen chronic respiratory conditions and increase the risk of heat stress. Extreme temperatures are projected to increase substantially with potentially large impacts in the very hot southern deserts, particularly the Las Vegas metro area where 70% of the state’s population resides.
Projected rising temperatures will increase the average lowest elevation at which snow accumulates (the snow level). This will increase the likelihood that precipitation will fall as rain instead of snow, reducing water storage in the snowpack, particularly at lower elevations which are now on the margins of reliable snowpack accumulation. Warmer spring temperatures will also result in earlier melting of the snowpack, further decreasing water availability during the already dry summer months.
Projections of annual precipitation are uncertain over the 21st century (Figure 9), but warmer temperatures are likely to decrease the amount of water in the mountain snowpack and increase the demand for water, creating challenges for water management. Higher temperatures will also increase evaporation which will reduce streamflow and soil moisture. Thus, the intensity of future droughts is likely to increase. More intense future droughts will also increase the risk of wildfires. Increases in urban population and potentially decreased water flow from the Colorado River may lead to water security issues in the future.