CLimate Change in Nevada

Nevada’s climate is changing. This is now being observed across the diversity of its climates, from the cool high mountains of the eastern Sierra Nevada and the Spring Mountains, to the uplands of the Humboldt River and the blistering heat of the Mojave Desert in the south. In fact, Nevadans say, they are already noticing and impacted by these changes. Climate change has come home.

Climate change has come home.

Just as the current climate varies from place to place in the state, future climate change will also vary in its particulars. Its impacts will manifest in different ways for different communities, economic sectors, and ecosystems. For example, flooding of the Humboldt River has different risks and impacts than flooding in Reno due to different population densities, economies, and infrastructure.

The table of climate impacts in Nevada (Table 1) provides an overview of historical trends and future projections for some major climate variables and how they may affect public health, water resources, the environment, hospitality, and agriculture. The remainder of this section describes in more detail what is known about these past and future changes in Nevada’s climate and what they imply for the state.

The current release of carbon into the atmosphere is unprecedented and more rapid than at any time over the past 56 million years.

Climate Change in a Geological Context: Are the Changes Today Like Those in the Past?

How does the observed rate of greenhouse gas (GHG) increases compare to the past? To provide some perspective on the current and future rates of climate change, a geological context is useful. Perhaps the best analog of a past rapid release of carbon to the atmosphere happened about 56 million years ago during the Paleocene-Eocene Thermal Maximum (PETM). At that time, a large amount of carbon (2,500 to 4,500 Pg (1015) of carbon) stored in the ocean was released to the atmosphere over a duration of at least 4,000 years (Zeebe et al., 2016). This corresponds to a release rate of ~0.6 to 1.1 Pg of carbon per year. For comparison, currently about 10 Pg of carbon are being released each year associated with fossil-fuel combustion and related processes (Hayhoe et al., 2017). These data indicate that the current release of carbon into the atmosphere is unprecedented and more rapid than at any time over the past 56 million years. Further, the magnitude of human-driven climate drivers in the modern era may be the largest Earth’s climate system has experienced over the past 420 million years (Foster et al., 2017). The current atmospheric concentration of CO2—about 413 ppm (and rising)—is similar to what was last experienced during the Pliocene period about 3 million years ago when global sea levels were at least 30 feet higher and global temperatures were 3.6 to 6.3˚F higher than they are today (Hayhoe et al., 2017). Future warming is expected to lag behind the rising emissions as the climate system equilibrates. This increased warmth in the Earth’s atmosphere will persist for many tens of millennia after carbon emissions have stopped (Clark et al., 2016; Lyons et al., 2019). This warming is ‘locked in’, and the climate system may take many thousands of years to return to temperature levels prior to the 20th century unless GHG emissions are rapidly curtailed.

The changes in climate are expected to interact with each other in ways that exacerbate the impacts. For example, warmer temperatures will result in more precipitation falling as rain rather than snow, leading to more-frequent and -intense extreme storms. Individually, these projected changes are likely to increase flooding, but together their impacts on flood risks are likely to be more than the sum of the two. What is known about past and future climate changes and how they will impact Nevada is outlined in more detail in the following sections.

Table 1. Climate Impacts in Nevada

Table 1. Climate Impacts in Nevada

GHG Emissions Scenarios Over Different Time Horizons

The future climate—particularly the long-term future climate of the globe or of Nevada—is not written yet. There are significant uncertainties, in part dependent on what society chooses to do to reduce the causes of climate change. We can bracket these uncertainties about the future by focusing on two different GHG emissions scenarios and two different time horizons.

GHG Emissions Scenarios

High GHG Emissions Scenario:

In this scenario, global GHG emissions continue to grow at more or less the recent historical pace throughout the century, so that GHG concentration continues its recent growth rates unabated. In the scientific literature, this is referred to as RCP 8.5. That same label is used here.

Reduced GHG Emissions Scenario:

Under this scenario, global GHG emissions begin to be reduced from their current rate of growth by midcentury and return to late-20th century rates by about 2075. This yields GHG concentrations that level off in the last half of this century. In the scientific literature, this is referred to as RCP 4.5.

Time Horizons

Near Term: Planning time horizons for many of today’s decisions are often focused on the next several decades. For this reason, we will discuss changes and impacts projected for a near-term period, 2030–2059. For this report, we will use a high-emissions scenario (RCP 8.5) to illustrate near-term changes and impacts because this is the current emissions trend. Projections using low- vs. high-end emissions scenarios do not significantly diverge until the latter part of the century.

Long Term: Climate change will continue for centuries. The differences between GHG emissions scenarios (and their impacts) grow over time. Depending on society’s emissions-related choices today, climate projections past 2060 diverge significantly. To illustrate the benefits of GHG mitigation, impacts are shown under both GHG scenarios at the end of the century, 2070–2099.

Both time periods cover 30 years, because it is a long-enough sample of data to allow the climate changes due to GHG emissions to be separated from natural weather variations that will continue from year to year, and sometimes decade to decade, even as the average character of the climate changes.

The data used here to develop these climate projections originates from some of the most-sophisticated global climate models currently available. The data has been downscaled using the localized constructed analogue (LOCA) method. For more information, please see http://loca.ucsd.edu/ and the references in the bibliography.

TEMPERATURE INCREASE, URBAN HEAT ISLAND, AND HEAT WAVES

Increased temperatures and the associated heat waves are particularly important to public health. In Nevada, average temperatures have been increasing and 8 of the 10 warmest years since 1895 have occurred since 2000 (Figure 1). Although temperatures throughout the state are increasing, the rate of warming is not the same everywhere. Urban areas, for example, are getting hotter faster relative to rural areas.

Figure 1. Nevada’s annual average temperature has increased about 2°F since the early 20th century. Data from NOAA Climate at a Glance.

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The amount of warming that Nevada will face in the future depends on whether GHG emissions are allowed to continue growing or whether they are reduced rapidly over the coming decades.

The amount of warming that Nevada will face in the future depends on whether GHG emissions are allowed to continue growing or whether they are reduced rapidly over the coming decades. Warming projections of 4–6°F throughout Nevada are expected in the near term. In the long term under a low-emissions scenario, warming is projected to reach 6–8˚F in all except the Clark County region (which is expected to see slightly less warming) and 10–12°F in most of central and northern Nevada under a high-emissions scenario (Figure 2). Simply, a certain degree of short-term warming is essentially locked in if GHG emissions continue, but a high-emissions scenario could result in about 50% more warming than a low-emissions scenario.

Nighttime temperatures—particularly important for human health—are projected to warm most, particularly in August and September, across much of the state. Daytime temperatures are projected to warm mostly in summer and fall. Increased temperatures affect multiple sectors, including increasing public health risks, in part by exacerbating poor air quality, stressing water resources by increasing water demand for irrigation and native vegetation, and creating a flashier streamflow regime by contributing to snow loss and leading to longer growing seasons (discussed below).

Figure 2. Temperatures 4 to 6ºF warmer are projected by midcentury across the State, increasing to 6 to more than 10ºF warmer by the end of this century, depending on which emissions scenario is followed in coming decades. This figure shows projected near-term changes in annual average temperature relative to historical average temperature for 256 hydrographic basins in Nevada, based on the averages of 2030–2059 and 1970–2000 simulations by 10 different global climate models responding to a high global GHG emissions scenario (RCP 8.5) (left). Average temperature projections for the long term (2070–2099) relative to the historical 1970–2000 average is shown on the right for a lower-emissions scenario, RCP 4.5 (top), and a high-emissions scenario, RCP 8.5 (bottom). Daily projections are downscaled and then aggregated within hydrographic basin boundaries.

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Projections of increasing average temperatures are punctuated by more-frequent and more-severe heat.

These projections of increasing average temperatures are punctuated by more-frequent and more-severe heat. The term heat wave generally refers to weather spells much hotter than normal, sufficient to be unpleasant or even unsafe. Extremely high temperatures pose a danger to human life and physical and mental health (Bandala et al., 2019; Zuo et al., 2015), to transportation and power infrastructure (Chapman et al., 2013), and to ecosystems. Extreme heat increases fire risk for some vegetation types (Zuo et al., 2015) and can also negatively impact wildlife (Albright et al., 2017). Extreme heat also impacts air quality, as higher temperatures are associated with increased ozone levels (Wise & Comrie, 2005).

What level of heat extreme is “problematic” depends on the impact in question (e.g., human health, infrastructure performance, or ecosystem health) and varies from place to place. There are different ways of defining heat waves that may take into account day and/or nighttime temperatures, humidity, and/or duration of the hot spell (Smith et al., 2013; AMS glossary 2020). For simplicity here, we use heat wave metrics that simply count the number of days per year where daytime temperatures exceed 95°F and nighttime temperatures remain above 65°F.

The number of very warm summer (June–August) days when daytime temperatures exceeded 95°F has increased across the state, with the largest increases in southern and northwestern Nevada (Figure 3), consistent with published analyses documenting increasing heat-wave frequency and/or severity across the Southwest (Allen & Sheridan, 2016; Gershunov et al., 2009) using a variety of heat-wave metrics. Increases in very warm nighttime temperatures (> 65°F) were larger in the southern portions of Nevada (Figure 3).

Figure 3. Most counties have experienced increases in the number of days each year when daytime high temperature exceeds 95°F (left) and when nighttime low temperature remains above 65°F (right). Southern Nevada experienced much larger changes in the number of warm nights than northern Nevada did. Changes between the periods 2001–2019 and 1981–2000 are shown. Maps use daily PRISM data, summarized in SC-ACIS. County boundaries are from the U.S. Census Bureau.

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As Nevada’s climate continues warming generally (Figure 2), the severity and number of extreme heat days and nights are also projected to increase markedly (Garfin et al., 2018; Jones et al., 2015; Mora et al., 2017). In the near term, much of Nevada is expected to experience 30 or more days per year when the daytime high exceeds 95°F, with the largest increases in west-central Nevada (Figure 4). Very warm nights are also expected to increase in frequency, with southern Nevada in particular experiencing 25 or more days each year when the nighttime temperature remains above 65°F (Figure 4). In the long term, projected increases in heat extremes are significantly different depending on the trajectory of future GHG emissions. Much of the state (Figure 5) is projected to experience 30 or more days of extreme heat days as defined above per year under the higher-emissions scenario compared to the reduced-emissions scenario

Figure 4. As in Fig. 2, but for near-term changes in the annual number of days when the daytime high temperature will exceed 95°F (left side) and when nighttime low temperatures will not drop below 65°F (right side). Figures use the higher-emissions scenario, RCP 8.5. By midcentury, an extra four weeks of hot days are projected for many parts of the State, and an extra two to three weeks (especially in the south) of hot nights are projected.

Near Term 2030-2059
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Figure 5. As in Fig. 2, but for long-term changes in the number of days per year when daytime high temperature will exceed 95˚F. By end of century, six extra weeks of hot days are projected in most of the State under the reduced GHG emissions scenario, RCP 4.5 (left) and a scorching 10 to 12 more weeks of hot days under high GHG emissions scenario, RCP 8.5 (right)

Long Term Days
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In addition to the warming documented statewide, Reno and Las Vegas have both experienced greater warming of annual temperature by 5°F and 4°F, respectively, than nearby rural areas (Figure 6). In particular, nighttime temperatures (the minimum daily temperatures) are increasing much more rapidly in the urban centers, the result of paving, buildings, and other land-use changes, called the urban heat island (UHI) effect (Box 1). Both Reno (Hatchett et al., 2016) and Las Vegas (Kamal et al., 2015; Miller, 2011) are known to have UHIs, which add to the broader-scale warming trends (Kamal et al., 2015). This urban heating can be expected to continue, leading to greater warming in cities beyond what is seen in regional climate projections.

Figure 6. Trends in average annual temperature at the Las Vegas McCarran International Airport and the Desert National Wildlife Refuge (left), and Reno International Airport and Boca Reservoir, CA (right). Data from SC-ACIS. Bottom colored plot shows the temperature difference between the airport stations and the rural stations, with red coloring above the average temperature difference for the 1950 through 1979 period. The trend lines are for the 1970–2019 periods. Gaps in the data are for years in which 36 or more daily observations were missing.

Annual Temperature
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Box 1. Urban Heat Islands

Urban heat Islands occur in developed areas that retain heat, leading to higher temperatures relative to more-rural, non-developed surrounding areas. Heat is released from vehicles, power plants, and other machine and equipment, along with the stored solar energy in buildings and other infrastructure. Together this causes the increased temperatures. This is in part illustrated by the photo below, where the black lizard is recording a higher temperature due to absorbing more solar energy. Urban heat islands often show a stronger nighttime temperature trend compared to rural areas because heat in urban areas does not dissipate due to the infrastructure.

 

Temperature white vs black

Photograph shows the effect of albedo on temperature. A thermometer in the white lizard reads 95.7ºF and the black lizard sculpture reads 142.5ºF. Photo from Springs Preserve, Las Vegas (Lachniet).

PRECIPITATION AND DROUGHT

Nevada is the driest state in the nation in terms of annual average precipitation (combined rain and snowfall). Las Vegas and Reno rely on water supplies that come primarily from mountains outside of Nevada (Box 2). The Rocky Mountains account for approximately 90% of the water supply to the Las Vegas Valley via the Colorado River, and the Sierra Nevada provides most of the water to Reno and surrounding areas. Elsewhere, local precipitation is critical for Nevada’s natural ecosystems, as mountains block clouds and cause local precipitation, which in turn recharges valley aquifers and springs to maintain healthy rangelands, forests, and riparian zones. Such local precipitation also provides snowpack and water supplies to smaller rural communities.

Nevada is the driest state in the nation. Nevada’s precipitation has historically been among the most variable from year to year in the United States.

Nevada’s precipitation has historically been among the most variable from year to year in the United States (Dettinger et al., 2016; Dettinger et al., 2011). In large part because of this high variability, no trends have been detected; any trends are indistinguishable from the large range of year-to-year differences. Projected changes in precipitation remain quite uncertain, as not all models agree on the direction of change—some models project a wetter outcome, others a drier future, and still others project almost no change. The difference between the models is in part a result of the highly variable nature of the precipitation in Nevada (Deser et al., 2012). Using the average projections (combining outputs from many different climate models) helps to minimize the impact natural variability has on the future projections. The average projections lean towards a possible small increase in precipitation across all but the southern tip of Nevada in the near term (Figure 7). The largest seasonal increase is projected to be in winter, with an average 15–30% precipitation increase across Nevada. The Clark County region is projected to dry during all other seasons. Despite these uncertain projections of small increases in precipitation, droughts (discussed below), snow loss, and flooding (discussed in the next sections) are all fairly likely to increase in intensity and frequency because these increases are due directly or indirectly to warming, which is confidently projected.

Figure 7. As in Fig. 2, but for near-term changes in water year (October–September) total precipitation (left side) and for long term changes (right side). Only seven global climate models are used here (vs. ten models for Fig. 2). Across much of the State (except in the far south), 5-15% more precipitation is projected, as the average of 10 global climate models, some of which project some drying and some of which project more precipitation.

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Nevadans are no strangers to drought. While much of the region is generally arid or semi-arid, precipitation shortages combined with growing losses due to evaporation have already led to hydrologic (water supply) droughts being more common than not since the start of the 21st century (Figure 8).

Figure 8. U.S. Drought Monitor weekly time series showing how much of Nevada (in % area of the state) falls into each drought category over the past 20 years (January 1, 2000 through October 13, 2020): D0 (abnormally dry), D1 (moderate drought), D2 (severe drought), D3 extreme drought, or D4 (exceptional drought). Source: https://droughtmonitor.unl.edu/

Nevada Percent Area
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Droughts still become more likely in the future… stressing Nevada’s water-limited ecosystems.

Projections of future droughts depend not only on changes in precipitation, but also on evaporative demand. Evaporative demand—the atmospheric thirst driven by temperature, wind, humidity, and solar radiation—plays an important role in droughts and can be particularly impactful in water-limited regions like Nevada (Hobbins et al., 2017). There is an imbalance between precipitation supply and evaporative demand across nearly all of Nevada (with the exception of high-elevation mountains) such that more water could be evaporated than actually falls as rain and snow. Therefore, it is critical to consider both precipitation and evaporative demand to understand drought in Nevada. When evaporative demand is higher than normal, soils dry out faster and vegetation (both live and dead) becomes drier, leading to increased fire risk, degraded ecosystems, and snow is lost more rapidly.

Over the past 40 years, evaporative demand has strongly increased in Nevada, with the fastest increases in the west-central part of the state (Figure 9). Climate projections indicate this trend will continue through the end of the 21st century (Figure 10). Despite projected (if uncertain) increases in precipitation across the region (Figure 8), droughts still become more likely in the future due to increased evaporative demand primarily as a result of increased temperature (Figure 2), stressing Nevada’s water-limited ecosystems. One measure of drought that accounts for both precipitation and evaporative demand is the Standardized Precipitation Evapotranspiration Index (SPEI; Vicente-Serrano et al., 2010) which has been found to be a good indicator of low streamflows and reservoir levels in Nevada (McEvoy et al., 2012). SPEI projections indicate that what counts as a moderate drought (D1 on the U.S. Drought Monitor scale, or about two events per decade) under today’s climate will become 3–4 times more common by mid-century for much of the state (Figure 11).

Figure 9. Observed trends in water year (October 1–September 30) total evaporative demand for the period 1980–2020. Evaporative demands have increased almost everywhere in the State, amounting to between 20 and 200 extra mm of demand over the 40-year period shown here. Source of trend computations: app.climateengine.org.

Water Year Evaporative
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Figure 10. As in Fig. 7, but for near-term changes in average total evaporative demand (left side) and for long-term projections (right side) Evaporative demand is projected to increase everywhere in the State this century, and could increase by as much as 20% (of historical totals) in some places, by end of century.

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Figure 11. As in Fig. 7, but for near term changes in water year standardized precipitation evapotranspiration index (SPEI)(left side) and for long term (right side). The SPEI represents normalized difference between precipitation and evaporative demand. Negative SPEI values (yellow, browns, and reds) indicate drought and positive SPEI values (blues) indicate wetness. U.S. Drought Monitor classifications of abnormally dry (D0), moderate drought (D1), severe drought (D2), extreme drought (D3), and exceptional drought (D4) are labeled on the color scale. The SPEI index of combined precipitation-evaporation drought is projected to decline (get drier overall) across the State, until by end of century, broad areas are on average in a D3-level drought condition under the higher emissions scenario (RCP 8.5).

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Box 2. Projected Changes in Southern Nevada’s Most Important Water Source: The Colorado River Basin

Since 2000, the Colorado River Basin has experienced an extended dry period in which the average annual water supply has been 18% lower than the historical average, contributing to depletion of water storage in the major reservoirs to less than half of capacity. This recent drought, along with the increasing recognition that rising temperatures impact the hydrology of the basin, has led to concerns about the long-term reliability of the basin’s water supplies. Research findings described in Colorado River Basin Climate and Hydrology: State of the Science (Lukas et al., 2020) demonstrate that the concerns are warranted. There is very high confidence regarding both future warming in the basin and in the role of emissions in leading to greater warming. Human-caused warming is already impacting droughts in the Colorado River Basin: an attribution study of the recent 2000–2018 drought indicates that it was made more severe by human-caused warming (Williams et al., 2020).

The future of precipitation in the basin is projected less confidently, so that it is not clear whether there will be more or less precipitation in the future overall. Studies have shown increasing variation from year to year, and on storm-to-storm scales, of basin precipitation. Consensus projections of overall shifts in hydroclimate driven by a warmer climate suggest a shift toward lower spring snowpacks, earlier melt and runoff, lower annual runoff volumes, and increasing water demand. Projected runoff changes are expected to lead to less streamflow overall (Udall & Overpeck, 2017), with the largest streamflow reductions projected for the Lower Basin downstream from Lees Ferry, Utah (i.e., the part of the basin from which Las Vegas extracts Nevada’s allotment of Colorado River water). A long-term perspective from tree rings and other paleoclimate data suggests that the Colorado River Basin has experienced droughts lasting many decades to many centuries, even in the absence of human-caused climate change (Lachniet et al., 2020; Routson et al., 2019; Williams et al., 2020; Woodhouse et al., 2010). These studies suggest that long-term drying (aridification) in the Colorado River Basin is a threat to water supply in southern Nevada and elsewhere, and the magnitude of future aridity in the Southwest will depend on the future trajectory of GHG emissions (Williams et al., 2020) and links with climate changes happening elsewhere (Lachniet et al., 2020).

For more information, please visit https://wwa.colorado.edu/publications/reports/CRBreport/

SNOW LOSS & MELTWATER RUNOFF CHANGES

In Spanish, “Nevada” refers to “snow capped” and many of Nevada’s mountains are indeed snow-covered during most winters. With more warming in the coming decades, though, more and more storms will drop rain rather than snow, even at high altitudes. In the near term, some 5–10% more of total precipitation is anticipated to fall as rain rather than snow, with basins around Tahoe and northwestern Nevada projected to experience 10–15% more rain rather than snow (Figure 12). By the end of the century, under the reduced-emissions scenario, Nevada could see approximately 10–15% more of its precipitation falling as rain rather than snow. Under the higher-emissions scenario, the proportion falling as rain could be 15–25% higher than today (Figure 12). The largest changes at the end of century are in northern and western regions of Nevada, while southern Nevada is not projected to receive much snow beyond mid-century.

Figure 12. As in Fig. 2, but for near-term changes in the fraction of annual precipitation falling as snow (left side) and for long-term changes (right side) The fraction of annual precipitation that falls as snow is projected to decline everywhere that snow falls historically.

Projected Changes in Snowfall
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In the near term, some 5–10% more of total precipitation is anticipated to fall as rain rather than snow.

With less precipitation falling as snow, and with snowpacks also more inclined to melt earlier due to the warming winters, the amount of water in April snowpacks—the time when snowmelt normally begins to swell the state’s streams and rivers—is projected to decline 30–50% by the end of century in most basins in the state (Dettinger, 2020). Less water in April snowpacks, less precipitation falling as snow and earlier precipitation runoff are all trends already being witnessed across the northern parts of the state and the Western United States, and constitute consistent and confident findings in the scientific literature (Figure 13) (Fritze et al., 2011; Knowles et al., 2006; Mote et al., 2018, Stewart et al., 2005). This leaves the state’s highlands and riparian areas drier by the time summer arrives (Harpold et al., 2014; Fritze et al., 2011).

Figure 13. The April 1 snowpacks measured across the West (including in Nevada) has been declining for the past 60 years. This map shows measured trends in 1 April snow-water equivalent (SWE, the amount of liquid water that would result if all the snow on the ground was melted) at 699 snow courses in the Western U.S. during periods of record during 1955–2016; diameters of circles are proportional to percentage change during this 62-year period, with red for declining SWE and blue for increasing SWE (Mote et al. 2018).

Observed SWE Trends
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Less snow and earlier snowmelt runoff  affect water management in Nevada, as snow serves as a natural reservoir. Furthermore, the loss of snow has implications for winter recreation, which would impact quality of life for many residents as well as winter tourism. A shorter snow season and/or a less-reliable winter snowpack imply a shorter ski season, which would impact tourism directly (i.e., at Nevada-based resorts) and indirectly,  as some communities in Nevada may benefit from visitors to ski resorts located in neighboring California communities.

By the end of the century, the higher-emissions scenario projects approximately 3 additional weeks of growing season relative to the lower-emissions scenario. However, because of the longer growing seasons, plants will likely demand more water overall, and with more of the year’s runoff occurring during the winter months, the growing season water demands and surface-water availability are expected to be increasingly out of sync, further challenging water management in Nevada.

The warming temperatures also are projected to lead to longer growing seasons for native plants and crops alike by an estimated 3–6 weeks in most basins in the near term (Figure 14). By the end of the century, the higher-emissions scenario projects approximately 3 additional weeks of growing season relative to the lower-emissions scenario (Figure 14, right). A longer growing season may provide some benefit to farmers in terms of season extension and crop diversity. However, because of the longer growing seasons, plants will likely demand more water overall, and with more of the year’s runoff occurring during the winter months, the growing season water demands and surface-water availability are expected to be increasingly out of sync, further challenging water management in Nevada. Longer growing seasons may also propagate the expansion of invasive species and pests in many places.

Figure 14. As in figure 2, but for near-term changes in temperature-based growing season lengths (left side) and in long-term changes (right side). . Growing-season length is estimated here as the number of days between the last springtime occurrence of 6 days with temperatures below 50ºF and the first autumn occurrence of 6 days with temperatures below 50ºF. (https://www.extension.purdue.edu/extmedia/nch/nch-40.html)13 Across most of the State, excepting only the far south where cool temperatures do not limit growing season, growing seasons are projected to last 20 to 40 days longer by midcentury and as much as 50 to 80 days longer by end of century under the higher emissions scenario (RCP 8.5).

Project Change in Growing Season Days
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Floods

While Nevada is the driest state in the Union, the state has experienced many catastrophic floods. In northern Nevada, the worst floods typically are associated with warm, very wet storms that deposit copious amounts of rain over much larger areas than do more typical cold storms (Albano et al., 2016). The winters of 1997 and 2017 were particularly severe examples of these conditions and caused major flooding and flood damages in Reno and along both the Truckee and Carson Rivers (Figure 15). In southern Nevada, intense summer thunderstorms have unleashed flash floods that have crashed through neighborhoods and the resort corridor along the Las Vegas Strip with devastating effects (Figure 15).

Figure 15. Historical flooding in (a) downtown Reno, January 2, 1997 (photo credit: https://www.reddit.com/r/ImagesOfHistory/comments/2z3c8f/the_great_reno_flood_of_1997_the_whole_valley/ , and (b) across the Strip at Caesar’s Palace in Las Vegas, July 3, 1975 (photo credit: http://water.nv.gov/home/pdfs/the%20flood%20of%201997%20final%20report.pdf).

Flood Images
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A warmer atmosphere can carry more water. When atmospheric conditions conspire to wring the water from storms, future storms are projected to become more severe more often. As a result, the most-extreme storms are expected to become even more extreme.

The projected precipitation increases, with more coming as rain than as snow, and earlier runoff will result in shifts in the annual sequences and peaks of streamflow and aquifer recharge to earlier in the spring and winter in many of the state’s basins. Because a warmer atmosphere can carry more water, when atmospheric conditions conspire to wring the water from storms, future storms are projected to become more severe more often (Gershunov et al., 2019; Kunkel et al., 2013). As a result, the most-extreme storms are expected to become even more extreme. This applies to both winter storms and summer monsoon rains. As a result of the projected changes, much of the projected increases in winter surface-water flows will come in the form of a much “flashier” flood flows regime for the state’s streams and rivers with drier intervening periods. In the southern parts of the state, where snowmelt is less of an issue, more-intense monsoon thunderstorms in the future are expected to result in more-severe flooding risks.     

Projected near-term and long-term changes in peak annual runoff rates (the maximum daily runoff rate occurring during the average year) are shown in Figure 16. Generally speaking, peak runoff rates are projected to increase more than 25–50% above historical peak rates across much of the state (especially in and around many mountain ranges) in the near term. In the long term, peak-runoff projections under a lower GHG emissions scenario (RCP 4.5) do not increase that much more compared to midcentury projections. Projections under the higher GHG emissions scenario (RCP 8.5) yield large additional increases (compared to midcentury) in peak runoff across nearly all of the state. A few locations, however, emerge as peak runoff “hot spots” that are projected to experience very large increases in the maximum runoff rates (e.g., around Las Vegas Valley, in various parts of the Walker River area extending up through the Carson River to the Sierra Nevada catchments of the Truckee River) (Figure 16).

Figure 16. As in figure 2, but for near-term changes in annual-peak daily runoff rates (left side) and for the long term (right side).. With few exceptions, peak runoff rates are projected to increase by from about 25 to 50 or more percent of historical rates by midcentury, and by end of centiury, will have increased by more than 50% in much of the State under the higher emissions scenario. The green spots on each map are “hot spots” where peak runoff rates (and thus flood risks) are projected to increase substantially more (see discussion above).

Projected Changes in Annual Peak Daily Runoff
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Wildfire Risk

During the period 1984–2017, 4 of the 5 years with the largest area burned have occurred since 2005.

Wildfire risk is influenced by land use, habitat, weather, and climate (Westerling et al., 2003) and regardless of risk, every wildfire needs some sort of ignition. Ignition is usually either human-caused (e.g., campfires, unextinguished cigarettes) or natural (e.g., lightning). Weather conditions prior to fires explain 27–43% of the variations in the area burned in the Great Basin (Pilliod et al., 2017), highlighting how climate can synergistically act with other factors to increase wildfire risk. When a wet winter is followed by a dry spring and summer, it is likely that more area will burn, suggesting that a seasonal drought is a larger factor that multi-year droughts in the Great Basin (Pilliod et al., 2017). During the period 1984–2017, 4 of the 5 years with the largest area burned have occurred since 2005 (Figure 17). Fire also creates a reinforcing feedback loop whereby cheatgrass more-commonly occurs and is more prevalent after fires, but it also increases fire risk (Bradley et al., 2018; Williamson et al., 2020). However, on the decadal timescale, if there is not a recurrence of fire, native sagebrush has been shown to return to areas once dominated by cheatgrass (Morris & Leger, 2016).

Figure 17. Acres burned in 1,000s of acres for large fires for the Nevada portion of the Great Basin Geographic Area Coordination Center (GACC), the focal point for coordinating the mobilization of resources for wildland fire. Large fires are defined as those of 1,000 acres or more in extent. Data from the Monitoring Trends in Burn Severity: https://www.mtbs.gov/.

Nevada Portion Great Basin GACC Acres Burned
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Spring and summer evaporative demand increases the wildfire risk by faster drying of vegetation. Evaporative demand in both seasons is projected to increase by 5–15% in the near term.

Changes in climate can affect the fire risk largely through variations in drying and warming. As mentioned above, winter precipitation is projected to increase throughout Nevada, which can increase wildfire risk through more vegetation and fuels growth (particularly grasses and small shrubs). Spring and summer evaporative demand increases the wildfire risk by faster drying of vegetation. Evaporative demand in both seasons is projected to increase by 5–15% in the near term (McEvoy et al., 2020). Moreover, the number of days with extreme evaporative demand each summer, which is largely indicative of increases in extreme temperatures, is projected to increase by 25–35 days (out of 92 possible days, or about 30% of the time) in the near term (Figure 18). By the end of the century the number of days with extreme evaporative demand is projected to increase by 10–20 days or more, depending on the GHG emissions scenario.

Figure 18. As in Fig. 7, but for near-term changes in the number of days with extreme (top 5% of all days) evaporative demand, indicative of fire weather conditions (left side) and for long-term changes (right side). The days are identified based on the 2-week Evaporative Demand Drought Index (EDDI). (More details in McEvoy et al., 2020 and Hobbins et al., 2016). Wildfire risks (as indicated by this measure) increase dramatically across the State.

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In addition to wildfire’s direct risk to residential and commercial properties, infrastructure, and to business operations, wildfire can pose widespread risks to life and public health. Smoke from wildfires can travel hundreds of miles, impacting the health of Nevadans well beyond the immediate threat from the fire itself (Moeltner et al., 2013). Wildfire smoke is associated with respiratory issues and hospitalization, especially for the elderly and children under four (Delfino et al., 2009). Emergency room visits for those with asthma increase as a result of wildfire smoke as well (Kiser et al., 2020).

REDUCING CLIMATE CHANGE THREATS TO NEVADANS

The most-effective way to forestall or reduce the projected impacts of climate change is to help minimize climate changes themselves. Nevada is actively pursuing reductions in GHG emissions (mitigation) and is poised to also take on climate change preparedness and adaptation to build the resilience of its sectors and communities.

To this end, one aspect of this effort would be increasing technical capacity at the state level for climate-informed decision-making, including increased in-house climate expertise. These resources and expertise can be focused on working directly with state agencies and counties to support the development of climate resilience strategies designed to reduce the impacts of climate change on Nevada’s economy, communities, and ecosystems.

A first step in this effort would be to build off this preliminary assessment into a comprehensive and regular assessment effort for Nevada that examines potential climate change impacts on the issues Nevadans most value and are concerned about. Such an assessment would allow resources to be more-effectively allocated in supporting the resilience of the ways of life and places most important to the people of Nevada.

Below we list examples of risks associated with different climate impacts, resilience-building efforts that have been adopted or are under consideration elsewhere, and insights that could be gleaned from research that would help characterize risks more usefully and identify additional opportunities to build community resilience. This list is not comprehensive, but rather illustrative of the scope and scale of considerations necessary to support climate resilience and adaptation planning. Future assessments could more-comprehensively assess potential risks and resilience-building efforts.

Public Health Risk Profile

  • Extreme heat killed more than 150 Nevadans in 2017 and 2018, and puts outdoor workers at risk.
  • Extreme heat is likely to increase in already-warm locales and affect parts of the state that have not historically experienced regular, very warm temperatures.
  • Decreased air quality via increased ozone levels associated with higher temperatures (Wise and Comrie 2005).
  • Higher wildfire risk and the potential for increased wildfire risk to lives and property, along with health risks from smoke exposure to fires within and from outside Nevada.
  • Increased frequency and/or severity of drought, along with mental health impacts, particularly in agricultural areas (OBrien et al., 2014; Vins et al., 2015).
  • Air quality degradation from PM10 particles may become a public health hazard in areas near desert terminal lakes as lake levels decline and lake beds begin to dry.

Examples of Resilience-Building Actions
  • In Nevada’s cities, urban planners and public health officials can work together to help build resilience in the face of more-extreme urban heat and greater flooding potential by managing green spaces and increasing bright reflective surfaces in the built environment (Georgescu et al., 2014) to reduce the urban heat island effect and flooding risks.
  • Enhanced situational awareness of events building off national, state, and county programs that already provide forecasting for these events and information about how people can limit their exposure (particularly for shorter-term heat and poor air quality events).
  • Communication of insurance programs designed to mitigate the impact of drought on farms and ranches, which might offset some stress.
  • Enhanced long-term, high-quality, spatially distributed monitoring of temperature and air quality with timely reporting to public health officials.

Research to Support Risk Assessment & Resilience-Building
  • Assess why Nevada cities have such large urban heat islands.
  • Evaluate specific strategies or combinations of strategies that are most effective in mitigating urban heat islands in Nevada without creating negative side effects.
  • Examine how the monsoon exacerbates or moderates public health impacts from extreme heat.
  • Assess how dust from drought affects public health.

Water Resources Risk Profile

  • Rising temperatures are likely to strain Nevada’s water resources, even if precipitation increases or does not change. 
  • Nevada is already experiencing earlier snowmelt and longer growing seasons. 
  • Nevada is already experiencing more droughts from increases in evaporative demand. 
  • Snowpacks will decline as temperatures warm and a flashier surface-water flow regime is expected to develop in coming decades. 
  • Desert terminal lakes in Nevada will likely have lowered lake levels and increased salinity, endangering fisheries and culturally sensitive species, such as the Cui-ui in Pyramid Lake.

Examples of Resilience-Building Actions
  • Fill weather and climate monitoring network gaps that have historically characterized Nevada to provide information critical to recognizing, measuring, and ultimately managing the changes that are projected to emerge this century.
  • Maintain and, where feasible, enhance water-, land-, and flood-management practices and upgrade infrastructures to better accommodate future climate extremes and impacts. 
  • Begin to consider and test options for slowing stream discharges and increasing upland recharge with a flashier, more heavily runoff-dominated system. This would hold precipitation (water) in basins longer and slow passage from the uplands to the basin floors. 
  • Project and assess the likely impacts of climate change on water availability, water law and allocations, and perennial yields of basins throughout Nevada.

Research to Support Risk Assessment & Resilience-Building
  • Identify and track the specific stresses and impacts drought causes on different sectors, with attention to how these impacts will change in the future.               
  • Reevaluate flood risks in light of expected changes towards a flashier runoff regime in all parts of the state.      
  • Develop strategies and programs for slowing the passage of runoff from the uplands to the basin floors and for increasing deep percolation in groundwater recharge areas to counteract the projected trends towards earlier runoff and more flooding. 
  • Research directed towards improving projections of the future monsoon regime and storm intensities in southern Nevada.     
  • Reassess historical water-supply sources and qualities in the context of future climate changes in cooperation with management agencies.   
  • Conduct research into adaptation strategies that could alleviate the growing risks to recreation and tourism industries in Nevada. 

Recreation and Hospitality Risk Profile

  • A shift from snow to rain and earlier snowmelt is expected to reduce the winter tourism season and potentially expand the summer outdoor recreation season.
  • Increased wildfire risk and occurrence could lead to increasing public lands closures due to both fire risk and post-fire debris flow risk, affecting other forms of recreation and tourism.
  • Increasing wildfire activity can also degrade air quality, which could discourage visitation.
  • Increased heat in Reno, and particularly in Las Vegas, might impact tourism.
  • Flooding can and has impacted tourism (for example, see the 1997 New Year’s Flood in Reno).

Research to Support Risk Assessment & Resilience-Building
  • Assess the timing and predictability of the changing frequency of snowy winters in the near term.
  • Determine the sub-seasonal to seasonal forecasts of winter snow needed for hospitality and outdoor recreation planning.
  • Determine the type of heat waves that are most impactful on the hospitality industry.

Agriculture and Ranching Risk Profile

  • Increased drought intensity and/or frequency may limit crop and forage production.
  • Longer growing seasons may benefit producers, particularly in cooler areas of the state.
  • Large wildfires can cause economic harms for ranchers, due to livestock losses and damage to grazing lands.

Examples of Resilience-Building Actions
  • Evaluate and connect existing tools and guidance (e.g., National Drought Mitigation Center, “Managing Drought Risk on the Ranch”) for Nevada ranchers and farmers. 
  • Enhance and expand current efforts of researchers and producers working toward sustainable grazing management and crop production in water-scarce environments.
  • Encourage rangeland resilience to prevent overgrazing, (e.g., grazing rotation).
  • Improved drought monitoring to better inform application of existing drought policies and drought remedies.

Research to Support Risk Assessment & Resilience-Building
  • Determine barriers for implementation of forecast tools useful applicable to agriculture and ranching. (e.g., Grass-Cast grassland productivity forecast expansion).
  • Assess what drought-tolerant crops can be grown successfully in Nevada and the market outlooks for those crops.
  • Evaluate irrigation efficiency improvements.
  • Understand plant uptake of water for Nevada-specific soils and the associated soil moisture relationships to crop vitality.