The current climate of the RMGB varies widely by season and location. As a region that encompasses nature's extremes, much of the area consists of ecological boundaries, ranging from hot, dry lands that push the limits for sustaining life, to alpine areas perched on the region's high mountains. At present, much of this region is semi-arid and subject to variable precipitation that can result in drought and floods.

Given the region's normal conditions and the projected climate changes that could occur, extreme weather events could be of concern. Although there are limitations in using global and regional climate models to project future patterns of extreme events, some historical records, observations, and model simulations suggest that such events could increase in frequency and/or intensity. Changes in extreme events that could be of special importance in this region include increased occurrence of lightning-induced fires, increased duration of drought conditions, and floods caused by precipitation increases and inability of the western hydrologic network to accommodate expanded run-off. Because extreme events can have distinct environmental impacts, each is discussed separately below.

Variations in amount and timing of snow and rain have important effects on fire frequencies in this region, with the drier years making the mountain forests more fire prone and wet years increasing fire frequencies in arid, low elevations due to increased amounts of burnable materials. Fire is a major factor altering the shrub-steppe vegetation of the Great Basin because it removes native plants and allows the invasion of exotic annuals; often one species of exotics can become dominant under such circumstances.

These ecological transitions are often semi-permanent in the burned areas, and this will become more likely in the future because the altered climate will be different than that which favored the development of the previous plant community. In recent years, the RMGB region has experienced forest and range fires of unprecedented intensity and extent. Some of this increase in fire occurrence was probably caused by fire-suppression and associated management practices that allowed unusually high build-ups of plant materials. Others are occurring because of the high temperatures and drought conditions that have created tinder-dry vegetation that is more vulnerable to intense fires. In addition, a trend toward more fires being caused by lightning has characterized some of the region's rangelands and forests.

Beyond these current situations, however, is a potentially more disturbing problem -- the possibility of climate-induced changes that lead to higher temperatures, increases in the probability of more intense and frequent storms, and consequently increased wildfire frequency.

Much of the RMGB is characterized by especially low levels of soil moisture and relative humidity. Changes in the climate of such semi-arid regions that would promote higher temperatures, increased evapotranspiration (loss of water by the soil and by plants), decreased precipitation, or increased variations in precipitation could result in prolonging the droughts that do occur, and/or increasing the frequency of drought occurrence. The extreme heat waves associated with drought can have devastating effects on all life forms. Drought and heat can cause waterways to warm or evaporate and affect the fish and wildlife dependent on those habitats. Heat and drought can increase community demands for water (at a time when there is less) and for hydropower, because the maximum electric energy demand tends to occur on hot summer days for cooling.

Although the semi-arid character of the RMGB region first brings to mind the prospects for drought, projections of climate models also indicate the potential for sharp increases in precipitation over the 21^st century. The weather records indicate that such precipitation increases over much of the region started during the 20^th century. Also, those records not only show increases in the yearly averages of precipitation, but also increased variability between years which is an indication of more extreme, precipitation events. This trend toward increased variability thus also raises the concern of more-frequent flooding for the future, a risk that is emphasized by the fact that most human settlements in the region originally were established along streams and in riparian zones (i.e., on the banks of natural waterways). Today, much of the expansion of these settlements continues to occur in and near riparian zones where building sites are aesthetically pleasing.

Flooding occurs under two sets of circumstances in the RMGB. Over most of the region, floods tend to occur in springs following exceptionally high winter snowfalls and snowpack, especially during early, warm springs that are accompanied by heavy, warm rainfall. The Teton River dam in southern Idaho failed on June 5, 1976 under such conditions and flooded several small Idaho towns. Water also flowed down the sandbagged, downtown main streets of Salt Lake City in the spring of 1984 following a record winter snowfall and overflow of City Creek, which carried the meltwater out of the Wasatch Mountains.

Flooding also occurs in the southern portion of the RMGB in late summer during the monsoon. Rainstorms during these events can be sudden, intense, and localized. For example, in July 1999, three inches of rain fell in Las Vegas in just two hours -- that is three-quarters of the rain Las Vegas normally receives in an entire year. The flash floods that resulted caused at least two deaths and some property damage. Some infrastructure damage was avoided due to a partially completed flood-control program.

How the probability of flooding in the RMGB will be affected by the projected changes in climate is not clear at this point in time. Precipitation in fall and early winter falling as rain rather than snow would allow that moisture to run-off in winter rather than contributing to the snowpack and spring run-off. Earlier spring warm-up would convert precipitation that would normally fall as snow to rain further limiting snowpack. The snowpack would be reduced further by occurring higher up the mountains. The net effect of these factors would be reduced snowpacks, higher fall and winter streamflows, and lower spring run-off peaks than are now the case.

However, if precipitation should double with the major increase occurring in winter, which is likely an extreme of the projected changes, this could lead to increased snowpacks despite shorter seasons and higher temperatures. Such conditions would result in the snowpack occurring higher in the mountains, and would maintain or even increase risk of springtime flooding. Careful hydrologic (water cycle) modeling is needed to better understand this issue.

The potential for increased risk of forest fires could endanger the life and property of home owners in and adjacent to forested land. Recreational users, such as hikers, campers, hunters, bird-watchers, and anglers could also be at increased risk.

Drought destroys crops, limits forage for rangeland animals, and distresses livestock -- conditions that would add significant further financial stress to farmers and ranchers who are already operating near the margin. Also, drought would increase demand for the already over-allocated water resources of the RMGB, increasing the competition between agricultural and other users.

Drought conditions and extreme heat would also be likely to increase energy needs due to rising air-conditioning demand for electricity, especially in heavily populated urban centers. Existing energy sources might not be sufficient in some areas to meet such additional peak needs. Action will be needed to ensure that the health of vulnerable populations susceptible to drought and extreme heat conditions -- especially without access to adequate water supplies and air-conditioning -- are not put at risk.

At the opposite end of the weather continuum, flooding would present significant increased risk. Floods could damage infrastructure critical to maintaining health, such as waste-treatment services and delivery of potable (drinkable) water, which could be tainted by waste and runoff from agricultural lands. Buildings and transportation infrastructure -- roads, railways, and air services -- also could be affected by extreme precipitation events and the flooding that could result in high-risk areas.

Insurance companies are watching closely to monitor how possible changes in climate will increase, or decrease, the frequency and intensity of extreme events. Anticipated cost increases would be passed on to consumers in the form of higher insurance premiums. Individual households and businesses could also expect to pay more for air-conditioning if more days of extreme heat become the norm.

If fire, drought, and flooding increase in frequency and/or intensity health impacts could be expected to rise due to increased human exposure. In addition, if infrastructure must be re-engineered or rebuilt in response to the challenges these conditions could create, economic costs would be certain to accrue. Forest fires, as an example of an extreme event, have economic impacts through the cost of fire-fighting, the loss of commercial timber sales and recreational opportunities, the health effects of smoke and stress, the economic impact of disruption of regional businesses, and the destruction and possible loss of natural erosion control and water- and air-cleansing benefits that forests provide.

Because the RMGB is typified by extremes, in some ways, the ecosystems are already resilient to extreme events. However, climate change is likely to increase the frequency and intensity of some types of extremes, while alleviating others. Strategies that seek to increase the resiliency of both the built environment and the natural environment to extreme events could include efforts to:

• Keep vulnerable land-uses out of flood and fire-prone areas.
• Design buildings so they can be heated or cooled more efficiently and so they retain that heating/cooling more effectively.
• Organize social services to provide relief to vulnerable populations unable to afford protection from extreme events.
• Plan developments that make use of historical climate records as well as long-term climate forecasts, and factor climate information into policy and infrastructure decisions.
• Support research for distributed and alternative energy sources and policies to encourage use of distributed and alternative energy products and energy-conservation practices since traditional sources and transmission systems may be vulnerable to increases in extreme events.
• Intensify the focus on long-term sustainability and resiliency-building in all uses of the natural and the built environment.