In addition, there may be an increase in the number of extreme hot days in the summer and decrease in the number of cold days in the winter. A report from a January 25, 1999 meeting sponsored by the U.S. Global Climate Change Research Program (USGCRP) report notes that "increased droughts are to be expected in a warmer world, and so are increased floods."

In April, 1999, the National Center for Atmospheric Research predicted an increase in wintertime precipitation in the Southwest as a result of climate change. According to the IPCC most climate change models predict drier soil surface in the summer for the Southwest (and most of North America), primarily because of increased evaporation.

A number of social and economic trends and environmental conditions make the Southwest especially vulnerable to climate change.

• An economy now focused largely on tourism, retail, and other service sectors, and a rapidly growing population could be affected by extreme weather conditions.
• Municipal and industrial users, irrigated agriculture, and natural ecosystems are all dependent on an assured water supply, and the supply is already stressed.
• Other land uses, including ranching, non-irrigated agriculture, and forestry, are dependent on soil moisture and timing of rainfall.
• Much agricultural land is being developed or shifting to new crops-often more water intensive.
• There are many political and value conflicts over land and water use in the region.
• Unresolved water rights issues with Native American tribes and water-related treaty obligations with Mexico pose unique challenges.

The landscapes and ecosystems of the Southwest have continually changed in response to natural variations in climate. The landscape and ecosystems are also expected to change with human-induced climate change and increased human use of the land and natural resources. This might include changes in the composition of ecosystems and changes in the availability of water and other resources.

Some types of areas within the region which are likely to be particularly vulnerable to climate change include:

• Semi-arid and arid ecosystems, such as those in much of the Southwest. The areas are considered to be among the most sensitive to change in part because of limited water and generally only marginal nutrient reserves.
• Mountaintop "island" habitats (small patches of a particular ecosystem unconnected to larger tracts of the same type of ecosystem) where species have little room to adapt and migrate.
• Desert ecosystems that are heavily dominated by a few plant species. Harm to the dominant species could significantly affect the whole ecosystem.
• Areas where landforms are unstable and climate is normally can be vulnerable to landslides or erosion if there is more precipitation.
• Riparian habitats near streams and lakes. These areas are extremely important for wildlife and many of these areas have already been significantly impacted by competing water use and development.

Water

Water supply in the Southwest is about half surface water and half groundwater. Irrigated agriculture is the largest user of water (using almost 80 percent of available water). Municipal use is second and growing rapidly.

The Colorado and Rio Grande Rivers are the most important surface water supplies and the flows are heavily dependent on snowmelt. The Colorado River provides water for about one million people and more than one million acres of farmland in Arizona, California, and Nevada. It also generates about 12 billion kilowatt-hours of electricity through hydroelectric plants on the river. In New Mexico, many streams run dry at some time during the year, however, reservoirs on the larger rivers, such as the Rio Grande, Pecos, and San Juan rivers, store water which helps reduce the variation.

The waters of the major rivers in the region are managed under compacts. For example, the Colorado River is managed under the Colorado River Compact (known as the Law of the River). The compact was established in 1922 between the seven Colorado River Basin states (Arizona, California, Colorado, New Mexico, Nevada, Utah, and Wyoming) to apportion the water from the Colorado River and its tributaries. In 1945, the United States and Mexico signed a treaty involving the Colorado River waters.

Under these agreements, more water has been committed to users than sources can supply-in part as a result of debate over how much water flows in the Colorado River each year. A number of tribal water claims are still unresolved which could impact future water demand and management.

Possibly the most severe effects of climate change in the Southwest relate to water supply, in part because of the extreme demands already being put on the system. According to the U.S. Geological Survey (USGS), the most inadequate water supplies in the United States are in the Southwest, including the lower Colorado River basin and the Great Plains river basins south of the Platte River. In these areas on the average, 70 percent of the water taken out of the stream is not returned to it (an example of such off-stream uses is irrigation, where water may or may not be returned to the stream via drainage ditches). In addition, the amount of water being pumped out of groundwater sources in many areas is greater than the amount being replenished, thus depleting the groundwater supply.

While there is uncertainty about specific climate change impacts, a recent report from a panel of experts notes several likely impacts on water, including increasing evaporation, early melting of snowpack, altered seasonal cycles of runoff, increased frequency and severity of extreme events, increase in river and stream flow variability.

These changes, and policy approaches to them, can potentially have significant impacts on such things as:

• Water supply - The water supply is already overtaxed.
• Ability to generate hydroelectric power - Lower rainfalls in the summer and fall would mean production of less hydroelectric energy and the need to shift to more expensive alternatives.
• Levels of reservoirs - Less precipitation or particular seasons with less rain could reduce the levels of reservoirs and reduce their ability to help mitigate effects of droughts and floods.
• Level of salinity - The levels could increase as a result of increased flooding or drier soils. Increased salinity is already a problem in the lower Colorado River and a source of conflict between Mexico and the United States. Higher salinity levels can make the water less usable or even unusable.
• Pollution levels in streams and rivers - Increased frequency of severe storms would likely increase polluted runoff entering the rivers from farms and urban areas. Periods of decreased river flows could lead to more concentrated pollution.
• Sediment load - The amount of sediment in rivers and streams could increase as a result of more frequent flooding or landslides.
• Timing of snow melts - A warmer climate would likely result in earlier snow melts and greater stream flows in the spring and early summer, but reduced flow and water supply later in the summer.

In addition, warmer temperatures will likely result in increases in demand for water for various uses including irrigation for agriculture; industrial cooling; municipal uses; and instream uses to protect the stream ecosystems. One recent study of the Colorado River basin found that a warmer, drier climate could lead to 10 percent decrease in delivery of water and would cost users an estimated $200 to $300 million per year.

Despite the impacts and costs, one recent study concluded that even a 19-year worst case severe drought would not drain the reservoirs of the Colorado River. They also found that non-consumptive and environmental water uses are more vulnerable to drought under the current Law of the River than are the consumptive uses.

Land

Drier soil and reduced vegetative cover resulting from drought or increased evaporation leaves the soil vulnerable to dust storms and irreversible wind erosion. With other factors constant (wind, stability, and relative humidity), the potential for evaporation increases by about 5-10 percent for each degree (C) increase in temperature.

Dust in the atmosphere can play a role in visibility, air quality, nutrient supply to plant communities, weather, and climate. Dust storms can cause visibility hazards and degrade visibility over great distances. Dust in the air can also block solar radiation from reaching the Earth - effectively cooling the lower atmosphere. When fine sediments are removed by the wind, it can destroy the nutrient base for vegetation. On the other hand, dust storms can provide plants with nutrients when the dust is carried into leafy canopies.

The Southwest could also experience more landslides as a result of more precipitation or more severe rains. (See map of landslides and susceptibility at http://geochange.er.usgs.gov/sw/impacts/geology/landslides/.) According to the USGS, two of the four most landslide-prone mountainous regions in the United States are in the Southwest: the Colorado Plateau, and the Southern Rocky Mountains. USGS also notes that "existing, relict landslides may be reactivated or new landslides generated by construction activity, excessive natural loading, or by unusual natural or artificial wetting (such as irrigation), or erosion."

Forests and Vegetation

Forests could undergo various changes depending the nature of climate change. For example, with a hotter, drier climate forests could begin to give way to grassland or rangeland. Even in a wetter climate, some tree and plant species could be replaced by those that can better survive in that climate. As a result of warmer temperatures, forest ecosystems are generally expected to move northward.

In recent years, forest fires in the Southwest have increased in intensity and extent, according to EPA. Several factors have contributed to the trend including many years of allowing dead wood to accumulate through fire suppression efforts, climate variability, and periods of prolonged drought. Warmer temperatures could result in even longer fire seasons and more frequent fires.

According to the USGS, vegetative cover in the Southwest responds dramatically to rainfall variability. Decreased precipitation and increased evaporation leave drier soil, which causes a decline in the vegetative cover. Warmer temperatures also increase plant transpiration-the loss of water to the atmosphere from plants.

In a warmer climate, some plants of the Southwest could migrate or spread into other areas. For example, the USGCRP suggests that the saguaro cactus "could shift dramatically north through the Great Basin." This kind of shifting could have significant effect on existing habitat and wildlife.

One example of change already being observed as a result of warmer temperatures on the plains of Colorado and eastern New Mexico was reported by researchers from the University of Nebraska and Colorado State University. Since 1970 average annual temperatures in the area have risen 2 degrees F and most of that is the result of increases in nighttime temperatures. The researchers found that blue grama grass on the plains of Colorado and eastern New Mexico is already being replaced by spring grasses. The researchers reported a close relationship between the shift in grasses and the increase in nighttime temperatures. This shift in grasses could greatly impact ranchers because the blue grama grass is key for cattle grazing because it can thrive on the hot plains in late summer. The cattle could still graze on the spring grasses but they would have little to eat in late summer.

Another change already observed that is potentially the result of warmer temperatures and earlier springs is the rapid tree growth in areas like the crest of the Sandia Mountains.

Wildlife

Many already threatened or endangered species in the Southwest could be further affected by even small changes in the climate and ecosystems. In the Colorado River Basin, many fish, plants, and animal species have already been severely affected by development, dam construction, water withdrawal, and non-native fish species.

Particularly important are the dense riparian habitats along rivers, streams, or wetlands, which are critical to the survival of many species, particularly birds. For example, the endangered southwestern willow flycatcher only breeds in narrow riparian habitats, and any loss of surface water or change in the vegetation could further endanger the species. The riparian habitats along the San Pedro River host more species than any other river in the United States. Any decrease in available water would significantly impact riparian habitats.

As with plant species, some animal species may become extinct or shift to other areas. At the same time, new species may migrate into the region from other areas affected by climate change. However, in some cases human development has cut off natural migration routes which some species may need to respond to a changing climate.

Agriculture

In other areas of the country, agriculture may benefit from warmer temperatures and longer growing seasons. But in the Southwest, most of the impacts of climate change on agriculture are expected to be harmful. If climate change brings a drier and warmer climate, when water supply would be more limited, irrigated agriculture would require more water. The majority of agriculture in the Southwest is irrigated and irrigation is the largest user of water. Crop yields would also be reduced if temperatures rose above levels crops could tolerate.

A drier climate would also increase soil salinity. In addition, milder winters and hotter summers would likely increase farmers' problems with pests and mold. Organic farmers would tend to be more vulnerable to such impacts.

The crops most vulnerable to climate change include cotton (the largest user of water), wheat, alfalfa, vegetables, and orchard crops. Some examples of potential impacts on crop production, according to EPA estimates, include:

• In Texas, cotton and sorghum yields could be reduced by 2-15 percent, and wheat yields by 43-68 percent.
• In Arizona, cotton yields could drop by 5-10 percent, and wheat yields by about 70 percent.
• In New Mexico, wheat yields could drop by 10-30 percent, and sorghum yields by 7-9 percent.

Ranching

Almost two thirds of the land in Arizona and New Mexico is used for ranching.

Ranching is vulnerable to climate change in part because it is dependent on a good forage base, which requires the proper timing and amount of rainfall. Purchasing alternative feeds is generally too expensive for farmers.

Ranchers would suffer if less water were available for their animals. According to the IPCC, climate change may also add to existing stresses on rangelands and make them more sensitive to droughts, floods, and insect outbreaks. Smaller ranchers are likely to be more affected by climate variability and droughts. During the drought of 1996 many small ranchers went out of business and many others had significant financial losses.

Climate change and related impacts can potentially affect human health directly and indirectly.

• Water quality and quantity - If climate change brought lower streamflows, water quality could suffer in ways that might harm human health. Increased runoff could increase the presence of waterborne disease organisms such as giardia and cryptosporidium.
• Heat - Heat related health problems could increase as a result of higher temperatures and more frequent heat waves. Those least able to afford air conditioning would likely be the most affected.
• Air pollution - Ground level ozone, already a problem in major urban areas in the Southwest, is expected to increase with increased temperatures. Ozone can cause respiratory problems and exacerbate asthma and other respiratory diseases.
• Disease - Ecological shifts accompanying climate change could increase the presence of disease-carrying insects, rodents, and other "vectors."
• Natural disasters - Extreme weather and related impacts, such as flooding, fire, and wind damage, can pose a risk to people and property.

The Role of El Niño One other potential factor is the impact of the El Niño Southern Oscillation (ENSO) on weather. The Southwest experiences increased severity and frequency of floods and more landslides during El Niño years. For example, Arizona and New Mexico have had on average more than 50 percent more rain in El Niño years than normal years, according to NOAA. In some areas of the Southwest, such as Arizona, the largest floods of the last 100 years have occurred in El Niño years. Some scientists say that climate change will increase the frequency of El Niños. If El Niños increase with climate change, then winter flooding would be expected to increase.

A 1998 report from the University of Arizona, notes that "in some ways the Southwest has already adapted to the warmer, drier, and more extreme climates that could be a result of global warming. Adaptations include large-scale water transfers and air conditioning, but these can be costly in terms of federal subsidies, environmental impacts, and individual water and energy bills. New, more sustainable strategies are needed to assure a high-quality lifestyle for the current population and future generations."

• For ranching and rangelands, management techniques that reduce the risk of harm from extreme events can be implemented. This might include such things as changes in the season that land is used, changes in the type or number of livestock, or the addition of new fencing or watering locations.
• Implementation of water conservation measures and sustainable water management concepts could go a long way toward addressing water supply problems.
• Land use regulations could minimize development in vulnerable or sensitive areas, such as steep hillsides or riparian habitats.
• Improved forest management, including reduction of fire-suppression efforts, can help maintain healthy forest ecosystems, more able to withstand minor changes and more extreme weather events.
• Actions can be taken to reduce air and water pollution to reduce weather-related deaths.