Environmental Health Center
Of course, "hot and dry" is something of a stereotype for southwestern climate. The Southwest United States is a region of great diversity and many unique characteristics. The highest temperatures in the country are found in the deserts and plains of the Southwest where it routinely exceeds 100 degrees Fahrenheit. The Southwest has during the past 45 years, been drier and had more droughts than any other region in the United States. The region has also experienced rapid population growth-the population of Arizona has tripled in the last 30 years, for example.
For purposes of this overview, the Southwest is defined as including all of Arizona and New Mexico and parts of Colorado, Nevada, Texas, and Utah adjacent to those two states. (See basic map of the region, http://geochange.er.usgs.gov/sw/resources/sw_basemap/.)
According to projections from the Intergovernmental Panel on Climate Change (IPCC) and the United Kingdom's Hadley Centre model :
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.
Some types of areas within the region which are likely to be particularly vulnerable to climate change include:
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
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.
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."
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.
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.
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:
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.
One other potential factor is the impact of the El Niņo Southern Oscillation Some scientists say that climate change will increase the frequency of |
| June 23, 2000 | | Disclaimer/Policy |
