Southwest may see warmer, wetter climate in the future

News Release, April 10, 2001

Hotter temperatures and heavier winter rains may be in store for the American Southwest over the next century, according to a report recently issued at the University of Arizona.

The report, "Preparing for a Changing Climate," brings together research on the climate of the Southwest, how it might change in the future, and the potential consequences for the region's economy, environment, and quality of life.

"We know that climate has changed and varied in the past, and it is in the process of changing today," said William A. Sprigg, principal investigator for the study. He is the deputy director of the UA Institute for the Study of Planet Earth.

The study focused on Arizona, New Mexico, southeastern California, and much of Nevada, Colorado, and Utah. It was part of a national assessment by the U.S. Global Change Research Program on the effects of climate change.

Researchers estimate that temperatures in the region may rise by as much as 5 degrees Fahrenheit in the next 30 years, with increases between 7 and 12 degrees Fahrenheit predicted for the year 2090. The estimates come from computer models that account for many features of the Earth's climate system, Sprigg said, including human-induced factors such as carbon dioxide emissions.

Three separate climate scenarios were considered in the report, including a high-resolution model focused on the Southwest. Each of the models shows increasing temperatures in the future.

Model results were less consistent for precipitation, but some estimates show a doubling of annual rainfall for parts of the Southwest at the end of the century. Increases up to 5 mm per day (about two-tenths of an inch per day) were projected for the winter months, with California, southern Nevada, and Arizona seeing the largest gains.

Sprigg acknowledged that there are possible inaccuracies in the models results. "We are well aware of the uncertainties—the things that we don't know—that these models are not yet able to handle," he said.

Still, he believes the climate scenarios can aid in the long-range planning for fresh water use, energy production, range management, flood control, and wildfire management. "Given the desire for compatible and sustainable cities and natural landscapes, this is imperative," Sprigg said.

In addition to modeling experiments, the assessment team looked at the climatic history of the Southwest to get a better idea of the range of possibilities for the future. Analysis of tree rings and other records show several droughts occurred in the region during the last millennium, including extensive dry periods in the late 1500s, late 1800s and the 1950s.

The records also indicate that rising temperatures in the past correlated to a greater frequency of El Niño—a periodic climate phenomenon that occurs when sea surface temperatures rise in the Pacific Ocean. El Niño brings heavier rains to the Southwest, while the opposite phenomenon, La Niña, is correlated with drier conditions in the region. Increased frequency in either of these two events could result in a more variable climate, with alternating wet and dry periods.

"Understanding that our population and demand for water and energy is increasing dramatically, we would be wise to prepare for a range of future climates," Sprigg said.

Population in the region may increase by 13.5 million people during the next 25 years, according to projections from the U.S. Census Bureau. Already, Phoenix and Las Vegas are among the fastest growing cities in the nation.

"This report opens the door to answering an important question: Will global climate change act to limit economic and population growth in the Southwest?" Sprigg said.

The study was supported by the U.S. Geological Survey and the National Oceanic Atmospheric Administration. The assessment team included people from private industry, local and national agencies, and universities with a stake in the region.

Team members assessed the potential consequences climate change may have for key sectors in the area. These include:

Water - Warming in the Southwest may increase evaporative loss within the region's water delivery systems. Increased rainfall may help to offset this, but heavy winter rains might also cause flooding. El Ni-o events can also bring severe floods, increasing soil erosion and threatening property and lives. Even without significant climate change, rising population in the Southwest may create water shortfalls.

Ranching - Rangelands in the region are not irrigated; therefore changes in temperature and rainfall directly affect the amount of grazing material for cattle. Higher rainfall may increase rangeland carrying capacity. However, a more variable climate with fluctuating wet and dry periods may leave ranchers in a vulnerable position.

Natural Ecosystems - Climate change may cause shifts in the structure, composition, and functioning of the Southwest's ecosystems. Sand dunes in the Colorado Plateau region may shift position, affecting local residents, as well as grazing and farming activities. Increased rainfall can enhance the growth of grasses and other vegetation, which can later become fuel for forest fires in drier years. Therefore, increasing frequency of El Ni-o and La Ni-a cycles may heighten wildfire hazards.

Mining - Successful mine operations depend on the availability and management of water. Higher rainfall may increase the risk of overflow of storage reservoirs and holding ponds containing contaminants.

Human Health - Climate variations in the region may trigger outbreaks of Hantavirus and create a culture for fungus and mosquito-bearing diseases such as dengue fever, encephalitis, and valley fever. Warmer temperatures may also cause greater numbers of heat-induced illnesses.

Urban Areas - Increased flooding may lead to the decay of sewage systems, pipelines, roadways, and other urban infrastructure. Rising temperatures may also reduce air quality and intensify the "urban heat island" effect—when cities retain more heat than surrounding rural environments.

Energy - Higher temperatures will likely increase electricity demand in the hot summer months. This will be compounded by the Southwest' s growing population, so a greater number of power plants may be needed to meet demand. More rainfall may help alleviate this problem by increasing the potential for hydroelectric power.

[Writer: Shoshana Mayden, Institute for the Study of Planet Earth, (520) 792-8712.