BNA’s climate blog is taking a closer look at U.S. regions covered in the draft National Climate Assessment. In addition to the Southwest, the assessment covers the Southeast and Caribbean, Midwest, Northeast, Great Plains, Northwest, Alaska and the Arctic, and Hawaii and U.S. Affiliated Pacific States.

Climate change poses a significant risk to the Southwest's most “precious” resource, water, according to the draft National Climate Assessment. The region also will continue to be plagued by an increasing number of wildfires, posing more risk to communities across extensive areas.

Furthermore, the Southwest—where a higher proportion of people live in cities compared to the national average—will face damage to infrastructure as the result of higher temperatures, more droughts, floods, and sea-level rise.

As detailed in an Energy and Climate Report article published Jan. 11, the draft, released by the National Climate Assessment and Development Advisory Committee, is not yet an official federal document. It will become so once it is submitted to the National Science and Technology Council sometime this fall. The council is expected to review it, modify it, and approve a final draft by Jan. 30, 2014.

A national assessment on climate science and climate change impacts is due to Congress every four years under the Global Change Research Act of 1990. However, only two reports have been completed since the law was passed, one in 2000 and the other in 2009.

The draft national assessment defines the Southwest as Arizona, California, Colorado, Nevada, New Mexico, and Utah.

Temperature, Precipitation

Temperatures in the Southwest by 2041-2070 are expected to rise by 2 to 6 degrees Fahrenheit if greenhouse gas emissions decrease substantially and by 5 to 9 F under current emission trends, according to the draft assessment.

In comparison, the draft estimates that, on average, U.S. temperatures will rise between 2 and 4 F by mid-century under current emission trends. However, the amount of warming by 2100 is expected to be about 3 to 5 F under a scenario involving substantial reductions in emissions after 2050, and 5 to 10 F assuming continued increases in greenhouse gas emissions.

The Southwest, with its high-density urban populations, will be especially impacted by the urban heat effect, according to the draft assessment, posing an even greater threat to public health, increasing the need for air conditioning, and likely causing disruptions to electricity and water supplies. Urban heat islands, composed of pavement and buildings that absorb heat, tend to have higher temperatures than areas with more vegetation.

The draft said urban infrastructure is especially vulnerable to increasing temperatures. As an example, the draft points to an 11-minute power system disturbance in September 2011 that cascaded into outages that left 1.5 million San Diego residents without electricity for 12 hours and also disrupted pumps and water service, causing 1.9 million gallons of sewage to spill near beaches.

In addition to increasing public health risks, extreme heat and other weather events are expected to impact the region’s production of high-value specialty crops, including fruits, nuts, and vegetables, which account for half the nation's supply. Farmland in the region depends mostly on irrigation, making it highly susceptible to projected drought and higher temperatures, the draft said.

Changes in precipitation are less certain, but the draft predicted that if greenhouse gas emissions continue to increase, the southern part of the region will see less rainfall. While the region is prone to drought—for example, it has experienced several severe 50-year-long-mega-droughts over the past 2,000 years—future droughts are expected to be substantially hotter, and for river basins, such as the Colorado River basin, “drought is projected to become more frequent, intense, and longer lasting than the historical record.”

In the northern part of the region, however, climate models disagree, projecting a mix of increases and decreases in precipitation during winter, summer, and fall. The models are in agreement for spring, showing a decrease in precipitation.

In addition, the draft said, “airborne moisture” that flows into California’s coastal ranges and the Sierra Nevada—also referred to as “atmospheric rivers”—will likely lead to an increase in damaging, widespread winter floods that have been known to reach inland as far as Utah and New Mexico.

The amount of winter snowpack, which provides fresh water during spring and summer thawing, is expected to continue its 50 years of decline due to less late-winter snow, earlier snow melt, and earlier arrival of most of the year’s stream flow, according to the draft. Decreased snowpack also is expected to be exacerbated by an accumulation of dust and soot that absorbs the sun’s energy, leading to earlier snowmelt and evaporation, the draft assessment found.

The result will be increased risk to water supplies, which will affect cities, ecosystems, and especially crop production, which may shift northward where water supplies are expected to be more stable, according to the draft.

Sea-Level Rise in California

In the past 100 years, sea levels have risen along California's coast  by 6.7  to 7.9 inches, according to the draft, and sea-level rise is expected to speed up in line with the global expected average of between one and four feet by 2100.

The risks of sea-level rise during the next 30 years will be particularly evident during high tides and storm events, eroding bluffs and beaches, causing floods, and inundating  infrastructure, including airports, power plants, wastewater treatment facilities, drinking water infrastructure, roads, bridges, and ports.

The draft said  420,000 people will be at risk of a once-in-100-year flood by the end of this century compared to the current 140,000 people who are at risk in the region.

Wildfires

While periodic natural wildfires maintain healthy ecosystems in the Southwest, climate change outweighed other factors, such as the accumulation of wood and other fuels and insect infestations, when accounting for burned areas in the Western United States from 1916 to 2003, according to the draft assessment. The determination was based on a 300-year-long reconstruction of Southwestern fire history.

The draft said climate change is expected to more than double the extent of burned areas in the southern Rockies and increase burned areas in California by up to 74 percent.

Adapting to Climate Change

Adaptation measures that could reduce the impact of wildfires include prescribed burning, mechanical thinning, and retention of large trees, the draft said.

The societal and economic impacts of reduced water supplies could be alleviated by “functioning water markets” that allow for the transfer of water across state and local jurisdictions, the draft said. However, it remains uncertain whether legal and institutional restrictions can be addressed and infrastructure investments made to allow for water transfers, it said.

The Southwest also is poised to produce more energy with less water. “The Southwest’s abundant geothermal, wind, and solar power-generation resources could help transform the region’s electric generating system into one that uses substantially more renewable energy,” the draft said, adding that large increases in renewable energy generation may be feasible at “reasonable costs.”