News•January 14, 2013

Southwest Faces Looming Threats From Climate Change

The American Southwest, which is already the hottest and driest region of the nation, is likely to become even hotter and drier in the next few decades thanks in part to the ongoing effects of human-generated greenhouse gases. That’s the verdict of the draft National Climate Assessment report, the product of a federal advisory committee charged with assessing how climate change has already affected the U.S., and what the future holds.

Projected changes in average temperature (°F) from observed average temperatures between 1971 and 1999. Top row shows projections assuming heat-trapping gas emissions continue to rise (A2), Bottom row shows projections assuming substantial reductions in emissions (B1). (Figure source: NOAA NCDC / CICS-NC. Data 9 from CMIP3.)Click image to enlarge. Credit: National Climate Assessment.

The report’s chapter on the Southwest — one of 30 chapters in the nearly 1,200-page document released for public comment on January 11 — tells an ominous tale of disappearing water supplies; reduced crop yields; increased wildfires; coastal flooding and erosion; a drop in recreational tourism and serious public-health consequence for a regional population that already stands at 56 million and is projected to climb by another 38 million by mid-century.

Based on observations and scientific papers assembled since the last such report, in 2009, the chapter’s eight authors note that the period since 1950 has been hotter than any span of similar length in at least the past 600 years. Of that approximately 60-year period, moreover, the most recent full decade, from 2001-2010, was the hottest since modern record-keeping began in about 1900.

Overall, says the report, the decade was just short of 2°F hotter than the historical average, thanks to a combination of fewer and shorter cold spells combined with more frequent and more intense heat waves.

In desert areas already prone to severe dry spells, warmer winters have led to less mountain snowpack to keep streams and rivers flowing in spring, while hotter springs and summers have helped boost the effects of naturally occurring drought. The ripple effects of this reduction in water are almost too numerous to mention. To begin with, heat and drought put trees under stress, setting them up for attack by predatory beetles. Stressed and dying trees are perfect fodder for wildfires, according to a Climate Central report. That report showed how fires have already gotten much larger, on average, thanks to heat and drought.

The loss of water resources will also put enormous stress on agriculture, says the report, which notes that excluding Colorado, “more than 92 percent of the region’s cropland is irrigated, and agricultural uses account for 79 percent of all water withdrawals . . . ”

Description of a “vicious spiral” of warming in Southwest cities that could lead to serious increases in illness and death due to heat stress.

Click image to enlarge. Credit: National Climate Assessment.

As streams and rivers begin to dry out earlier in the summer, this overwhelming dependence on the water they carry could lead to massive crop failures — especially since human populations frequently draw on these same water supplies for drinking and other needs.

While heat and drought are likely to have an enormous impact inland, the California coast faces a different sort of threat: rising seas will threaten what the report calls “. . . the nation’s largest ocean-based economy, estimated at $46 billion annually . . . If adaptive action is not taken, coastal highways, bridges, and other transportation infrastructure (such as the San Francisco and Oakland airports) are at increased risk of flooding with a 16-inch rise in sea level in the next 50 years.”

In the Los Angeles area, meanwhile, rising seas are likely to push salty water in from the coast, contaminating both estuaries and groundwater.

As for the public-health impacts of climate change, the report fingers heat stress, which puts people — the sick and elderly, especially — at increased risk of death. That is especially true in cities, where temperatures are higher than in rural areas thanks to the so-called “urban heat island effect.”

Since an increased demand for air conditioning can lead to blackouts and no air conditioning at all, an overall rise in temperatures over the next 50 to 100 years could lead to an increase in mortality as well. And as if that weren’t bad enough, the report goes on to note that “Increased temperatures and longer warm seasons will also lead to shifts in the distribution of disease-transmitting mosquitoes.”

The report also says that those increased temperatures are pretty much inevitable: even if heat-trapping greenhouse-gas emissions around the globe are “substantially reduced” — a prospect that doesn’t yet appear to be on the horizon — the region could see another 2°F of warming by somewhere between 2040 and 2070.