Home > Articles > Climate change and drought in the American Southwest
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Rudy Baum | October 2015

 

Introduction

Drought is a common occurrence in the American Southwest.1,2 Since the beginning of the 20th century, the region has experienced four major multiyear droughts: the Dustbowl (1929-1940), the Southwest/Great Plains drought (1946-1956), a turn-of-the-century drought that lasted from 1999-2005, and the ongoing 2012-2015 drought.3,4 Other, shorter dry spells occurred during the same period as well.3 During the current drought, many states in the region have seen record or near-record persistent high temperatures, as well as extreme lows in precipitation, streams flows, and soil moisture.5 California has been hit especially hard. 2012-2015 have been the driest four years in the state’s recorded history,5 and in 2015 the Sierra Nevada snowpack hit a 500-year low.6 Drought conditions in California in 2014 represented a 200-year event,7 while the cumulative 2012-2015 drought may be completely unprecedented within the past 10,000 years.8

There is a strong consensus that rising global temperatures forced by anthropogenic CO2 emissions will increase the frequency, intensity, and duration of droughts across the American Southwest.9,10,11,12,13 Increased evaporation due to warmer surface temperatures and decreased precipitation will drive significant reductions in soil moisture across the region.12,13,14 This poses serious environmental and economic risks for the Southwest. Between 1980 and 2003, droughts caused an estimated $190 billion dollars in economic damage across the United States (adjusted to 2015 dollars), making it the most costly form of natural disaster over that period.3 Droughts can decrease the amount of water available for agriculture,15 reduce power generating capacity,16 and increase the risk of forest fires.17,18,19 Between growing demand for water and the predicted increase in drought frequency, the Southwest’s water infrastructure may struggle to adapt to a new, drier climate regime.3

 

Climate and Paleoclimate of the Southwest

The American Southwest, here defined as the area between 95°W and 125°W and 25°N and 40°N,9 covers over four million square kilometers. Four of western North America’s major watersheds lie within its boundaries: the Colorado River basin, the Rio Grande basin, the Sacramento-San Joaquin watershed, and most of the Great Basin. Annual mean precipitation across much of the region ranges between 10 and 60 centimeters per year,20 low enough to qualify as arid or semi-arid.21 Some areas, such as the Sierra Nevada in California and the high peaks of the Colorado Plateau, receive significantly more precipitation, mostly in the form of large quantities of snow.20 Much of the Southwest’s summer water supply comes from the melting of these alpine snowpacks.6,22

The combination of low annual precipitation and hot, dry summers makes the Southwest especially drought-prone. 10-15% of the years between 1895 and 1995 saw severe or extreme drought conditions in the region.1 At most, these modern droughts last about ten years; however, reconstructions of the Southwest’s paleoclimate have shown that the region is also susceptible to extraordinary megadroughts that can last for decades or even centuries. A fifty-year drought struck the Southwest, Northern Mexico, and the Rocky Mountains between about AD 1540 and 1590, which contributed to the abandonment of nearly a dozen native pueblos23 and may have aggravated an outbreak of an unknown, indigenous hemorrhagic virus that killed 7 to 17 million people.24 Three hundred years before, the Great Drought of AD 1267-1299 was at least partially responsible for the abandonment of Ancestral Pueblo sites throughout the Colorado Plateau.3,25,26

Some of the most severe megadroughts to strike the Southwest occurred in California. Between about AD 1250 and 1350, a century-long drought struck the Sierra Nevada region.27 Another, longer drought in the region has also been identified between AD 900 and 1100,27 although subsequent research has suggested that this earlier interval may not have been a single, uninterrupted drought, but rather a series of decade-long droughts following one after another.28 Precipitation was so low during this period that the shorelines of many lakes throughout the Sierra Nevada were tens of meters lower than they are today, which allowed mature pine trees to grow for decades in areas that today are underwater.27,29 These century-scale droughts caused significant disruptions for native cultures in California and the Great basin, leading to major migrations, site abandonments, and the collapse of long-distance trade routes.28

Both the California megadroughts and the Great Drought (which was part of a larger pattern of increased aridity in the Colorado Plateau) occurred during a period of elevated global temperatures known as the Medieval Climate Anomaly.3,26,27 In both cases, the droughts were caused by persistent, anomalously cold sea surface temperatures in the eastern Pacific consistent with the La Niña phase of the El Niño-Southern Oscillation (ENSO).3,23,26,27,30

 

Causes of Future Drought

Although drought is often thought of as the result of a shortfall in precipitation, most studies of drought instead look at deficits in the total moisture content of soils. Drought is therefore most often measured by comparing moisture supply into soils (precipitation) minus moisture demand from soils (evaporation).

21st century drought in the Southwest will primarily be driven by increased evaporation due to warmer global surface temperatures. Relative humidity will decrease as temperatures rise, which will lead to increased evaporative demand from soils.9,10,12,13 Enhanced evaporation due to global warming will reduce soil moisture in the Southwest by an average of 3 cm/year.9 By 2099, soils in the region will be 10-20% drier than they are today, which will increase the risk of drought by at least 20%.13 One of the characteristic features of climate change is that, for almost all weather phenomena, the risk of extreme events increases more quickly than it does for moderate events.13,31,32,33 Accordingly, the risk of a severe, multidecade drought occurring in the Southwest between 2050 and 2099 is more than 80% greater than it is today.34 Soil desiccation will be exacerbated by an increased likelihood of extreme daily high temperatures. Extreme hot days are already five times more likely than they were prior to the Industrial Revolution, and with another 1°C of warming they will become five times again more likely.33 In some cases, evaporative demand from warmer surface temperatures will exceed available soil moisture—in other words, there will be periods in the coming century during which some soils in the Southwest dry out completely.12 This excessive drying may feed back into even warmer surface temperatures: there is evidence that as soil moisture content reaches zero, the loss of cooling heat fluxes due to evaporation will cause additional warming.31

Changes in precipitation will also play a role in determining the future severity of drought conditions in the Southwest, although not as great as that of evaporation.12,13,14  As the planet warms, the downward arms of the Hadley Cells will expand poleward. Hadley Cells are a form of atmospheric circulation where air rises at the equator and descends at about 30° latitude on either side (about the latitude of the U.S.-Mexico border in North America). The downward arms of the Hadley Cells are associated with high atmospheric pressure and arid climates. Hadley cell expansion over the American Southwest is predicted to decrease precipitation there by 3-15% by the end of the 21st century.13,14 The increased high atmospheric pressure over the region will also reduce cloud formation, which will worsen evaporation by allowing more solar energy to reach earth’s surface.13 The decrease in overall precipitation will be exacerbated by changes in the way precipitation falls: more intense, less frequent precipitation will lead to longer dry spells.13 Increased winter rains, reduced snow packs, and earlier snow melts will also lead to less moisture being available during summer months, when evaporation is most intense.34,35

One major source of uncertainty in predicting future Southwestern droughts is the response of ENSO to rising global temperatures. La Niña (the cool or negative phase of ENSO), which is characterized by cool sea surface temperatures in the eastern equatorial Pacific Ocean, is strongly associated with drought conditions across the American Southwest.3,9,26,36 Indeed, the megadroughts of the 11th-13th centuries are thought to have been caused by unusually persistent La Niña-like conditions.3,23,30 However, it is unclear how ENSO will react to a warming climate.11,37 Climate models are split over whether ENSO will become more intense,37,38 or remain relatively unchanged.37,39 There is also the possibility that ENSO will evolve non-linearly over the course of the 21st century, initially becoming more intense before returning to normal variability after 2040.40 The uncertainty surrounding ENSO’s response to global warming thus makes it difficult to predict the precise details of future climate regimes in the Southwest.

 

Predicted Effects

As global temperatures continue to rise, perhaps by as much as 2-4°C by 2100,11 the Southwest will become increasingly dry.  Average surface temperatures in the region have already risen by at least 1°C since the beginning of the 20th century.2 At present, record hot days are occurring at twice the rate of record cold days.41 By the end of the 21st century, much of the region will experience at least moderate drying.12 This will lead to significant changes in regional climate types and plant communities, with many temperate climate zones in the region transitioning to semi-aridity.14

Changes in precipitation and evaporation will have significant impacts on snow pack levels, water availability, stream flow levels, and soil moisture. Warmer temperatures mean that more winter precipitation will fall as rain instead of snow.14 Earlier snowmelts have already been observed across the southwest, and in some basins rainwater has come to replace snowmelt as the dominant water source.35 Given that snowpack serves as a particularly important reservoir during the Southwest’s hot, dry summer months—snow melt accounts for between 30% and 75% of the region’s stream flows, depending on the basin6,22,42—a decrease in total snowpack has serious economic consequences. Further, warmer temperatures will result in earlier snow melts, which in turn will lead to earlier peak stream flows, as well as more extreme low flows later in the year.2,42,43 By the end of the 21st century, stream flows in the Sierra Nevada are predicted to drop at least 25% below their historical averages;2 essentially every stream in the Colorado River basin will have below-normal summer flows, with many previously perennial streams developing intermittent flows;43 and the Rio Grande’s upper basin will see total stream flow reductions of over 400 million m3/year.42

The economic and environmental impacts of increased Southwestern drought are substantial. The Colorado River is a major source of drinking water for 40 million people,44 while the Sierra Nevada snowpack, which accounts for 30% of California’s water supply, is a crucial reservoir not just for the state’s 38 million residents, but its lucrative agricultural industry as well.6 The 2012-2015 drought has cost California’s economy $2.74 billion (with $1.84 billion in losses coming from the agricultural industry alone) and 21,000 jobs.15 As surface water supplies decrease, groundwater is being increasingly relied on to make up the difference. Faced with a surface-water deficit of 10.7 km3, California will extract 7.4 km3 of groundwater in 2015 alone.15 In the Colorado River basin, groundwater is currently being depleted at a rate of 5.6 km3/year.45 (Note that, as California draws some water from the Colorado River, there will be some overlap between these numbers.) Reduced stream flows also have implications for power generation in the Southwest. By the mid-21st century, hydropower capacity along the Colorado River and in California could drop by 2-5%.16

On the environmental side, extended droughts may increase tree mortality, and, as a consequence, fuel loading in forests.19,46 This can lead to increased forest fire frequency and intensity, which can damage human health and property,17,18 as well as alter the ecological composition of burn areas.19,47 Research suggests that forest fires in the region are already becoming more common. Since 1986, the frequency of large-scale forest fires in the western United States has increased by a factor of four, and the total area burned has increased by a factor of six.48 Drought also imperils the viability of regional stream ecologies. As a result of increased surface temperatures and low stream flows, warmer annual mean temperatures are predicted in many streams across the Southwest.2,43 This could prove especially damaging for coldwater fish species such as trout. Total suitable habitat for all trout species found in the American west could decline by 47% by the end of the 21st century.49

 

Conclusion

Drought is a regular feature of the American Southwest’s climate regime, as can be seen in both the modern record, and through reconstructions of the region’s paleoclimate.1,3,26,27 Over the course of the 21st century, the Southwest will experience significant increases in drought frequency and overall aridity as a result of anthropogenic global warming.9,10,12,13,14 Warmer surface temperatures will increase evaporative demand, leading to reduced soil moisture content.9,12,13 Decreased precipitation across the region will also contribute to the increase in drought frequency, although its role will be secondary to that of evaporation.12,13 The effects of increasing regional aridity are expected to include changes in plant communities,14 reduced snowpacks and earlier snow melt,22,35 lower stream flows,2,42,43 warmer stream temperatures (Reynolds 2015), increased groundwater extraction,15,45 reduced hydropower generating capacity,16 increased tree mortality,19,46 and increased wildfire risk.17,18,19

In short, in the future the American Southwest will be hotter and drier than it is today. Drought conditions, as defined relative to 20th century averages, will become the norm—a permanent drought, in essence. There will still be wet years, as well as some truly exceptional dry years, but that natural variability will fluctuate around a mean climate state that, overall, will be much drier than the previous century’s.9

Human adaptation to a dryer Southwest will be a challenging proposition. As populations throughout the Southwest continue to grow,50 so too will the demand placed on the region’s diminishing water resources. The Southwest is a marginal landscape, characterized by rugged terrain, low precipitation, and high summer temperatures,20 and civilization in the region has often balanced on a hydrological knife edge. Within the past 1,000 years, several severe, multidecade droughts have struck the region, causing societal collapse, mass migrations,3,25,26,28 and in extreme cases perhaps even widespread human mortality.24 Future droughts in the region will be as or more severe than the megadroughts of the past.8,9 Due to its wealth, the United States has substantial capacity for climate change adaptation,51 but the scale of the challenges facing the Southwest in the coming century should not be underestimated. In the past, native civilizations in the region experienced major societal reorganizations or even collapsed in the face of severe, long-term aridificatoin—civilizations that, although less technologically advanced than the modern United States, were nevertheless very well adapted to life in the water-scarce Southwest.3,25,26,28 The possibility that climate change will fundamentally alter the economy and social structure of the Southwest cannot be ignored.

 

References

 

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