THIRTEEN
Future Climate Change and the American West
Many scales of climate change are in fact natural, from the slow tectonic scale, to the fast changes embedded within glacial and interglacial times, to the even more dramatic changes that characterize a switch from glacial to interglacial. So why worry about global warming, which is just one more scale of climate change? The problem is that global warming is essentially off the scale of normal in two ways: the rate at which this climate change is taking place, and how different the “new” climate is compared to what came before.
ANTHONY BARNOSKY, Heatstroke
LILACS AND SNOWMELT: WARNING SIGNS
A SURE SIGN OF THE beginning of spring in the American West is the annual bloom of wildflowers: they blanket hillsides and fields, setting them ablaze with color. During the 1990s, Dan Cayan, Susan Kammerdiener, Mike Dettinger, and Dave Peterson, all climate researchers at the U.S. Geological Survey, took an interest in the fragrant lilac. They were looking for plant data that would provide a key to interpreting satellite imagery of the mountain snowpack, a critically important feature of the water supply in the West. They did not start out wanting to know more about flowers; rather, they wanted to know more about snow cover. As it happens, the two are intimately linked.
The idea began in 1957, when Montana’s state climatologist Joe Caprio distributed lilac plants (Syringa vulgaris) to people across the West, requesting that they send him annual postcard reports with the dates of when the flowers sprouted and bloomed. Lilacs were chosen because they are found throughout the United States, they thrive in all types of soil and climate conditions, and they bloom in response to the increase in spring temperatures.
Four decades later, when Cayan and his fellow climatologists inherited these accumulated data on western spring wildflowers, they found surprising evidence of subtle changes occurring in the West. The researchers pored over the records, teasing out patterns that the casual observer might have easily overlooked. They found that the lilacs had begun blooming throughout the West earlier and earlier over time. By the mid-1990s, the flowers were blooming two weeks earlier than when record-keeping began, signaling that the arrival of spring was coming earlier. The climate researchers compared these data with their stream-flow data across the western United States—from New Mexico to Alaska—and found that mountain streams fed by snowmelt were flowing one to two weeks earlier as well. The researchers set out to investigate why these changes were occurring and what they would mean for water resources in the West.
Why Does Spring Come Earlier?
Initially, Cayan and his colleagues thought that the earlier onset of spring might be associated with a natural climate oscillation, perhaps related to changes in ocean temperature in the eastern Pacific. This was something that the climatologists had seen before in their climate records. But, as the evidence mounted, they realized that this was not just another climate cycle—it was a trend that had started decades before. Its beginnings were tied to the small but significant rise in global temperatures that began in the mid-twentieth century and to the steady rise in heat-trapping greenhouse gases, like carbon dioxide, in the atmosphere. This global temperature increase is reflected in more local records from the American West (see figure 32).
After comparing global temperature data from the mid- to late twentieth century with climate records spanning the past thousand years, most of the world’s climate scientists now agree that the period between 1960 and 2006 was substantially warmer than during the previous millennium. Nine of the ten warmest years in the West have occurred since the year 2000, with new records being set every year. This warming is the result of changes in the earth’s atmosphere: carbon dioxide levels have risen to 390 parts per million—an increase of approximately 75 parts per million since 1960.
A Changing Storm Track
Satellite observations reveal that, between 1987 and 2006, global precipitation has also increased. Warmer temperatures increase the rate of water evaporation, placing more water as vapor in the atmosphere. Eventually, this additional vapor condenses, resulting in increased precipitation. However, the relationship between warmer temperatures and precipitation is not straightforward. Climate models predict that some areas will experience net losses in moisture because of increased evaporation whereas other areas may experience net gains. Scientists are predicting that the already dry regions, like the American West, will become even drier and that wet regions farther north will become even wetter.
FIGURE 32. Annual mean atmospheric temperature (in degrees Fahrenheit) in the western United States from 1895 to 2010. (Figure courtesy of Kelly Redmond, Desert Research Institute, Nevada.)
These predictions are increasingly supported by observation. For example, climate models indicate that warming will cause a northward shift of the Aleutian Low and the North Pacific Storm Track, which will intensify precipitation over the Pacific Northwest, leaving the southerly regions, including much of the American West, drier. Researchers at the University of Arizona have observed that, since 1978, the storm track during wintertime (February to April) has been shifting northward, meaning that fewer rain-bearing storms are reaching the southwestern states—that is, Southern California, Arizona, Nevada, Utah, western Colorado, and western New Mexico. These researchers believe the trend will continue into the future if warming continues. Such shifts in the regional water regime may seem subtle in and of themselves, but the impacts are anything but subtle.
The Disappearing Snowpack
The American West depends on snow-bearing winter storms for its water supply. In fact, the region is unique in the contiguous United States in its dependence on the winter snowpack in the high mountains for a natural water reservoir. The snow begins melting in the late spring and continues into the summer, filling streams, lakes, and reservoirs that sustain ecosystems and humans throughout the dry summer months. The snowpack has supported growth in cities and irrigated agriculture. In California, agriculture is an industry that produces 55 percent of the nation’s produce, making this semiarid region an important source of food for the nation. Snowpack provides up to 80 percent of the annual water supply in this region.
A team of climatologists at the Scripps Institution of Oceanography, located near San Diego, predicts that global warming will affect this critical snowpack in several ways. For example, less precipitation will fall as snow as the region warms. For each degree of warming, the snowline moves up the mountainsides by 500 feet. Considering the climate-warming projection for the year 2100 of 5.4°F, the change means snow would fall 1,500 feet higher on the mountainsides; the lower elevations would receive rain. Regions like Northern California that have relatively low mountain elevations in the Sierra Nevada (averaging 7,000 to 8,000 feet) would lose half of their area of snowpack.
Studies have shown that the amount of the snowpack in California has declined by 10 percent during the twentieth century. Climate-modeling predictions suggest that, by the end of the twenty-first century, the snowpack will be reduced by at least 40 percent and perhaps as much as 80 percent. (The amount ultimately depends on just how much warming the region experiences in the future.)
In addition to changes in the volume and location of snow, there will be a change in the timing of snowmelt. The snow that does fall will melt earlier in the spring, rather than during the late spring and early summer when it is so critically needed. The decrease in summer snowmelt will only exacerbate the future drying that will come with warmer conditions during the summer months.
In the 1990s, the U.S. Geological Survey, Department of Water Resources, and other California agencies began continuous monitoring of streamflows, temperature, and chemistry of the Tuolumne and Merced rivers in Yosemite National Park. Changes in stream chemistry provide information about the links between the larger-scale atmospheric circulation over the Sierra Nevada and the hydrology in those mountains. For instance, the amount of silica dissolved in the water provides information about chemical weathering (dissolution) of rocks and minerals in the watershed, and nitrate and nitrite concentrations reflect pollution levels in the atmosphere. Among other results, these studies reveal that the spring snowmelt has indeed been starting earlier.
Earlier snowmelt has propagated impacts downstream as well, such as on water quality and ecosystems in the San Francisco Bay. Monthly monitoring efforts in the bay by the U.S. Geological Survey over decades have shown that earlier snowmelt is causing the salinity of bay waters to rise during the late spring and summer.
Wildfires on the Rise
As the mountain slopes across the great mountain ranges of the West dry, their forests—the pine, Sequoia, and juniper—will be increasingly susceptible to insect infestations, disease, and, inevitably, fire. Diminished winter snowpack, earlier spring snowmelt, and longer, hotter summers will weaken the entire forest ecosystem, leaving the trees fragile and flammable, increasing the number of forest fires across the West.
Residents of the West are all too familiar with this lethal effect of warming and drying. For instance, Southern California residents experienced a catastrophic fire season in 2007 in which close to a million people were evacuated and two thousand homes were destroyed, resulting in $1 billion in damages. For the climatologists at Scripps, this fire season was a painful reminder of the precarious conditions of life in the West. The region had suffered from a lack of adequate precipitation for seven years, including a deep drought in early 2007. Weakened by the dry conditions, millions of acres of spruce, piñon, and ponderosa pine succumbed to bark beetle disease, leaving vast stands of dead trees across the region that acted as kindling during the summer heat waves (see figure 33).
The following year, over two thousand wildfires raged throughout the state of California, scorching 900,000 acres. Subsequently, during the fall of 2008, communities in Los Angeles and Santa Barbara were scorched as hurricane-strength winds roared through the region.
The 2007 and 2008 fires were devastating, but were they a sign of a system out of balance? After all, fire has been a natural phenomenon in California and the West for thousands of years. Climate researchers, including Dan Cayan, wanted to find out whether Southern California was entering into a new fire regime. In a study of twentieth-century wildfires, they found that wildfire frequency did indeed increase over the century, with four times as many fires after the mid-1980s as before.
FIGURE 33. Forest fire in Sedona, northern Arizona, in June 2008. (Photo by B. Lynn Ingram.)
California is not alone in its plight: fires have been on the rise throughout the American West. Thomas Swetnam, co-director of the Laboratory of Tree-Ring Research at the University of Arizona, sees increased wildfires as one of the first big indicators of climate change in the United States. Warming, reduced precipitation, and earlier spring onset are all linked with larger forest fires. Swetnam notes that these impacts are not probable events in the future; rather, they are happening today.
The Drying Colorado River Basin
In the Southwest, a vast desert region that relies heavily on the Colorado River, similar effects concerning snowpack are predicted. Climate modeling of the Colorado River basin predicts that the snowpack there could decrease by 30 percent as early as 2050. The headwaters of the river lie in the high elevations of Colorado, Wyoming, and New Mexico. The river then flows through the high Colorado Plateau, ultimately bringing precious water to the lower desert states of Nevada, Arizona, and California. Under natural conditions, the river would then drain through its delta into the Gulf of California. Today, all of that water is spoken for and used before it ever reaches the once-thriving delta, as discussed in chapter 12. For decades, every drop has gone to sustaining a growing population and expanding agriculture throughout this arid region.
The stretch of the Colorado River in northern Arizona, between the two largest reservoirs on the river (Lake Powell and Lake Mead), supplies water to over 30 million people in major Southwest cities, including Los Angeles, Phoenix, Las Vegas, and San Diego. Climate models of the region predict that, over the next thirty to fifty years, the Colorado River system will experience 10–30 percent reductions in runoff as the result of climate change. Marine physicist Tim Barnett and climate scientist David Pierce at Scripps have incorporated these predictions into a water budget analysis assessing the impacts of natural climate variability, future climate warming, and water usage on the Colorado River basin. Their results show a net deficit of almost one million acre-feet of water per year in that basin. At this rate, they conclude, there is a 50 percent chance that both Lake Mead and Lake Powell could reach “dead pool,” rendering them useless for hydroelectric power generation or useful water storage, by as early as 2021. In the decade between 1999 and 2009, the level of Lake Powell dropped by 60 percent of its “full pool” capacity. The falling reservoir level has exposed enormous white calcium carbonate coatings, rising as high as ten-story skyscrapers on the canyon walls, and these are likely to grow higher in the future (see figure 34).
ECOSYSTEM IMPACTS
As difficult as these changes to our water supply will be on cities and agriculture, we must also consider the profound impacts of altered hydrology on the West’s ecosystems. At the University of California, Berkeley, ecologist John Harte has been studying the consequences of global warming in the American West. He has warned that, if all of the snow in mountains of the West were to melt by May, there would not be any water supply during the mid- to late summer, severely affecting the animals and plants that depend on that water.
FIGURE 34. Lake Powell, in 2009, showing a white calcium carbonate “bathtub ring” exposed after a decade of drought lowered the level of the reservoir to 60 percent of its capacity. (Photo courtesy of U.S. Bureau of Reclamation.)
In an effort to predict the possible ecological consequences of these changes, Harte’s research team has been conducting empirical experiments artificially heating wildflowers in a Colorado meadow near Crested Butte since the early 1990s. This research documents the fact that wildflowers bloom earlier when temperatures rise, and it indicates the consequences of that trend. In many cases, wildflowers depend on, and sustain, migrating butterflies that pollinate them. But the pollinators are not arriving earlier: their migration is triggered by day length, not temperature. This has dire consequences for the wildflowers (not to mention for the butterflies as well), threatening an enormous decline in their abundance.
Dozens of plant and animal species are now threatened by climate-induced changes in their food and water sources, habitats, and natural temperature ranges. In prehistoric periods of climatic warming, plants and animals had the time and space to gradually shift their habitats to higher elevations or latitudes. Today, many of these habitats have been reduced to mere remnants—islands in the midst of human development—leaving threatened populations with nowhere to go. Many are already on the verge of extinction from other pressures, such as habitat loss, over-exploitation, and environmental degradation.
A RENEWED DUST BOWL
As the climate warms and dries, the Southwest also faces an increase in atmospheric dust levels from both natural and human causes. Under natural conditions, the desert soils of the Southwest are stabilized by “desert crust”—a mixture of soil, cyanobacteria, and lichens. The cyanobacteria secrete a sticky material that holds soil particles together and retains water and nutrients important for the sparse desert vegetation.
As in the Great Plains of the 1920s, when farmers ploughed the soil too deeply, leaving it vulnerable to wind erosion and leading to the Dust Bowl drought of the 1930s, humans today are disrupting the natural protective surface of desert crust with vehicles, livestock, housing developments, and recreational activities. These effects are exacerbated during droughts (like the decade-long drought between 1999 and 2009) because the bacterial filaments between the soil particles become dry and more fragile, making them more easily crushed. As microbes and lichen decline and the desert crust erodes, the unstable soils are more susceptible to wind erosion, leading to an increase in atmospheric dust.
In lakes near the desert Southwest, researchers have collected cores from lakebeds and analyzed the sediments to calculate the levels of atmospheric dust over the past several centuries. The cores show a six-fold increase in dust after 1850, when livestock were first introduced to the West. The dust levels then declined somewhat after the 1930s, when the government passed the Taylor Grazing Act to reduce overgrazing in the West. But dust levels have been on the rise in recent decades as the human population has grown.
The increased dust levels also have other unforeseen negative impacts on hydrology in the West. For example, as dust settles onto snow in adjacent mountains, it darkens the surface, lowering its reflective properties and causing it to absorb more sunlight, which causes the snow to melt faster, leaving less snow for the dry summer months.
INCREASED FLOOD RISKS
Ironically, even as drought has become the most pressing concern in the American West, climate scientists and hydrologists are warning about increased flood risks throughout the region. Climate researchers Tapash Das, Mike Dettinger, Daniel Cayan, and Hugo Hidalgo have described how, even in a West with reduced precipitation, warming conditions can lead to more frequent flooding in mountain watersheds. Only a few degrees in temperature can determine whether precipitation falls as rain or snow over a watershed. In the mountains, as we have seen, these few degrees translate to a shift of several hundred feet in snow-line elevation. As temperature across the region rises, the total area above the snow line is reduced, with a greater area receiving precipitation as rain. The increase in rain during the short winter months generates runoff that will overwhelm flood control systems and overtop the banks of river channels, leading to floods. A particularly hazardous combination includes early winter storms that drop snow in the mountains followed by early spring storms that bring warm rains in these higher elevations, resulting in rapid and catastrophic snowmelt.
Das’s research team has simulated floods on the western slope of the Sierra Nevada, using projected temperature and precipitation patterns and assuming that greenhouse gas emissions will increase throughout the twenty-first century—the so-called “A2” scenario. Their results indicate that, between 2050 and 2100, both the northern and southern Sierra Nevada will experience larger and more frequent floods. The causes include increased frequencies of storms, greater amounts of precipitation, and a greater proportion of precipitation falling as rain rather than as snow.
Mike Dettinger has also independently explored the possible effects that warming in the twenty-first century could have on atmospheric river storms. His analysis shows that, under the A2 greenhouse gas emissions scenario, the nature of atmospheric river storms may change, with increasing frequency of such storms and increases in water vapor transport rates as compared to the historical record. Predicted storm temperatures will be higher, meaning more rain will fall during these events. Dettinger is concerned that these changes will lead to more frequent and larger floods in the future. He predicts a lengthening of the atmospheric river storm season, meaning floods could potentially occur over a longer portion of the year. This will make it difficult for water managers to operate reservoirs, where they are balancing flood control during the wet season with water storage for the dry summer and fall months.
COASTAL IMPACTS
By the year 2050, when the average global temperature is predicted to be about 2°F higher than it was at the turn of the twenty-first century, scientists are forecasting a number of other changes in the West that will directly or indirectly affect the water supply. For instance, current predictions indicate that sea levels along the Pacific coast will rise about thirteen inches or more due to melting ice and thermal expansion resulting from the increased heat absorption by the ocean surface. Rising sea levels will inundate coastal wetlands. The more protected parts of the coastline—bays and estuaries like San Francisco Bay and Puget Sound—will grow larger and saltier with the rising sea, encroaching on the low-lying bordering cities. Around these estuaries, the tidal marshes that had so long been part of the natural history will be squeezed out.
More insidious, though, will be the intrusion of saline water into underground freshwater aquifers that now provide much of the region’s drinking and irrigation water. Coastal ground waters are also at risk of becoming saltier as sea levels rise.
In California’s Sacramento–San Joaquin Delta, higher sea levels may cause levees to fail, drowning the surrounding low-lying valleys. By 2050, two-thirds of the state’s projected 50 million residents may live in the southern Central Valley and Southern California, areas that depend on the freshwater that passes through the delta and is pumped southward through the California Aqueduct. Should the levees fail, two-thirds of the state will be left without a significant amount of their potable water.
INCREASED RISK OF DAM FAILURES
Those who live on floodplains are no longer the only ones at risk of catastrophic floods. The twentieth-century dams hold back vast reservoirs of water, and they have the potential to fail, with catastrophic results. Southern California already experienced a taste of this in 1928, when the St. Francis Dam collapsed, resulting in hundreds of deaths. It was the worst civil engineering disaster of the century in the United States.
Although the twentieth-century era of extensive dam construction (the “hydraulic era,” as discussed in chapter 12) essentially came to an end in the 1970s with the rise of the environmental movement, residents of the West will have to contend with the risk of dam failures in the future because aging dams are increasingly stressed by the changing hydrology of the West. As the climate warms, more precipitation will fall as rain and less as snow, and this means more runoff during the winter and more pressure on the reservoirs.
Most of the region’s dams were built more than half a century ago—and some are considered disasters waiting to happen. For instance, Glen Canyon Dam on the Colorado River, located on the Arizona-Utah border, began to show signs that its foundation was deteriorating in June 1983. This project had been controversial when it was built in 1956. The resulting reservoir submerged 186 miles of magnificent canyon lands in northern Arizona and southern Utah, including over a hundred side canyons and over two thousand Ancestral Pueblo archaeological sites. After the wet El Niño winter of 1983, unusually large amounts of meltwater entered the reservoir from the watershed. The excess water that poured through the overflow spillways began to turn red, indicating that the spillways were beginning to erode the red sandstone bedrock that supported the dam. James Powell vividly described the unfolding disaster in his book Dead Pool: Lake Powell, Global Warming, and the Future of Water in the West:
Leaks sprang from joints in the outlet works. High pressure popped up manhole covers all over the dam, as if a master magician had levitated them. Everything leaked that could. The water rumbling through the spillways and the vibrating dam produced a cacophony of sound. Standing in the access tunnels in the dam abutments was like being inside a factory in a rainstorm, as the enormous pressure forced water through the porous sandstone. Those approaching the dam from downstream could hear the noise from four miles away. At two miles, large waves stirred the surface of the river and a violent rainstorm fell from the mist emitted from the spillways. Springs spurted from the sandstone walls of Glen Canyon. Closer to the dam the jets from the spillways began to eat into the protective apron that led back to the base of the dam where the generator releases emerge. To one making the trip upstream, that the largest dam disaster in human history might be under way did not seem far-fetched. (p. 14)
Fortunately, the dam remained standing. According to Powell, its collapse would have resulted in a 580-foot wall of water travelling 300 miles downstream, destroying Hoover Dam and Lake Mead, primary sources of water and power for 30 million people in Los Angeles, Phoenix, Tucson, and Las Vegas.
HUMAN-CAUSED OR NATURAL CLIMATE CHANGE?
Climate researcher Tim Barnett and his colleagues have analyzed the climate and hydrology of the American West over the past sixty years, particularly river flow data from the Columbia River, Colorado River, Sacramento River, and San Joaquin River, draining watersheds in the Cascades, northern Rockies, and Sierra Nevada across nine western states. After the research team demonstrated that their global climate model could realistically portray the natural climate in the West, they then ran climate simulations of the West to predict changes in snowpack and river flows.
Using these data and some very long (greater than 800-year) simulations of natural variations (without the human-caused greenhouse gas additions) of climate and river flows from the models, Barnett and his team showed that changes in temperature and snowpack over the last half of the twentieth century were not simply the result of the natural, long-term climatic shifts of the Pacific Decadal Oscillation (PDO). Although the PDO moved to a warmer phase in 1976–77, which was consistent with warmer temperatures and earlier spring snowmelt in the West, it then swung back to a cool phase starting in 1999. Yet this phase shift did not reverse the trends of increasing temperatures and earlier snowmelt. The researchers also showed that volcanic eruptions and solar activity occurring over the past several decades could not have been responsible for changes in snowpack and temperature—in fact, such events would have caused an opposite response: a decrease in temperature and an increase in snowpack. Their conclusion was that only anthropogenic increases in greenhouse gases—that is, human causes—could account for the rising temperature and diminishing snowpack trends they were seeing.
The days of water “on demand” will surely end in the West, replaced by desperate efforts to capture and control what water is left. The Southwest will become a region of extremes as the early spring brings massive flooding and the relentless summer bakes the region dry. While climate scientists have presented the potential risks facing our society, environmental scientists and innovators have explored the various steps we can take to better prepare for a hotter, drier (and sporadically much wetter) future in the West. For example, implementing more effective policies of water conservation and limiting new growth and development in threatened regions will greatly improve the chances of surviving and even flourishing in the future. The next chapter will discuss some of these alternatives as well as the lessons we have learned from the past.