In 2015, scientists reported that a five-year drought in California had been intensified 15 percent to 20 percent by rising global temperatures. They projected that drought is almost certain to get worse as temperatures continue to rise. Melting ice in the Arctic may play a role in anchoring high pressure off the North American west coast and diverting storms in winter (as well as the usual summer pattern), a matter that is open to scientific debate. There is little question, however, that warmer air accelerates evaporation ( Gillis 2015 ). A. Park Williams and colleagues wrote in Geophysical Research Letters (2015) ,

“California is facing a new climate reality,” wrote Noah S. Diffenbaugh and Christopher B. Field in The New York Times (2015) . “Extreme drought is more likely. The state’s water rights, infrastructure and management were designed for an old climate, one that no longer exists.” Their research, they said,

An August 2015 analysis of tree rings from blue oaks in California’s Central Valley indicated that snowfall in nearby mountains was lower by 2015 than it had been in 500 years. By that time, the Sierra Nevada was entirely bereft of snow for the first time in 75 years of recorded history. “The results were astonishing,” said Valerie Trouet, an associate professor at the University of Arizona, an author of the analysis in Nature Climate Change ( Belmecheri et al. 2015 ). “We knew it was an all-time low over a historical period, but to see this as a low for the last 500 years, we didn’t expect that. There’s very little doubt about it,” she said ( Fears 2015 ).

California’s drought caused $2.2 billion in agricultural loses as fields were left fallow in just 2014 alone. More than 12 million trees had died in the five-year drought by 2015. This may be a foretaste: “Future droughts will be compounded by more-intense heat waves and more wildfires. Soaring temperatures will increase demand for energy just when water for power generation and cooling is in short supply. Such changes will increase the tension between human priorities and nature’s share” ( AghaKouchak et al. 2015 ).

During the winter of 2014–2015, rain teased California for a few December days but evaporated in January with record heat and low rainfall. By March, temperatures in Southern California had risen to nearly 100°F in some places as mountain snowpacks eroded to the lowest on record. On March 3, statewide snowpack was only one-fifth of usual. Rising temperatures were shortening the period in which snow could accumulate and were melting snow at lower altitudes. What little precipitation did fall came more often as rain, which melted existing snow and then soaked into the ground before reaching reservoirs. Winter 2013–2014 was California’s warmest on record, at 45.6°F, until the winter of 2014–2015 averaged 47.4°F. Higher temperatures increase evaporation and human demand for water. The winter of 2016–2017 brought record rainfall.

“The normal cyclical conditions in California are different now from what they used to be, and that’s not because the long-term annual precipitation changed,” said Noah Diffenbaugh, a senior fellow at the Stanford Woods Institute for the Environment. “What is really different is there has been a long-term warming in California,” he said. “And we know from looking at the historical record that low precipitation years are much more likely to result in drought conditions if they occur with high temperatures” ( Nagourney 2015 ).

Since the 1990s, an epic drought has seized the western United States from the Navajo reservation (which has been inundated by sand dunes) to California, where the continent’s most productive agricultural valleys have been parched. With a few exceptions, the dominant jet stream pattern, locked in place by advancing Hadley cells in the upper troposphere, has moved rain-bearing storms to the north, even during the usual winter rainy season. The winter of 2010–2011 was an exception. Snow was so plentiful that the Missouri flooded for months all summer long. However, the drought soon locked in once again.

“Widespread annual droughts, once a rare calamity, have become more frequent and are set to become the ‘new normal,’ ” wrote Christopher Schwalm, Christopher A. Williams, and Kevin Schaeffer in The New York Times (2012) . Schwalm is a research assistant professor of Earth sciences at Northern Arizona University, Williams is an assistant professor of geography at Clark University, and Schaefer is a research scientist at the National Snow and Ice Data Center.

“It sounds like the plot of a disaster movie that somehow fuses the dangers of drought, earthquakes, and overpopulation—‘Mad Max’ meets ‘Soylent Green’,” wrote Justin Moyer in the Washington Post in 2015.

With snow scarce in western North America and temperatures abnormally warm, intense wildfires broke out early in May 2015 as far north as the Canada’s Northwest Territories. The fires were so numerous that the NASA Earth Observatory described “[heavy] smoke from the growing blazes merged with lingering smoke from previous days, making air quality a major concern.” “Fires this early are very unusual,” said Mike Flannigan of the University of Alberta. He noted that the Northwest Territories and northern Alberta usually see the most active part of the fire season in July, and British Columbia’s fire season typically runs from the end of July through August. Some of the fires were so large and intense that they generated their own pyrocumulonimbus clouds and thunderstorms. In Alberta, NASA said, more than 60 fires displaced thousands of residents and led some energy companies to suspend oil operations and evacuate staff, which reduced tar-sands production ( “Intense Fires” 2015 ; “Alberta Wildfires” 2015 ).

Wildfires across Canada during the summer of 2015 exported smoke to urban areas. Across the western half of North America, from Alaska to California, record heat and drought provoked an unprecedented fire season. During the first week of July, smoke that had risen into the stratosphere from fires in Alberta and Saskatchewan obscured the sun across the U.S. upper Midwest, including in Omaha, Des Moines, and other cities. Smoke was so thick some days in Denver that flights were delayed at its international airport. Colorado state health officials warned people with asthma to stay indoors on days when the smoke was thickest.

By the end of July 2015, as many as 300 wildfires, most started by lightning, were burning at one time across Alaska as record temperatures prompted early snowmelt and drought turned forests to tinder. Nearly 5 million acres had already burned, making the season among Alaska’s worst, with several weeks to go until the first snows of early fall snuff residual cinders. During the record year 2004, 6.59 million acres burned ( Mooney 2015 ).

More land burned in Alaska in 2015 than in the rest of the United States combined, producing smoke so dense on some days that many outdoor events were canceled in Fairbanks. The intense burning was adding carbon dioxide to the atmosphere in two ways: the torched trees themselves, as well as the smoldering of what was once frozen ground (permafrost), much of which is laced with carbon-rich organic peat. “The more severe the fire, the deeper that it burns through the organic layer, the higher the chance it will go through this complete conversion,” said Ted Schuur, a Northern Arizona University ecologist and permafrost specialist. “What happens in the summer of 2015 has the potential to change the whole trajectory of [the burned area] for the next 100 years or more” ( Mooney 2015 ). “Across the Arctic, permafrost in North America, Northern Europe, and Siberia contains twice as much carbon as Earth’s atmosphere. The permafrost that we degrade now in these forest fires might never return in our lifetimes,” Schuur said ( Mooney 2015 ).

Smoke from hundreds of fires burning across British Columbia swept into Vancouver on the coast and extended into nearby interior areas. NASA detected a thick pall of smoke July 5 and 6 that “settled over Vancouver and adjacent areas of British Columbia, leading some residents to wear face masks and health officials to warn residents and World Cup tourists against outdoor activities” ( “Metro Vancouver” 2015 ). “Tan and gray smoke almost completely obscured the Strait of Georgia and southern Vancouver Island. Winds shifted abruptly between July 5 and 6, driving the smoke plume toward the east, dispersing it in some places while fouling the air in areas to the east, such as the Fraser Valley” ( “Smoke Blankets” 2015 ). For a few days, officials in Vancouver compared its air quality to that of Beijing.

“When it comes to the skies overhead, it appears that orange is the new blue,” reported Yvonne Zacharias in the Vancouver Sun (2015) .

As smoke from at least 110 fires in the province of Saskatchewan poured southward into large areas of the United States, some 13,000 indigenous people of the Lac La Ronge and other bands were forced to evacuate their homes during the first week of July 2015. One hundred miles to the south of this evacuation, the Montreal Lake Cree Nation declared a state of emergency as its community was surrounded by a wall of fire so fierce that several hundred firefighters were forced to withdraw. Residents were forced out of their homes as several structures burned to the ground.

“There was no stopping it—four water bombers, two helicopters bucketing, crews on the ground doing whatever they could, but we just couldn’t do anything when the wind picked up,” said Chief Edward Henderson, describing the wall of flames in La Ronge. “I’ve never seen anything like this,” Lac La Ronge Indian Band Chief Tammy Cook-Searson told CBC News. “We’ve had to evacuate all six of our communities” ( “Saskatchewan” 2015 ). At the height of the fires, 2,500 firefighters and Canadian armed forces troops from Ontario, New Brunswick, Newfoundland, Alberta, and Saskatchewan battled the flames along with reinforcements from South Dakota.

As many as 80 million piñon trees (the state tree of New Mexico) died in Arizona and New Mexico between 2001 and 2005 because of intense drought. That represents some 90 percent of the area’s piñon trees ( Carlton 2006 , A1). Four Page 9  |  Top of Articlemillion have died in Santa Fe alone. The trees also have fallen victim to a bark beetle the size of a grain of rice that feasts on dying and diseased trees.

Confirming many anecdotal observations, a study of data collected over the past 50 years and published online on January 31, 2008, by the journal Science said that sharp declines in mountain snowpacks are being caused primarily by human-induced global warming, As temperatures warm, mountains receive more rain and less snow, which alters the hydrological cycle in many ways that are detrimental to nature and human activity. Snows have been melting earlier and more rivers have been running dry in the summer.

The study examined natural variability of temperatures and precipitation and the effect of sun-spot cycles as well as worldwide volcanic activity and human-induced greenhouse gas releases. “We’ve known for decades that the hydrology of the West is changing, but for much of that time people said it was because of Mother Nature and that she would return to the old patterns in the future,” said lead author Tim Barnett of the Scripps Institution of Oceanography at the University of California–San Diego. “But we have found very clearly that global warming has done it, that it is the mechanism that explains the change and that things will be getting worse” ( Kaufman 2008 ).

Gradual annual melting of snowpacks is vital for generating electricity, agriculture, drinking water, and to provide enough water for salmon to reach their breeding grounds. More abundant rain tends to run off too quickly to be useful, especially during dry summers, and may cause floods. “Our results are not good news for those living in the western United States,” the researchers wrote, adding that the changes may make “modifications to the water infrastructure of the western U.S. a virtual necessity” ( Kaufman 2008 ).

The pattern was not linear; some years (such as 2007–2008) were still notable for heavy snows. This study indicated, however, that since 1950 snowpack water content on April 1 has declined on average in eight of the nine mountainous areas—from 10 percent in the Colorado Rockies to 40 percent in the Oregon Cascades. The southern Sierra Nevada range was alone in not showing a decline.

Jonathan Overpeck, a climate scientist at the University of Arizona in Tucson, said the new study “closes the circle” in terms of understanding what is happening to the climate of the West. “Almost all of the models we’ve seen in recent years show the area becoming warmer and more arid due to climate change, but the question was always whether we could believe them,” he said. “Now someone has done the statistical analysis to connect the dots so they can say with real confidence that this is happening because of greenhouse gases” ( Kaufman 2008 ).

Paleoclimatic precedent exists for the kind of substantial drought that has become common in the United States West in recent years. Prevailing hot southwest winds during the growing season in the Great Plains of the United States 800 to 1,000 years ago spawned a drought that destroyed the Anasazi civilization in the Southwest. During this period, the prevailing winds shifted from the south to the southwest, bringing in moisture from the Gulf of Mexico, which is hot and dry like now.

A team of scientists (Sridhar Venkataramana, David B. Loope, James B. Swinehart, Joseph A. Mason, Robert J. Oglesby, and Clinton M. Rowe) from the University of Nebraska at Lincoln and the University of Wisconsin have discerned a major wind shift by studying the shapes of dunes in the Sand Hills, which once constituted a sea bottom, in north-central Nebraska. During the Medieval Warm Period, the drought killed the dunes’ vegetative cover. The winds caused the dunes to migrate, leaving today’s patterns, which are again anchored by grasses. As the team noted, “Longitudinal dunes built during the Medieval Warm Period (800 to 1,000 years before now) record the last major period of sand mobility. These dunes are oriented northwest to southeast and are composed of cross-strata with bipolar dip directions” ( Sridhar et al. 2006 , 345).

“Such a westward shift [in prevailing winds] would … greatly reduce the flow of moist air into the central Great Plains, thereby generating severe drought,” the scientists wrote in July 2006 ( Sridhar et al. 2006 , 346). Although the “dry line” now brings spring and summer thunderstorms that traverse Nebraska from the Rocky Mountains, “during the Medieval warm Period the mean position of the dry line moved much further east, such that the Sand Hills were most often in the dry, hot air with greatly reduced precipitation” ( Sridhar et al. 2006 , 346). A similar wind shift coupled with depletion of the Ogallala Aquifer could make much of Nebraska too dry for agriculture in coming decades. Large parts of this aquifer, the largest body of underground water in North America, are being drawn down by feet per year while precipitation only recharges it in inches per year.

Such dune patterns can be used as proxies for atmospheric circulation that governs weather and climate over periods decades to centuries long but leaves no records. Approximately half the Sand Hills’ annual precipitation falls during May, June, and July, nourishing a grassland ecosystem. During the Dust Bowl years of the 1930s (and to a lesser extent during droughts in the 1950s), part of this ecosystem broke down, producing what scientists call “isolated blowouts” ( Sridhar et al. 2006 , 345). Historical accounts indicate that some dune crests lost their grass to drought occasionally even during the generally cooler 19th century. These accounts paled, however, beside the sustained drought that accompanied the Medieval Warm Period.

In early 2015, NASA issued a warning: if current rates of increase in greenhouse gas emissions continue, most of North America, except for the Arctic and near-Arctic, would suffer a “megadrought” during the balance of the 21st century that will make what the U.S. Southwest has thus endured look like an appetizer. According Page 12  |  Top of Articleto NASA, the drought between 2050 and 2100 will be the worst in at least 1,000 years since the Chaco drought that devastated the Anazasi and Mayan civilizations between 1100 and 1200 CE. Jason Smerdon, a climate scientist at Columbia University’s Lamont–Doherty Earth Observatory in Palisades, New York, coauthored this study and said the impending megadrought will make the Chaco drought look “quaint” ( Underwood 2015 ). In addition, in the midst of the United States’ worst wildfire season in 2015, NOAA released a study projecting conditions six times as bad by 2050 because of global warming, with droughts and longer fire seasons aggravating fire conditions ( Rice 2015 , August 28).

“Future precipitation trends, based on climate model projections for the coming fifth assessment from the Intergovernmental Panel on Climate Change, indicate that droughts of this length and severity will be commonplace through the end of the century unless human-induced carbon emissions are significantly reduced,” the team wrote. Drought tends to compound the warming of the atmosphere because plants stressed by lack of water photosynthesize less and take in only half of the carbon dioxide they would if they were adequately hydrated ( Schwalm et al. 2012 ).

The effects of the great drought in the U.S. West exceed that of any other on record, including the Dust Bowl of the 1930s, which mainly gripped the center of the continent. “While that drought saw intervening years of normal rainfall, the years of the turn-of-the-century drought were consecutive. More seriously still, long-term climate records from tree-ring chronologies show that this drought was the most severe event of its kind in the western United States in the past 800 years,” Schwalm and colleagues wrote (2012) .

Most significantly, this drought is probably not cyclical. A warming atmosphere is locking in changes in upper-air circulation that may freeze it in place, with a few exceptions (see “Atmospheric Circulation Changes” and the discussion of Hadley cells). As Schwalm and colleagues wrote, “These climate-model projections suggest that what we consider today to be an episode of severe drought might even be classified as a period of abnormal wetness by the end of the century and that a coming megadrought—a prolonged, multidecade period of significantly below-average precipitation—is possible and likely in the American West” ( Schwalm et al. 2012 ).

Scientists are measuring reductions in snowpack across western North America’s higher elevations, which are important because they provide agricultural and urban sustenance. As noted by another team writing in Science in 2011,

Other studies support these conclusions. In 2010, Jonathan Overpeck and Bradley Udall wrote in Science (2010) ,

Mortality rates have risen substantially even in previously healthy conifer forests of the western United States averaging more than 200 years of age, doubling during two to three decades because new trees often fail to replace those that die. Drought provoked by rising temperatures is a major reason for rising death rates of pine, fir, hemlock, and others, according to a study released early in 2009. The increasing mortality of trees also reduces the ability of the forests to absorb carbon dioxide ( Van Mantgem et al. 2009 ; Pennisi 2009 ).

If greenhouse gas emissions continue to increase along current trajectories throughout the 21st century, there is an 80 percent likelihood of at least one decades-long megadrought in the Southwest and Central Plains between the years 2050 and 2099 ( “NASA Study” 2015 ). The high-emissions scenario projects an atmospheric carbon dioxide level of 1,370 parts per million (ppm) by 2100, whereas the moderate emissions scenario projects 650 ppm by 2100, compared to around 400 ppm in 2015 ( “NASA Study” 2015 ). “We can’t really understand the full variability and the full dynamics of drought over western North America by focusing only on the last century or so,” Cook said. “We have to go to the paleoclimate record, looking at these much longer time scales, when much more extreme and extensive drought events happened, to really come up with an appreciation for the full potential drought dynamics in the system” ( “NASA Study” 2015 ).

“Summers are getting longer,” said Nathan L. Stephenson, of the U.S. Geological Survey (USGS), a coauthor of the study with Phillip van Mantgem, also of USGS. “Trees are under more drought stress” ( Navarro 2009 ). The recent warming in the West “has contributed to widespread hydrologic changes, such as a declining fraction of precipitation falling as snow, declining water snowpack content, earlier spring snowmelt and runoff, and a consequent lengthening of the summer drought,” the scientists wrote ( Eilperin 2009 ). “It’s very likely that mortality rates will continue Page 14  |  Top of Articleto rise,” said Stephenson, adding that the death of older trees is rapidly exceeding the growth of new ones, which is analogous to a human community in which the deaths of old people surpass the number of babies being born. “If you saw that going on in your home town, you’d be concerned” ( Eilperin 2009 ).

The Colorado River by 2004 was carrying only half as much water compared to the Dust Bowl drought of the 1930s. Utah’s Lake Powell—the second biggest human-made lake in the United States—had lost nearly 60 percent of its water by 2004. Assuming current trends, the lake may lose its ability to generate electricity in three years’ time ( Lean 2004 , 20). After nine years of drought that caused Lake Powell’s water level to fall more than 100 feet, heavy snows during 2006–2007 recouped 50 feet of water ( Clark 2008 , 1D).

If global warming continues to intensify, the danger exists that decreasing winter snowpack, intensifying summer dry conditions, and increasing forest fires may reduce vegetation cover and regional soil-biosphere water-holding capacity. If these conditions reach sufficient intensity and geographical scale, they may become self-perpetuating; if so, we will have suddenly entered a long-term megadrought in the western United States. Weather would continue to fluctuate from year to year, but water supplies would be much more limited than in prior decades and dust storms may become frequent. We cannot say what level of global warming is needed to cause such a megadrought, but the likelihood increases with increase of greenhouse gases and global warming ( Hansen 2006 , 31–32).

In Canada’s province of Alberta, experts have warned that the area is headed for a massive drought worse than the Dust Bowl conditions of the 1930s, largely because of dwindling supplies from mountain snowpack coupled with a rising number of people and livestock using water. “I see a disaster shaping up in Alberta and it’s not a question of if, it’s a question of when,” said University of Alberta ecologist David Schindler, a water researcher and longtime critic of Alberta’s water policies. “There is going to be a major drought coinciding with global warming and record numbers of people and livestock on the landscape. It’s something that gives me nightmares” ( Semmens 2003 ). Glacier cover in the Canadian Rockies is nearing its lowest point in at least 10,000 years. Statistics Canada reported that 1,300 glaciers in the country have lost between 25 percent and 75 percent of their mass since 1850. Most losses, however, have been recorded in the last 50 years. The report said that most of losses can be attributed to global warming ( Paraskevas 2003 ).

Schindler cited research by several Alberta scientists that indicates the average temperature for western Canada is the highest in at least 10,000 years and is expected to continue climbing. Such conditions will lead to continued glacial recession in the Rocky Mountains, less rain and snowfall, and increasing water evaporation. With some of Alberta’s rivers already running as much as 80 percent lower than 100 years ago, Schindler said Alberta is also overdue for a drought that tends to hit the province every 100 to 150 years ( Semmens 2003 ).

Even at 140 feet below its capacity level of 1,220 feet, Lake Mead—formed by the Colorado River backing up behind Hoover Dam—is a formidable body of water. Even in 2014, it drew 7 million tourists. It still covers more than 242,000 acres and reaches a depth of 300 feet in some places ( Rojas 2015 ). Las Vegas, Nevada, which draws 90 percent of its water from Lake Mead, has been reducing its usage even as its population has risen; usage went down 30 percent between 2002 and 2015. “We came into the 21st century with Lake Mead essentially full, and over the last 14 years, it has declined about 130 feet,” said John Entsminger, general manager of the Southern Nevada Water Authority. “What we’ve learned here in Las Vegas is it sure looks like the 21st century in the Western U.S. is a time when everyone is going to have to use less water” ( Rojas 2015 ).

With weather patterns in a warming world favoring a drier American West, a study by scientists at the Scripps Institution of Oceanography indicates that Lake Mead, which spans the border of Nevada and Arizona and is a major source of water for Phoenix as well as Las Vegas, could run so low that water pumps may become useless. The study has become a center of controversy between scientists at Scripps and others at the U.S. Bureau of Reclamation who assert that the institution’s climate models are too crude to forecast the future water level of a single large lake.

The Scripps study found that Lake Mead’s water supply has a 50-percent chance of becoming unusable by 2021 if the demand for water remains at current levels and if global warming trends conform to midrange models. Researchers Tim P. Barnett and David W. Pierce of Scripps said that demand for the lake’s water exceeds the amount added each year by runoff, even with an occasional snowy winter (such as 2007–2008 and 2015–2016). “We were really sort of stunned,” Barnett said. “We didn’t expect such a big problem basically right on our front doorstep. We thought there’d be more time.” He added, “You think of what the implications are, and it’s pretty scary” ( Barringer 2008 ). By 2015, Lake Mead was down to 38 percent of its capacity, the lowest in its history (it was filled during the 1930s). In California in 2015, hydropower contributed 7 percent of the state’s electricity, down from 23 percent in 2011 because of persistent drought. A new generation of solar and wind power technologies were filling that gap ( “Lake Mead’s Level” 2015 ; “Energy Is Another” 2015 ).

Other research has found that the Colorado River watershed, of which Lake Mead is a part, has had a long-standing tendency toward drought that makes the last century look unusually wet. Climate models also indicate that a warmer climate favors persistent drought in this area. The projected drying trends “are really quite robust and in many cases scary when you compare them to even the severe megadroughts of the 1100s ands 1200s,” said Benjamin Cook, of the NASA Goddard Institute for Space Studies in New York City ( Underwood 2015 ).

“Natural droughts like the 1930s Dust Bowl and the current drought in the Southwest have historically lasted maybe a decade or a little less,” said Ben Cook, climate scientist at NASA’s Goddard Institute for Space Studies and lead author of the study. “What these results are saying is we’re going to get a drought similar to those events, Page 16  |  Top of Articlebut it is probably going to last at least 30 to 35 years,” Cook said ( Rice 2015 , February 13).

“The study is strong scientifically,” said Jonathan Overpeck, at the University of Arizona’s Institute of the Environment. “It strengthens our understanding of what is ahead, and it isn’t pretty.… The future megadroughts will be hotter and more severe” than those in the past ( Underwood 2015 ). Under such conditions, even large rivers such as the Colorado may run dry, according to this study. “They do the best job yet in linking likely future change in drought severity to that of the last 1,000 years as recorded by tree rings,” Overpeck said. “The results are striking, and highlight how future temperature increases will trump precipitation change in driving totally unprecedented levels of drought unless we make dramatic reductions in greenhouse-gas emissions” ( Rice 2015 , February 13).

See also: Atmospheric Circulation ; Australia, Heat and Drought in ; Desertification ; Drought and Deluge, Scientific Analysis of ; Drought, Worldwide ; El Niño and La Niña ; Heat Waves, Evidence of ; Heat Waves, Forecasts of ; Wildfires