From November 2013 through January 2014, the jet stream took on a remarkably extreme and persistent shape over North America and Europe. This global river of eastward-flowing winds high in the atmosphere dipped farther south than usual across the eastern U.S., allowing the notorious “polar vortex” of frigid air swirling over the Arctic to plunge southward, putting the eastern two thirds of the country into a deep freeze. Ice cover on the Great Lakes reached its second-greatest extent on record, and two crippling snow-and-ice storms shut down Atlanta for multiple days.

At the same time, a stubborn ridge of high pressure hunkered down over California, creating the warmest winter on record there. Although the balminess may sound nice, the resulting drought became the worst since record keeping began in the late 1800s, causing billions of dollars in agricultural losses.

The jet stream's contortions also pummeled Europe, where a succession of intense storms led to additional billions of dollars of damage. In England and Wales the winter was the wettest since at least 1766. Much of the rest of Europe basked in exceptional warmth: Norway suffered unprecedented January wildfires, and Winter Olympics officials in Sochi, Russia, struggled with melting ski slopes. In May nearly one third of the entire country of Bosnia was flooded by a massive, swirling rainstorm.

Ordinarily the jet stream resembles a band of air blowing across the middle latitudes. As we see on television weather forecasts, it often has mild bends from north to south and back to north again, looking somewhat like a sine wave on an oscilloscope. The bends are called planetary or Rossby waves and typically progress across the U.S. in three to five days. They deliver much of the day-to-day weather we experience.

During the 2013–2014 winter, however, the waves became amplified with gigantic, steep sides, resembling an erratic electrocardiogram printout. This configuration of winds also moved across the earth much more slowly than usual, at times stopping in place for weeks and bringing remarkably long periods of uncommon weather. A May study led by Shih-Yu (Simon) Wang of Utah State University found the jet stream pattern over North America during that time was the most extreme ever recorded.

Was the radical jet stream an anomaly? Apparently not, because it seems to be happening more and more. In 2010 Russia baked through its most oppressive heat wave in written history, one that killed more than 55,000 people. At the same time, intense rains deluged Pakistan, its most expensive natural disaster on record. In 2011 Oklahoma endured the hottest summer any American state has ever had. U.S. drought conditions in 2012 were the most extensive since the 1930s.

The bends in the jet stream during those particular events shared a common feature, according to an April 2013 paper by scientists at the Potsdam Institute for Climate Impact Research in Germany, led by Vladimir Petoukhov. The usually eastward-moving waves “ground to a halt and were greatly amplified,” two of the authors wrote in a blog post about their research. In some cases, the bends remained stuck for days or even months at a time. The scientists also showed that the extreme configurations were twice as common during summers from 2001 to 2012 as they were during summers of the prior 22 years.

As Bob Dylan sang, “You don't need a weatherman to know which way the wind blows.” Something is clearly up with the jet stream, and it is not hard to see the probable reason why. The base state of our climate has changed dramatically over the past 150 years, and that change is starting to alter the jet stream's behavior. Atmospheric levels of heat-trapping carbon dioxide, for example, have increased more than 40 percent, primarily because of the burning of coal, oil and natural gas. The extent of summer sea ice in the Arctic is down nearly 50 percent since 1900, affecting heat flow in the atmosphere and ocean. Solar energy reflecting off the earth's surface has changed significantly because we have modified more than half of the planet's landscape with crops, pastures and cities. Massive clouds of sunlight-reflecting and sunlight-absorbing soot and pollution belch forth from power plants, vehicles, buildings and industries. A huge ozone hole disrupts upper-level winds over the Antarctic.

Humans have kicked the climate system hard, and physics demands that the earth's fundamental weather patterns change as a result. Indeed, Wang and his colleagues concluded that the jet stream's configuration most likely could not have grown so strange without the influence of human-caused global warming.

The danger is that climate is not linear. A modest level of global warming can suddenly create a step change to a new regime with wildly different weather. Climate scientists are intensely debating whether climate as a whole and the jet stream in particular have crossed a tipping point into a new long-term state. They are also debating a controversial theory put forth by the Potsdam researchers and others that says that the changes in the jet stream stem largely from events occurring in the fastest-warming portion of the planet—the Arctic.

If indeed the jet stream is entering a new state, that bodes ill for civilization. An August paper published in Nature Climate Change by James Screen of the University of Exeter in England and Ian Simmonds of the University of Melbourne in Australia went so far as to pinpoint the potential effects. (Scientific American is part of Nature Publishing Group.) If jet stream waves “are amplified in response to anthropogenic [human-caused] climate change, as has been proposed,” they wrote, “our results suggest that this would preferentially increase the probabilities of heat waves in western North America and central Asia, cold waves in eastern North America, droughts in central North America, Europe and central Asia, and wet extremes in western Asia.”

This new normal would mean more terrible summer droughts for midwesterners. Winters featuring strings of snowstorms like the 2010 “snowmageddon” that closed Washington, D.C., would blast eastern U.S. residents more often. And people worldwide would see food prices go up, a consequence of intense and persistent droughts in central North America, Europe and Central Asia.

Natural Variations
Climate change would revise the jet stream indirectly by acting on big forces in the atmosphere that ultimately shape it. The ever present river of wind, nine to 14 kilometers high, circles the globe in both hemispheres and acts as a guide along which precipitation-bearing low-pressure systems ride. The jet stream typically has two branches: a polar jet that acts as the boundary between cold air near the poles and warm air closer to the equator and a less vigorous subtropical jet that lies closer to the equator. Henceforth, when I discuss the jet stream, I mean the dominant polar jet.

That jet's latitude rises and falls a bit with the seasons: it is typically over the central U.S. in winter and near the U.S.-Canadian border in summer. The flow, however, is chaotic, and large Rossby waves are always present. In the Northern Hemisphere, when the jet stream bulges northward as a ridge of high pressure, warm air flows up from south to north. Where the jet loops to the south as a trough of low pressure, cold air spills southward.

The jet stream is created by three major interlocking cells of circulating air over each hemisphere. Although the cells help to shape the jet stream, other forces in the sky can contort it further. The atmosphere actually resonates because of energy from the sun, the shape and location of the continents and ocean currents, the presence of mountain ranges, and the amount of heat-trapping greenhouse gases and reflective dust in the air. Just as a guitar resonates differently when various strings are plucked, as these factors change, the atmosphere resonates with multiple tones, called teleconnection patterns. These natural resonances can reshape the jet stream, complicating the determination of whether its recent behavior is a sign of a permanent change.

In the Northern Hemisphere, the two most important teleconnection patterns are the El Niño/Southern Oscillation and the Arctic Oscillation. The El Niño/Southern Oscillation is a three- to eight-year cycle in tropical atmospheric pressures. It drives warmer than average ocean waters toward the eastern Pacific during an El Niño event and cooler than average waters during the opposite phase, La Niña. The jet stream typically dips farther to the south over the eastern Pacific during El Niño but bulges to the north there during La Niña. The Arctic Oscillation is caused by week-to-week fluctuations in sea-level pressure between the Arctic and the midlatitudes. If this pressure difference is small, the jet stream winds tend to weaken, allowing large-amplitude loops to form; in winter, a small pressure difference typically allows cold air to spill far to the south over the eastern U.S., western Europe and East Asia.

A Revelation in California
The atmosphere's teleconnection patterns are intertwined. They can cancel one another out or reinforce one another. Changing the base state of the atmosphere in which these patterns arise could alter them so that they cause jet stream weirdness. I thought about this possibility in 2011, when an extreme jet stream persisted with a weak to moderate La Niña in place for only part of the year, which was odd. At the time, there were no published theories detailing how this situation might arise. But in December of that year at the American Geophysical Union meeting in San Francisco, the world's largest gathering of climate scientists, Rutgers University atmospheric scientist Jennifer Francis presented intriguing new findings related to the event. At one point, she said, “The question is not whether [Arctic] sea-ice loss is affecting the large-scale atmospheric circulation..., it's, How can it not?” Francis pointed out that the Arctic is warming two to three times faster than the rest of the Northern Hemisphere—a phenomenon known as Arctic amplification—and that this phenomenon could significantly disrupt the flow of the Northern Hemisphere jet stream.

The assertion makes perfect sense. One of the main causes of Arctic amplification in fall and winter is sea-ice loss. The Arctic Ocean has lost a stunning amount of its ice in recent years because of melting and unfavorable winds. In September 2012, 49 percent of the ice cover went missing—an area 43 percent of the size of the contiguous U.S.—compared with the mean value from 1979 to 2000. When sea ice melts, it exposes dark water, which absorbs more solar energy than white ice. The ocean and atmosphere then heat up, driving additional warming and more sea-ice melt in a vicious cycle.

The exposed water releases its stored heat in fall and winter, resulting in a massive, months-long perturbation to the base state of the Arctic atmosphere. Unusual Arctic amplification in summertime has also been occurring as Arctic snow cover diminishes. Global warming has caused spring to arrive earlier by about three days per decade, melting the snow cover and exposing dark soil sooner. The soil absorbs heat and dries out, jump-starting an early continental heating season.

The Arctic amplification caused by sea-ice loss and reduced springtime snow cover, along with other factors, has significantly decreased the temperature difference between the Northern Hemisphere's midlatitudes and the North Pole. This reduction can make a big difference to the jet stream; if the temperature difference decreases, less energy is transferred between two of the large atmospheric circulation cells, and the jet stream winds slow down. Francis and Stephen Vavrus of the University of Wisconsin–Madison have documented a roughly 10 percent reduction in upper-level winds since 1979 in autumn over North America and the North Atlantic, in concert with a reduction in the temperature difference.

Slower flow allows the jet stream to make large, meandering loops, and Francis has documented a sizable increase in the amplitude of the troughs and ridges in the polar jet since 2000, in summer and winter. The bigger kinks tend to allow warm air to flow much farther poleward than usual on one side of the jet stream, with cold air pushing far to the south on the other side. Such a pattern occurred during this past January's cold air outbreak in the eastern U.S.—the much ballyhooed polar vortex invasion—and simultaneous record warmth and drought in California. Mathematical theory shows that a slower-flowing jet stream also causes the Rossby waves to progress eastward more slowly, allowing the abnormal weather in the high-amplitude loops to last longer in any particular location. These ridges and troughs might also be more prone to stalling in place completely and forming “blocks” that stop wave movement, the way that back eddies in a river create a dead spot with no flow.

Disagreement over the Arctic's Role
The research linking Arctic amplification to jet stream craziness has stirred up a blizzard of turmoil in the climate science community. A September 2013 workshop on the subject at the University of Maryland, convened by the National Research Council, attracted more than 50 climate scientists who engaged in spirited debate. Although a large number of such experts agree that the jet stream seems to be changing, many of them question whether the relatively short period that Arctic amplification has been strong—about 15 years—is enough to link the two phenomena.

Some experts also question the hypothesis based on energy arguments. Because the high-volume flow of the jet stream contains a lot of energy, a lot of energy should be needed to change it. The amount of heat energy that has been added to the Arctic through Arctic amplification is an order of magnitude less than the energy in natural El Niño/Southern Oscillation–driven changes to the jet stream that have been studied, observes Kevin E. Trenberth of the National Center for Atmospheric Research. He co-authored a paper published online in August in Nature Climate Change showing that the large energy changes that have occurred naturally in the tropical Pacific Ocean in recent years because of a teleconnection pattern called the Pacific Decadal Oscillation could have caused the unusually wavy jet stream we have observed. Yet the paper also concluded that the nature of the changes to the oscillation during the past 10 years could mean that natural variability itself is being altered by climate change.

Trenberth was one of five leading climate scientists who published a critique of Francis's research in the journal Science this past February. The research linking Arctic warming to excessive jet stream waviness “deserves a fair hearing,” they wrote. But they concluded that they did not “view the theoretical arguments underlying it as compelling.”

Some scientists even question whether the amplitude of jet stream waves is increasing. In a 2013 paper, Screen and Simmonds measured jet stream bends using a different definition than Francis did and found few statistically significant changes in amplitude—although they did note a weak general tendency toward higher-amplitude waves. Yet critics have offered little else to explain the jet stream's extremes. One idea published in August in the Proceedings of the National Academy of Sciences USA by Dim Coumou of Potsdam and his colleagues noted that the dwindling difference in temperature between the midlatitudes and the poles, alone, could be enough to amplify the jet stream and cause it to get stuck, at least in summer.

Too Late to Wait
Although scientists may not agree on an explanation yet, the weather data are eye-opening. Some of the most iconic and destructive weather events in U.S. history—the “supertornado” outbreak of 1974, the Dust Bowl heat and drought of 1936, and the great Mississippi River flood of 1927—were all matched or surpassed in 2011 and 2012 alone. Our recent jet stream behavior could well mark a crossing of a threshold into a new, more threatening, higher-energy climate.

As the planet continues to warm, hotter temperatures will drive more intense heat waves and droughts where high-pressure ridges ripple along the jet stream. Stronger storms with heavier downpours will occur where the jet bends toward the equator into troughs of low pressure, as increased evaporation from the oceans puts more moisture into the atmosphere. If the jet stream continues to exhibit slower-moving, higher-amplitude waves, these harsh weather conditions will grow even more intense and stay in place longer, multiplying their potential for death and destruction. If the theories presented by Francis and her colleagues are correct, there is no going back to our old climate unless we find a way of growing more Arctic sea ice. Given that the amount of heat-trapping carbon dioxide in the atmosphere continues to increase at about 0.5 percent a year, no scientists who study Arctic sea ice are expecting a long-term recovery.

Drought is the greatest threat because it affects the two things we need most to survive: water and food. If a high-amplitude jet stream pattern with eccentric ridges of high pressure were to stay stuck for an entire summer over the grain-producing areas of Russia and the U.S., the precipitation that these crops rely on would not arrive. The resulting droughts could cause huge spikes in food prices, widespread famine and violent unrest. During the great Russian drought and heat wave of 2010, a massive and impenetrable ridge of high pressure settled over the country. That shunted the low-pressure systems that usually bring rain to Russian crops over to Pakistan, causing catastrophic floods there. The drought and heat wave was Russia's deadliest and most expensive natural disaster in history. It forced the country to cut off wheat exports, which drove up global grain prices and helped to foment the “Arab Spring” unrest that toppled multiple governments in 2011.

Clearly, the world cannot safely wait to act until scientists fully understand how and why the climate is changing. According to the Intergovernmental Panel on Climate Change, we must act swiftly, forcefully and globally to keep warming below the dangerous two degree Celsius threshold. Energy sources such as solar, wind and nuclear that emit low or zero levels of carbon dioxide, along with technologies that can capture and store carbon, must at least triple by 2050, and greenhouse gas emissions must fall by 40 to 70 percent, compared with 2010 levels. The shift might be surprisingly affordable, cutting global economic growth by only 0.06 percent a year, the panel has said. But if we wait until 2030, the necessary actions will be much more expensive, and it may become impossible to avert the threshold.

That is also the year that summertime Arctic sea ice will essentially disappear, according to several leading climate scientists. If Arctic changes are truly to blame for wacky jet stream behavior, losing the remaining 50 percent of the Arctic sea-ice coverage between now and 2030 will bring even greater antics. If the Arctic is not involved, that is worrisome as well because it means jet stream changes are being triggered by an unknown mechanism, leaving us with no idea how the jet stream will respond as climate change progresses. Thus, my forecast for the next 15 years: expect the unprecedented.