According to a study in Science by Princeton University researcher Andrew Dobson and others, global warming is causing the spread of bacteria, viruses, and fungi that cause human diseases into areas that were formerly hostile to them. "Climate change is disrupting natural ecosystems in a way that is making life better for infectious diseases," said Dobson. "The acc.u.mulation of evidence has us extremely worried." Another coauthor of the study, Richard S. Ostfeld, said, "We're alarmed because in reviewing the research on a variety of different organisms, we are seeing strikingly similar patterns of increases in disease spread or incidence with climate warming."

Although the prevalence of international travel has increased dramatically and some disease-carrying insects have been unwittingly transported from the mid-lat.i.tudes to other regions, the shifting climatic conditions are contributing to the spread of diseases like dengue fever, West Nile virus, and others. The Union of Concerned Scientists wrote that, "Climate change affects the occurrence and spread of disease by impacting the population size and range of hosts and pathogens, the length of the transmission season, and the timing and intensity of outbreaks."

They also noted, "Extreme weather events such as heavy rainfall or droughts often trigger disease outbreaks, especially in poorer regions where treatment and prevention measures may be inadequate. Mosquitoes in particular are highly sensitive to temperature." Improvements in public health systems are crucial to control the spread of these migrating diseases, but many lower-income countries are pressed to find the resources needed for hiring and training more doctors, nurses, and epidemiologists. They also warned that in many of the areas to which these pathogens and their hosts spread with warmer temperatures, "The affected populations will have little or no immunity, so that epidemics could be characterized by high levels of sickness and death."

In the summer of 2012, the United States experienced the worst outbreak of West Nile virus since it first arrived on the Eastern Sh.o.r.e of Maryland in 1999 and spread rapidly to all fifty states in only four years, during a period of unusually warm temperatures. Dallas, Texas, was the first to declare a public health emergency and began aerial spraying of the city for the first time since 1966. As concern peaked, public safety officials issued an appeal for people to stop calling 911 when they were bitten by mosquitoes. The disease eventually spread by the end of 2012 to forty-eight of the fifty states, killing at least 234 people.

The late Paul Epstein, a professor at Harvard Medical School and a close friend, wrote in 2001 about the relationship between West Nile virus and the climate crisis. More recently, he said, "We have good evidence that the conditions that amplify the lifecycle of the disease are mild winters coupled with prolonged droughts and heat waves-the long-term extreme weather phenomena a.s.sociated with climate change."

According to Christie Wilc.o.x with Scientific American: They have been predicting the effects of climate change on West Nile for over a decade. If they're right, the US is only headed for worse epidemics.... Studies have found that mosquitos pick up the virus more readily in higher temperatures. Higher temperatures also increase the likelihood of transmission, so the hotter it is outside, the more likely a mosquito that bites an infected bird will carry the virus and the more likely it will pa.s.s it along to an unwitting human host. In the United States, epicenters of transmission have been linked closely to above-average summer temperatures. In particular, the strain of West Nile in the US spreads better during heat waves, and the spread of West Nile westward was correlated with unseasonable warmth. High temperatures are also to blame for the virus jumping from one species of mosquito to a much more urban-loving one, leading to outbreaks across the US.... Record-breaking incidences of West Nile are strongly linked to global climate patterns and the direct effects of carbon dioxide emissions.

In 2010, the world experienced the hottest year since records have been kept, and ended the hottest decade ever measured. Last year, 2012, broke even more high temperature records. October 2012 was the 332nd month in a row when global temperatures were above the twentieth-century average. The worst drought since the Dust Bowl of the 1930s ravaged crops and dried up water supplies in many communities. Many farmers have already been forced to adjust to the drying of soil. The lack of water has caused a buildup of toxins in corn and other crops unable to process nitrogen fertilizer.

WORLD FEVER.

In order to pinpoint the difference between global warming and natural variability, Dr. James Hansen, the single most influential climate expert in the scientific community, produced with two of his colleagues, Makiko Sato and Reto Ruedy, a groundbreaking statistical a.n.a.lysis of extreme temperatures all over the world from the years 1951 through 2010 that compared the more normal baseline period of 1951 through 1980 to more recent decades, 1981 through 2010, and especially the last several years when the impacts of global warming have been more prominently manifested, 1981 through 2010.

By breaking down the surface temperatures of almost the entire world into blocks of 150 square miles each, Hansen was able to calculate the frequency of extremely high temperatures (and all other temperatures) during the last sixty years. The results-which do not rely on climate models, climate science, or any theories of causation-demonstrate clearly that there has been up to a 100-fold increase in extreme high temperatures in recent years compared to earlier decades. The statistical a.n.a.lysis shows that in the last several years, extreme temperatures have been occurring regularly on approximately 10 percent of the Earth's surface, while during the earlier decades such events occurred on only 0.1 to 0.2 percent of the Earth's surface.

Hansen's chosen metaphor to explain the difference consists of two dice, each with the requisite six sides. The first die, which shows the range of temperatures over the years between 1951 and 1980, has two sides representing "normal" seasons, two other sides representing "warmer than normal" seasons, and the final two sides representing "cooler than normal" seasons. That used to be the "normal" distribution of temperatures. The second die, however, showing the range of temperatures in more recent years, has only one side representing a normal season and only one side representing a cooler than normal season, but three sides representing warmer than normal seasons and the remaining side now representing extremely hot seasons-seasons that are way outside the boundary of the statistical range that used to prevail.

In the language of statisticians, a standard deviation quantifies how far the range, or spread, of a particular set of phenomena differs from the average spread. Extreme-in this case, either unusually hot or unusually cold-seasons naturally occur far less frequently than average or near-average seasons. Because seasons with extreme temperatures used to be so much less frequent, they nevertheless often surprised us, even though they fell within the normally expected range. Seasons that are three standard deviations from the average were exceedingly rare, but still did occur from time to time as part of the normal range.

The average temperature is warmer overall even though extremely cold events still continue to occur, though rarely. In other words, the entire distribution of temperatures has moved to much warmer values, and the bell curve of distribution has widened and flattened slightly, so that there is much more temperature variability than used to occur. But the most significant finding is that the frequency of extremely hot temperatures has gone up dramatically.

Hansen infers that the cause is global warming-and indeed, these results turn out to be perfectly consistent with what global warming science has long predicted. (In voluminous other studies, Hansen and climate scientists around the world have proven causality to a degree judged "unequivocal" and "indisputable" by virtually all of the world's scientific community.) But the results themselves are based on observations of real temperatures in the real world. They cannot be argued with, and the implications are powerfully clear.

As the old saying has it in Tennessee, if you see a turtle on top of a fence post, it is highly likely that it didn't get there by itself.* And now we are seeing turtles on every tenth fence post in every field in the world. They didn't get there on their own. It is now abundantly obvious that all the extreme temperatures and the extreme weather events a.s.sociated with them are like turtles on a fence post. They didn't happen without human interference in the climate.

In 2012, new World Bank president Jim Yong Kim released a study showing temperatures will likely rise by 4 degrees C (7.2 degrees F) without bolder steps to reduce CO2, and that there is "no certainty that adaptation to a 4 degree world is possible." Gerald Meehl, of the National Center for Atmospheric Research, uses a different metaphor to explain what is happening: if a baseball player who takes steroids. .h.i.ts a home run, it's possible that he might have hit the home run even without the steroids. But the fact that he took the illegal performance-enhancing drug makes it much more likely that he will hit a home run in his next at bat. Within Meehl's metaphor, the 90 million tons of global warming pollution that we are putting into the atmosphere every twenty-four hours are like steroids for the climate. An innovative 2012 study of the previous decade's climate predictions showed that the "worst case" future projections are the ones most likely to occur.

The increases in the global average temperature and the greater frequency of extremely high temperatures that Hansen and others are doc.u.menting are also melting all of the ice-covered regions of the Earth. Only thirty years ago, the Arctic Ocean was almost completely covered by ice in summer as well as winter. Remember? Some called it the North Polar Ice Cap. Tell your grandchildren how it used to separate Eurasia from North America and the Atlantic from the Pacific all year round. Last year's record low in the volume and the area it covered marked an acceleration of a melting pattern that has led to a 49 percent loss in three decades and could, in the view of many ice scientists, produce a 100 percent loss in as little as a decade.

Some shipping companies are excited that the fabled Northern Sea Route is now open for several months a year. A Chinese ship, the Snow Dragon, traversed the North Pole to Iceland and back in the summer of 2012. A high-speed fiber optic cable is now being installed to link the Tokyo stock markets with their counterparts in New York City so that computer-driven trades can be executed more quickly. Fishing fleets are preparing to exploit the rich biological resources of the Arctic Ocean, which until now have been protected by the ice. Navies from some countries are discussing the movement of military a.s.sets into the region, though discussions have also begun on the possibility of agreements to foster the peaceful resolution of issues involving the safety, sovereignty, and development of the Arctic Ocean as it becomes ice-free in summer.

Several oil companies are thrilled at the prospect of new drilling opportunities and some are already moving their rigs into place. But the consequences of an accidental wellhead blowout at the bottom of the Arctic Ocean similar to BP's disaster in 2010 would be far more catastrophic and far more difficult to deal with than in the Gulf of Mexico, or in any of the other numerous deepwater locations where wellhead blowouts have produced large oil spills. The relatively new and unperfected technology used for deepwater drilling involves more risk than conventional drilling because the pressures at the ocean's bottom are so great. Drilling for oil at the bottom of the Arctic Ocean, and running the risk of a large spill in a pristine ecosystem where repair and rescue operations are all but impossible for much of the year, is an absurdly reckless endeavor. The CEO of the French multinational oil company Total broke ranks with his industry in 2012 and expressed his view that drilling for oil in the Arctic Ocean posed unacceptable ecological risks and should not be carried out.

The ecology of the Arctic Ocean is already experiencing significant changes. Scientists were shocked in 2012 at the discovery of the largest algae bloom ever recorded on Earth extending from open areas of the Arctic Ocean underneath the remaining ice cover-a phenomenon that has never been seen before and was considered impossible. The researchers explained that the most likely cause of this new occurrence was that the remaining ice is now thin enough, and has so many pools of water dotting its surface, that enough sunlight was penetrating to the ocean below to provide energy for algal growth.

The consequences of melting the North Polar Ice Cap will include large impacts on weather patterns extending far southward into the heavily populated temperate zones. The dramatically increased heat absorption in an Arctic Ocean that is ice-free in summer will have consequences for the location and pattern of the northern jet stream and storm track through the fall and winter seasons, modifying ocean currents and weather patterns throughout the northern hemisphere, and perhaps beyond. Moreover, if the world's long familiar pattern of wind and ocean currents is pushed into a completely new design, the old one may never reemerge.

The land area surrounding the Arctic Ocean is also heating up, thawing frozen tundra that contains enormous amounts of carbon embodied in dead plants. They warm up and rot as the tundra thaws. Microbes turn the carbon into CO2 or methane, depending on the amount of soil moisture. Huge deposits of methane are also contained within frozen ice crystal formations called clathrates in the tundra, at the bottom of the many shallow frozen lakes and ponds surrounding the Arctic, and in some parts of the seabed underneath the Arctic Ocean. The bubbling methane carries heat energy upward, melting the underside of the ice-which then increases the heat absorption by the water when the sun's rays are no longer reflected off the ice.

Scientists are struggling to quantify the amount of CO2 and methane that could be released, but the area involved is so vast that their work is extremely difficult. Already, however, they have found outga.s.sing under way that exceeded what they expected at this early stage of global warming.

Moreover, scientists discovered in 2012 that there are likely to be enormous deposits of methane underneath the Antarctic ice sheet, in amounts that may be as large as the methane presently trapped in Arctic tundra and coastal sediments. Since the clathrates are kept in place by cold temperatures and high pressures, the thinning of the Antarctic ice sheets could, scientists fear, reduce pressures underneath the ice enough to trigger the release of methane.

The changes under way in Antarctica and Greenland are the focus of intense study by scientists who are trying to calculate how much sea levels will rise, and at what rate. Both ice sheets are being destabilized and are losing ma.s.s at an increasing rate, which is leading to a much faster sea level rise than was predicted just a decade ago.

Throughout the history of urban civilization, the seas have been slowly and gently rising, as the warmer temperatures of the interglacial period have caused thermal expansion of the ocean's volume, and the melting of some terrestrial ice. But with the rapid acc.u.mulation of CO2 and other greenhouse gases in the atmosphere during the last half century, global warming has accelerated and so has the melting of ice almost everywhere on the planet.

Predictions of the rate of sea level rise have been notoriously difficult, in part because many scientists use models calibrated using data derived from their studies of retreating glaciers at the end of the last Ice Age when conditions were very different from the ones we are now confronting. New real-time satellite measurements of ice ma.s.s in Greenland and Antarctica will soon improve scientific understanding of this process, but these measurements have been made for only a few years and more time is required to build confidence in what they are telling the scientific community. Recent observations in both west Antarctica and Greenland, however, already confirm a rapid and accelerating loss of ice. After a highly unusual melting event that affected 97 percent of Greenland's surface in July 2012, Bob Corell, chairman of the Arctic Climate Impact a.s.sessment, said, "It shocked the h.e.l.l out of us."

James Hansen, for one, surmises that we are witnessing an exponential process of ice ma.s.s loss, and that, as a result, the most relevant statistic is the doubling time of the observed loss. Based on his preliminary a.n.a.lysis of the data, Hansen believes it is likely that we will see a "multi-meter" sea level rise in this century. Others note that the last time temperatures on Earth were consistently as high as they are now, sea level was twenty to thirty feet higher than the present-although it took millennia for the seas to rise that much.

Because so many countries were settled by migrants, and in some cases colonialists, arriving by ship-and because trade and supply routes rely so heavily on oceangoing vessels-a disproportionate percentage of the world's largest cities are located near the sea. In fact, 50 percent of the world's population lives within fifteen miles of the coast, and according to the U.S. National Academy of Sciences, "Coastal populations around the world are also growing at a phenomenal pace. Already, nearly two-thirds of the world's population-almost 3.6 billion people-live on or within 100 miles of a coastline. Estimates are that in three decades, 6 billion people-that is, nearly 75 percent of the world's population-will live along coasts. In much of the developing world, coastal populations are exploding."

Those in low-lying areas are therefore especially vulnerable to the increases in sea level produced mainly by the melting and breakup of large ma.s.ses of ice in Antarctica and Greenland. A recent study by Deborah Balk and her colleagues at the CUNY Inst.i.tute for Demographic Research showed that approximately 634 million people live in low-elevation coastal zones and that the ten nations with the most people in threatened areas are: China, India, Bangladesh, Vietnam, Indonesia, j.a.pan, Egypt, the United States, Thailand, and the Philippines. Moreover, two thirds of the world's cities with more than five million people are at least partly in vulnerable low-elevation areas.

Some of the populations who live on low-lying islands in the Pacific and Indian oceans and in coastal deltas are already beginning to relocate. Large island populations are also at risk in the Philippines and Indonesia. The number of climate refugees is expected to grow and could potentially involve more than 200 million people in this century, especially because of those who will have to move away from the mega-deltas of South Asia, Southeast Asia, China, and Egypt. Refugees from coastal areas of Bangladesh have already crowded into the capital city of Dhaka, and many have moved farther north across the border into northeastern India, where their arrival has contributed to the worsening of preexisting tensions based on religious and complex tribal conflicts. In 2012, these conflicts generated contagious fear that was spread by text messaging and email into cities throughout India.

All these regions and others are also threatened by climate-related flooding during storm surges as stronger cyclones (known as hurricanes in the U.S.) gain energy from warmer seas. Even small vertical increases are magnified by storm surges that carry the ocean inland. And with stronger storms, these surges are already having a bigger impact. In 2011, for example, New York City was put on emergency alert as a hurricane threatened to flood its subway system. In 2012, Superstorm Sandy did. London has long since built barriers between the ocean and the Thames River that can be closed to protect the city against such surges-at least for a while; the city is already discussing plans for further steps.

As noted in Chapter 4, the surge in population growth in the balance of this century will be completely in urban areas. The cities with the highest population at risk from rising seas are, in order: Calcutta, Mumbai, Dhaka, Guangzhou, Ho Chi Minh City, Shanghai, Bangkok, Rangoon, Miami, and Hai Phong. The cities with the most exposed a.s.sets vulnerable to sea level rise are: Miami, Guangzhou, New York/Newark, Calcutta, Shanghai, Mumbai, Tianjin, Tokyo, Hong Kong, and Bangkok.

In addition, as the chief scientific advisor in the United Kingdom, Sir John Beddington, recently noted, many climate refugees have migrated to low-lying coastal cities vulnerable to increased climate-related flooding and rising seas. They are unknowingly relocating into areas from which they may once again become climate refugees.

Contrary to most popular thinking, the rate of sea level rise is not uniform around the world, because some of the tectonic plates on which the landma.s.ses rest are still slowly "rebounding" from the last Ice Age. Scandinavia and eastern Canada, for example, were pushed down by the weight of the last glaciation and are still moving slowly upward long after the ice retreated. Conversely, areas at the opposite ends of the same tectonic plates-the coastal nations of Western Europe and the mid-Atlantic states of the U.S., for example-are slowly sinking, in a kind of seesaw effect. Cities like Venice, Italy, and Galveston, Texas, are also sinking-for a mixture of complicated reasons.

Because warmer oceans expand when their molecules push apart from one another (thermal expansion of the oceans has contributed significantly to the relatively small increases in sea level we have experienced thus far), areas of the ocean with large acc.u.mulations of warmer water are experiencing more rapid sea level increases-the coast of the U.S. between South Carolina and Rhode Island, for example. But all the increases in sea level thus far are nothing compared to what scientists warn is in store for the entire world as Antarctica and Greenland are affected by the sharp increases in global temperatures now in store.

Many agricultural areas in low-lying coastal regions and areas adjacent to river deltas are already suffering impacts from rising seas because of salt.w.a.ter invasion of the freshwater aquifers on which their farms depend. In 2012, the combination of sea level rise and sharply diminished flows in the Mississippi River, due to the drought in the U.S., led to salt.w.a.ter intrusion into drinking water wells and aquifers in southern Mississippi.

The characteristics of the seawater itself are also being profoundly altered by global warming. Approximately 30 percent of human-caused CO2 emissions end up in the ocean, where they dissolve into a weak acid, building up in such enormous volumes that it has nevertheless already made the world's oceans more acidic than at any time in the last 55 million years, which was during one of the five previous great extinction events in the history of the Earth. And the rate of acidification is faster than at any time in the last 300 million years.

One of the immediate concerns is that the higher levels of acidity are reducing the concentration of carbonate ions that are essential to species that make sh.e.l.ls and coral reefs. All such structures are made from various forms of calcium carbonate, which the coral polyps and sh.e.l.l-making creatures scavenge from seawater. But the increasing acidity of the ocean interferes with the solidifying of these hard structures. The director of the U.S. National Oceanic and Atmospheric Administration, Jane Lubchenco, calls ocean acidification global warming's "evil twin."

The warmer ocean temperatures-also caused by man-made global warming-are especially stressful to the specialized algae that form the brightly colored skin of coral reefs and live in an intricate symbiosis with the coral polyps. When water temperatures rise too high, these specialized algae-known as zooxanth.e.l.lae (also called zoox)-leave the skin of the coral, rendering it transparent and revealing the white bony skeleton underneath. These events are known as coral bleaching. Reefs can and do recover from bleaching events, but several events in the s.p.a.ce of a few years can and do kill the reefs.

Coral reefs are particularly important because, according to experts, approximately one quarter of all ocean species spend at least part of their lifecycles in, on, and around reefs. Shockingly, scientists warn that the world is in danger of killing almost all of the coral reefs in the ocean within a generation. Between 1977 and 2001, 80 percent of the coral reefs in the Caribbean were lost. All of the rest, experts say, are threatened with destruction before the middle of the century. And the same fate threatens reefs in every ocean, including the largest of all, the Great Barrier Reef off the eastern coast of Australia. In 2012, the Australian Inst.i.tute of Marine Science announced that half of the Great Barrier Reef corals had died in just the previous twenty-seven years.

The most visible and familiar reefs are warm-water reefs at relatively shallow depths. However, there may be an equal or even larger number of deeper, cold-water reefs. Because of their depth, they have been less studied and doc.u.mented, but scientists say that since colder water absorbs more CO2 than warmer water (just as a cold container of soda stays more carbonated than a warm one), many of the cold-water reefs may be in even greater danger. Some scientists hold out hope that coral reefs might yet survive, but many of their colleagues are now convinced that virtually all corals are likely to be killed off by the combination of higher ocean acidity, higher temperatures, pollution, and overfishing of species important to reef health.

The growing absorption of CO2 in the oceans also interferes with the reproduction of some species. And among the sh.e.l.l-making creatures at risk are tiny zooplankton with very thin sh.e.l.ls that play an important role at the bottom of the ocean food chain. Although much research remains to be done, many scientists are concerned about what has been happening to this crucial link that lies at the base of the ocean food chain.

Some areas of the ocean, including some off the coast of Southern California that have been sampled, are actually corrosive. In coastal areas of Oregon, newly corrosive seawater is killing commercially valuable sh.e.l.lfish. Experts have noted that even if human-caused CO2 emissions were somehow ended in the near term, it would take tens of thousands of years before the chemistry of the oceans returned to a state comparable to that which existed prior to the last century.

Global warming and CO2-caused acidification are exacerbating declines in fisheries and marine biodiversity that have already been caused by other human activities, such as overfishing. According to the United Nations, almost a third of all fish species are presently overexploited. Overfishing, described in Chapter 4, has already led to the dangerous depletion of up to 90 percent of large fish like tuna, marlin, and cod.

Some fishing techniques such as dynamite fishing (which still takes place in some developing countries with coral reefs) and bottom trawling (the northeast Atlantic has been particularly damaged by this practice) do extra damage to the ocean ecosystems important to the survival of sea life. Although there have been some notable success stories in some ocean fisheries, the overall picture is still extremely troubling. The combination of many factors poses a synergistic threat to the continued health of the oceans.

Along with coral reefs, critical ocean habitats like mangrove forests in many coastal areas and so-called sea gra.s.s meadows are also at risk. In addition, the number of dead zones growing in the oceans near the mouths of major river systems is doubling every decade. The heavy concentrations of nitrogen and phosphorus contained in agricultural runoff water and wastewater feed algae growth and when the algae are consumed by bacteria, the large areas of the ocean are completely depleted of oxygen, leading to the dead zones.

Ironically, the historic North American drought of 2012 reduced the flow of water from the Mississippi into the Gulf of Mexico so much-and the nitrogen, phosphorus, and other chemicals normally carried with the water-that the large dead zone spreading from the mouth of the Mississippi began to temporarily clear up.

A conference of ocean experts meeting at Oxford University in the summer of 2011 reported their conclusions as a group: "This examination of synergistic threats leads to the conclusion that we have underestimated the overall risks and that the whole of marine degradation is greater than the sum of its parts, and that degradation is now happening at a faster rate than predicted.... When we added it all up, it was clear that we are in a situation that could lead to major extinctions of organisms in the oceans.... It is clear that the traditional economic and consumer values that formerly served society well, when coupled with current rates of population increase, are not sustainable."

MITIGATION VERSUS ADAPTATION.

For at least three decades, there has been a debate in the international community about the relative importance of reducing greenhouse gas emissions to mitigate the climate crisis compared to strategies for adapting to the climate crisis. Some of those who try to minimize the significance of global warming and oppose most of the policies that would mitigate it often speak of adaptation as a subst.i.tute for mitigation.

They promote the idea that since humankind has adapted to every environmental niche on the planet, there is no reason to believe that we shouldn't merely accept the consequences of global warming and get busy adapting to them. For example, the CEO of ExxonMobil, Rex Tillerson, recently said in an exchange provoked by longtime activist David Fenton, "We have spent our entire existence adapting, OK? So we will adapt to this."

FOR MY OWN part, I used to argue many years ago that resources and effort put into adaptation would divert attention from the all-out push that is necessary to mitigate global warming and quickly build the political will to sharply reduce emissions of global warming pollution. I was wrong-not wrong that deniers would propose adaptation as an alternative to mitigation, but wrong in not immediately grasping the moral imperative of pursuing both policies simultaneously, in spite of the difficulty that poses.

There are two powerful truths that must inform this global discussion about adaptation and mitigation: first, the consequences that are already occurring, let alone those that are already built into the climate system, are particularly devastating to low-income developing countries. Infrastructure repair budgets have already skyrocketed in countries where roads, bridges, and utility systems have been severely damaged by extreme downpours and resulting floods and mud slides. Others have been devastated by the climate-related droughts.

And the disruptions of subsistence agriculture by both the floods and the droughts have led to skyrocketing expenditures for food imports in many developing countries. Also, as noted earlier, some low-lying nations are also already struggling to relocate refugees from coastal areas affected by the early stages of sea level rise, while other nations are struggling to integrate arriving refugee groups into already fast-growing populations.

Since these and other developments will not only continue but worsen, the world does indeed have a moral duty and practical economic necessity to a.s.sist these nations with adaptation. Disturbingly, the world has yet to fully realize the effects of the global warming pollution already in the atmosphere. Even if we drastically reduce our emissions today, another degree Fahrenheit of warming is already "in the pipeline" and will manifest itself in the coming years. In other words, so many harmful changes are already built into the climate system by the enormous increase in the emissions, and particularly the increased concentration, of greenhouse gases in the atmosphere that adaptation is absolutely essential-even as we continue building the global political consensus needed to prevent the worst consequences from occurring. We have no choice but to pursue both sets of policies simultaneously.

But the second truth that must inform this debate is still by all odds the most powerful imperative: unless we quickly start reducing global warming pollution, the consequences will be so devastating that adaptation will ultimately prove to be impossible in most regions of the world. For example, higher greenhouse gas emissions are already beginning to cause large-scale changes in atmospheric circulation patterns and are predicted to bring almost unimaginably deep and prolonged drought conditions to a wide swath of highly populated and agriculturally productive regions, including all of Southern and south-central Europe, the Balkans, Turkey, the southern cone of Africa, much of Patagonia, the populated southeastern portion of Australia, the American Southwest and a large portion of the upper Midwest, most of Mexico and Central America, Venezuela and much of the northern Amazon Basin, and significant portions of Central Asia and China.

The scientific reasoning behind this devastating scenario requires some explanation. The basic nature of the global climate system, when viewed holistically, is that it serves as an engine for redistributing heat energy: from the equator toward the poles, between the oceans and the land, and from the lower atmosphere to the upper atmosphere and back again. The large increase in heat energy trapped in the lower atmosphere means-to state the obvious-that the atmospheric system is becoming more energetic.

In the northern hemisphere, this climate engine transfers heat energy from south to north in the Gulf Stream-which is the best known component of the so-called ocean conveyor belt, a Mobius Striplike loop that connects all of the world's oceans. Other components include deep currents that travel along the bottom of the ocean, redistributing cold water from the poles back to the equator, where they return to the ocean surface. The largest of these are the Antarctic circ.u.mpolar current, which travels around the Antarctic continent and feeds the shallower Humboldt current, which flows from the Southern Ocean northward along the west coast of South America and upwells-laden with nutrients-to nourish the rich concentration of sea life off the coast of Peru; and, less well known, the deep cold current that travels north to south from an area of the North Atlantic in the vicinity of southern Greenland, underneath the Gulf Stream, back to the tropical Atlantic waters.

Energy is also redistributed by cyclones, by thunderstorms, and by multiyear patterns such as the alternating El Nino/La Nina phenomenon (known to scientists as the ENSO, or El Nino/Southern Oscillation). Moreover, all of these energy transfers are affected by the Coriolis effect, which is driven by the spinning of the Earth on its axis, from west to east.

THE HADLEY CELLS.

Until recently, relatively less attention has been paid to the relationship between global warming and the atmospheric patterns that move energy vertically up and down in the atmosphere. The so-called Hadley cells spanning the tropics and subtropics are enormous barrel-shaped loops of wind currents that circle the planet on both sides of the equator, like giant pipelines through which the trade winds flow from east to west.

Warm and moist wind currents rise from the ground vertically into the sky in both of these cells at the edge of each respective loop that is adjacent to the equator. When their ascent reaches the top of the troposphere (the top of the lower atmosphere, approximately ten miles high in the tropics), each loop turns poleward-which means northward in the northern hemisphere cell and southward in the other. By the time these currents reach the top of the sky, much of the moisture they carried upward has fallen back to the ground as rain in the tropics.

At the apex of its ascent, each of these air currents starts flowing poleward along the top of the troposphere and travels about 2,000 miles (approximately 30 degrees of lat.i.tude), until it has discharged most of its heat. Then it descends vertically as a cooler and much drier downdraft. When each loop reaches the surface again, it turns back toward the equator, recharging itself with heat and moisture as it travels across the surface of the Earth. As it returns to the equator, it completes its loop and repeats the cycle by rising vertically once again, laden once more with heat and water vapor.

As a result of the dry downdrafts of the Hadley cells, the areas of the Earth 30 degrees north and 30 degrees south of the equator are highly vulnerable to desertification. Most of the driest regions of the Earth, including the largest of the planet's deserts, the Sahara, are located under these dry downdrafts. (Other factors contributing to the location of deserts include the "rain shadows" of mountain ranges-the areas downwind from mountain peaks-because the prevailing winds rise when they hit the windward side of the mountains and lose their moisture before descending as dry downdrafts on the leeward side. In addition, the location of deserts is influenced by what geographers call continentality-which means that the areas in the middle of large continents typically get much less moisture because they are farther away from the oceans.) But on a global basis, the most powerful desertifying factor is the downdraft of the Hadley cells.

The problem-which climate scientists have long predicted with computer models and are now observing in the real world-is that the ma.s.sive warming of the atmosphere is changing the locations of these great global downdrafts, moving them farther away from the equator and toward the poles, thus widening the subtropics and intensifying their aridity. Indeed, in the northern hemisphere, the downdraft has already moved northward by as much as 3 degrees lat.i.tude-approximately 210 miles-although measurements are still imprecise. The downdraft of the Hadley cell south of the equator has also moved poleward.

There are several theories for why global warming is causing a shift in the Hadley cells, none of which are as yet confirmed. The solar heating of the lower atmosphere in the tropics and subtropics is much greater than anywhere else on the planet for obvious reasons: the sunlight strikes the Earth at a more direct angle all year round. On a percentage basis, surface temperatures are rising faster in the higher lat.i.tudes because the melting of ice and snow is dramatically changing the reflectivity of the surface, thereby increasing the absorption of heat energy. This means, among other things, that the difference in average temperatures between the tropics and the polar regions is diminishing over time-which also has consequences for the climate balance.

However, the much larger amounts of overall heat energy absorbed in the mid-lat.i.tudes is still much greater, and causes the warmer (and thus less dense) air in the tropics to rise higher. As a result, the extra heat raises the top of the troposphere, where the wind currents deflect at a right angle from their vertical trajectory and begin traveling poleward.

The widening of the Hadley cells moves the downstroke of its circular path farther north in the northern hemisphere and farther south in the southern hemisphere. As with many of the realities connected to global warming, while this one sounds technical and can seem abstract, the real consequences for real people, animals, and plants are extremely severe.

For the areas now subjected to this downdraft, it's a bit like being under a giant hairdryer in the sky. The results include not just more frequent and more severe droughts, but consistent drought patterns likely leading to desertification in many of the countries in the line of fire. Moreover, most of the areas affected, like Southern Europe, Australia, Southern Africa, the American Southwest, and Mexico-are already on the edge of persistent water shortages anyway.

The word "desert," by the way, is derived from the relationship of people to the land involved: deserts are deserted by people. Consider the significance of Greece, Italy, and the Fertile Crescent-the cradles of Western civilization-turned into deserts by human alteration of the same natural climate feature that created the Sahara Desert beginning 7,300 years ago.

The jet stream that controls the location of storm tracks in most of North America and Eurasia is also being affected by the impact of global warming on atmospheric circulation patterns and the unusually chaotic weather patterns in these lat.i.tudes in recent years. There are actually two jet streams in both hemispheres-a subtropical jet stream flowing from east to west along the poleward margin of the barrel loop of the Hadley cells (the trade winds), and the so-called polar jet stream-which flows from west to east on the poleward side of a second set of barrel loop atmospheric currents known as the Ferrel cells.

The location of the northern polar jet stream (which North Americans and north Eurasians typically call the jet stream) is determined in part by the wall of cold air extending southward from the Arctic Circle. But in recent years, the melting of the Arctic ice cap has led to so much extra heat absorbed there that the northern boundary of the jet stream flowing across North America and Eurasia appears to have been profoundly and radically dislocated-changing storm tracks, pulling cold Arctic air southward in winter, and disrupting precipitation patterns.

All of these energy transfer mechanisms-the wind and ocean currents, storms and cyclones, and atmospheric cells-define the shape and design of the Earth's climate pattern that has remained relatively stable and constant since shortly before the Agricultural Revolution began. Yet global warming is changing all of the energy balances that have given definition to this climate envelope, and is both intensifying and changing the locations of the weather phenomena we are used to.

Some of these balances are being changed to such a degree that scientists worry that they could be pushed far enough out of the pattern we have always known that they could flip into a very new pattern that would produce weather phenomena with intensity, distribution, and timing that are completely unfamiliar to us and inconsistent with the a.s.sumptions upon which we have built our civilization.

By way of ill.u.s.tration, take a leather belt and hold one end in either hand; push your hands together until a loop forms sticking upward. As you move your hands and change the inflection of your wrists, the shape of the belt loop will vary but it will remain in the same basic shape. But if you inflect your wrists a little more, it will suddenly flip into a new basic pattern with the loop pointing downward instead of upward. The variations in climate that we have always known, large as they are, are like the variations in the belt loop pointing upward. There would still be similar variations if the loop pointed downward, but if we push the boundary conditions of the loops to a point that causes it to adopt an entirely new pattern, the consequences for our climate would be extreme indeed.

We have already been confronted by unwelcome surprises in our experimentation with changing the chemical composition of the Earth's atmosphere. The sudden appearance of a continent-sized stratospheric ozone hole above Antarctica in the 1980s raised the specter of a deadly threat to many forms of life on Earth, because it allowed powerful ultraviolet radiation normally blocked by the stratospheric ozone layer to reach the surface. And except for the fact that the progressive destruction of the stratospheric ozone layer was arrested, scientists say it would have spread to the stratosphere above highly populated areas.

Even though the Antarctic ozone hole lasted each year for only approximately two months, it had already begun to produce a slight thinning of ozone in the stratosphere surrounding the entire planet. Scientists warned at the time that if the concentrations of chemicals causing ozone destruction continued to build, this dangerous thinning process would accelerate, and an even more dangerous ozone hole above the Arctic might form on a more regular basis.

Luckily, almost immediately after this frightening discovery, President Ronald Reagan and Prime Minister Margaret Thatcher helped to organize a global conference in 1987 to negotiate and quickly approve a treaty (the Montreal Protocol) that required the phasing out of the group of industrial chemicals-including the best-known, chlorofluorocarbons (CFCs)-that two scientists, Sherwood Roland and Mario Molina, had proven conclusively in 1974 were interacting with the unique atmospheric conditions in the cold stratosphere above Antarctica to produce this progressive destruction of the protective ozone layer that shields humans and other life-forms from deadly ultraviolet radiation.

EVEN THOUGH THE Montreal Protocol has been a historic success, it is important to understand the precise mechanism through which these chemicals led to the stratospheric ozone hole in the first place-because of new threats to the ozone layer from global warming. To begin with, there is a third and final set of barrel loop atmospheric cells at both the North Pole and the South Pole, called polar cells, within which the winds form a vortex around each pole.

The south polar vortex is much stronger and more coherent, especially in the austral winter, because Antarctica is land surrounded by ocean-whereas the Arctic is ocean surrounded by land-and while the Arctic Ocean is covered, at least in winter, by a thin layer of ice only several feet thick, Antarctica is covered year-round by two kilometers of ice. That also makes it the continent with the highest average alt.i.tude, which means it is closer to the top of the sky and radiates the reflected sunlight back into s.p.a.ce more powerfully. Consequently, the air above Antarctica is much colder than anywhere else on Earth, which produces an unusually high concentration of ice crystals in the stratosphere there.

The tight vortex formed by the Antarctic circ.u.mpolar wind currents during winter holds the CFCs and ice crystals in place above the continent, almost like a bowl. And it is on the surface of these ice crystals that the CFCs react with stratospheric ozone. One other crucial ingredient must be present before the chemical reaction that destroys the ozone starts taking place: a little bit of sunlight.

At the end of the southern hemisphere winter, around the middle of September, when the first rays of sunlight strike the ice crystals held in this "bowl," the chemical reaction is ignited. Then it quickly spreads, destroying virtually all of the stratospheric ozone inside the bowl. As the atmosphere absorbs more heat, the vortex formed by the wind currents weakens and the bowl breaks up, signaling the end of the ozone hole for that year. Some large blobs of ozone-free air sometimes move northward, like the blobs in an old lava lamp from the 1960s-exposing populated areas in the southern hemisphere like Australia and Patagonia to high levels of ultraviolet radiation when air with low concentrations of ozone is no longer able to provide a screen for those at the surface.

Stratospheric ozone depletion and global warming have always been considered almost completely separate phenomena, but in 2012 scientists discovered that global warming is producing an unexpected and unwelcome threat to the stratospheric ozone layer-this time above highly populated areas in the temperate zone of the northern hemisphere.

Just as the extra heat energy absorbed in the tropics is causing the updraft of the Hadley cells to nudge the top of the troposphere higher, the extra heat energy being absorbed in the temperate zone of the northern hemisphere is causing more powerful thunderstorms to punch through the top of the troposphere, injecting water vapor into the stratosphere, where it freezes into a new and dangerous concentration of ice crystals-thus creating the conditions for triggering stratospheric ozone loss by providing the surfaces on which the CFCs still in the atmosphere can come into contact with stratospheric ozone and sunlight to destroy the protective ozone layer. This new phenomenon has begun to appear at a time when the stratosphere is also getting colder, in inverse proportion to the warming of the lower atmosphere. Long predicted by climate models, stratospheric cooling is a result of the Earth's atmosphere attempting to maintain its energy "balance." Much more work will need to be performed before this troubling surprise is fully understood, but it already ill.u.s.trates the recklessness of this "planetary experiment" that humanity has under way. We are not only playing with fire, but ice as well. As Robert Frost wrote, "Some say the world will end in fire; some say in ice." Either one, he added, "would suffice."

THE RISKIEST OF EXPERIMENTS.

The idea that we are engaged in an unplanned experiment with the planet was first articulated by Roger Revelle, who was my teacher and mentor on global warming. In 1957, Revelle wrote with his coauthor, Hans Suess, that, "Human beings are now carrying out a large scale geophysical experiment." They also noted, "The increase of atmospheric CO2 from this cause [combustion of fossil fuels] is at present small but may become significant during future decades if industrial fuel combustion continues to rise exponentially."

The word "experiment" is worth a little reflection. There are ethical prohibitions against human experimentation that puts lives at risk or seriously damages those who are subjects of the experimentation. Since there are millions of lives put at risk by the "unplanned experiment" that is radically changing the Earth's atmosphere and threatening the future of human civilization, surely the same ethical principle should apply.

Climate science began more than 150 years ago when the legendary Irish scientist John Tyndall discovered that carbon dioxide traps heat. The actual mechanism by which this occurs is more complicated than the popular metaphor of a "greenhouse effect"; the bonds holding together the atoms of the CO2 molecule absorb and radiate energy at infrared wavelengths, impeding the flow of energy from the surface outward toward s.p.a.ce much like a blanket.

But the consequences are the same-the CO2 in the atmosphere, like the gla.s.s in a greenhouse, retains heat that comes in from the sun. Tyndall's historic finding occurred the same year, 1859, as the drilling of the first oil well by Colonel Edwin Drake in Pennsylvania.

Thirty-seven years later, in 1896, the Swedish chemist Svante Arrhenius cited Tyndall in a landmark paper in which he addressed the following question: "Is the mean temperature of the ground in any way influenced by the presence of heat-absorbing ga.s.ses in the atmosphere?" Arrhenius performed more than 10,000 calculations by hand in order to arrive at his conclusion that a doubling of CO2 concentrations in the atmosphere would raise global average temperatures by several degrees Celsius.

In the second half of the twentieth century, in the midst of the postwar burst of industrialization, research into global warming picked up considerably. The International Geophysical Year of 195758 led to the establishment by Roger Revelle and Charles David Keeling of a historic project to begin the long-term systematic measurement of CO2 concentrations in the global atmosphere. The results were astonishing. After only a few years of measurements it became obvious that the concentration was increasing steadily by a significant amount, a result confirmed in the following years by installation of observation stations all over the world.

Because most of the landma.s.s and deciduous vegetation is in the northern hemisphere, the CO2 concentration shows an annual cycle of CO2 intake and outga.s.sing by the terrestrial biosphere, which is so much larger north of the equator than south. As a result, the CO2 concentration in the northern hemisphere goes up in winter (when uptake of CO2 by leaves and plants is low) and down in summer (when the trees and gra.s.ses are once again pulling CO2 from the air).

But the observations also showed clearly that the overall concentration of CO2 throughout this yearly seasonal cycle was being shifted steadily upward. After the first seven years of the iconic measurements contained in what is now known as the Keeling Curve, the low point in the annual cycle was already higher than the high point when the measurements began. Fifty-six years later, these measurements still continue every day-from the top of Mauna Loa; at the South Pole; in American Samoa; in Trinidad Head, California; and in Barrow, Alaska. In addition, there are sixty other "distributed cooperative" sets of measurements, including aircraft profiles, ship transects, balloons, and trains. The project is now overseen, by the way, by an outstanding scientist, Ralph Keeling, who happens to be Dave's son. He is also now monitoring the small but steady reduction in the concentration of oxygen in the atmosphere-not a cause for concern in itself, but yet another validation of the underlying climate science, which has long predicted this result, and an effective cross-check on the accuracy of the CO2 measurements.

Ten years after Revelle and Keeling began measuring CO2 in the atmosphere, I had the privilege of becoming Revelle's student in college and was deeply impressed by the clarity with which he described this phenomenon and the prescience with which he projected what would happen in the future if the exponential increase in fossil fuel combustion and consequent CO2 emissions continued.

A decade after leaving college, I began holding hearings about global warming in Congress, and in 198788, I first ran for president in order to focus more attention on the need to solve the climate crisis. In June of 1988, NASA scientist Jim Hansen testified that the evidence of human-caused global warming had become statistically significant in observations of rising global temperatures. Six months later, in December, the United Nations established a global scientific body-the Intergovernmental Panel on Climate Change (IPCC)-to provide authoritative summaries of the evidence being found by scientific studies around the world.

Today, a quarter century after the IPCC began its work, the international scientific consensus confirming the dominance of human activities in causing global warming is as strong as any consensus ever formed in science. The threat is real, is linked primarily to human activities, is serious, and requires an urgent response in the form of sharply reduced greenhouse gas emissions. Every national academy of science and every major scientific society in the world supports the consensus view.

In a joint statement in 2009, the national academies of the G8 nations and five other nations declared, "The need for urgent action to address climate change is now indisputable." According to a peer-reviewed study published in the Proceedings of the National Academy of Sciences in the U.S., "9798 percent of the climate researchers most actively publishing in the field support the tenets of ACC (anthropogenic climate change) outlined by the IPCC."

It is also significant that virtually all of the projections made by scientists in recent decades about the effects of global warming have been exceeded by the actual impacts as they later unfolded in the real world. As many have noted, scientists in general and the scientific process in particular are inherently cautious in coming to a conclusion, even, you might say, conservative. Not conservative in the political sense of the word, but conservative in their methodology and approach. This tradition and long-established culture of caution is reinforced by the peer-review process, which demands convincing proof of any claims that are published. The same culture discourages statements about even seemingly obvious implications that may reflect common sense but cannot be adequately proven to the degree required for publication in a peer-reviewed journal.

Nevertheless, in spite of this conservative culture, the global scientific community has loudly and publicly warned policymakers that we must act quickly to avert a planetary calamity. Yet even with the mounting toll from climate-related disasters and the obvious warming of the Earth that is now viscerally apparent to almost everyone, there have been very few significant policy changes designed to confront this existential threat.