A map of the Arctic showing the reduction in summer sea ice that has taken place since 1980. Data from the University of Illinois, Urbana-Champaign

The area of sea ice lost since 1980 was half again greater than the entire area of the United States east of the Mississippi River. Taking into account changes in both area and thickness, almost 90 percent of the ma.s.s of summer sea ice over much of the twentieth century had disappeared by 2007. In 2006-7 the French ship Tara Tara replicated the late-nineteenth-century drift of the Norwegian ship replicated the late-nineteenth-century drift of the Norwegian ship Fram Fram across the Arctic Ocean, locked in sea ice. But across the Arctic Ocean, locked in sea ice. But Tara Tara completed the drift in only five hundred days, compared to the three-year drift of completed the drift in only five hundred days, compared to the three-year drift of Fram Fram; the shorter drift time of Tara Tara was attributed to the greater ease with which thinner sea ice can move across the ocean. In 2008, perhaps for the first time in many centuries, both the Northwest and Northeast pa.s.sages were open at the same time. was attributed to the greater ease with which thinner sea ice can move across the ocean. In 2008, perhaps for the first time in many centuries, both the Northwest and Northeast pa.s.sages were open at the same time.

The current rate of summer sea ice loss is exceeding all projections,99 and there is a very real possibility that in only a few decades the Arctic Ocean may be ice-free in the summer, for the first time in fifty-five million years. Climate scientist Ian Howat of Ohio State University remarked that the loss of Arctic sea ice might be the largest change in Earth's surface that humans have ever observed. and there is a very real possibility that in only a few decades the Arctic Ocean may be ice-free in the summer, for the first time in fifty-five million years. Climate scientist Ian Howat of Ohio State University remarked that the loss of Arctic sea ice might be the largest change in Earth's surface that humans have ever observed.100

ARCTIC SEA ICE typically takes the brunt of punishing ocean waves and storms, but as the ice has all but disappeared in the late Arctic summer, the exposed coastline has become vulnerable to severe erosion. Villages in the Arctic have traditionally been built close to the sea to enable easy pursuit of seals, walruses, and fish. But the loss of sh.o.r.eline protection has taken a toll on structures built near the sea. Shishmaref, Alaska, a small community with fewer than six hundred residents built on a small island in the Bering Strait, is facing the possibility of forced evacuation. Shishmaref has had human inhabitants for several thousand years, but it has always had the protection and convenience of surrounding sea ice during the storm season. In recent years, however, storms have time and again eroded the island. Shishmaref is literally losing ground, and the residents have had to relocate houses farther from the sh.o.r.eline. Travel to the mainland for hunting moose and caribou, formerly easily accessible over the sea ice, has been impeded because the winter freeze of the sea has recently been delayed by as much as six weeks.

Just east of the Alaska-Canada border, along the coast of the Beau-fort Sea, sits Tuktoyaktuk, a Canadian port town. The terrain of the area is low-lying, with little topographic relief. Sea ice formerly offered protection from big waves and storm surges, but today, erosion in Tuktoyaktuk is an increasing problem. As in Shishmaref, buildings are being abandoned to the encroaching sea. Multiple attempts at engineering sh.o.r.eline protection have failed, in a losing battle with the advancing surf. Most of the coast surrounding the Arctic Ocean is increasingly vulnerable to destabilization, because the seawater accelerates the melting and destruction of the coastal permafrost and leads to the development of thermokarst. The temperature of seawater is clearly above the melting point of ice, and so contact between seawater and permafrost leads to rapid degradation.

It is not only the hunters on Shishmaref who have seen the loss of sea ice cut them off from large game. Polar bears are also affected. For years sea ice has served as a widespread platform for polar bears to move about in search of food, particularly the seals of the Arctic Ocean. Pregnant female bears spend winter in dens of snow and ice, emerging in spring after a half year without food, and with a new family of hungry cubs. An early breakup of the sea ice impedes their ability to hunt, forcing them to walk less and swim more in search of seals, a bad trade-off in terms of energy expended. Reproductive success is closely tied to the fitness of the mother-weakened females produce fewer and smaller cubs. Multidecadal studies of polar bears in the Hudson Bay area reveal declines both in average parental weight and in the number and weight of cubs. And the sea ice is breaking up a week earlier each decade.

Though stronger swimmers than the bears, the seals are faring no better. They, too, use the sea ice as a platform at calving time, and an early breakup of the ice leads to a premature separation of parents from the newborn, before the young seals are ready to face the world on their own. Walrus also rest on sea ice as they scavenge the shallow near-sh.o.r.e seafloor, but as the edge of the sea ice moves to deeper water, the walrus is faced with more arduous seafloor foraging.

MEANWHILE, AT THE BOTTOM OF THE WORLD . . .

Palmer Station, one of three U.S. research stations in Antarctica, was built on Anvers Island, along the Antarctic Peninsula, in 1968. It comprises a handful of prefabricated buildings that host research projects, a small boat anchorage, fuel storage tanks, and living and dining quarters for some forty people. The station, named for the early-nineteenth-century American sealer and explorer Nathaniel Palmer, is chiseled into a rocky outcrop, squeezed between the large Marr Glacier and the open sea. From the air the station is dwarfed by the vastness of the peninsular ice and the ocean that surround it. The research focus at Palmer is largely biological, but includes a seismographic installation and a meteorological station.

Since 1950, the Antarctic Peninsula has warmed by more than six Fahrenheit degrees, mostly due to a remarkable rise in wintertime temperatures of more than ten Fahrenheit degrees. This warming has reduced the extent of winter sea ice along the western edge of the peninsula by 40 percent and shortened the duration of sea ice cover there by almost three months. These changes in the sea ice along the peninsula are leading to changes in the ecology of the rich marine life of the region. But the changes begin not with the big animals-the whales, seals, and penguins-but rather at the bottom of the food chain, with the marine phytoplankton. Phytoplankton are single-cell plants that fuel their growth with sunlight. They flourish near the margin of the sea ice that grips much of the Antarctic Peninsula each winter. When spring arrives and the sea ice begins to break up, the phytoplankton bloom pro lifically. They are then devoured by small shrimplike crustacean called krill, which are in turn the diet of choice for penguins and whales farther up the food chain.

If the warming trend of the peninsula continues, by 2050 midwinter temperatures will remain warmer than the freezing point of seawater, and sea ice will not form. Jim McClintock, a professor of polar and marine biology at the University of Alabama at Birmingham and a long-time researcher at Palmer, says that the warming will lead "to a regime change in the ecosystem. Krill are highly dependent on sea ice; without it they cannot complete their life cycle and breed successfully." The decline of sea ice in area and duration is particularly ominous for the krill, and for the penguins that eat them, and for the predatory seabirds that eat the penguin eggs and chicks, and so on, up the food chain. At the top of the list are the whales, which don't depend upon or even bother with the intermediate links in the food chain-they eat krill directly, by the ton every day.

One long-term study at Palmer has focused on the population dynamics of the three penguin species in the area-the Adelie, Gentoo, and Chinstrap. McClintock and colleagues Bill Fraser and Hugh Ducklow have pieced together the interdependencies of these penguins with the changing environment.101 Because each species interacts with sea ice in different ways, princ.i.p.ally in their access to food, the changes in the ice are driving the populations in different directions. Adelie penguins, the cla.s.sic little tuxedo-clad seabirds, prefer wintertime foraging at offsh.o.r.e locations where nutrient-rich marine upwellings occur, sites made more accessible to them by the existence of widespread sea ice. By contrast, Chinstrap and Gentoo penguins prefer to forage along open sh.o.r.elines. The declining sea ice has therefore been hard on the Adelies but generous to the Chinstraps and Gentoos. Because each species interacts with sea ice in different ways, princ.i.p.ally in their access to food, the changes in the ice are driving the populations in different directions. Adelie penguins, the cla.s.sic little tuxedo-clad seabirds, prefer wintertime foraging at offsh.o.r.e locations where nutrient-rich marine upwellings occur, sites made more accessible to them by the existence of widespread sea ice. By contrast, Chinstrap and Gentoo penguins prefer to forage along open sh.o.r.elines. The declining sea ice has therefore been hard on the Adelies but generous to the Chinstraps and Gentoos.

I have been to Palmer Station four times over eighteen years. When I first visited in 1991 there was a big Adelie population on the islands around Palmer-noisy, smelly, coming, going. But on my most recent stop, in early 2008, the Adelies were largely gone. Indeed, all along the northern half of the peninsula, Adelie populations are in decline. Deciphering these trends would not have been possible without year-in, year-out monitoring of the penguin populations, the sea ice extent and timing, and temperature and precipitation changes. The U.S. National Science Foundation has designated Palmer Station a Long-Term Ecological Research (LTER) site, a designation that a.s.sures funding for the long-term observational programs that track trends in the ecosystems. The NSF has established twenty-six LTERs, three of which are in polar regions-one on the north slope of Alaska and two in Antarctica.

Adelie penguins of Antarctica ENCROACHMENT OF THE SEA.

The most dramatic consequence of a world losing its ice will be the steady-and perhaps not so steady-rise of sea level. For the entire twentieth century and continuing into the twenty-first, sea level has been creeping upward, due to two princ.i.p.al causes. The first relates to some simple physics-seawater expands as ocean temperatures rise. The second is the return of melt.w.a.ter to the ocean, after spending thousands of years as ice in temporary residence on the continents. Both causes lead to encroachment of the sea onto the land. There is already more water on Earth than can be accommodated in the deep ocean basins, and the excess has spread out to cover part of the continental shelves. As sea level continues to rise, foot by foot, sh.o.r.elines will relocate inland.

The IPCC has estimated the likely sea level rise in the present century to be in the range of ten to thirty inches, which, when added to the eight-inch rise of the previous century, indicates that we are facing an increase of some two to three feet above the historic average sea level. Such numbers may seem modest, but in fact changes of that magnitude will have nontrivial consequences in sh.o.r.eline erosion, infrastructure destruction, and population displacement. In making these estimates, the IPCC considered only the effects of continued thermal expansion of ocean water, the addition of new water by melting and runoff of continental ice, and the return of some groundwater to the sea. But the IPCC hedged their projections with one important caveat that I will return to shortly.

Low-lying places on Earth-those barely a foot or two above today's sea level-will be the first to feel the encroachment of the sea. The barrier islands and very gently sloping coastal plains along the Atlantic and Gulf coasts, many coral-fringed atolls of the Pacific, and major river deltas the world over are particularly vulnerable to a rising sea. These are the places that already take the brunt of storms and hurricanes. Almost every structure on the barrier islands at Galveston on the Texas Gulf coast was leveled by the winds and storm surge of Hurricane Ike in 2008, and as sea level creeps upward, these islands will be increasingly vulnerable to even lesser storms.

Along the Atlantic coast of the Carolinas, Georgia, and Florida there is a continuing struggle between the erosion and shifting sands of the barrier islands and the ever-increasing number of people who build houses on them. The houses are expensive, and so is the struggle-dredging sand to keep established channels open, trucking in sand to replace the beaches lost in storms. And there is always the temptation toward "hard stabilization"-the building of protective seawalls and other types of barriers-which almost always fail, leaving a ma.s.sive jumble of concrete debris with bent steel reinforcing rods to degrade the landscape, in mute testimony to the futile attempt at forestalling nature.

Well before rising waters force abandonment of coastal and barrier island homes, the residents will also notice that their well water has turned briny, as a result of seawater pushing into coastal aquifers. Shortly thereafter sewage systems will begin to fail as seawater degrades bacterial action. Little of Florida from the Everglades south through the Keys sits more than three feet above sea level. During the storm surge of Hurricane Wilma in 2005, many of the Keys were under three feet of water and more than half the houses in Key West were flooded. Every part of this region is vulnerable to even a modest rise in sea level.

The levees that channel the Mississippi River through New Orleans are an example of hard stabilization. These mounds of earth and concrete walls are ma.s.sive constructs designed and engineered by the U.S. Army Corps of Engineers. New Orleans does not sit a foot or two above sea level-much of the city is several feet below sea level. The life of New Orleans has for decades been entrusted to the levees, a trust that was betrayed during Hurricane Katrina in 2005. Katrina forced the evacuation of some 150,000 residents of New Orleans, and many never returned. Despite some post-Katrina repairs, there was another near-betrayal during Hurricane Gustav in 2008, when the storm surge lapped at the very top of New Orleans's floodwalls, splashing over with any slight provocation from the wind. Gustav was not the storm that Katrina was, but had sea level been only a foot higher, Gustav would have replicated what Katrina wrought. The slowly rising seas of the twenty-first century will make any hurricane a danger to New Orleans.

Venice is built on several islands in a marshy lagoon at the north end of the Adriatic Sea. The city proper and other occupied islands in the lagoon are home to one hundred thousand people. The ca.n.a.ls of Venice lead to the open sea, and are only one indication that the city and the sea are like hand and glove. Piazza San Marco, perhaps Venice's premier tourist destination, is vulnerable to flooding from a mult.i.tude of causes-stiff winds from the sea, high tides, or intense rainfall. In early December of 2008 all three conspired to place St. Mark's thigh deep in water, a level within the range of the IPCC estimates of sea-level rise this century. With sea level creeping higher, Venice will be visited more frequently by flooding, even in the absence of a triple conspiracy. Authorities in Venice have long dreamed of building a flood barrier at the mouth of the city's lagoon to hold back the Adriatic at times of high tides and wind-driven water, but the costs to build the barrier have escalated faster than the available money. And meanwhile, sea level keeps rising.

Kiribati is an island nation in the south Pacific, with some ninety thousand citizens scattered on more than thirty atolls straddling the equator. Its princ.i.p.al community is on Tarawa atoll, the site of a ferocious struggle between the j.a.panese and Americans during World War II. Kiribati will be another of the countries that, like Tuvalu, will lose territory to rising sea level. Already many places are experiencing flooding, erosion, and salt.w.a.ter contamination of the groundwater at the time of the twice-monthly high spring tide. In 2008, Kiribati asked Australia and New Zealand to open their doors to Kiribati citizens as permanent climate refugees. Kiribatians worry that no matter how the large industrial countries might attempt to prevent future climate change the oceans will continue to warm and sea level will continue to rise because of the long-lived greenhouse gases already in the atmosphere.

The Maldive Islands are an archipelago of twenty-six atolls in the Indian Ocean, south of the southern tip of India. Most are about three feet above today's sea level; none reaches more than seven feet. That topographic fact gives the Maldives the honor of being at the bottom of the list of nations ranked by their maximum elevation. Some 350,000 people reside in the Maldives, about one third in the capital city of Male. Seawalls completely surround Male for protection. Perhaps more prosperous than many island nations, the Maldives are thinking about creating an investment fund to buy land in India, Sri Lanka, or elsewhere as a destination for climate refugees. The Maldives recently experienced the devastation of higher seas when the tsunami a.s.sociated with the 2004 Indonesian earthquake literally rolled over many of the islands.

THE DELTAS.

Most rivers flow to the sea, completing the hydrological cycle by returning water evaporated earlier from the oceans. But big rivers also carry vast amounts of sediment eroded from the land. Deltas are geologically dynamic meeting places-where rivers and their sediment load meet the sea. When a river flows into the sea its current is slowed and the sediment settles. Over time the channel wanders back and forth over the acc.u.mulating pile of sediments to form a delta. The Greek letter delta () has the shape of a triangle, and is an appropriate descriptive term for the broad fan of sediment that rivers dump into the sea at their mouths. The deltas of Earth's major rivers-the Amazon, Ganges, Nile, Mississippi-extend over hundreds of miles, some parts slightly above sea level, others slightly below. Over time, some mud banks and sand-bars will slowly sink below the surface, while other areas receive enough sediment to rise slightly above the sea and feel the sunshine for a few decades.

The Nile Delta meets the Mediterranean Sea along a 150-mile sh.o.r.eline, about a hundred miles north of the apex of the delta where the river emerges onto the delta from its narrow floodplain after a long meandering journey across the Egyptian desert. Along the coast of the delta at its western edge sits the ancient and modern city of Alexandria, home to more than four million people. Much of the historic ancient city is now submerged beneath the sea, largely due to tectonic subsidence of the land a.s.sociated with regional earthquakes. Alexandria's El Nouzha International Airport itself sits six feet below sea level, isolated from the sea by natural barriers.

The soil of the Nile Delta is very rich, and accordingly, rural parts of the Nile Delta are heavily agricultural. Over much of the history of the Nile, the sediment of the delta was replenished by the annual flood of the river, but since the completion of the Aswan High Dam in 1970, six hundred miles south of the delta, the annual flood no longer occurs along the lower Nile. The soil of the delta now requires fertilizers to maintain productivity. But the delta itself, no longer replenished with sediment from Africa's interior, is slowly retreating as sh.o.r.eline erosion along the Mediterranean inexorably eats away at the delta's margin. And as seawater pushes its way into the subsurface aquifers, the groundwater in the delta is becoming more brackish. The erosion of agricultural land and the intrusion of salt.w.a.ter will only accelerate as sea level rises due to climate change.

The Bengal region of eastern India and the adjacent country of Bangladesh sit astride the immense delta built by the Ganges and Brahmaputra rivers. These two rivers, both with headwaters in the Himalayas, drain a sizable fraction of Asian territory and deliver to the delta a heavy load of sediment eroded from the mountains. This area, with its large cities of Calcutta and Dhaka, has one of the world's highest population densities, on some of the lowest terrain. Calcutta, located some sixty miles from the Indian Ocean, has more than eight million residents, and its lower precincts are only five feet above sea level. Much of the coastal south of this huge delta is only a foot or two above sea level. It is a great oversimplification to speak of a coastline-the actual margin of the land with the ocean is like a giant frond, with bays and marshes extending far inland, separated by tongues of land that do not qualify for the adjective dry dry. Seasonally, about 10 percent of Bangladesh is underwater.

The terrain of the Ganges Delta is about as flat as one can find, rising only a foot above the sea for every twelve miles inland from the coast. By comparison, the Great Plains stretching across Kansas and Nebraska to the mountains of Colorado are a steep ramp, rising by about eight feet for each mile they sit westward of the Missouri River. A slope of one degree, which we might imagine to be a very small slope, rises ninety feet in a mile. A sea-level rise of a foot or two in this century-the conservative projection of the IPCC-will force a devastating displacement of millions of people living on the Ganges Delta.

The numbers are sobering-11,000 Tuvaluans, 60,000 Marshall Islanders, 90,000 Kiribatis, 100,000 Venetians, 150,000 people in the Big Easy, and millions in the deltas of the Nile and Ganges-all facing displacement by only a modest rise in sea level. A recent study102 of where people live on Earth showed that 108 million humans, equivalent to the population of Mexico, live on terrain no more than three feet above sea level. Those numbers translate into 1.6 percent of Earth's population living on less than 1 percent of the land surface-that part of the land barely above sea level. In the United States along the Atlantic and Gulf coastal plain and barrier islands, more than 2.5 million people live no more than three feet above sea level. of where people live on Earth showed that 108 million humans, equivalent to the population of Mexico, live on terrain no more than three feet above sea level. Those numbers translate into 1.6 percent of Earth's population living on less than 1 percent of the land surface-that part of the land barely above sea level. In the United States along the Atlantic and Gulf coastal plain and barrier islands, more than 2.5 million people live no more than three feet above sea level.

Hurricane Katrina displaced about 150,000 people from New Orleans, and the 2008 California wildfires displaced almost 1 million-and both events severely tested our nation's ability to address the needs of environmental refugees. Cyclone Sidr, a November 2007 tropical storm in the Indian Ocean, killed more than 3,000 Bangladeshis and left more than one million homeless, with the full consequences of that population dislocation still to be determined. A three-foot rise in sea level this century, creating more than one hundred million climate refugees, is equivalent to about seven Katrina-scale evacuations every year, although much of the population displacement will occur later in this century.

In rich nations, much effort and treasure will be expended to forestall and avoid the advance of the sea. Levees will be strengthened, seawalls will be built, but most of the effort will be futile-it is almost impossible to hold back the sea along an extended coastline. The masters of that craft, the Dutch, already have recognized the near impossibility of the task, and have made plans to abandon some of their land to the rising sea. In poor nations, people will simply be forced to move to higher ground, ground already occupied by other people, who are unlikely to welcome the newcomers. If immigration now seems to be a th.o.r.n.y, controversial, and emotional problem in the United States and Europe, after three feet of sea-level rise and one hundred million climate refugees worldwide, today's immigration complexities will in retrospect look like a Sunday school picnic.

It is entirely possible that sea-level rise will not continue at the slow and steady rate we have experienced in the twentieth century. As seawater warms, its ability to take in CO2 from the atmosphere diminishes because the solubility of CO from the atmosphere diminishes because the solubility of CO2 in seawater decreases as the temperature of the water increases. The likely result will be that in the present century, as the oceans continue to warm, they will be less of a sponge for atmospheric carbon dioxide. More CO in seawater decreases as the temperature of the water increases. The likely result will be that in the present century, as the oceans continue to warm, they will be less of a sponge for atmospheric carbon dioxide. More CO2 will remain in the atmosphere and enhance the heat-trapping greenhouse-thus accelerating the melting of land ice, and the warming and thermal expansion of the ocean water. And the physical mechanisms that cause water to expand are more effective at higher temperatures than at cooler temperatures. The physics and chemistry of the oceans and atmosphere make inescapable the conclusion that a warmer Earth will lead to sea level rising at an accelerating pace. Already the rate at which the sea was observed to rise in the period 1993-2003 exceeded the average rate over the longer period 1961-2003 by more than 50 percent. will remain in the atmosphere and enhance the heat-trapping greenhouse-thus accelerating the melting of land ice, and the warming and thermal expansion of the ocean water. And the physical mechanisms that cause water to expand are more effective at higher temperatures than at cooler temperatures. The physics and chemistry of the oceans and atmosphere make inescapable the conclusion that a warmer Earth will lead to sea level rising at an accelerating pace. Already the rate at which the sea was observed to rise in the period 1993-2003 exceeded the average rate over the longer period 1961-2003 by more than 50 percent.

THE THIRD TRENCH OF DENIAL.

The climate contras have occupied several successive defensive trenches in their campaign of denial of anthropogenic climate change. The first trench was simply that of disputing the many observations that climate has been changing. The second was that if climate has been changing, the change has been due entirely to natural causes-that humans have had no essential role in climate change. Although some contras remain active along these lines of defense, many have retreated to a third trench: Who, they ask, will be unhappy with a CO2-rich atmosphere and a warming world? They point out that CO2 acts as a fertilizer, stimulating agriculture around the world, and that a warmer world will extend the growing season and enable more food production. And they muse about the pleasures of milder winters, free from the ha.s.sles of snow and ice. In their more rhapsodic moments they speak of a return to the Garden of Eden. acts as a fertilizer, stimulating agriculture around the world, and that a warmer world will extend the growing season and enable more food production. And they muse about the pleasures of milder winters, free from the ha.s.sles of snow and ice. In their more rhapsodic moments they speak of a return to the Garden of Eden.

Indeed, the growing season is growing-in Alaska it is already two weeks longer than a half century ago. And it is true that CO2 is a necessary ingredient for photosynthesis. But CO is a necessary ingredient for photosynthesis. But CO2 alone is not sufficient for increased plant growth. Frequently in natural settings it is the availability of other nutrients-particularly water and nitrogen-that are the limits to additional bioma.s.s, not the availability of CO alone is not sufficient for increased plant growth. Frequently in natural settings it is the availability of other nutrients-particularly water and nitrogen-that are the limits to additional bioma.s.s, not the availability of CO2. Controlled experiments at Duke University's Duke Forest showed that elevated levels of CO2 produced no incremental growth in trees rooted in nutrient-poor soil. produced no incremental growth in trees rooted in nutrient-poor soil.103 Moreover, in nature, CO2 does not vary in isolation. In a greenhouse-forced climate change, higher temperatures and changed geographic patterns of precipitation and soil moisture accompany an increase in atmospheric CO does not vary in isolation. In a greenhouse-forced climate change, higher temperatures and changed geographic patterns of precipitation and soil moisture accompany an increase in atmospheric CO2. In one of nature's own experiments, the boreal forest in Alaska has not shown dramatic growth as the atmospheric CO2 has increased because it is also experiencing heat stress from the warming climate. And the pine forests of western North America, far from flourishing in a higher CO has increased because it is also experiencing heat stress from the warming climate. And the pine forests of western North America, far from flourishing in a higher CO2 atmosphere, are suffering terribly from a resurgence of the pine bark beetle, a pest formerly kept under control by colder winters. atmosphere, are suffering terribly from a resurgence of the pine bark beetle, a pest formerly kept under control by colder winters.

CO2 is not a discriminating fertilizer-it fertilizes weeds as well as foodstuffs. And there is nothing to guarantee that the crop growth we might want to enhance will be able to outpace unwelcome invasive insects and plant species that also like the warmer and CO is not a discriminating fertilizer-it fertilizes weeds as well as foodstuffs. And there is nothing to guarantee that the crop growth we might want to enhance will be able to outpace unwelcome invasive insects and plant species that also like the warmer and CO2-richer atmosphere. If a greater use of herbicides and pesticides to control invasive species follows, the negative effects on public health and the general ecosystem will diminish whatever benefits might come from CO2 fertilization. fertilization.

The rhetorical question "Who wouldn't welcome warmer winters?" is both parochial and simplistic. It implicitly a.s.sumes that a warming that would ameliorate winter cold, snow, and ice in the lat.i.tudes where those phenomena occur would be welcomed also by the majority of Earth's population who do not live in that environment. Higher temperatures in already warm regions are not high on the wish list of those who live there. And of course a long-term trend toward warmer winters does not occur independently of changes in the other seasons-such a trend is often accompanied by a similar trend to warmer summers, with more heat waves and soil desiccation. The IPCC a.s.sessment of impacts accompanying a warming world shows that the tropics will fare poorly by almost any measure.

Even were there to be some regional benefits in the early stages of moderate climate change, as there well might be in some mid-lat.i.tude settings, those selective and limited benefits hardly outweigh the large-scale hardships and human displacement a.s.sociated with even a modest rise in sea level. Higher seas will affect almost all of the world's coastlines adversely-large nations and small, rich nations and poor. There is no way that rising sea level can be described as beneficial.

THREE FEET OF SEA-LEVEL rise, however, may be just the beginning. The ongoing breakup of Antarctic ice shelves-the La.r.s.en, Wilkins, and Ronne along the Antarctic Peninsula, and the Ross Ice Shelf farther around the continent to the west-has ominous implications for sea levels globally. These floating shelves of ice, in place for thousands of years, form b.u.t.tresses that bottle up glacial ice on the land of the peninsula and West Antarctica, slowing spillage of ice into the sea. Those long-standing b.u.t.tresses are now breaking away, and the glaciers are speeding up their delivery of land ice to the ocean.

When ice flows off the land and enters the sea for the first time, it raises sea level. It is nothing more than a large-scale replication of the process that takes place when you drop an ice cube into a gla.s.s of water. The submerged part of the floating cube displaces water and moves it aside, and up, in the gla.s.s. Once afloat in the gla.s.s, however, ice causes no further change in the water level as it melts. The same holds true for floating ice shelves-when they break up and melt, they do not change sea level because the effect of the floating ice on sea level occurred when the ice originally spilled out onto the sea. But the breakup of the shelves is an early warning sign, an icy "canary in the coal mine," because the breakup allows new land ice to spill into the sea, and the new ice does raise sea level.

The ma.s.s of ice bottled-up on the Antarctic Peninsula, were it to slide into the sea, would raise the global sea level about ten inches, roughly equivalent to the c.u.mulative sea-level change observed during the twentieth century. However, the twentieth-century sea-level change was due princ.i.p.ally to the warming and thermal expansion of the seawater globally, and to some new water added to the oceans from the melting of mountain glaciers and permafrost. Those were relatively slow processes in the twentieth century. But when ice is dropped directly into the ocean, the sea-level change takes place right away, before the ice melts. That is why an acceleration of ice loss from Antarctica and Greenland is worrisome-sea-level changes can come much more quickly than the relatively slow increases arising from warming alone.

The amount of ice on each of the Greenland and West Antarctic landma.s.ses is equivalent to about twenty feet of sea level change, and that on East Antarctica, about two hundred feet. Because much of the ice surface on East Antarctica is remote from the sea and very high, the temperature is extremely cold. It averages about -50 to -60 Fahrenheit over the year, and never gets close to melting, even in summer. Of all the ice on the globe, that in East Antarctica is thought to be the most stable. But Greenland and West Antarctica present a more ominous scenario.

GREENLAND.

As the Arctic has warmed, summertime melting has been creeping to higher elevations on the Greenland ice cap. Some of the melt.w.a.ter runs to the sea, and some ponds up in slight depressions to form blue lakes on the surface of the white ice. Occasionally these melt.w.a.ter lakes disappear suddenly, draining into large fissures called moulins, which apparently penetrate the entire mile-thick ice sheet. Melt.w.a.ter plunges to the base of the ice and makes its way along the interface between the rock and ice, much like subterranean rivers that flow deep within limestone caves.

The consequence of this deep subglacial flow is profound-the water lubricates the base of the ice sheet, allowing the ice to slip toward the sea at a faster rate.104 The Jakobshavn Glacier, on the western coast of Greenland, is the largest drainage channel for interior ice-it alone accounts for about 6.5 percent of Greenland's ice delivery to the sea. Between 1983 and 1997 the ice moved down valley at a speed of three to four miles per year, but then it rapidly increased, reaching about seven to eight miles per year in 2003. The Jakobshavn Glacier, on the western coast of Greenland, is the largest drainage channel for interior ice-it alone accounts for about 6.5 percent of Greenland's ice delivery to the sea. Between 1983 and 1997 the ice moved down valley at a speed of three to four miles per year, but then it rapidly increased, reaching about seven to eight miles per year in 2003.105 An investigation of all the glaciers draining Greenland An investigation of all the glaciers draining Greenland106 revealed widespread acceleration of ice loss between 1996 and 2005, first in the south, then steadily moving north. In that decade, Greenland revealed an annual ice deficit-yearly ice losses that exceeded the annual replenishment by snowfall-that more than doubled. revealed widespread acceleration of ice loss between 1996 and 2005, first in the south, then steadily moving north. In that decade, Greenland revealed an annual ice deficit-yearly ice losses that exceeded the annual replenishment by snowfall-that more than doubled.

The accelerated ice movement is redefining the phrase "at a glacial pace." The slow flow of ice is similar to the slow creep of Silly Putty down a gentle slope. Both ice and Silly Putty, although solid, yield to the pull of gravity through slow plastic deformation in their interiors. But ice complains when "a glacial pace" is scaled upward-it literally creaks and groans as gravity pulls it downward at a faster pace. Already this disequilibrium is manifesting itself with the appearance of ice quakes, small seismic disturbances created as the ice adjusts internally to the changing velocity of flow, and as it squeezes its way over or past irregularities in the rock surface below.

WEST ANTARCTICA.

West Antarctica is capped with an immense mound of ice about a mile high and a thousand miles across. From the top of the mound the ice flows in three directions-to the Ronne Ice Shelf in the Weddell Sea, to the Ross Ice Shelf, and to the Amundsen Sea. Both the Ronne and Ross ice shelves, even though losing some ice on their seaward margins, have remained effective b.u.t.tresses to the ice streams feeding them. But the two princ.i.p.al glaciers carrying most of the ice flow from West Antarctica to the Amundsen Sea are showing dramatic acceleration. The Pine Island and Thwaites glaciers are not typical Alpine glaciers that flow a few tens of miles in valleys a mile or so across. Multiply those dimensions by ten to envision the immense scale of the Pine Island and Thwaites glaciers. These ice streams, each tens of miles wide and up to a mile thick, flow for hundreds of miles from the top of the West Antarctic ice pile to the sea. The scale is difficult to appreciate-imagine the entire Mississippi River floodplain from Memphis to New Orleans filled with ice several thousand feet thick, slipping southward to the Gulf of Mexico at the glacial pace of about a mile each year. These glacial streams drain an area that would contribute about five of the twenty feet of the potential sea-level rise represented in all the ice of West Antarctica. And the discharge from these glaciers has been speeding up.

The ice shelf fed by these glaciers, like shelves elsewhere in the Arctic and Antarctic, is breaking up, allowing the ice from the interior of West Antarctica to drain more quickly to the sea. An understanding of what leads to the breakup of ice shelves, and the subsequent opening of the "floodgates" to upstream ice, is slowly emerging. It is a complex process involving not only melt.w.a.ter penetration through surface creva.s.ses and moulins, but also thinning and weakening along the bottom of the floating ice sheet by warmer seawater. These new observations are challenging our old understanding about the rate at which glacial ice flows.

Earlier in this chapter I mention that there was an important caveat attached to the IPCC estimate that the likely sea-level rise in the present century would be in the range of ten to thirty inches. In making these estimates, the IPCC considered only the effects of continued thermal expansion of ocean water, the addition of new water by melting and runoff of continental ice, and the return of some groundwater to the sea. Such estimates have proved conservative in the past-actual sea-level rise since 1990 has been at the upper limit of earlier projections by the IPCC. In their 2007 report, the IPCC purposely sidestepped evaluating the accelerating ice loss and the direct deposit of ice into the sea from Greenland, the Antarctic Peninsula, and West Antarctica, which would lead to both larger and faster sea-level rises.

The reason for this sidestep lies in part in the IPCC rules-very new research results, those that have not been around long enough to be digested and evaluated by other researchers, cannot be included in the reports. The IPCC was, however, very much aware of the implications that accelerated ice loss held for rapid sea-level changes, and posted a clear cautionary statement that their estimates "did not include the possibility of significant changes in ice-flow dynamics." But as the ice behavior on the fringes of both Greenland and Antarctica is showing, this possibility may already be contributing to an acceleration of the rise of sea level. Scientists in the field, and at conferences and workshops, have quickly turned to evaluating the changes in ice dynamics, and are revising projections of ice loss upward.

EARTH HISTORY IS replete with examples of different climates and higher sea levels, as evidenced in part by sedimentary rocks of various ages that had been deposited in shallow seas lapping onto the continents. The fossil-bearing sandstones, shales, and limestones that drape the older continental crust give testimony to higher levels of the sea several times during the past six hundred million years. In the Late Cretaceous period, ninety to one hundred million years ago, sea level was five hundred feet higher than today; warm ocean waters washed over interior North America from Alaska to the Gulf of Mexico, and dinosaurs flourished in the surrounding wetlands. And at other times in Earth's history there were extensive glaciations, including the possibility that Earth was at one time frozen over entirely.107 Higher seas or extensive ice at different times in the past, however, do not offer as much insight into the more recent climate change as one might hope for because in earlier eras, Earth was very different from Earth today. Continents and ocean basins were in different locations on the globe, and ocean currents that move heat around the globe had distinctly different patterns, because of the constraints placed on circulation by the position of the continents. Earth's modern climate system reflects these fundamental present-day constraints: an ocean at the North Pole, a continent at the South Pole; a Gulf Stream that transports tropical heat to the polar north, and an Antarctic Circ.u.mpolar Current that prevents the warming of the polar south.

But if there is disappointingly little guidance to be gleaned from the ancient past, there is much to be learned from the geological record of the more immediate past. Just prior to the beginning of the most recent glaciation, about 120,000 years ago, Greenland had only about half its present ice cover, and sea level stood some 10 to 15 feet higher than today. At the time there were, perhaps, a few hundred thousand people on Earth, mostly still in Africa. Today, upward of 400 million people live on terrain that would be inundated by that large a rise in sea level.

The ice cores extracted from the Greenland ice cap reveal some surprises in the ice from the very bottom of the ice sheet-fossil DNA of ancient trees, plants, and insects that lived in southern Greenland a half-million years ago,108 later to be obliterated by ice advances during more recent glaciations. This evidence of a forest in an area now covered with thick ice clearly indicates that the ice volume on Greenland has oscillated-sometimes less, sometimes more than in the present day- with obvious implications for increases or drops in sea level. But few of our ancestors lived anywhere close enough to Greenland to notice the comings and goings of the ice, and those living far from ice but closer to the sea were mobile enough to contend with moving sh.o.r.elines. Nomadic hunter-gatherers made no investments in permanent dwellings. later to be obliterated by ice advances during more recent glaciations. This evidence of a forest in an area now covered with thick ice clearly indicates that the ice volume on Greenland has oscillated-sometimes less, sometimes more than in the present day- with obvious implications for increases or drops in sea level. But few of our ancestors lived anywhere close enough to Greenland to notice the comings and goings of the ice, and those living far from ice but closer to the sea were mobile enough to contend with moving sh.o.r.elines. Nomadic hunter-gatherers made no investments in permanent dwellings.

Earth also experienced a significant warm interval some three million years ago, in the middle of the Pliocene epoch of geological time, and sea level was as much as one hundred feet higher than today. The Atlantic and Gulf coastal plains of the United States were partially flooded, with the sh.o.r.eline close to one hundred miles inland from its present position. If Augusta, Georgia, and Richmond, Virginia, were Pliocene cities, seaside beach homes would have been only a few minutes away. The Florida peninsula was entirely submerged. The sediments deposited in this shallow sea include shark teeth, the fossils of sea turtles, and pollen, which indicated the proximity of nearby land.

In contrast to the climate systems that governed ice and sea levels much earlier in Earth's history, the mid-Pliocene climate is an excellent a.n.a.log of today's global climate system-the locations of the continents have changed only slightly over the past three million years. The uplifting of the Isthmus of Panama had just closed the connection between the Atlantic and Pacific oceans, the Drake Pa.s.sage between South America and Antarctica had been open for several million years, and oceanic currents had achieved a distinctly modern pattern. The microscopic single-cell marine foraminifera in the deep-sea sediments beyond the continental shelf show the periodic oscillation in oxygen isotopes, indicating that the coming and going of modern-style ice ages had begun. These micro-fossils from three million years ago also revealed a surprise-that the glaciations of the Pliocene were dominated by the 41,000-year cycle in the tilt of Earth's axis, unlike the three most recent glaciations paced by the 100,000-year cycle in the orbital ellipticity. The more rapidly oscillating climate of the Pliocene may have been the stimulus for the evolutionary change from the earlier Australopithecines to the modern genus h.o.m.o h.o.m.o. In the mid-Pliocene there was no ice in the Northern Hemisphere, and reduced ice on Antarctica. The greatly elevated mid-Pliocene sea level and the much-diminished ice indicate how much the ice can change and the seas can rise, within the constraints of the modern climate system.

The overarching lesson of the Pliocene is sobering: an ice-free Northern Hemisphere, with no sea ice covering the Arctic Ocean and no ice sheet on Greenland, is a possible condition of the modern climate system. When this happened in the Pliocene the global average temperature was only about four to six Fahrenheit degrees warmer than today. And that may be where we are headed again. If at times in the past ice ruled the world, then in the warm centuries of the future, seawater-ice's playmate on the global hydrological seesaw-will be the formidable adversary of human life on Earth.

CHAPTER 8.

CHOICES AMID CHANGE.

If you do not change direction, you may end up where you are heading.

-LAO TZU Chinese philosopher, sixth century BC

Where indeed are we headed? The ark of humanity seems dangerously adrift in the sea of climate change, with no apparent navigational charts, or even a captain, on board. Can we prevent a world without ice? Can we avoid flooded coastlines? Are there pathways to the future that are less calamitous? These are straightforward questions, but ones that do not yield simple answers.

CONFRONTING UNAVOIDABLE CHANGE.

Unfortunately, there is no course of action that will freeze today's status quo and forestall any further changes. Change is under way and is certain to continue because of inertia in both the climate system and the global industrial economy; it is impossible simply to pull the plug and stop these systems in their tracks. They each have momentum a.n.a.logous to that of an aircraft carrier trying to change course-for several seconds after the helmsman turns the ship's rudder to a new heading, the vessel plows ahead on its old course before slowly beginning to turn. In the global climate system, some of this inertia derives from the greenhouse gases we have already emitted into the atmosphere in times past, but with effects that extend far into the future. Carbon dioxide, the princ.i.p.al greenhouse gas, lingers in the atmosphere for more than a century, as it slowly dissolves into the ocean and is gradually consumed by green plants. These greenhouse gases will continue to warm the atmosphere and oceans even if new emissions could somehow be eliminated.

We have no illusions, however, that eliminating greenhouse gas emissions is within easy reach. Climate scientists have developed projections of how the climate might evolve if greenhouse gas concentrations in the atmosphere could be frozen at their current level, to see what changes might accompany a stabilized greenhouse. In climate science and policy circles, this idealized conceptualization of the future is called a "climate commitment."109 Even after stabilizing the greenhouse, the inertia of the climate system will continue to drive climate change for several centuries into the future. Even after stabilizing the greenhouse, the inertia of the climate system will continue to drive climate change for several centuries into the future.

So what is it that we have already and unavoidably committed ourselves to? Were we able to immediately stabilize the atmospheric greenhouse at current concentration levels, Earth's atmosphere would still warm by about one Fahrenheit degree by the end of this century. This warming would be followed by continued loss of Arctic sea ice, shrinking of the ice caps in Greenland and West Antarctica, ocean waters warming to greater depths, changes in the geography and intensity of storms and drought, and sea level rising at a rate almost twice that experienced in the twentieth century. Because this climate commitment is the outcome of an unachievable a.s.sumption-an immediate and complete stabilization of greenhouse gases in the atmosphere-we must recognize that the projected outcomes const.i.tute an underestimate of the changes that will actually take place. In other words, the consequences that will actually unfold in this century, while still veiled in some uncertainty, will exceed those just mentioned. It is imperative, therefore, to begin planning for changes that are unavoidable, an endeavor broadly termed adaptation.

STRATEGIES OF ADAPTATION.

Adapting successfully to a changing climate will require fundamental and sweeping rea.s.sessments. We need to ask how a changing climate will affect everything we do, wherever we do it. For example, in agriculture the questions might include: What problems and opportunities will a longer growing season present? What will be the impact of warmer soil on seed germination? What problems will changes in water availability and timing create? Will different crops be better suited to the future climate than the current crops? Will different pests and weeds replace those present today? Will there be a need for more or fewer, or different, fertilizers?

Already researchers are looking to wild relatives of some domestic foodstuffs in search of the genetic makeup that has given these wild plants natural resiliency to drought, heat, and changes in the salinity of water.110 Other research has focused on building resiliency to extended flooding, with some strains of rice now able to survive up to two weeks underwater, compared to only a three-day survival of earlier varieties. And of course careful water management is an essential in any setting-researchers and farmers are always studying and testing new methods of irrigation. Other research has focused on building resiliency to extended flooding, with some strains of rice now able to survive up to two weeks underwater, compared to only a three-day survival of earlier varieties. And of course careful water management is an essential in any setting-researchers and farmers are always studying and testing new methods of irrigation.

In the public health arena, pract.i.tioners will need to think about how to cope with increased frequencies of extremely hot days and nights in crowded cities, how to safeguard munic.i.p.al water supplies against bacterial blooms, how to modify sewage systems to cope with more extreme rainfall events, and how to prevent more frequent food contamination in a warmer world.

The list has no end. The transportation infrastructure of the Arctic region, long dependent on hard frozen ground or thick ice, must adapt to a softer, mushier foundation for much or all of the year. Emergency preparedness agencies will need to reshape their responses to more frequent floods, hurricanes, and wildfires, and make plans for refugees from rising seas. Electric utilities will face higher peak demands during the more intense and more frequent heat waves, while at the same time adjusting to different sources of electricity generation. City managers, urban planners, and architects will need to rethink what is required to build or reconstruct climate-resilient and energy-efficient cities, and governments of oceanfront munic.i.p.alities will face zoning and building code issues along beaches and barrier islands, and infrastructure changes to their harbors. The private sector will find opportunities to provide new materials, products, and technologies.

The marine fishing industry will have to antic.i.p.ate where the fish will be found as the temperature structure of the oceans changes. Freighters on the Great Lakes, in response to lower lake levels resulting from increased evaporation, will have to lighten their loads to access shallower harbors, and modify navigation to avoid hazards that once lay safely below the surface. The insurance industry is already facing a revision of its risk and rate tables as floods become more frequent, wildfires more widespread, hurricanes more intense, the storm season longer, and coastal areas more vulnerable to storm surges as sea level rises. Some insurance carriers have already pulled out of the home insurance market in Florida because climate-related threats-and the consequent cost of claims-are too great. And educational inst.i.tutions at all levels will bear new responsibilities to prepare students for the demands of a changing world.

The unavoidable future also includes issues that humans have never had to think about in the past. Near the top of the list is open access to the Arctic Ocean. There is a very real possibility that in only a few decades the Arctic Ocean will be free of ice in the summertime, giving people unimpeded access to this vast region for the first time in human history. In 2007, the extent of summer sea ice diminished to the lowest ever recorded since comprehensive synoptic data have been available. In 2008, for the first time in at least a half century and probably much longer, a ring of open water encircled the Arctic-both the Northwest and Northeast pa.s.sages were open simultaneously.

This physical opening of the Arctic Ocean leads to an opening of important geopolitical issues as well-the claiming of territory, the exploration for mineral and energy resources, and the exploitation of biological resources-issues that were more or less moot when the Arctic was inaccessible. The nations bordering the Arctic already have begun jockeying for position. In 2007, a Russian submersible planted the national flag in the ocean floor at the North Pole, an action reminiscent of when, a half century earlier, the USS Nautilus Nautilus surfaced at the North Pole and the United States opened a scientific research station at the bitterly cold and windswept South Pole. While largely symbolic, occupancy of the pole with its central 360-degree range of vision is a geopolitical statement of control. surfaced at the North Pole and the United States opened a scientific research station at the bitterly cold and windswept South Pole. While largely symbolic, occupancy of the pole with its central 360-degree range of vision is a geopolitical statement of control.

In 2008, the U.S. Geological Survey released a study of oil and gas potential in the Arctic Ocean. This study indicates possible Arctic oil reserves equal to three years of current global consumption, and perhaps a decade of natural gas reserves. The latter amount equals the vast land-based natural gas reserves in the Russian Arctic. Fortunately, from the point of view of potential conflicts, the USGS said that most of the reserves are located in areas on the continental shelf where national sovereignty is well established, princ.i.p.ally in offsh.o.r.e Alaska and offsh.o.r.e Russia. The biggest exception is the Lomonosov Ridge, a relatively high submarine topographic feature that extends from Asia toward Arctic Canada and Greenland, bisecting the Arctic Ocean. Russia, Greenland, and Canada all have a.s.serted that it is part of their continental shelf, and are seeking exclusive mineral rights under the provisions of the United Nations Convention on the Law of the Sea.

It is not just energy resources that can generate tension in the Arctic. At a time when the historical fisheries of the world have been severely depleted, the biological resources of the Arctic are becoming increasingly attractive and accessible. The countries and peoples bordering the Arctic Ocean have a long-standing dependence on food from the sea, and will welcome access to new resources. But so will many others; j.a.pan, Korea, and China, nations with large fishing fleets, are sure to cast their eyes on-and their nets in-the Arctic Ocean. The European Union has already had discussions about how to oversee the development and exploitation of the Arctic, as well as to provide protection of the environment and the indigenous people.111 NATO representatives meeting in Iceland in 2009 discussed security challenges that might develop as the Arctic opens, and Canada has already revealed plans to build a deepwater port and a military training center in the high Arctic. Both the Pentagon and the United States polar research community have called for building a much larger icebreaker fleet to enable greater access to and better control of U.S. polar waters. NATO representatives meeting in Iceland in 2009 discussed security challenges that might develop as the Arctic opens, and Canada has already revealed plans to build a deepwater port and a military training center in the high Arctic. Both the Pentagon and the United States polar research community have called for building a much larger icebreaker fleet to enable greater access to and better control of U.S. polar waters.112 Not to be outflanked, Russia, too, has announced plans to deploy military forces to protect its national interests in the Arctic. Not to be outflanked, Russia, too, has announced plans to deploy military forces to protect its national interests in the Arctic.

To many people adaptation means planning for the long-term future, but there are some places where adaptation is already a necessity. About halfway south along the western coast of Greenland is the small town of Ilulissat, home to some five thousand residents and an equal number of sled dogs. Ilulissat, known also by its Danish name of Jakobshavn, is the third largest town on Greenland. Tom Henry, a reporter for the Toledo Blade Toledo Blade, wanted to help his Ohio readers appreciate the consequences of climate change in the Arctic, to make sure they knew that this was as much a story about people as it was about polar bears. In 2008, Henry persuaded his publisher to send him to Ilulissat to get a look at a place where both climate change and adaptation are current realities. In the Inuit language, Ilulissat Ilulissat means "icebergs," appropriately so because the town sits near the mouth of the fjord that hosts the Jakobshavn Glacier-Greenland's most prolific ice stream. This glacier alone accounts for more than 6 percent of the ice loss from Greenland's interior ice cap, an amount that has doubled in only the past decade. means "icebergs," appropriately so because the town sits near the mouth of the fjord that hosts the Jakobshavn Glacier-Greenland's most prolific ice stream. This glacier alone accounts for more than 6 percent of the ice loss from Greenland's interior ice cap, an amount that has doubled in only the past decade.

Once in Ilulissat, in many conversations around town, Henry learned of both changes and adaptations. Most Inuits in the surrounding areas travel by dogsled over the flat surface sea ice, because much of the rugged inland topography makes for difficult, if not impa.s.sable, sledding. Unfortunately, the diminishing sea ice has effectively isolated residents of outlying settlements for much of the year. Fishermen docked in Ilulissat are finding the halibut more elusive and the catch more expensive, as the warming ocean water has driven the fish to greater, and cooler, depths. But the tourist industry is booming as visitors come to see ma.s.sive icebergs break from the glaciers, and to watch whales that now cruise the area, consuming large quant.i.ties of fish species unknown around Ilulissat prior to the warming of the water.

NAVIGATING AN UNCERTAIN FUTURE.

Given that it will be effectively impossible to hold greenhouse gases in the atmosphere at current levels, what can we expect from more realistic scenarios? The answer to this question, like many other attempts to illuminate the future, are burdened by many uncertainties-uncertainties in our knowledge of how the climate system works in all its complexity, in our ability to transcribe what we do know about climate into detailed computer models, and in how we humans, major players in today's changing climate, will respond to the challenge it presents.

The nonscientific public often has been impressed with the successes of science: the highly precise and accurate predictions of solar and lunar eclipses centuries into the future, of the comings and goings of Halley's comet, and of the transits of Venus across the face of the Sun. Science has enabled the realization of ballistic missile intercepts far out in s.p.a.ce, the launch and management of satellites that have revolutionized mobile communications, and the pinpoint navigation of the Phoenix Phoenix Lander to Mars. Given these achievements, it's not surprising that many people have high confidence that scientists will lead us smoothly and accurately into the future, with few surprises. Lander to Mars. Given these achievements, it's not surprising that many people have high confidence that scientists will lead us smoothly and accurately into the future, with few surprises.

But in reality it is not an easy task to forecast the future, particularly the future of a system as complex as Earth's climate. Such a large natural system is generally far more complicated than the relatively simple physics governing the orbits of celestial bodies and the trajectories of s.p.a.cecraft. There is also a big difference between predicting the future of inanimate systems such as planetary orbits and that of a system in which humans play a very important role. When human behavior is part of the equation, the uncertainty of the outcome escalates substantially. So, when the IPCC scientists declare that global average temperature at the end of the twenty-first century will likely exceed the temperature at the beginning of the century by 3.2 to 7.2 Fahrenheit degrees, the range expresses the uncertainty not only about the climate science, but also about how the people and governments of the world will address the challenges of a warming climate.

The latter uncertainty, a.s.sociated with the human responses to climate change, is sometimes called behavioral or social uncertai