This extraordinary terrain arrayed across Washington is called the Channeled Scablands. The scale of the features in the landscape and of the processes that formed them-like continent-wide ice sheets themselves-is beyond anything in the modern human experience. These landforms stand there today-the channels carry little water, the basalt boulders tumble no more, and the falls are dry. They offer mute testimony to a time when glacial melt.w.a.ter roared across this lava plateau in a ma.s.sive flash flood.

Archeological evidence now doc.u.ments a human presence in Oregon some fourteen thousand years ago, perhaps the earliest humans ever in western North America. Imagine the reaction of these early beings to the periodic walls of water that churned through the valleys and spread catastrophically across the landscape. The estimates of the recurrence interval of the floods are around fifty to sixty years, well within the lifetimes of early residents of the region. These floods would be the stuff of legend and oral history, pa.s.sed on through many generations, similar to the historical memories of major tsunamis that have affected coastal populations in earthquake-p.r.o.ne regions.

When J. Harlan Bretz, a geologist at the University of Chicago, first offered the giant flood hypothesis in the early 1920s, he became an object of derision. What he had proposed was an example of catastrophism in the geologic record, a concept that gave special emphasis to the role of infrequent and improbable events in the shaping of Earth's surface. The conventional geological wisdom of the time favored the concept of unifor mitarianism, whereby the landscape could be interpreted in terms of observed and reasonably well understood processes acting slowly over long periods of time. But Bretz persisted in offering more and more field evidence to support the flood concept, and eventually his perspective was accepted.

THE THICK PILE of ice sitting on the surface of North America and Eurasia had another remarkable effect-it formed such a ma.s.sive load that the crust of Earth actually sagged beneath it. In central Canada the depression was sufficiently deep that when the ice melted and sea level rose, ocean water filled the depression, producing the large marine embayment now called Hudson Bay. Today, with the load of the ice gone, the crust is slowly rebounding upward, and spilling the seawater in Hudson Bay back into the deeper ocean basins. Just as Lake Bonneville became smaller because of evaporation, Hudson Bay is also shrinking as more and more of the continental crust reemerges above sea level.

In Europe, the center of the ice pile was in the Gulf of Bothnia, between Sweden and Finland. As in North America, Earth's crust was depressed beneath the load of ice, and following the melting of the ice, the rock surface is slowly emerging from beneath the sea. These long-term geological processes are slowly adding territory to the maps of Canada and Fennoscandia (the geological name for the Scandinavian peninsula), a peaceful restoration of land following the invasion and occupation by the glacial ice. But whether the restoration will continue is not so clear. Post-glacial rebound will continue to uplift the still-depressed crust, but the uplift is now meeting compet.i.tion as anthropogenic climate change diminishes polar ice and raises sea level. If sea level rises faster than the crust does, the sea may once again conquer the land.

ICE AGES LEAVE THEIR MARK IN THE SEA.

In the ocean, the signature of an ice age can be found in the chemistry of seash.e.l.ls that form layers on the ocean floor. As water evaporates from the ocean and falls as snow on the growing continental ice sheets, the chemicals dissolved in the remaining seawater become more concentrated. Since small marine creatures use the ocean's chemicals to grow their sh.e.l.ls, the composition of their sh.e.l.ls during an ice age reflects the more concentrated chemistry of the oceans in which they grew. When these creatures died and fell to the ocean floor, the sh.e.l.ls acc.u.mulated as a layer of history representing a time when higher chemical concentrations indicated less water in the oceans. These stratified marine cemeteries reveal that no fewer than twenty ice ages have rhythmically paraded over the continents in the past three million years, each time borrowing water from the oceans, which forced the remaining seawater to carry a heavier chemical burden.

The particular signature of water withdrawal is in the relative abundance of one of the uncommon isotopes of oxygen: 18 18O. This isotope of oxygen has two extra neutrons in its nucleus, making it about 12 percent heavier than the very common isotope 16 16O. That extra weight makes it harder to vaporize (evaporate) water containing 18 18O as compared to 16 16O, with the result that as water leaves the ocean on loan to continental ice, 18 18O becomes more concentrated in the remaining seawater, leading to an increase in the ratio of 18 18O/16O. Consequently, the marine creatures that use the chemicals in the ocean for sh.e.l.l construction will record this higher concentration of 18 18O in their sh.e.l.ls, which in turn is the signal of less ocean water and, by implication, that ice volumes are higher. Very clever indeed.

Under the shallow waters that now cover the continental shelves, other topographic features also indirectly point to lower sea levels. Along the eastern coast of the United States, off the present-day mouths of the Hudson River and of the Susquehanna River emptying through Chesapeake Bay are deep valleys cutting a few hundred miles across the continental shelf into the Atlantic Ocean Basin. When sea level was lower during the last ice age, the continental shelves were exposed, and these rivers had farther to go to reach the sea. As the ice began to melt, the rivers carried much more water and had greater erosive power than they do today. This enhanced river discharge and strong erosion across the exposed continental shelf cut deep valleys, which can be imaged with marine geophysical methods, the same tools that are used to explore for oil in the rocks of the continental shelf. Today these valleys, now completely submerged beneath the ocean surface, are called the Hudson Canyon and the Baltimore Canyon.

At the edge of the continental shelf, the submarine topography drops steeply into the true ocean basin. Most of the water in the oceans of course resides in these low-lying areas. The fact that today there is also ocean water atop the continental shelves is a statement that there is more water on Earth than the ocean basins can accommodate, so some of it laps onto the margins of the higher-standing continents. In geological terms, oceans are defined not by where marine waters are found, but rather by the deep basins that surround the elevated continents. Indeed, if all the ice now present on Earth were to melt, sea level would rise onto the continents another 250 feet above today's level.

At the maximum of the last ice age, some twenty thousand years ago, sea level was lower than today by some six hundred feet, and the sh.o.r.eline of eastern North America was at the edge of the continental shelf, in places several hundred miles away from today's coastline. Icebergs discharged into the Atlantic would float along that margin, and sometimes their deep keels would drag along the bottom, carving gouges into the rock and sediment. These long scour marks can be seen on the ocean floor today off the coast of South Carolina.26 WHEN MELt.w.a.tER REACHES THE SEA.

In financial markets, transfers of capital between one market segment and another can be accommodated without upsetting the market unduly, provided the transfers are in small enough parcels spread out over reasonable periods of time. An "orderly" market follows such a pattern. But if huge blocks of stock in one sector are dumped onto the market all at once, turmoil can overwhelm the marketplace.

This financial a.n.a.logy is apt for thinking about transfers between the ice and ocean reservoirs in Earth's hydrological system. As glacial ice melts in response to a slowly changing climate, melt.w.a.ter forms streams that merge into rivers, and the rivers eventually reach the sea. And the freshwater of the rivers is gradually mixed into the salt.w.a.ter of the oceans via the action of wind and ocean currents. The great hydrological accounting book will show the balance in the ice account slowly going down and the balance in the ocean account creeping up. In essence, there is an "orderly" shift of hydrological capital from one reservoir to another. On occasion, however, there can be abrupt shifts in the movement of hydrological capital that cause chaos in the exchange. One such moment of turmoil occurred during the melting of the last North American continental ice sheet.

Just prior to the abrupt shift, the melt.w.a.ter along the southern margin of the North American ice sheet was drained by the Missouri and Ohio rivers and delivered to the Mississippi River for final pa.s.sage to the Gulf of Mexico. There it mixed with warmer and saltier water, eventually making its way into the open Atlantic Ocean, where it was stirred into the general circulation pattern of the Atlantic current system. But by about 12,800 years ago, the front of the North American ice sheet had melted back to a point where the melt.w.a.ter found a new and shorter pathway to the sea-the topographic lowlands that would suddenly become the St. Lawrence River Valley. Large volumes of cold melt.w.a.ter no longer went the Mississippi way, but instead coursed northeastward, past the Gaspe Peninsula and into the Atlantic Ocean. There it interjected itself into the slow northward drift of the Gulf Stream, inserting a cold, fresh barrier into this warm surface current, and interrupting the delivery of heat to the far North Atlantic. A big chill fell over the region, and people living farther to the north must have wondered what happened for the heat to be turned off so abruptly.

This cold period, which chilled Iceland, Greenland, and Western Europe for more than a millennium, has been called the Younger Dryas, because in the regions affected by the sudden chill, a cold-climate flower, the Dryas, was reestablished. It had grown previously in the regions a few thousand years earlier, when these lat.i.tudes were emerging from the last glacial maximum. The end of this thousand-year cold wave came when the melt.w.a.ter was reduced to a volume inadequate to maintain a thermal barrier to the Gulf Stream, which then resumed its delivery of heat to the far north, accompanied by a substantial warming of Iceland and Western Europe.27 WHAT CAUSES AN ICE AGE?.

Why do ice ages come and go? What factors lead to a periodic acc.u.mulation of thick ice on the continents? These questions are more difficult to answer than simply a.s.sembling the evidence that shows that ice ages have occurred. Because continental glaciations are relatively rare occurrences in the geological record, we know that the conditions producing ice ages do not occur very often. The previous occurrence of widespread glaciation (prior to the multiple ice advances of the past three million years) was in Gondwa.n.a.land during the late Paleozoic, about 275 to 300 million years ago.

Some generalities, however, can be garnered from the geologic record. One obvious factor that plays an important role in glaciations is the location of landma.s.ses on the globe. Land that is situated at high lat.i.tudes, where it is colder, is a more probable setting for snow and ice acc.u.mulation. Today Antarctica lies entirely within the Antarctic Circle, and much of Greenland lies north of the Arctic Circle. Conversely, where there is only open ocean at high lat.i.tudes, as with the present-day Arctic Ocean that surrounds the North Pole, there will be no thick acc.u.mulations of ice on the sea surface. Sea ice, of course, does form over the Arctic Ocean, but it is a thin and fleeting cover, with marked seasonal variations in extent, and a lifetime measured in years or perhaps decades. By contrast, acc.u.mulated land ice can survive hundreds of thousands of years. The oldest ice in Antarctica is about eight hundred thousand years old,28 and in Greenland, a little more than one hundred thousand years old. and in Greenland, a little more than one hundred thousand years old.29 Other factors, such as changes in the patterns of atmospheric and oceanic circulation, play a role in making Earth susceptible to ice acc.u.mulation in the high lat.i.tudes. One such event (discussed in chapter 1) was the climatologic isolation of Antarctica brought about by the opening of the Drake Pa.s.sage. Other factors, such as changes in the patterns of atmospheric and oceanic circulation, play a role in making Earth susceptible to ice acc.u.mulation in the high lat.i.tudes. One such event (discussed in chapter 1) was the climatologic isolation of Antarctica brought about by the opening of the Drake Pa.s.sage.

During the last three million years, conditions developed for "a perfect storm" that has led to multiple oscillations of ice over the planetary surface, with the most recent ice sheets retreating only ten to twenty thousand years ago. What happened three million years ago that set the stage for recurring acc.u.mulations and dissipations of ice? One important change was the tectonic uplifting of the Isthmus of Panama, to link North and South America. But more important than the connecting of two continents was the disconnecting of two oceans, the Atlantic and Pacific. Prior to the formation of Panama, the Atlantic and Pacific exchanged water via east-west currents that flowed through the gap between North and South America. But when Panama blocked that exchange, the Atlantic surface currents became predominantly south to north, and carried warm water into the Arctic, water that altered the atmospheric circulation and precipitation patterns in the far north. More snow began to fall, and not all of it melted in the subsequent summers. The modern cycle of ice ages had begun.

A very elementary question about the causes of ice ages focuses simply on the issue of acc.u.mulation: What conditions lead to winter snowfall greater than summer melt-off? When year after year more snow falls than melts, there is a growing acc.u.mulation-that is the scenario laid out at the opening of this chapter. It is a.n.a.logous to your savings account-when year after year you spend less than you earn, there is an acc.u.mulation in the piggy bank. The conditions that favor an acc.u.mulation of snow are related to changes in the seasons on Earth, particularly at high lat.i.tudes, where it is already cold enough to make summertime melting a short-lived phenomenon. The crucial arena is near the Arctic Circle, in the Northern Hemisphere, which pa.s.ses through Alaska just north of Fairbanks, across northern Canada and the southern tip of Greenland, near Iceland, through Scandinavia, across northern Russia, all the way to the Bering Strait. Anything that can slightly alter the compet.i.tion between winter and summer, that leads to shorter, colder summers at that lat.i.tude, may trigger the onset of an ice age.

So what causes the seasons on Earth? Why do we have winter, spring, summer, autumn, and then another winter? There are two princ.i.p.al and unequal effects that lead to seasonality. The greater effect is called the tilt season, and the lesser, the distance season. The tilt season is a.s.sociated with Earth's rotational axis, that imaginary line running through the planet from pole to pole, around which Earth spins daily (and 365 times plus a fraction in its yearly journey around the Sun). The rotational axis is not, however, perfectly upright relative to the plane of Earth's...o...b..t about the Sun-it is tilted a little more than twenty-three degrees away from upright. That means that in the course of the year, first one hemisphere and then the other will get slightly more sunshine, because it is tilted toward the Sun. In the annual journey around the Sun, when a hemisphere is tilted toward the Sun, it experiences summer, and when it is tilted away from the Sun, it experiences winter.

The second and lesser cause of seasons relates to the fact that Earth's...o...b..t about the Sun is not a perfect circle, but rather an ellipse, a slightly squashed circle with a long dimension and a short one. And the Sun does not sit exactly in the middle, but rather a bit off-center, closer to one end of the long dimension than to the other. This means that as Earth follows the elliptical path around the Sun, its distance from the Sun is always changing. As Earth comes closer to the Sun, it gets a little extra sunshine, and as it moves away from the Sun, it gets a little less. This variation in solar heating during every trip around the Sun also contributes to warmer and colder periods during the year-giving rise to the distance seasons.

The tilt season and the distance season combine to produce the actual seasonal variation in sunshine, but the sum is different in each hemisphere. When the Northern Hemisphere is tilted directly toward the Sun on June 21, Earth is at the most distant part of its...o...b..t. So, while tilting gives the North some extra sunshine, the distance effect gives it a little less, thus diminishing the heating from the tilt. Six months later, on December 21, when the Southern Hemisphere is tilted directly toward the Sun, Earth is making its closest approach to the Sun, so the tilt and distance reinforce each other. This produces asymmetry in seasonality between the hemispheres-the Southern Hemisphere has stronger seasonal variation than does the Northern Hemisphere.

That is the basic picture of how a typical year's sunshine gets spread over the hemispheres to produce seasons, but it only shows us how seasons are established-it does not tell us what changes to this picture of seasonality would allow acc.u.mulations of snow at high lat.i.tude year after year, and initiate an ice age. That story is more complicated.

There are very small periodic variations in the amount of seasonal sunshine that the hemispheres receive through the tilt and distance effects, because the tilt of Earth's rotational axis and the elliptical shape of Earth's...o...b..t about the Sun are themselves changing, albeit very slowly. These slight changes are imposed by the gravity fields of the other planets in the solar system, but the princ.i.p.al effects come from Jupiter, the largest and most ma.s.sive planet in the solar system, some 318 times more ma.s.sive than Earth.

There are three perturbations to Earth's seasonal story that result from these planetary gravitational tugs, one that affects the distance season, a second that affects the tilt season, and a third that determines the geography of where the interactions of the other two add to or subtract from each other. These slowly changing effects are called the Milankovitch cycles, in honor of Milutin Milankovitch, a Serbian geophysicist who early in the twentieth century pointed out their importance for climate change. In the longest of the Milankovitch cycles, the shape of the ellipse becomes slightly more elongated, then slightly more circular, and then back again, while at the same time the ellipse is slowly rotating around the Sun to trace out an orbit that over a long period looks like petals of a flower. A full oscillation in the elongation takes about 100,000 years, and changes the distance contribution to the seasons.

In a second cycle the tilt of Earth's rotation axis, currently about 23.4 degrees, oscillates between 22.1 and 24.5 degrees every 41,000 years. At greater tilts, the seasons become more extreme, and at lesser tilts, more uniform. A third cycle arises from the precession of the rotational axis-a familiar effect seen in the slow wobble of a spinning top's axis of rotation. This changes the orientation of the rotation axis, and determines the hemisphere in which the distance and tilt season reinforce each other, as in the Southern Hemisphere today. The precession of Earth's rotational axis makes a complete cycle with respect to the seasons in about 23,000 years-so, in half that time, the reinforcement will occur in the Northern Hemisphere, before shifting back south of the equator to complete the cycle. The maximum amplification of the seasonal contrast takes place when the orbit is most elongate and the tilt is at its maximum. The precession determines which hemisphere will receive the maximum reinforcement of the distance and tilt effects.

The effects at these three periods-100,000; 41,000; and 23,000-combine at any given time to produce a composite increment or decrement to the sunshine received at a given place on Earth. The composite is like listening to sound from an electronic synthesizer, which uses only three tones with different volume settings. The combination is usually some gentle cacophony, but from time to time there is some harmony between two of the tones, and on occasion with even one tone dominating, coming through loud and clear. The right combination of these Milankovitch factors sets the stage for snow acc.u.mulation at high lat.i.tudes and the beginning of an ice age.

Two lines of evidence suggest that for the last several hundred thousand years it has been the 100,000-year oscillation in the distance effect that is dominating the variations in sunshine and seasonality at high lat.i.tudes on Earth. One line is found in the sea, in the layers of marine fossils that indirectly provide the history of ocean water volume by way of the 18 18O/16O ratio. These sedimentary layers show four recent low-water stands separated by an average of just over 100,000 years. The second line of evidence comes from deep ice cores from the Russian Vostok borehole through the East Antarctic ice sheet. These cores show the same 100,000-year periodicity in the polar air temperature, with the lowest temperatures occurring at the same time that the oceans show their lowest sea level. It can be no coincidence that the coldest temperatures occur at the same times that the oceans display minimum water volume-the ocean water has been transferred to the land and frozen into an ice blanket, which is sitting on the cold continents. Earlier glacial cycles revealed in the Antarctic ice show the 41,000-year periodicity. This indicates that the oscillations in the tilt were then the dominant factor in the three-note tone poem composed by the bobbing and weaving of Earth in its long-term relationship with the Sun and other planets.

While it is clear that the Milankovitch climate cycles are the pacemakers of the ice ages, other factors come into play as ice acc.u.mulates. The albedo, or reflectivity, of Earth begins to increase, and vegetation is overridden by the spreading ice, thus altering both the polar radiation budget and carbon cycle. These changes amplify the polar chill initiated by the Milankovitch influences on seasonality.

HUMANS ON THE MOVE.

The closure of the Isthmus of Panama three million years ago brought changes to more places than just the poles. The adjustments in the circulation of the oceans and the linked effects on the atmosphere led to redistribution of global precipitation. Africa, the cradle of human development and evolution, was no exception. An aridity set in that led to a cooling and drying of the continent, particularly north of the equator. Forests gave way to gra.s.slands, and the Australopithecines, the progenitors of the modern human genus h.o.m.o h.o.m.o, struggled to adapt. Evolutionary pressure favored new tools and skills. The emergence of stone tools marked the beginning of the so called Paleolithic period of human evolution-and the makers and users of these tools were early representatives of the genus h.o.m.o h.o.m.o. These early humans spread widely through Africa between 2.5 and 1.5 million years ago, and later into Europe and southwest Asia. There they learned-or, to their detriment, did not learn-to cope with a slowly oscillating climate, and with ice sheets that periodically moved across northern Europe and Asia.

The archeological site at Atapuerca, in northern Spain near the city of Burgos, is known as the home of the first Europeans,30 with evidence of occupancy dating back to 1.2 million years before the present. Dr. Josep Pares is a member of the scientific team at the National Research Center on Human Evolution in Burgos (and my colleague at the University of Michigan). These anthropologists and geologists have for more than a decade been engaged in reconstructing the human history at this site. Pares's role has been to create a time frame for the human presence at Atapuerca, using a wide array of geological and geophysical techniques to determine the age of the occupancy. In 2007 he took me to the dig and guided me into a cool dark cave and through layer after layer of sediment containing the bones and fossils and tools that tell of this early distant outpost of humanity, on its journey out of Africa and to the far corners of the world. with evidence of occupancy dating back to 1.2 million years before the present. Dr. Josep Pares is a member of the scientific team at the National Research Center on Human Evolution in Burgos (and my colleague at the University of Michigan). These anthropologists and geologists have for more than a decade been engaged in reconstructing the human history at this site. Pares's role has been to create a time frame for the human presence at Atapuerca, using a wide array of geological and geophysical techniques to determine the age of the occupancy. In 2007 he took me to the dig and guided me into a cool dark cave and through layer after layer of sediment containing the bones and fossils and tools that tell of this early distant outpost of humanity, on its journey out of Africa and to the far corners of the world.

For the most part, this journey was on foot. It is not easy to walk around the world, but as the ancient proverb says, "A journey of a thousand miles begins with a single step." And as various early representatives of our genus h.o.m.o h.o.m.o left Africa, little by little they expanded into new regions of Europe and Asia, always in search of abundant food. In the last half million years the Heidelberg species of left Africa, little by little they expanded into new regions of Europe and Asia, always in search of abundant food. In the last half million years the Heidelberg species of h.o.m.o h.o.m.o, and later the Nean derthals, became established in Europe, while at the same time, back in Africa, another human species, h.o.m.o sapiens, h.o.m.o sapiens, was in ascendancy. Midway through the last ice age, some seventy thousand years ago, was in ascendancy. Midway through the last ice age, some seventy thousand years ago, h.o.m.o sapiens h.o.m.o sapiens felt the winds of climate change in Africa-a drying out that pushed food gathering to an untenable situation and thinned human populations almost to extinction. felt the winds of climate change in Africa-a drying out that pushed food gathering to an untenable situation and thinned human populations almost to extinction.31 Thus began the final great human migration to all the habitable continents of the globe. Thus began the final great human migration to all the habitable continents of the globe. h.o.m.o sapiens h.o.m.o sapiens-our immediate genetic ancestors-were on the move to distant places. Their pathways around the world were opened, ironically, by the glaciation of the last ice age, a large-scale conversion of ocean water into continental ice.

What happens between water and ice that opens pathways for migration? Water and ice are two faces of the same material: H2O. Earth's total endowment of H2O is more or less stable, but during an ice age the proportions of the H2O in ice and in water change. As noted in the earlier discussion of the isotopic chemistry of the oceans, large expanses of ice sitting on continents during an ice age represent large withdrawals of water from the oceans. One expression of the withdrawal is the lowering of sea level. This adjustment in the water budget of Earth leads to exposure of the continental shelves, and provides a dry land human migration path that in more temperate times would be covered by shallow seas. The broad, shallow platform between Southeast Asia and the many islands of Indonesia, Papua New Guinea, and Australia provided a partial walkway of migration that led to human occupancy of Australia by fifty thousand years ago.

Another migration pathway of early h.o.m.o sapiens h.o.m.o sapiens led eastward across Asia to the Kamchatka Peninsula and the Chukotka region of the Russian Far East, some thirty thousand years ago. The maximum of the most recent ice age was still ten thousand years in the future, and sea level would drop still farther. The peopling of the Americas was a direct consequence of exposing the seafloor across the Bering Strait and along the Aleutian Islands arc. The Bering route, some thousand miles across at its widest, brought people to the Arctic regions of Alaska, Canada, and into the mid-continent of North America by fourteen thousand years ago. led eastward across Asia to the Kamchatka Peninsula and the Chukotka region of the Russian Far East, some thirty thousand years ago. The maximum of the most recent ice age was still ten thousand years in the future, and sea level would drop still farther. The peopling of the Americas was a direct consequence of exposing the seafloor across the Bering Strait and along the Aleutian Islands arc. The Bering route, some thousand miles across at its widest, brought people to the Arctic regions of Alaska, Canada, and into the mid-continent of North America by fourteen thousand years ago.

The flow of people along the Aleutian route went to southern Alaska and southward along the western coast of North America. Evidence of humans in coastal North and South America32,33 also dates from around fourteen thousand years ago. It's a long walk from Alaska to Patagonia, around nine thousand miles. But over a thousand years, it really amounts to a slow diffusion into new territory, some ten miles or so each year. There would be ample time for these folks to pick the berries and smell the roses, and marvel at the new world they were occupying for the first time. With the spread of humans into the Americas, the dispersal of also dates from around fourteen thousand years ago. It's a long walk from Alaska to Patagonia, around nine thousand miles. But over a thousand years, it really amounts to a slow diffusion into new territory, some ten miles or so each year. There would be ample time for these folks to pick the berries and smell the roses, and marvel at the new world they were occupying for the first time. With the spread of humans into the Americas, the dispersal of h.o.m.o sapiens h.o.m.o sapiens into all the habitable continents had reached completion. into all the habitable continents had reached completion. 34 34 And just in time, for the great melt-off of the glacial ice sheets had begun, and the land bridges from Asia created by lowered sea level were disappearing fast. The newcomers to the Americas were on their own. And just in time, for the great melt-off of the glacial ice sheets had begun, and the land bridges from Asia created by lowered sea level were disappearing fast. The newcomers to the Americas were on their own.

The last ice age was nearing an end, and the ice front continued to retreat northward. Yes, there were interruptions to the fast warming of the climate: the sudden drainage of the large transient melt.w.a.ter lakes-the Younger Dryas event that cooled the Northern Hemisphere for a millennium between 11,500 and 12,500 years ago, and another later, shorter drop in temperature 8,200 years ago, when Lake Aga.s.siz emptied much of its water quickly into Hudson Bay and on into the Atlantic. But these were the last gasps of the ice age-temperatures around the globe reached levels similar to those of today, and remained more or less at that thermal plateau for the next 8,000 years. It was a remarkable span of climatic stability that enabled humans to flourish and multiply.35 Sedentary agriculture largely replaced a nomadic lifestyle, and communities developed that recognized the advantages of occupational specialization. Sedentary agriculture largely replaced a nomadic lifestyle, and communities developed that recognized the advantages of occupational specialization.

THE RECURRING ICE ages of the last three million years dramatically shaped the landscape, and stressed the small human population who, out of necessity, learned to cope with climate change and its consequences. Ice sheets overran vegetation that the early humans and their animal cohabitants both relied on. Water supplies were continually shifting. Encroaching ice forced humans to be on the go, and lower sea levels opened up avenues of human migration. Ice ruled the world, and humans simply reacted to its advances and retreats. By the end of the last ice age, the ingenuity of humans had been honed by the stress of multiple glaciations, and the human species, equipped with enhanced technical skills, was poised for a rapid growth in population.

The subsequent ascendancy of h.o.m.o sapiens h.o.m.o sapiens, both in numbers and in capabilities, was beginning to leave a mark on the planet. The large land mammals, such as the mammoths and mastodons, became extinct, in part due to the pressures of human hunting-an early demonstration of what would come later, when the bison of North America were hunted nearly to extinction. And as human communities developed agriculture, the changing uses of land and water slowly altered the web of life in their environment. But little did anyone envision that such habits would eventually lead to large-scale deforestation of the continents-in Europe from 1100 to 1500, and in North America in the nineteenth and early twentieth centuries.

By 1800, Earth's population had grown to 1 billion people, some 250 times bigger than only 10,000 years earlier. The human population today is nearly 7 times bigger yet than in 1800, and with far greater technical capabilities. Because of human activities, ice, the force majeure of the planet only 20,000 years ago, is today in retreat, and perhaps on a trajectory to disappearance. No longer are humans pa.s.sive adapters to the natural world-today we have become the princ.i.p.al agents of large-scale changes in the global environment.

CHAPTER 4.

WARMING UP.

Some people change their ways when they see the light; others when they feel the heat.

-CAROLINE SCHOEDER

The spreading of the last continental ice sheets over North America and Europe reached a maximum some twenty thousand years ago, and then the ice blanket began to melt back. The melting was the result of a warming of the climate over the next ten to twelve thousand years, an ameliorating change that brought Earth's average surface temperature upward by about fifteen to twenty Fahrenheit degrees. That change took Earth from the chill of the Last Glacial Maximum to a level a bit warmer than today, a warm thermal plateau called the Mid-Holocene Optimum. The ascent was not always smooth, as surges of fresh melt.w.a.ter from the shrinking ice sheets spilled into the ocean, temporarily disrupting the currents transporting heat from the tropics to more remote and colder parts of the globe.

Reconstructing historic climate by reading the effects that a changing climate has on the natural world is both art and science, an endeavor known as paleoclimatology. It is not unlike piecing together a jigsaw puzzle, but with many pieces not quite fitting, and others missing altogether. And there is no picture on the box to guide you. The "pieces" that climate scientists work with come from both natural archives and human record-keeping, and are called climate proxies. A climate proxy subst.i.tutes imperfectly for a measuring instrument such as a thermometer or rain gauge. For a proxy to be useful in a historical reconstruction, it must also give an indication of the time when a climatic effect is being recorded.

For climate reconstructions we seek long records that encompa.s.s many years, and we prefer proxies that give year-by-year information. Trees turn out to be good proxies because they grow a little bit each year, and add a new ring of material around their trunks. The thickness of the ring indicates whether it has been a "good" or "bad" year for the tree. A thicker ring grows in a year when conditions are just right-not too hot, not too cold, not too wet, not too dry. In times of drought or thermal stress, growth is suppressed and the annual ring is thinner. Trees that are "on the edge," so to speak, at the very margins of the temperature range in which they can survive-in the polar lat.i.tudes or high up on mountains-are most sensitive to stress and therefore better proxies. And because the rings correspond to years, they can be counted and easily dated.

Snow acc.u.mulations that compress into ice are also good annual proxies, because there is usually a seasonal rhythm to snowfall that makes yearly acc.u.mulations appear as distinct layers in a glacier or polar ice sheet. Thicker layers, of course, indicate more snowfall, and the temperature at which the snow precipitated can be teased out of the oxygen and hydrogen isotopes in the ice's H2O. In the marine environment, corals show an annual addition to their framework that reveals the temperature of the seawater in which the coral has been growing. Geographically well distributed proxies, over land and in the oceans, are necessary to reconstruct a global average temperature.

Wind-blown dust is an indicator of both aridity and wind patterns. As dust falls from the atmosphere it settles into the ocean, into lakes, and onto glacial ice. In all three settings it is incorporated into sedimentary or ice layers. The varying amount of dust in the layers indicates the changing susceptibility of Earth's land surface to wind erosion, which usually increases at times of drought. And the composition and mineralogy of the dust often indicate where the dust originated, so climatologists can reconstruct the pattern of atmospheric circulation from the dust source to the depositional site. Today satellite photos frequently reveal huge plumes of dust blowing westward off the Sahara Desert in North Africa-and this dust is slowly acc.u.mulating in sedimentary layers at the bottom of the Atlantic Ocean.

Human doc.u.ments, such as records of agricultural production, human health, and sea ice extent, are also useful proxies. Just as trees grow better with enough sunshine and rainfall, so do agricultural crops. Year-by-year records of the wheat or maize yield, or of the grapes in vineyards, can serve as local proxies of weather conditions. Likewise, public health records can be ma.s.saged to reveal cold, damp years from warm, sunny ones. And for hundreds of years, European fishing fleets have kept good records of the geography of the sea ice that they encountered in the far North Atlantic.

So what do proxies tell us about the post-glacial climate? After reaching the thermal plateau some eight to ten thousand years ago, Earth slowly cooled about two Fahrenheit degrees on its way to the twentieth century. To be sure, there were some fluctuations superimposed on that slow descent, one known as the Medieval Warm Period, extending from about AD 950 to 1200, and a cooler event called the Little Ice Age, from about AD 1400 to 1850. But these were small departures in a long period of relative stability, a period that offered a golden opportunity for humanity to grow and spread. It was the time of the well-appointed, well fed (and well-preserved) Iceman mentioned in chapter 2, who until the day of his demise was leading the good life of a central European who had already discovered the advantages of a varied diet and metal tools.

The Medieval Warm Period is best remembered as the time when Europeans first established settlements in Newfoundland and southern Greenland, complete with domestic animals that could graze on limited gra.s.s during the still short summer growing season. At the peak of settlement, Greenland hosted a few thousand people on a few hundred farms. These hardy settlers and their descendants remained in Greenland for a little more than three hundred years, until the climate shifted to a cooler phase that eliminated summer pasture and cereal crops and ended the European presence. The maximum temperature during the Medieval Warm Period has been reconstructed through proxy methods to be about at the temperature level of the mid-twentieth century-warm, but not as warm as early twenty-first-century temperatures.

The slow cooling from the peak of the Medieval Warm Period continued for some five hundred years and led directly into the Little Ice Age. In the North Atlantic region, the effects were well observed: The tenuous growing season in Greenland disappeared, and the seasonal sea ice extended much farther south into the Atlantic, making Iceland a much more difficult destination to reach. Alpine glaciers, rejuvenated with greater snowfall, advanced downward in their valleys, and crop failures became more common.

Both the Medieval Warm Period and the Little Ice Age are best doc.u.mented in the North Atlantic region-in the Greenland ice cores, European tree rings, and the abundant historical records. But did these events mark only a regional change in climate, or were they global events, with effects seen elsewhere? In a sense the answer must be global, because even if the main expression of these climatic excursions occurred in the North Atlantic, the oceanic and atmospheric circulation would slowly "export" the effects to all parts of the globe, albeit in diminished scale and without simultaneity.

THE INSTRUMENTAL RECORD OF A CHANGING CLIMATE.

In the early 1600s a major technological advance occurred that would transform our ability to monitor climate: the invention of the thermometer. This new technology put determinations of temperature changes on a quant.i.tative basis, with an unprecedented uniformity and specificity. However, it was more than a century later, in 1724, that Daniel Fahrenheit, a German engineer, devised the scale of temperature that still bears his name, and two decades later, in 1744, when Anders Celsius, a Swedish astronomer, set forth his temperature scale. These two temperature scales survive to the present day, with 180 Fahrenheit degrees and 100 Celsius degrees separating the freezing and boiling points of water-thus each Fahrenheit degree is only five ninths as big as each Celsius degree. All countries that have adopted the metric system of weights and measures use the Celsius temperature scale. Of the world's major countries, only the United States continues to use the Fahrenheit scale.

The invention of the thermometer and the widespread adoption of either the Fahrenheit or Celsius temperature scale enabled temperature to be measured virtually anywhere and compared with measurements elsewhere. Thus the foundation for a network of calibrated instruments was laid, one that would provide the observations that show that Earth's average temperature, from the middle of the nineteenth century onward to the present day, has been rising.

At a few sites in Europe, such as the Klementinum Observatory in Prague, temperatures have been recorded daily for more than two hundred years. But from a global perspective, for a very long time after the invention of the thermometer, there were not enough measurement sites well distributed around the world to be able to make any global p.r.o.nouncements. In particular, there were few measurements in the southern continents and vast regions of the oceans. Antarctica had no permanent meteorological station until 1903, when the Scottish National Antarctic Expedition established one in the South Orkney Islands. Since 1904 the station has been operated continuously by Argentina.

It was not just temperatures that were going un.o.bserved in the seventeenth and eighteenth centuries-science in general was not exported to these regions for another two centuries. The religious brothers of the Society of Jesus, better known as the Jesuits, were an exception. They established meteorological stations and later seismographic observatories at several locations in Asia, South America, and North America. They appreciated that long historical records were necessary for the understanding of the environment in which they worked. But they were also early advocates of what today is called "liberation theology," and their attempts to educate indigenous peoples with such ideas did not sit well with the Spanish and Portuguese royalty and the Church authorities of the time. The Jesuits were recalled from their remote outposts and remained absent for two centuries, and the climate in many parts of the world went un.o.bserved and undoc.u.mented.

At sea, save for the few meteorological stations established on islands, the record of sea surface temperatures had to be extracted from the log-books of ships. Measurements of sea surface temperatures were made frequently aboard sailing ships by scooping up a bucket of seawater and sticking a thermometer in it. Later the measurement process shifted to the intake ports for seawater that was used to cool marine engines.

Today the taking of Earth's temperature occurs at thousands of locations around the globe, day in, day out, year in, year out. These observations of temperature take place on all the continents; at the sea surface by merchant, military, and scientific ships monitoring the temperature of the seawater as they traverse the global ocean; and by big arrays of fixed and floating buoys that automatically relay their temperature readings via satellite telemetry. The annual average of these many millions of individual thermometer readings around the globe is called the instrumental record of Earth's changing temperature. However, the measurement of temperature has had this global geographic coverage only over the past 150 years or so.

The merging of such large amounts of data, gathered in different ways at different times, requires careful attention to detail and the exercise of considerable quality control. Meteorologists sometimes change thermometers at weather stations, as new instrumentation is developed. They occasionally change the location of stations, as developing urban areas gradually surround formerly rural stations. The methods of measuring sea surface temperatures by ships at sea changed from buckets to water intake ports on ship hulls. And in doing the averaging, climatologists must be careful not to give too much weight to regions with many measurement sites as compared to other regions with far fewer sites. The good news is that the several organizations around the world that have independently developed methodologies to address these issues have produced very similar results.

THE VERDICT: EARTH IS WARMING.

So what have the many millions of thermometer readings over some 150 years-the instrumental record-revealed? The fundamental result: they show that Earth's surface has on average warmed about 1.8 Fahrenheit degrees.

It has not been an unbroken climb for 150 years-there have been year-to year ups and downs, some decades in which the temperature increased rapidly, and other decades when the warming slowed or was interrupted by some slight cooling. But one does not need to be a climate scientist to readily see that the graph of global average temperature over the past century and a half indicates a warming trend-a trend that is in fact accelerating. The warming trend over the past 25 years of the instrumental record is four times greater than that of the full 150 years. Earth's fever is rising rapidly.

Changes in the global average temperature, shown as departures from the mean temperature over the years 1951-80.

Data from the NASA G.o.ddard Inst.i.tute for s.p.a.ce Studies

In a geographical context, not every region displays the average behavior. Some parts of the globe have warmed more than average, some less, and a few areas have not warmed at all, or have even cooled. But the instrumental record is very clear: the average temperature of our planet's surface has increased significantly over the past 150 years. For the past half century it has been warmer than during the Medieval Warm Period. And during the last three decades, Earth's temperature has been rising faster than at any earlier time in the instrumental record.

Warming, moreover, is not confined to Earth's surface. Yes, the surface has warmed, but so has the lower atmosphere, the ocean water below the surface, and the rocks beneath the surface of the continents. Temperature measurements at depth within the oceans have been gathered from a number of different sources. Knowledge of the thermal structure of the oceans below the surface is important to submariners trying to cruise clandestinely and to fishing fleets seeking the habitats of favorite fish. Because sound travels faster in warmer water, an understanding of the temperature patterns in the ocean improves the accuracy of depth soundings-determinations of water depth based on how long it takes a pulse of sound to travel from the surface down to the seafloor and bounce back to the surface.

Much of this temperature data, collected over many decades, now resides in the U.S. National Oceanographic Data Center, and has proved to be a treasure trove of information about how the oceans have responded to the warming that has taken place at the surface. These data show that since about 1950 the oceans have been absorbing heat at a measurable rate, to depths of about 10,000 feet, with about two thirds of the heat stored in the upper 2,500 feet.36 The temperature of the rocks beneath the surface of the continents also shows the effects of a changing climate. Much of my own scientific work over the past two decades has been devoted to collecting and a.n.a.lyzing subsurface temperatures from around the world, to reconstruct the climate history the rocks have experienced. The principles behind this geothermal method are straightforward: a rock placed next to a campfire in the evening will still be warm in its interior in the morning, long after the campfire has burned out. The warm temperatures in the interior can, with the help of a little mathematics, reveal when, how long, and how hot last night's campfire burned-in other words, it can reveal the "climate history" that the rock's surface was exposed to the previous evening.

In the Earth, we measure temperatures at intervals down deep boreholes that have been drilled into the rocks of Earth's crust, or into the thick ice sheets of Greenland and Antarctica. The rock holes, penetrating to depths of 1,000 to 2,000 feet, have usually been drilled in search of minerals or water, or, in a few cases, for scientific research. The profiles of temperature down the holes reveal depth ranges over which the temperatures are either higher or lower than the temperature expected at that depth in the absence of any changes in the climate. These anomalous zones are the remnant signatures of past temperature fluctuations at the surface that have propagated downward into the subsurface. The Little Ice Age can be "seen" in the temperatures 500 feet down in the Greenland ice sheet, and the warm plateau of the mid-Holocene at depths between 1,500 and 2,500 feet.

How long can rocks and ice "remember" their thermal history? The pace at which heat is transferred through these materials is very slow, so slow that any fluctuations of surface temperature, either increases or decreases, since the last glacial maximum 20,000 years ago will have propagated no deeper than a mile or two into the subsurface. So the uppermost part of Earth's continental crust is effectively a thermal archive of climate change over the past several thousand years.

My colleagues and I have studied more than eight hundred borehole temperature records from around the world in some detail, and have been able to show that five centuries ago, Earth's average temperature was about two Fahrenheit degrees (a little more than one Celsius degree) lower than today.37,38 Since the year 1500, Earth's rocks have warmed, slowly at first and more rapidly later-fully half of the warming occurred in the twentieth century alone. And the surface temperature changes interpreted from the rocks are fully consistent with, but totally independent of, the instrumental record on the continents during the period of overlap, 1860 to the present. Scientific conclusions are always more persuasive when the same conclusion is reached by more than one independent method. Since the year 1500, Earth's rocks have warmed, slowly at first and more rapidly later-fully half of the warming occurred in the twentieth century alone. And the surface temperature changes interpreted from the rocks are fully consistent with, but totally independent of, the instrumental record on the continents during the period of overlap, 1860 to the present. Scientific conclusions are always more persuasive when the same conclusion is reached by more than one independent method.

Just as the water ma.s.s of the oceans and the rocks of the continental crust are warming, so also is the atmosphere above the surface. Temperatures taken by instruments aboard weather balloons since the 1950s and from orbiting satellites since the late 1970s have provided a picture of the temperature trends at various levels in the atmosphere, albeit over a considerably shorter time than is available with the instrumental record at the surface. And the task of a.s.sessing the temperature from a satellite looking down at its target from above, rather than being immersed in it, is not an easy one. Initially there was a suggestion that the satellite record was at odds with the measurements at the surface, but as the technical difficulties of the satellite measurements were recognized and resolved one by one, the differences largely disappeared. Today these two independent estimates of the surface temperature trends are very similar.

The measurements of temperature with scientific instruments the world over-at Earth's land and sea surface, in the deeper waters of the ocean, in the rocks of the continents, and in the thin atmospheric envelope above the surface-all are telling the same story: planet Earth is, without question, warming.

THE TRENCHES OF DENIAL.

In the decades sandwiching the end of the twentieth century and the beginning of the twenty-first, probably no other scientific topic was more in the news, and more contentious, than Earth's changing climate. It achieved prominent coverage and editorial commentary in the New York Times New York Times, the Washington Post Washington Post, and the Wall Street Journal Wall Street Journal, as well as cover story status in Time Time, Newsweek Newsweek, The Economist The Economist, BusinessWeek BusinessWeek, Vanity Fair Vanity Fair, The Atlantic Monthly The Atlantic Monthly, Skeptical Enquirer Skeptical Enquirer, New Scientist New Scientist, Scientific American Scientific American, Wired Wired, Sports Ill.u.s.trated Sports Ill.u.s.trated, and many other magazines. Former vice-president Al Gore produced his film An Inconvenient Truth An Inconvenient Truth, and the Weather Channel has its weekly Forecast Earth Forecast Earth. Both the U.S. Senate and House of Representatives have held formal climate change hearings.

Initially the media presented climate change as a "he said, she said" story, with little a.n.a.lysis of conflicting positions. It became apparent in this "fair and balanced" coverage that there was a not-so-subtle subterfuge taking place, in which prominent players in the carbon-based energy industry39 (e.g., Peabody Coal and ExxonMobil) had invested quietly to interject misinformation and uncertainty about climate science into the discussion, describing it with terms such as "unsettled science" and "uncertain science," or more boldly attempting to discredit the acc.u.mulating scientific results as "unsound science" or "junk science." A handful of contrarian scientists-many who were financially supported by the fossil fuels industry-took issue with the emerging climate science consensus and became known as skeptics or climate contrarians. (e.g., Peabody Coal and ExxonMobil) had invested quietly to interject misinformation and uncertainty about climate science into the discussion, describing it with terms such as "unsettled science" and "uncertain science," or more boldly attempting to discredit the acc.u.mulating scientific results as "unsound science" or "junk science." A handful of contrarian scientists-many who were financially supported by the fossil fuels industry-took issue with the emerging climate science consensus and became known as skeptics or climate contrarians.

Although the attacks on the developing scientific consensus about terrestrial climate change seemed in some ways to come from a shotgun, the essential position of denial could be distilled into four main elements: 1. The instrumental record of surface temperature change was flawed.2. The causes of climate change were entirely natural.3. The consequences of climate change would be beneficial.4. The economic cost of addressing climate change would not be worth the effort.

These four elements were in effect sequential trenches of defense occupied by those ideologically opposed to the concept of anthropo-genic climate change, or blind to its reality. If any one of these a.s.sertions could be persuasively demonstrated or proven, the rest of the list would be rendered irrelevant. The defensive a.r.s.enal was (and continues to be) well stocked with misinformation, irrelevancies, half-truths, misunderstandings, oversimplifications, and outright falsehoods, but underlying all was a notion of seriality: the climate contras viewed the entire climate change argument as a long chain of evidence, and if any link could be broken, then the chain could no longer carry any weight and the climate change concept would fall apart. In reality, the scientific story of climate change is much more like a net hammock of interwoven strands of evidence-if one strand proves weak, there remain many that continue to support the growing reality of the climate change saga.

The climate contras recognized that if the first trench could be successfully defended-if they could make the case that there were no compelling observations of a changing climate-the war would effectively be over. So, they rolled out mortars that lobbed argument after argument to a puzzled and largely scientifically illiterate public, attacking the instrumental record of a warming Earth: "We shouldn't be placing much credence in data from weather stations in cities, because the 'urban heat island' effect is contaminating the record." "You can't trust a century-long record of thermometer readings when they change thermometers every couple of decades." "How can climatologists argue that Earth is warming when we here in Graniteburg, New Hampshire, are experiencing the worst winter in memory?" "How could anyone say the globe is warming when for the past umpteen years, Dry Valley Crossing, Nevada, has been cooling?" "Satellites taking Earth's temperature don't show any warming." "Maybe the continents are warming, but the oceans are cooling."

These arguments, sometimes raising interesting scientific or technical questions, have all been addressed: The urban heat island effect is real, but it has been corrected for, or sidestepped by using only rural meteorological data on land. And one must remember that there are no cities sitting on 70 percent of Earth's surface-the oceans. The effect of changing thermometers can be a.s.sessed by using both the new and old thermometers side by side for a time to be sure they give the same results. Whether someplace is having a very cold winter, or a very hot summer, is an irrelevancy-climate change is about long-term trends in temperature, not about year-to-year oscillations. That someplace may actually be cooling when the globe on average is warming is also irrelevant-as noted earlier, some places may be warming more than the average, some less than average, and a few might actually be cooling, but the overall average remains one of warming. It would be a rare cli mate system where every place did exactly the same thing. I have already mentioned that the early apparent differences between satellite and surface measurements have now been resolved. And are the oceans cooling while the continents are warming? Hardly. The long-term trend of temperature in all the oceans of the world is a very consistent warming for the past half century.

NATURE'S OWN THERMOMETERS Amidst the lunges and parries about the accuracy of the instrumental record, it is easy to lose sight of the fact that one need not rely at all on scientific instruments to make a persuasive case that Earth's climate is warming. Nature has her own thermometers-plants and animals that inhabit the land and the sea. Flowering plants that take their annual cues from the warming and cooling of the seasons are now sprouting and blooming earlier in the spring, and birds are laying their eggs earlier. Birds that time their annual migrations by changes in temperature are lingering longer in the fall before departing for their winter habitat-and some are no longer migrating at all because winters have become so mild. Insects have begun to migrate up mountains as the warming adds new terrain to their ecosystems, and some insect population dynamics have suddenly changed when mild winters no longer are cold enough to kill off most of the previous summer's residents. And as lake waters have warmed, their fish populations also change-coldwater species such as walleye and trout are being gradually replaced by warmer water ba.s.s and bluegills.

The timing of natural events is a part of the biological sciences called phenology, and observing the timing of seasonal arrivals and departures, of blooming and folding, of hatching and fledging, has long been a favorite activity of amateur naturalists. The routine collection of phenological and environmental data such as temperature and precipitation over long time intervals is vitally important to understanding the behavior of the climate system. But this type of scientific work is not glamorous. It often is done by unheralded people-some professional, some amateur-who receive no substantial reward or recognition save for the knowledge that they are contributing to a body of data that ultimately has immense scientific value. Euan Nisbet, an atmospheric scientist at Royal Holloway College of the University of London, has commented that "monitoring is science's Cinderella, unloved and poorly paid."40 Let me describe some of this Cinderella science. Let me describe some of this Cinderella science.

At the Mohonk Mountain House, a resort some eighty miles north of New York City, a meteorological observing station sits atop a rugged outcrop of rock. Since 1896 someone has trudged up the outcrop every day to read the thermometers and rain and snow gauges that are housed there.41 Over the full more-than-a-century period of observation, the "someone," in fact, has been only five individuals, with Daniel Smiley, Jr., a descendant of the founders of the resort, doing the duty for a half century. He made notes of many other phenomena, such as the first blooming of this or that flower each year, the first arrival of various birds in the spring, and the temperature and acidity of a nearby small lake, thus compiling a remarkable record of natural history and change at this location. At Mohonk Mountain, these thousands of daily observations show that since 1896 the average annual temperature has risen 2.7 Fahrenheit degrees and the growing season has been extended by ten days. Over the full more-than-a-century period of observation, the "someone," in fact, has been only five individuals, with Daniel Smiley, Jr., a descendant of the founders of the resort, doing the duty for a half century. He made notes of many other phenomena, such as the first blooming of this or that flower each year, the first arrival of various birds in the spring, and the temperature and acidity of a nearby small lake, thus compiling a remarkable record of natural history and change at this location. At Mohonk Mountain, these thousands of daily observations show that since 1896 the average annual temperature has risen 2.7 Fahrenheit degrees and the growing season has been extended by ten days.

ON A RESEARCH TRIP to Russia in 2001, I spent several days in Irkutsk, situated in southern Siberia, just north of the border with Mongolia. Irkutsk is just about as far to the east of the Greenwich prime meridian as my home state of Nebraska is west. It is a stop on the Trans-Siberian Railway, but it is not a new railway city like Novosibirsk, a thou