WARMING UP - A World Without Ice - Henry N. Pollack

A World Without Ice - Henry N. Pollack (2009)

Chapter 4. WARMING UP

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


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 meltwater 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 substitutes 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 encompass 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 latitudes 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 accumulations that compress into ice are also good annual proxies, because there is usually a seasonal rhythm to snowfall that makes yearly accumulations 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 accumulating in sedimentary layers at the bottom of the Atlantic Ocean.

Human documents, 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 massaged 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 grass 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 documented 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.


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 quantitative 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 pronouncements. 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 unobserved 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 unobserved and undocumented.

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.


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 Goddard Institute for Space 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 analyzing 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.

Just as the water mass 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 assessing 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.


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, the Washington Post, and the Wall Street Journal, as well as cover story status in Time, Newsweek, The Economist, BusinessWeek, Vanity Fair, The Atlantic Monthly, Skeptical Enquirer, New Scientist, Scientific American, Wired, Sports Illustrated, and many other magazines. Former vice-president Al Gore produced his film An Inconvenient Truth, and the Weather Channel has its weekly 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 analysis 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 accumulating 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 assertions could be persuasively demonstrated or proven, the rest of the list would be rendered irrelevant. The defensive arsenal 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 assessed 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.


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 bass 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.

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.

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 thousand miles west along the line of rail. In 1727, Vitus Bering, on his three-year overland trek on horseback to Kamchatka, to begin his voyage of discovery of the Bering Strait, wintered in Irkutsk because of its “amenities.” Irkutsk remains a place of stark contrasts—three-hundred-year-old small ornate wooden houses juxtaposed with Soviet-style concrete apartment blocks. Less than an hour from Irkutsk is the foot of Lake Baikal, the oldest and deepest lake with the largest volume of freshwater in the world. It is a narrow lake, but almost four hundred miles long and a mile deep, occupying a tectonic rift valley, similar to Lakes Tanganyika and Malawi in East Africa.

When I went to Lake Baikal in early April of 2001, it was still frozen tight with an ice lid three feet thick—the spring ice breakup was still six weeks away. In conversations with scientists at a solar observatory overlooking Lake Baikal, I learned of a remarkable family at a small biological research station just a few miles away. This family had been studying Lake Baikal for three generations. Mikhail M. Kozhov had come to Irkutsk State University shortly after the end of World War II, and began making temperature measurements and biological surveys in the waters of Lake Baikal.42 In summer he worked from a boat; in winter, through holes drilled through the ice. His daughter, Olga Kozhova, assisted him, and when he died in 1968, she continued the program of measurements, later assisted by her daughter, Lyubov Izmesteva. Olga died in 2000, but Lyubov, herself now a professor at Irkutsk State University, continues the measurements. This archive of temperature data shows that the surface waters of Lake Baikal have been warming at a rate of about one Fahrenheit degree every twenty-five years, and the warming is slowly penetrating to greater depths.

COOPER ISLAND IS a small low-lying barrier island a short distance off Point Barrow, Alaska, the northernmost point of the United States and indeed of North America. It sits some three hundred miles north of the Arctic Circle and is the nesting and breeding site of a colony of black guillemots, a not-so-common seabird of the Arctic. In 1972, a young ornithologist named George Divoky began a study of the breeding habits of these birds.43 Every summer, for the next thirty years, he spent on Cooper Island with the black guillemots and an occasional polar bear. Most summers were in “solitary confinement,” but occasionally he took a field assistant. Carefully he noted the dates when the guillemots returned to Cooper, the dates when they laid their eggs, the dates when chicks hatched and later fledged. What he has discovered is that the entire reproductive sequence has shifted more than ten days earlier in the Arctic summer. Guillemots will nest as soon as the snow melts, but no sooner, and so the earlier nesting of these seabirds serves as a proxy for the timing of the annual snowmelt. But there was also some bad news for the guillemots: their population began to diminish around 1990. Year by year the Arctic warming has been moving the sea ice much farther from the nests on Cooper Island. Because the margins of the sea ice are favorite feeding spots for these seabirds, the retreat of the sea ice has been slowly moving food almost out of their reach.


NATURE’S BEST THERMOMETER, perhaps its most sensitive and unambiguous indicator of climate change, is ice. When ice gets sufficiently warm, it melts. Ice asks no questions, presents no arguments, reads no newspapers, listens to no debates. It is not burdened by ideology and carries no political baggage as it crosses the threshold from solid to liquid. It just melts.


When I was a boy growing up in eastern Nebraska, the calendar of certain activities was set by the seasons. Neighborhood hockey started up when nearby George’s Lake froze over, and duck hunting began as the myriad channels of the Platte River became choked with ice. The family springtime fishing trip to the boundary waters of Minnesota was determined by the breakup of the winter ice six hundred miles to our north. In late May my father would be on the phone to friends in International Falls, Minnesota, checking whether the ice had moved out, and one week after the breakup, we were there trolling for walleyes. The rhythms of communities the world over have similarly been tied to the comings and goings of the annual ice.

Madison is the Wisconsin state capital and home to the University of Wisconsin. The city sits between Lakes Mendota and Monona, bodies of water that have provided recreational activities for residents ever since the city was founded in 1836, the same year the Wisconsin Territory was created. Perhaps not surprisingly, the dates of fall freezing and spring breakup have been dutifully recorded for almost a century and a half, and they tell a very interesting story. In 1850, Lake Mendota froze in early December and broke up in early April, but 150 years later, freezing had shifted to some nine days later and breakup occurred almost two weeks earlier.

Along the eastern shore of Lake Michigan is Grand Traverse Bay, another location with diligent record-keepers since the mid-nineteenth century. The long record compiled there shows that since 1851 the bay froze over completely at least seven times in each of the first twelve decades; this figure dropped to six times in the 1980s, three times in the 1990s, and only twice in the first decade of the twenty-first century.44For the years when the bay has frozen over, the period of winter ice cover has diminished by thirty-five days.

But it is not just lakes in North America that are showing trends toward shorter intervals of annual ice cover—in Scandinavia and Europe, in Asia and Japan, the long-term observations are telling the same story. And it is happening not just to lakes—major rivers leading to the Arctic Ocean, such as the Mackenzie in Canada and the Angara and Lena in Siberia, show similar trends. Wintertime ice in the freshwater of the Northern Hemisphere is becoming a much rarer commodity.45

Mountain glaciers everywhere—in New Zealand, the Andes, the Alps, Alaska, the Rocky Mountains, Central Asia, equatorial Africa—shrank over the twentieth century. The U.S. Geological Survey and the U.S. National Snow and Ice Data Center have collected air and ground photography of glaciers in the United States showing the extent of glaciers at various times in the past.46 In Glacier National Park in Montana, the melt-off has been dramatic—of the 150 glaciers present in 1850, fewer than 30 are still present today. At the present rate of melting, none will survive past 2030.

Mount Kilimanjaro sits just a few degrees south of the equator, in East Africa. The equator is an unlikely place to find natural ice, unless you go very high. Kilimanjaro reaches more than nineteen thousand feet above sea level, and for as long as anyone can remember it has had snow and ice at its peak. This iconic image of Africa was immortalized in Ernest Hemingway’s short story “The Snows of Kilimanjaro.” But throughout the twentieth century, Kilimanjaro has lost ice steadily. The volume of ice present in 2008 was less than 10 percent of what it was a century ago—and at the present rate of loss, ice will disappear from equatorial Africa by 2020.

The Athabasca Glacier in the Canadian Rockies of Alberta is perhaps the most visited glacier in North America, by virtue of its position between Banff and Jasper national parks, two of Canada’s favorite scenic treasures. The recession of this glacier is well marked by a succession of signposts installed over the years at the snout of the glacier, which show the glacier’s extent at various times in the past. Over the past 125 years the Athabasca has receded almost a mile from the first signpost.

The snowfields and glaciers of the European Alps are also shrinking rapidly, so rapidly in fact that the tourist industry is resorting to desperate measures to slow summertime melting, including laying reflective sheets over the glaciers, as a giant seasonal sunscreen. At the present rate of melting, Alpine glaciers will be only memories by the end of this century. In Asia the glaciers in the Himalayas each year are losing ice equivalent to the entire annual flow of the Huang He, China’s fabled Yellow River.


Getting around in the Arctic terrain is never simple, but it is easier, ironically, in winter than in summer. To be sure, the unending daylight of summer offers visibility of unimagined scale, and ease of navigation in an area with few human landmarks. But the broad vistas disguise the fact that in the summer, the ground becomes soft and spongy, depriving vehicles of a firm surface to traverse. The permafrost, the terrain that experiences an average annual temperature below the freezing point, undergoes some limited summertime melting in what is known as the “active zone.” This zone extends downward a foot or two or three, and turns a frozen-hard wintertime surface into summertime mush. Off road traffic (and there are very few roads) becomes impossible. Thus, overland transport of supplies to mineral and petroleum exploration camps, scientific stations, and remote settlements is, of necessity, confined mostly to winter.

The time of year when such transport can take place is known as the tundra travel season, and is measured in terms of the number of days that vehicle passage overland is possible. Tundra travel days are rapidly diminishing in number. In 1970 one could roll over the frozen surface of northern Alaska more than seven months of the year, but today such travel is possible during only four months, from early January until about mid-May. The overland travel window is closing at a rate of about one month per decade. The tundra surface is now an “active zone” two thirds of the year, and in another half century it may be impassable year-round.


Greenland is an Arctic island bigger than Mexico. It sits almost completely north of the Arctic Circle on the North American side of the Atlantic Ocean. It is a huge reservoir of ice, in volume second only to that of Antarctica. The ice on this large island, covering all but its coastal fringe, is equivalent to more than twenty feet of sea-level change, were it to return to the sea. The top of the ice pile is about twelve thousand feet above sea level, with another thousand feet below sea level because the ice load has depressed the rocky surface beneath it. Greenland’s ice slowly creeps downward, and spills into the sea in hundreds of glacial streams around the periphery of the island.

The glaciers are like small holes around the base of a rain barrel—some water escapes through each hole, and in the absence of precipitation, the water level in the barrel will slowly decline. When precipitation into the barrel equals the water losses through the holes, the water level in the barrel remains unchanged, and when the rainfall exceeds the losses at the bottom, the water level will go up. Were there no replenishment of ice in the interior from snowfall, Greenland would eventually be drained of ice.

Every year Greenland undergoes summertime melting around the perimeter of the ice sheet, where the seasonal temperatures at low elevations are sufficiently warm. This band of melting on the fringes has been more or less stable in areal extent and in elevation throughout most of the twentieth century, but toward the end of the century the zone of melting began to creep to higher elevations and over larger areas. The fraction of Greenland’s area that undergoes summer melting is 30 percent greater today than it was only thirty years ago, and now ice melts at elevations greater than six thousand feet above sea level.

In midsummer the melting areas are dotted with meltwater pools and lakes, beautiful blue jewels accenting the white backdrop. Some of these bodies of water will refreeze in winter, and thus do not represent a net loss of ice mass. Others lose their water in streams that run to the sea, and these do represent a net loss of ice mass and contribute to a rising sea level.

It is not an easy task to determine whether the ice budget of Greenland or Antarctica is in surplus or deficit. One technique that has been employed is called repeat-pass airborne laser altimetry, a method in which an aircraft flies over the ice surface at a low but steady altitude and repeatedly flashes a laser beam at the surface below. The beam is reflected from the surface back to the aircraft. The time it takes for the laser beam to go down to the ice and return can be translated into the elevation of the ice surface.47 A repeat of the measurement in a few months will show what changes have taken place in the elevation of the ice surface. If the surface has become lower, there is a deficit, and if it is higher, there has been an accumulation. But elevation changes can be misleading—a fresh snowfall might add three feet in elevation, but because the new snow is light and fluffy, it doesn’t represent the same mass as three feet of dense glacial ice.

Another technique to determine if an ice budget is changing makes use of detailed measurements of Earth’s gravity, as felt by scientific satellites as they orbit the planet. Very small changes in gravity are associated with the different densities of the various rocks that make up Earth’s crust. Compared to the average, a low-density rock has a mass “deficiency” and a high-density rock has a mass “excess.” The force of gravity increases over areas of excess mass and decreases over regions of mass deficiency. The paths of Earth-orbiting satellites are perturbed very slightly—sped up or slowed down a tad—by these small variations in local mass and gravity. Thus careful observations of satellite orbits can over time reveal whether a region is losing or gaining ice. A special satellite experiment known as GRACE (Gravity Recovery and Climate Experiment) has been operating since 2002, paying special attention to Greenland and Antarctica. In Greenland, GRACE determined that there is an ongoing ice mass loss tied to an acceleration of the glaciers draining the interior. The ice deficit for Antarctica has also increased, by 75 percent over the past several years, principally because of accelerating glacial flow following the disintegration of floating ice shelves around the continent.


The Arctic Ocean is a roughly circular ocean with the geographic North Pole at its center. The diameter of the ocean is about 2,800 miles, with North America and Greenland sitting on one side, and Europe and Asia on the other. The entire ocean lies north of the Arctic Circle, and thus experiences the annual extremes of solar illumination—including some days of around-the-clock darkness in winter and unending daylight in summer. For as long as people have been paying attention, much of the ocean has remained frozen year-round in a vast sheet of sea ice. In summer, some of the sea ice breaks up and melts to expose open water, but in winter it refreezes, in a layer about three to six feet thick. During the first half of the twentieth century, about one third of the sea ice melted and refroze each year, leaving two thirds of the ocean with older ice, up to about five years old in places. The older ice is also thicker, occasionally reaching a thickness of fifteen feet or more. Even though much of the Arctic Ocean has been covered with sea ice for at least as long as humans have observed it, the ice is not the same ice. Because sea ice is always on the move—drifting from the Far East, over the North Pole, on toward Scandinavia, and exiting into the Atlantic—no extensive region of the Arctic Ocean has ice much older than five years. The exceptions are in the narrow channels that surround the many islands of the Canadian Arctic, outside of the mainstream of the Arctic drift.

The Age of Exploration—roughly the sixteenth through the nineteenth centuries—coincided with the Little Ice Age cool interval. In the Arctic Ocean, ice formed in every nook and cranny, including in the many channels that wind their way through the archipelago of islands comprising the northern territory of Canada. This maze of waterways, were they to become ice-free, would allow a maritime shortcut from Europe to the trading nations of Asia, a route shorter by two thirds compared to the alternative routes around either Africa or South America. This passage, more concept than reality, was called the Northwest Passage.

For most of maritime history, however, this route has been closed with ice. Time and again the ice thwarted attempts to open this new trade route to the Orient. On his third and last voyage of discovery aboard HMS Discovery, Captain James Cook searched for the western entry to the passage. Sailing west along the Aleutian Islands in the summer of 1778, he crossed into the Bering Sea near Unalaska Island, and then along the western coast of Alaska to the Bering Strait, with still no hint of a pathway to the east. Northward he continued, through the Bering Strait to latitude 70º north, where the land began to ease off to the east. Was this the western portal? Cook excitedly began the mental calculations of how long it might take to reach Baffin Bay, that stretch of open water between Canada and Greenland some two thousand miles to the east.

But it was not to be. Two days later Cook saw ice blink, the reflection of a vast expanse of ice on the low clouds in the distance. In a few hours the ice came into view, a solid wall more than ten feet high, as far as the eye could see. Cook recognized the futility of continuing, and turned around to retrace his course back into the Pacific. It would be his last glimpse of ice ever—six months later Captain Cook was dead, killed in a battle with native Polynesians in Hawaii.

Others tried to navigate the Northwest Passage from east to west with no more success. The storied Franklin Expedition of 1845-47 became ice-locked about midway through the passage, and all aboard perished from starvation. It was not until 1906, when Roald Amundsen, the Norwegian explorer who would five years later gain fame as the first person to reach the South Pole, completed a three-year journey through the Northwest Passage to reach Alaska. But the ice he faced may already have been less of an obstacle than that encountered by the eighteenth-and nineteenth-century explorers—the Little Ice Age had reached its peak in the nineteenth century. By 1906, Amundsen was already benefiting from a warming climate.

The summertime retreat and winter refreezing of sea ice are regular cyclical occurrences of long standing, but in the latter decades of the twentieth century, the summer melting began to consume much more than the usual amount of ice, and the winter refreezing fell short of restoring it. By the end of the twentieth century, the summer sea ice had diminished by some 25 percent from its mid-century extent. And as the older ice was replaced by younger ice, the average thickness of the sea ice also diminished, to about half its mid-century measurement.

The Russian icebreaker Yamal for a number of years has ferried tourists to the North Pole, for a “picnic” on the ice. In August of 2000, everyone aboard was in for a surprise—when Yamal reached the pole, there was only open water. Occasional open water in sea ice is not uncommon—such an ice-free area is called a polynya, a Russian word now in the international lexicon. Polynyas come and go, vagaries of upwelling ocean currents beneath the ice and the wind above. A few polynyas are more or less permanent geographic features, reflecting the stability of the ocean currents, but many others are transient—here today, gone tomorrow. We do not know how common or how rare a North Pole polynya may be, but those who witnessed the occurrence in 2000 remarked that the sea ice had been very thin and peppered with polynyas all the way to the pole. Yamal’s captain said that in all the years he had been traveling to the pole, a polynya there was a first for him.

THE NORTHERN HIGH latitudes are not unique in signaling a changing climate. In the south, all around Antarctica, the ice is also growing restless. Big ice is the norm in Antarctica, and after coming to the white continent for eighteen years, my jaw does not drop easily. Yet in mid-December 2007, I was awestruck with what was unfolding on the horizon. We were at latitude 61º south, longitude 54º west, between Elephant and Clarence islands, at the tip of the Antarctic Peninsula, when the biggest piece of floating ice I had ever seen came into view. Thirty-one miles long, twelve miles wide, edged with sheer ice cliffs reaching more than a hundred feet above the sea surface, and another eight or nine hundred feet below—a massive island of ice adrift in the southern ocean.


This great slab may have broken away from the Filchner Ice Shelf deep in the Weddell Sea, or perhaps was a fragment of an even bigger mass that had separated from the Ross Ice Shelf some 2,500 miles south of New Zealand in 2001. Numbers cannot fully describe this floating behemoth. Sixty cubic miles of ice? Fifteen times the area of Manhat tan? The volume of water in Lake Erie? There it was, gigantic ice adrift on a journey to nowhere, pushed along by wind and currents at about two miles per hour.

Sailing alongside this floating ice island I found it impossible to capture the scale—a photo simply showed a cliff extending from the foreground to the horizon. One needs to step back—way back—to be able to see this slab in its entirety. Actually, one needs to step up about a hundred miles, to the viewpoint of an Earth-orbiting satellite, to capture this slab in a single frame. The experience is not unlike feeling the fierce wind of Hurricane Katrina on the ground in southern Louisiana, but needing a satellite image of a giant spinning pinwheel covering the entire Gulf of Mexico to see the full scale of nature’s atmospheric fury.

The breaking away of ice of this magnitude from the outer edge of an Antarctic ice shelf certainly begs for attention, particularly when it is not an isolated phenomenon. The Ross Ice Shelf, about the size of France, is the biggest of the huge floating ice sheets nestling along the margins of Antarctica. Others abut both sides of the Antarctic Peninsula, the long and narrow finger-like mountain chain that stretches toward South America. Along the peninsula, mountain glaciers drain ice from the high places, sending it to the sea, where it floats in giant sheets that extend tens and hundreds of miles away from the rocky coast. The Larsen, the Filchner, the Ronne, and the Wilkins ice shelves—named for whalers, scientists, and explorers of a century ago—also are showing wear and tear today.

In early 1996, when I was working aboard MS Explorer (the expedition ship that sank not far from Elephant Island a decade later), the captain and expedition leader revealed to the expedition staff that we were going to attempt the first-ever circumnavigation of James Ross Island, named for Sir James Clark Ross, a British explorer who navigated the region in 1842 (and for whom the Ross Ice Shelf is also named). James Ross Island lies near the tip of the peninsula, on the east side; it is the eleventh largest of the myriad islands that dot the fringe of Antarctica. It had been bound tightly by ice at least since any human had viewed it. Yet, there were hints that a circumnavigation might be possible.

Just to the south lay the Larsen Ice Shelf, one of the big ice shelves attached to the peninsula. Two years earlier, the northernmost segment of the Larsen, an area about the size of Luxembourg, had disintegrated, flushing great icebergs into the adjacent Weddell Sea. Could the icy handcuff holding James Ross Island also be loosening? We thought it was a possibility, and began our push into the ice. We were not to be rewarded, however, because the ice was not ready to yield its grip—but the very next year, Explorer succeeded making it around James Ross Island, through channels that had not seen open water for thousands of years.

Only five years later an even larger segment of the Larsen disintegrated, one as large as Rhode Island, in a spectacular one-month breakup that delivered so much floating ice to the region that ship navigation was substantially impeded and only ships with a scientific mission ventured into the area. And in late March of 2008 the Wilkins Ice Shelf, on the southwest side of the peninsula, an area about half the size of Scotland, began to disintegrate, shedding floating ice islands of Brobdingnagian scale into the sea. The initial “sliver” from the edge of the shelf was 25 miles long and 1 mile wide, and once it was separated, another 150 square miles behind it quickly broke up. New fractures in the remaining shelf appeared in a November 2008 photo, indicating that the breakup was still progressing, and by April 2009 the disintegration was complete.

The ice shelves around Antarctica are great sheets of ice that have ponded up around the mouths of glaciers that drain ice from the interior. Parts of the shelves may be grounded, but much of the ice is floating as large sheets on the sea. These massive, partially anchored shelves serve as buttresses that slow the outflow of the glaciers that nourish them—but when the shelves disintegrate, the glaciers find new freedom and speed up their delivery of ice to the sea. A recent survey48 of all the outlet glaciers around Antarctica shows little net loss of ice from East Antarctica (the bigger fraction of Antarctica, east of the Transantarctic Mountains), but substantial and increasing ice loss from West Antarctica and the Antarctic Peninsula.

WHAT DOES THIS accelerating ice loss from both Greenland and Antarctica mean? In a bathtub, the volume of water determines how high the water reaches, and the same holds true for Earth’s great natural bathtub—the ocean basins. The volume of water in the oceans rises and falls during the comings and goings of ice ages, and these hydrological transfers are accompanied by changes in sea level of several hundred feet. But in the period of general warmth and relative stability that we have experienced over the past ten thousand years, we have not seen dramatic changes in sea level. There has been a rough equilibrium between losses from the oceans through evaporation, and returns to the oceans via precipitation and the flow of rivers and glaciers to the sea. Over the past several millennia these withdrawals and deposits have continued to take place in the oceanic account, but the balance has remained pretty steady.

But the twentieth-century warming of Earth and the loss of ice from the continents are beginning to change the oceanic balance—in the upward direction. Two principal factors are at work. The first is the increased melting of ice and the return of the meltwater to the sea, or alternatively the direct deposit of ice into the sea from faster-flowing glaciers. The second factor is the volumetric expansion of seawater as the oceans warm. The volume of most liquids increases when the tempera-ture goes up—that is the fundamental principle behind liquid-in-glass thermometers that show the level of the liquid rising in the scaled glass tube as the temperature rises.

Changes of water level during a flood episode along a river are apparent in vertical changes—how high the water rises along a levee or the wall of a building—and in horizontal changes seen in the growth in the area covered by water. Sea level changes are apparent in these same ways. The measurement of the vertical change, the amount of sea-level rise, is accomplished by an instrument called a tide gauge, the primary purpose of which is to measure the amplitude of the high and low tides in a bay or harbor. This instrument records the changing water level during the daily rise and fall of the tide, but over years, decades, and centuries it also shows the long-term changes associated with slowly rising sea level. Data from thousands of tide gauges the world over have been collected and analyzed, and show that during the twentieth century, sea level on Earth rose about eight inches. One third of the rise comes from new deposits of meltwater and ice into the sea, and two thirds from the thermal expansion of the warming oceans.

But it is the horizontal incursion of the rising sea that is most apparent to the eye. On a gently sloping beach, a small rise of sea level will extend quite a ways inland to define a new shoreline. Eight inches of sea-level rise on a beach with a gentle slope of one degree will move the shoreline almost forty feet inland. And the same forty feet will be subject to the daily flooding of the high tide, and more vulnerable to the high-water surges from storms at sea.


1n 1988 the United Nations created an international scientific working group called the Intergovernmental Panel on Climate Change (IPCC). The charge to this group was to assess whether climate change was occurring, what the causes of such climate change might be, what the consequences of past and future climate change have been and might be, and what options might exist for mitigation of, and/or adaptation to, a changing climate on Earth. The task of organizing this panel fell to the World Meteorological Organization and the United Nations Environment Programme, both entities of the United Nations.

The IPCC is not a research organization itself, but rather an evaluator and summarizer of peer-reviewed scientific research published in scholarly journals the world over. It issues periodic assessment reports every five years or so, describing the state of knowledge about climate change. These assessment reports have appeared in 1990, 1995, 2001, and, most recently, 2007. Components of the reports are written by teams of scientists active in the various subfields of climate science, then collected into chapters by a group of “lead authors”; the chapters are assembled into a coherent and seamless assessment by an even smaller number of “coordinating lead authors.” Altogether, more than two thousand active climate scientists contributed to the Fourth Assessment Report published in 2007.

A few words about the process of peer review in the IPCC assessments: peer review is essentially a process of quality control in the world of serious scholarship. Publication of research results in a peer-reviewed journal means that an article has been read by other practicing researchers in the area, and assessed for originality, appropriate methodology, data quality, and sound conclusions. Most articles that appear in journals have been revised once or twice prior to publication in response to critiques from the reviewers. Submitted articles that fail the review are, of course, rejected.49

The published scientific results considered by the IPCC have already been peer-reviewed by the independent journals in which they were published, but that is not the end of peer review in the IPCC assessment reports. After a draft of an assessment report has been prepared, it is sent to a large pool of climate scientists not involved in its writing, but active in and knowledgeable about the fundamental science. They are asked to provide another layer of peer review to determine whether the draft assessment report is accurate, balanced, and free of distortion or exaggeration. Critiques can be formally expressed, and then forwarded to the assessment authors for a response. The authors are obliged to respond to each critique, either accepting and incorporating or rejecting and rebutting the essence of each commentary. More than thirty thousand written comments were submitted by more than six hundred individual expert reviewers of the Fourth Assessment Report’s volume on the physical science of climate change.

The revised assessment report is next forwarded for review to the governments of the member countries represented in the United Nations. At this level the issues discussed are a blend of science, economics, and policy, but ultimately the language of the assessment reports must be approved by the governments. Considerable debate, accompanied by nuanced word-crafting, takes place, sometimes requiring an “agreement to disagree,” but ultimately the text is approved and the assessment report becomes official and publicly available. For the Fourth Assessment Report in 2007 some 130 governments participated in this final stage of review.

The reason I have gone to great length to describe this review process is to make clear that, in the end, the IPCC report is a document that must, by any measure, be deemed conservative. The review process weeds out unbounded speculation, problematic science, and untested hypotheses. It carefully evaluates and states the uncertainties at every step of the way. In the end, what results is a lowest-common-denominator consensus of what the science is telling us. Moreover, the resulting reports are not policy prescriptive: that is, they do not tell governments what to do. They simply lay out various scenarios with attendant consequences: if you do X, you can expect Y; if you don’t do W, you can expect Z.

The thorough and systematic quality control exercised by the IPCC contrasts strongly with the communications of the climate contras. These “skeptics” choose newspapers, radio, and television for their “scientific” pronouncements, or publications subsidized by vested interests that want to discredit what the real science is revealing. The contras have little interest in persuading the mainstream scientific community, and care little about peer review. The audiences they aim to persuade are state legislators and members of Congress, and other governing bodies around the world, where climate policy will ultimately be shaped.


So what did the IPCC’s Fourth Assessment Report say about the evidence that Earth’s climate is changing? Here is its bottom line:

Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level.50

Ice everywhere is talking to us—not politically or emotionally or conventionally—but in a language that we must understand and heed. Ice is a sleeping giant that has been awakened, and if we fail to recognize what has been unleashed, it will be at our peril.

The IPPC’s use of the word unequivocal leaves little wiggle room. It means there can be no confusion about it. There can be no mistake about it. “Maybe, maybe not” is over. Significant climate change is happening. Seldom do we hear scientists make such an unambiguous pronouncement.

It is time to move on to other issues. Let us turn now to the causes of climate change. In the next two chapters we look first at the natural factors and then at the human factors that can cause Earth’s climate to change on the time scale of the twentieth-century warming.