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The Winds of Change places the horrifying carnage unleashed on New Orleans, Mississippi, and Alabama by Hurricane Katrina in context.
Climate has been humanity's constant, if moody, companion. At times benefactor or tormentor, climate nurtured the first stirrings of civilization and then repeatedly visited ruin on empires and peoples. Eugene Linden reveals a recurring pattern in which civilizations become prosperous and complacent during good weather, only to collapse when ...
The Winds of Change places the horrifying carnage unleashed on New Orleans, Mississippi, and Alabama by Hurricane Katrina in context.
Climate has been humanity's constant, if moody, companion. At times benefactor or tormentor, climate nurtured the first stirrings of civilization and then repeatedly visited ruin on empires and peoples. Eugene Linden reveals a recurring pattern in which civilizations become prosperous and complacent during good weather, only to collapse when climate changes — either through its direct effects, such as floods or drought, or indirect consequences, such as disease, blight, and civil disorder.
The science of climate change is still young, and the interactions of climate with other historical forces are much debated, but the evidence mounts that climate loomed over the fate of societies from arctic Greenland to the Fertile Crescent and from the lost cities of the Mayans in Central America to the rain forests of Central Africa. Taking into account the uncertainties in both science and the historical record, Linden explores the evidence indicating that climate has been a serial killer of civilizations. The Winds of Change looks at the present and then to the future to determine whether the accused killer is on the prowl, and what it will do in the future.
The tragedy of New Orleans is but the latest instance in which a region prepared for weather disasters experienced in the past finds itself helpless when nature ups the ante. In the closing chapters, Linden explores why warnings about the dangers of climate change have gone unheeded and what is happening with climate today, and he offers perhaps the most explicit look yet at what a haywire climate might do to us. He shows how even a society prepared to absorb such threshold-crossing events as Katrina, the killer heat wave in Europe in 2003, or the floods in the American Midwest in the 1990s can spiral into precipitous decline should such events intensify and become more frequent.
The Winds of Change places climate change, global warming, and the resulting instability in historical context and sounds an urgent warning for the future.
"Should be required reading for policy makers across the globe."
— Doug Macdougall, The Chronicle of Higher Education
"Linden is measured and takes a penetrating historical view.... Its scope and seriousness are impressive.... Its links between areas of study are very valuable."
— Janet Maslin, The New York Times
"Readers who wish to understand the reality of global warming and climate change should read Linden."
— Carl M. Bender, St. Louis Post-Dispatch
— Meryl Streep, The Green Guide
"For a quick window into how easily Mother Nature can wipe us off the mat for being too lazy to alter our habits, there is no book quite so relevant as Eugene Linden's The Winds of Change."
— John Freeman, Newsday
"Beautifully written, The Winds of Change is a very thought-provoking volume. Linden manages to weave history, science, and narrative together in a compelling way."
— Andrew Goudie, Science
"Linden uses in-depth research and expert opinion, rather than scaremongering and exclamation marks, to make his point.... He expertly and succinctly describes the natural cycles that control climate and the many ways they interact. He has a great knack for metaphor."
— David S. Reay, Nature
I've been writing about nature and the environment since the early 1970s. I've watched as some environmental catastrophes have materialized, while with others it has been "Never mind." Lake Erie did not die as some predicted when I was a college student, and, as skeptics of global warming point out constantly, we have not begun a plunge into a new ice age, which was the prediction of some climatologists in the 1970s. On the other hand, even decades of insistent warnings could not prepare Americans for the actual horrors that Hurricane Katrina unleashed in August 2005. Turbo-charged by complacency, folly, and incompetence, Katrina destroyed a great city, transforming New Orleans into a septic stew of floating bodies, roaming gangs, disease, and toxic slime. The storm launched a wave of refugees not seen in the United States since the Dust Bowl, and the damage inflicted on crucial energy and transport infrastructure sent ripples throughout the economy.
If there is a message to take away from a look back at past predictions of potential calamity, it is that the risks of erring on the side of caution tend to be fewer than the costs of dismissing predicted threats out of hand. Alarms about Lake Erie mobilized people and governments to take action, and in proving doomsayers wrong, the cleanup also created billions of dollars in value as the lake area became a draw for real estate and recreation. While in the United States officials took action to clean up the air and water beginning in the 1970s, elsewhere in the world environmental threats such as deforestation and extinction are more critical today than they were thirty years ago.
Perhaps the most revealing aspect of this look backward to the early 1970s, however, is the threats that were not there, but which have since risen to prominence. Most prominent would be the possibility of climate change. In the late 1970s, climate specialists first started worrying about the possibility (and indeed a report submitted to President Jimmy Carter in 1979 was right on the money about noticeable changes in climate by the year 2000 if nothing was done to check emissions of greenhouse gases), but with the Iranian hostage crisis and stagflation dominating public concerns, the warning got little notice. The possibility that humans might be altering climate (performing a global experiment with us in the test tube, as some scientists put it) only became an issue in 1988, when Washington sweltered during an abnormal heat wave at the same time that Senator Timothy Wirth held hearings on the issue.
Assignments have taken me to both polar regions and out into the Gulf Stream in attempts to keep pace with the science of this unfolding story. Since 1988, public concerns about climate change have waxed and waned with the weather, but in the United States at least, climate change has not been a pressing issue for the public despite periodic alarms raised by scientists. I have more to say about this in Chapter 18, but at least part of the problem is that, for all practical purposes, the threat is unprecedented. In this respect, our attitudes toward climate change are a little like American attitudes toward terrorism before September 11, 2001, or the attitude toward tsunamis of a tourist visiting Phuket, Thailand, before December 26, 2004. With regard to climate, it's hard to imagine that we puny humans could affect something so all-encompassing as climate itself; it's hard to imagine what it would mean if climate started changing everywhere on earth; and today even those Americans who view climate change as a threat see it as an event that lies far off in the future.
After all, Americans suffer extremes of weather all the time without any long-term disruption of the economy. If climate change brings more extreme weather, the economy will absorb that too. Similar attitudes and such confidence might well have characterized the Akkadian priests in 2200 b.c., the rulers of the Old Kingdom in Egypt at that same time, the Mayan elite in a.d. 900, the Anasazi in the American Southwest, the Norse settlers in Greenland before a.d. 1350, and many other societies and civilizations which would discover that climate, as oceanographer Wallace Broecker puts it, "is an angry beast."
We humans have a difficult time estimating risk. We spend disproportionate energy worrying about statistically insignificant risks — e.g., being attacked by sharks — and yet are blasé about the risks of getting behind the wheel of a car. We are probably at our worst when estimating the risk of something, such as global climate change, that has not yet happened, or happened long ago.
For present-day Americans, the threat of climate change may be abstract because it is unprecedented, but the impact of climate change on other civilizations is not without precedent. And so perhaps the best way to understand the risks might be to look back at the ways in which climate change has affected successful civilizations in the past. This is an undertaking that has become possible only in the last decade (although visionary climate historians like H. H. Lamb began writing about the impact of climate on history in the 1960s), since prior to the 1990s, the picture of past climate was spotty, and in many cases the resolution too crude to link particular historical events with weather at a given time.
Until very recently, climate has been viewed as static. It was only in the mid nineteenth century that scientists discovered the wrenching changes of the ice ages, but even after that, the prevailing attitude was that the present 10,000-year warm period that gave rise to civilization was monotonously stable. So long as climate was viewed as predictable and stable, there was no pressing need to consider it a factor in the fate of civilizations. (It was the discovery that the warm periods between glacial eras tend to last 10,000 years that in part prompted fears of a new ice age in the mid-1970s.) A scientific paper published in 1997 entitled "Holocene Climate Less Stable Than Previously Thought" shows that the notion that our present climatic era has been boring and predictable persisted until very recently.
Nor have historians and archaeologists greeted climate historians with open arms. Those reconstructing the fate of ancient civilization already have to deal with a full plate of competing factors that could bring down a civilization without any deus ex machina like climate. An archaeologist who has been studying a matrix of trade relations, warfare, internal strife, and political intrigue is not going to drop everything when a paleoclimatologist says, "The weather did it."
In certain cases, however, the evidence is pretty compelling, not just in linking weather to a particular event, but also specific ways in which a changing climate may have undermined the legitimacy of rulers. In some cases, climate change fostered the spread of disease; in others, climate change might have set in motion a chain of events that led to migration and warfare. In one well-documented case, the cold alone made life untenable. The interplay of climate, politics and economies is complex, but there is evidence from the past that helps us sort this out.
I offer evidence that we disregard the role of climate in history at our peril. I've structured the book along the lines of a case. The opening section presents the prosecution's argument that climate change has either killed off or at least been an accomplice in the fall of several civilizations. It quickly runs through the various victims (and a notable evolutionary beneficiary), and also details the weapons and methods of this civilization killer.
The first chapter of Part Two explores how environmental factors, including climate, have fallen in and out of favor as forces affecting history. Subsequent chapters in this section offer a brief description of the gears of our climate system and then look into the forensics of climate history, describing and assessing the various proxies that paleoclimatologists use to reconstruct past weather. The section also suggests some big unanswered questions about past climate and how climate works, questions that have a bearing on our assessment of the present threat of climate change.
Part Three revisits the cases introduced in the opening section; it presents dissenting opinions and digs deeper into the implications of the proxy evidence. Part Four looks at El Niño as a force in history. Although that familiar event is not nearly as disruptive as other climate events of the more distant past, this regular cycle has had huge impacts on humanity at different times. Some historians argue that a series of El Niños in the late nineteenth century killed more people than the two world wars of the twentieth century combined. Moreover, there is a detailed record of El Niño's role in various historical events that reveals both the resiliency of the modern market economy as well as new vulnerabilities to changing climate.
In Part Five, we return to the present. The first chapter looks in detail at the peculiarities of the climate-change story as it has unfolded since the threat first surfaced. In the next chapter, I join a research expedition in the Gulf Stream to check the health of one of the vital organs of the global climate system. The final section draws on what happened in the past and what is happening in the present to develop a scenario of what we may face in the future.
We have an advantage over past civilizations that were blindsided by climate change. We can learn from their misfortunes.
Copyright © 2006 by Eugene Linden
The Gears of Global Climate
At a Rockefeller Brothers Foundation conference in Pocantico Hills, New York, in 2004, scientists, activists, and media and advertising people concerned with environmental issues were each asked to name the most effective piece of environmental advocacy of the modern environmental movement. Without hesitation, I cited the images of the earth that were beamed back in the early days of space exploration. Though the impetus of the space race had nothing to do with environment, for the first time, humanity had an opportunity to view the planet whole, and see the vast oceans, air currents, and weather systems that tie the inhabitants of the planet into one system. Glimpsing the vulnerability of life on earth prompted a powerful protective response from millions of people.
Decades after these first images, I think it will still be years before we realize the true impact of seeing the home planet from space. For the first time, answers lay before scientist's eyes about some of the linkages that connect the land, seas, and sky. The new view of earth also prompted questions. Given the brutal and extreme forces that assault the planet continually, how is it that earth maintains the delicate balance necessary for life?
Those images and that question gave new impetus to a hypothesis first posed by an inventor-scientist named James Lovelock in the 1970s. Named the Gaia hypothesis after the ancient earth goddess of the Greeks, the idea holds that the planet is alive and functions as a superorganism in which living things interact with geophysical and chemical processes to maintain conditions suitable for life. In his first formulations, Lovelock imputed purpose to the interactions that maintained life on earth. He later accepted that even the exquisitely precise balances of the atmosphere (oxygen levels have remained at roughly 21 percent for 200 million years — if that percentage rose to 25 percent, fires would spontaneously break out; if it dropped below 15 percent most mammals would suffocate) might be perpetuated and stabilized by simple feedback. In a hypothetical scenario, Lovelock showed that a planet covered by light- and dark-colored daisies could control the heat received from the sun. In this self-regulating model, dark daisies would absorb sunlight and warm the planet, until it became too hot for the dark daisies and instead favored the proliferation of light-reflecting daisies. That would have the effect of cooling the planet until the cycle reversed itself again.
Regardless of whether earth amounts to a superorganism, the Gaia hypothesis offers a helpful model for envisioning the intricate interconnections that maintain and balance the climate system. Indeed, perhaps the biggest beneficiaries of space imagery were geophysicists and other climate specialists who could now see whole climate systems that previously had to be pieced together from data collected around the world. The tremendous bounty of a new perspective on global climate was as humbling as it was invigorating. Here before scientists was a system in which everything, from earth's position in its orbit around the sun to what's growing on the ground, influences climate. How the climate system balances these various inputs and feedbacks is a problem as complex as life itself.
Let us take a brief tour through the basic climate system. The tour starts with a description of the way the spinning globe distributes the sun's heat. Then it moves into the cosmic and planetary events that change climate over long timescales, and subsequently describes some of the shorter cycles that ripple through climate in response to these big events. The tour concludes with a description of where we are now in the interplay of climate cycles, big and small.
The basic engine of climate is the sun, which delivers a reasonably steady amount of energy to the earth each day. (That energy changes over time though on very long timescales.) Think of it as an allowance given daily to the planet, mostly at the equator. The amount of energy and its distribution vary according to factors that affect temperature, precipitation, and wind on timescales ranging from hundreds of millions of years to the change of seasons. Through blind physics and biophysical and geochemical feedback, the system maintains relative stability even as one or another variable changes. Some of these key factors include variations in earth's orbit, the spinning of the globe, movement of the continents and the resulting array of oceans and landmasses, the shape of the ocean floor, mountain ranges on land, sunspot activity, the composition of the atmosphere, the waxing and waning of ice sheets and glaciers, the color of land and water, as well as extraordinary events such as direct hits from giant meteors or episodes of volcanism. What is living on the planet can have big effects as well. Deforestation, for instance, can have large effects on regional climate and can impact rainfall patterns far away from the affected forests. Picking one's way through the interplay of climate cycles and the biosphere is like entering a wilderness of mirrors. It's hard enough to identify the various forces and conditions that impinge on climate; sorting out the myriad interconnections of these factors introduces a new level of complexity.
Today about 30 percent of the sun's incoming radiation bounces off light-colored surfaces such as ice, snow, clouds, and sand, and is reflected back to space without heating the earth (obviously, the amount reflected can vary with changes on the planet that affect the reflective surfaces of the planet ). Overall, the energy budget of the planet finds a balance. If more energy came in than was used on earth or radiated back to space, the planet would steadily warm until that budget came into balance. Conversely, if the energy used on earth or radiated back to space exceeds the amount coming in, the planet would steadily cool.
Of the energy that gets through, a good portion of the daily allowance of the sun's energy enters near the equator, where light rays hit the earth most directly. (The tropics can't get too hot because if sunlight becomes stronger, evaporation simply makes more clouds, which reflect heat before it hits the surface.) Were the earth not spinning, this energy would pile up, and, following the laws of physics, gradually spread north and south from the equator, rising in the atmosphere and spreading through water vapor in air currents, while fingers of warm water snaked toward the poles in the oceans. The world is spinning of course, which complicates this dry exercise in energy flows.
As the globe spins on its daily rotation, the atmosphere that cloaks the globe spins with it. If it didn't, winds at the equator would be 1,000 mph as the earth hurtled through the still air. In Stockholm and Anchorage, however, these winds would be about half as fast, and at the poles, there would be no wind at all. The spinning, and its different speeds at different latitudes, have large effects on the capture and distribution of the sun's energy as it warms land and water.
Imagine a child standing on the equator with a very large squirt gun pointed due north. When he squeezes the trigger, he launches a jet of water northward and, at first, as the stream of water leaves the gun, it heads due north. But as this jet is moving north, the earth is rotating toward the west underneath the water so that the path the water takes over the land seems to curve toward the right. That's because the only force impelling the water was the initial squeeze of the trigger. The only way the child could make the squirt move straight north would be to find a way to exert constant westward pressure on the jet to compensate for the earth's rotation. Otherwise, as the water moves north, its path trails off to the east as the initial force dissipates in response to friction and gravity. If he turned around and squirted his gun due south, the same thing would happen; only the stream would trail off to the left.
This apparent force is called the Coriolis effect or force, named for Gustave-Gaspard Coriolis, a French mathematician who first described it in 1835. It has major effects on climate. As heated air rises over the equator and begins spilling north, the Coriolis effect diverts its movement toward the east. Along its route, the air cools and begins sinking in the horse latitudes between 25 and 30 degrees north and south (so named because sailing ships would often become becalmed in these areas of light winds and the crew would sometimes throw horses overboard to lighten the load). The sinking air is drawn back toward the equator by the low pressure created there by rising heated air. As this air flows south and west, drawn back in as the rising air creates a partial vacuum, it forms the trade winds, so named because trading captains relied on these constant winds.
The whole loop, consisting of air rising in the tropics, sinking in the doldrums, and then flowing back toward the tropics, is called a Hadley cell. An idealized diagram of this cell as a vertical slice of the atmosphere would look like one big gear rotating counterclockwise. Horizontally, it would extend between the equator and 30 degrees north and vertically between sea level and 50,000 feet. North of the Hadley cell, neatly occupying the latitudes between 30 and 60 degrees north, would be another giant gear called a Ferrel cell. The Ferrel cell lies between the Hadley cell and a third cell that lies from 60 to 90 degrees north. This Polar cell circulates warm air northward that then cools and sinks at the poles to return southward at the surface. Caught between these two gears, the Ferrel cell circulates air toward the north at the surface, rises where it meets the Polar cell, and then returns air southward at high altitudes. The Coriolis effect bends this surface northward flow to the east, giving the northern hemisphere its characteristic westerlies. The southern hemisphere mirrors this three-cell system.
Driven by heat collecting at the equator and the spinning of the earth, this collection of atmospheric gears moves about half the excess heat collected at the poles toward the northern latitudes. Were it not for the Coriolis effect, which diverts this flow eastward, the poles would be much warmer (this is the situation on Venus, where the Coriolis effect is much weaker because that planet rotates very slowly). About 50 percent of this atmospheric transport of heat comes in the form of storms, which draw in warm, moist southern air and then move north. Storms carry much of that heat in water vapor and then release the energy as the water vapor condenses in the colder air. It takes 540 calories to evaporate a gram of water, for instance. That evaporated water travels until it hits cooler air, which has less capacity to hold water vapor. As the moisture condenses into droplets, a good portion of those 540 calories returns to the atmosphere, warming the air. Hurricanes can move massive amounts of heat, cooling the tropical waters and warming the air by releasing heat as they weaken while moving north.
The other big heat distributors on the planet are ocean currents — such as the Gulf Stream — that move the other half of the equator's excess heat toward the poles. The atmosphere and ocean are entwined, each constantly affecting the other. Oceanographers and atmospheric scientists use the phrase "coupled system" to describe the interplay of two of the dominant factors in the earth's climate. It's a system that distributes heat in three dimensions. Everywhere, when warm air is rising, cooler air is being pulled in to replace the rising air. Similarly, in the oceans, cool water surfaces to replace warmer water driven by winds along the surface.
The trade winds drive warm Atlantic Ocean water westward along the equator, where it eventually piles up when it encounters the geographical barrier of Panama (more on this later). This movement of surface water has two major effects on climate. On the one hand, as the surface water is blown westward, an upwelling of cooler, nutrient-rich deep water rises to replace it. Thus, just to the north and south of the equator in the Atlantic and Pacific are two zones of cool waters in spite of the fact that they receive the sun's most intense energy. The wind-driven warm water on either side of the equator eventually spills north or south, guided by the winds and the contours of the ocean basin.
The warm water spills north and south through a series of ocean currents that mirror to some degree the cells in the atmosphere above the water. Like the atmospheric cells, for instance, these giant oceanic gears — called gyres — fall under the influence of the winds and the Coriolis effect and form loops that move water first away from the equator, then parallel, and then back to rejoin the equatorial currents. The familiar Gulf Stream is one of these gyres, and because of its unique characteristics it has particular salience to the fortunes of many civilizations.
Alone among the equatorial gyres north of the equator, the Gulf Stream has extensions that deliver heat very far from the equator. The North Atlantic Current and North Atlantic Drift drag heat much farther north than any Pacific Ocean current. Thanks to this peculiarity of the Atlantic currents, temperatures along the Norwegian coast are about 25 degrees Fahrenheit warmer than places such as Nome, Alaska, that lie at equivalent latitudes. This relative warmth helps explain why European peoples developed material cultures far more elaborate than tribes inhabiting equivalent latitudes elsewhere: simply put, it's hard to build a civilization on lichens.
The Gulf Stream collects its warm water from equatorial currents that flow up from the Yucatán Channel through the Florida Straits and in from the north of the Windward Islands. Then, driven by southwesterlies and corralled by the continental shelf, the warm water heads northeast across the Atlantic. Most of these waters turn eastward and then head south to rejoin the equatorial flow, but a portion splits off and wanders up past the British Isles, Iceland, and Scandinavia.
The Gulf Stream is, in essence, a gigantic hot river moving through the Atlantic. It has been pumping a portion of its warm water to the far north for at least 2.5 million years. More than 60 miles wide in many places, and with an average depth of 3,300 feet, the Gulf Stream books along, moving as fast as five knots. It moves staggering amounts of energy. Water weighs 773 times as much as air, which means that a relatively small amount of water can store a huge amount of heat. There's as much heat in a cubic meter of water at the surface of the ocean as there is in the entire seven-mile-high column of air above it.
In an idealized climate system on a static and uniform globe, the Gulf Stream would be just another giant ocean gyre, and climate would be pretty predictable, and vary gradually and predictably as earth wobbled on its axis and moved through its orbit around the sun. But the globe is not static and its surface is not uniform. In fact, the peculiarity of the Gulf Stream that generously delivers heat to the far north is partly the result of one of the biggest and longest influences on climate.
If a habitable northern Europe is in part a gift of the Gulf Stream, the current's northward wanderings represent, in turn, a gift of continental drift. Of all the variables that can change climate, the position of the major landmasses is probably the longest cycle of all — on the order of 100 million years — and the shifting of landmasses over time has played an enormous role in creating the perfect climate for humanity. On these timescales the present path of the Gulf Stream is one of the more recent developments.
As noted in the first chapter, the Gulf Stream and its northern limb comprise crucial portions of what Wallace Broecker described as the Global Ocean Conveyor Belt (sometimes called the Great Ocean Conveyor), a continuous loop of deep ocean and surface currents that distribute enormous amounts of heat during its 1,000-year cycle. Before there could be a conveyor (at least in its present form), a number of pieces had to fall into place. One of them was the formation of ice sheets in Antarctica, and here too plate tectonics played a role.
The growth of the first ice sheets in Antarctica may have resulted in part from tectonic shifts that isolated the continent. A belt of globe-circling winds formed a polar vortex, which acted as a barrier to storms in the midlatitudes that might deliver heat to the continent. Nurtured by this thermal isolation (there are various theories for the specific mechanism that built the ice sheets, ranging from a shift in ocean currents from warm to cold to a precipitous 80 percent drop in CO2 in the atmosphere), ice sheets grew up to 3 miles thick (15,600 feet to be exact), entombing 70 percent of the world's fresh water and lowering sea level several hundred feet in the process.
It's hard to imagine the scale of the Antarctic Ice Sheets. When I flew from McMurdo Station to the South Pole in 1997, our route took us along the Transantarctic Mountains. Many of these peaks rise over 14,000 feet — as tall as the highest peaks in the lower forty-eight states. I could see the scale of these mountains as they rose above the Ross Ice Shelf, but on the other side of the mountains an endless sea of ice, starting just below the peaks as though the mountain range was the rim of a bathtub, extended for over 2,000 miles.
The changes that brought about the growth of ice sheets had other effects as well. In Antarctica, the seasonal appearance of sea ice every winter more than doubles the size of the continent. This has its own profound effects on climate. Instead of sunlight hitting dark, heat-absorbing seawater and land vegetation, it now bounces off the brilliant white of ice and snow. Apart from reducing the amount of energy captured by the oceans and atmosphere, the ice regime of the southern ocean also provides one crucial element of the Global Ocean Conveyor.
Here's how the Antarctic component of the conveyor works: As the south Atlantic portion of the conveyor moves into the Southern Ocean, the Coriolis effect diverts it to the east. Since this open ocean completely circles the globe at this latitude, there is no landmass to give the current a purchase to move farther south, except for very deep water that follows the contours of the sea floor. During the sea ice season, some of this already cool water freezes into sea ice, which has two effects. On the one hand, as it freezes, it releases a small amount of heat, and, secondly, as fresh water is captured in sea ice, the remaining seawater becomes saltier. This cooler, salty water is denser than surrounding waters and sinks to become what is called deep water.
The formation of what is called deep water is one of the main engines of this gigantic system of heat distribution in the oceans. As it sinks, this deep water pulls the conveyor as water moves in to replace the sinking water. There are only a few places on earth where such deep water forms. Two are off the coast of Antarctica, and the other two are in the far north, one spot between Iceland and Greenland, and the other in the Norwegian Sea. The way deep water is formed and sinks in Antarctica is thus slightly different than deep water formation in the North Atlantic, although the engine of both systems is the same.
Another major building block of the present form of the ocean conveyor and modern climate was the uplift of the Isthmus of Panama. This event entirely closed the ancient Central American Seaway between 2.5 million and 3 million years ago, blocking the westerly flow of water from the Atlantic to the Pacific. In turn, this set in motion a cascade of repercussions, creating the modern Gulf Stream as this new barrier deflected warm waters northward, while perhaps contributing to the glaciation of the north polar latitudes. Contemporaneous with these events was the onset of the most recent series of ice ages.
As the newly formed Gulf Stream moved northward, it warmed the climate of the North Atlantic. The main northern extension of the Gulf Stream is called the North Atlantic Current, and farther north this current becomes the North Atlantic Drift, a slow-moving wide expanse of water that significantly warms its surrounding area. Some estimates suggest that the heat released by the drift adds between 15 and 25 percent (depending on the latitude) to the energy northern Europe receives directly from the sun.
As the drift cools and as evaporation makes it saltier, the water becomes denser and heavier. In "normal" times, a good deal of the drift passes over a series of underwater sills, such as the underwater ridge that lies between Iceland and the Faroe Islands. Once over this sill, which averages several hundred meters in depth, the weight of the dense, cool, salty water causes it to plunge into the depths of the Norwegian Sea. Other lobes of this current sink in the Labrador Sea, and this cold, salty deep water then begins its journey south. As in the Antarctic, the "pull" of this sinking water is the engine that brings warm water north. Because the water entering the North Atlantic is so much warmer than that entering the Antarctic, however, it has a bigger impact on surrounding climate than the meager heat given up during Antarctic deepwater formation.
While most oceanographers stipulate the contribution of thermohaline circulation to Europe's climate, some oceanographers take issue with the degree to which this heat warms the continent. For instance, Richard Seager of Lamont-Doherty led a study published in the October 2002 issue of the Quarterly Journal of the Royal Meteorological Society. Based on observational data and climate models, Seager and his collaborators argue that almost all of the relative warmth of European winters can be explained by atmospheric transport of heat thanks to the disruption of the path of planetary winds by the Rocky Mountains (the mountains temporarily deflect the jet stream southward and the winds pick up heat and moisture released by the ocean in winter before they get to Europe), and by the release of ocean heat stored locally rather than transported by the North Atlantic Drift. In other words, Europe is warm because it lies on the edge of a continent on the eastern side of an ocean.
That being said, the authors of the study acknowledge that the THC does have important impacts above 60 degrees north, and that by secondary effects, such as dampening the formation of sea ice, it might impact weather farther south as well. Even the most vociferous proponents of the role of THC in Europe's climate do not ascribe all of Europe's clement climate to heat transported northward. Terrence Joyce of the Woods Hole Oceanographic Institution fixes the amount as between 15 and 25 percent, depending on the latitude, but notes that even at the lower end of that estimate the marginal warmth provides the difference between harsh frigid winters and the more manageable temperatures of more normal conditions.
There are many unanswered questions about the THC. No one is really sure why this deepwater formation only occurs in the Atlantic Ocean in the northern hemisphere. The North Pacific is less salty than the North Atlantic, which means that the ocean currents do not have the density to sink as they do in the Atlantic. Some climate modelers speculate that the rivers running back to the Pacific from the coastal mountain ranges of North America collect and return evaporation from the Pacific, continually pouring fresh water into the ocean. It's possibly that the bathymetry of the North Atlantic, with its underwater ridges and choke points, plays a role as well. Reconstructions of past movements of the North Atlantic Drift suggest that during the ice ages thermohaline circulation varied in intensity. At times it shut down or diminished, and during these cool periods, the North Atlantic Drift shifted to the south. These shifts played out repeatedly during the wild climate swings of the most recent ice ages, which began some 2.5 million years ago.
What caused the ice ages also remains an open question. Some experts on paleoclimate attribute the onset to the repercussions of the rising of the Isthmus of Panama (thereby diverting an enormous amount of water vapor northward, which provided the raw materials for ice sheets), while Mark Cane, an oceanographer based at Lamont-Doherty Earth Observatory (and one of Seager's coauthors), argues that uplift in the western Pacific laid the groundwork for these latest ice ages. Between 3 million and 4 million years ago, the northward drift of New Guinea and the surfacing of parts of the Indonesian archipelago would have rejiggered the ocean currents of the Pacific, replacing the flow of warm water from the South Pacific into the Indian Ocean with cooler waters from the North Pacific, while also changing the currents that formerly carried warm water into the upper latitudes. This latter change could have precipitated the ice ages, argue Cane and his MIT-based collaborator Peter Molnar, while the cooler waters entering the Indian Ocean would have reduced the rainfall for East Africa.
Whatever combination of events started the ice ages, the general cooling and drying was punctuated with great swings of climate, periods during which climate would remain unstable for 100,000 years or more. Ice ages tend to be more tumultuous than interglacial periods in no small measure because of the impact of ice on the ocean conveyor. But the fault lies in the stars as well, or at least in earth's orientation as it orbits the sun. Most likely, plate tectonics set the table for the beginnings of the ice ages, and earth's orbital dynamics supplied the chill.
The longest orbital pulse that changes climate involves the regular rounding and flattening of earth's orbit as it circles the sun. One full cycle takes about 100,000 years. This seems to coincide with the spacing of ice ages. As earth slowly chills at about .01 degree centigrade per century, ice builds up over a 90,000-year period. Then in the next 10,000 years, it all melts. Ice sheets have complicated and sometimes counterintuitive dynamics of their own. In The Two-Mile Time Machine, for instance, Richard Alley argues that big ice sheets can melt more rapidly than small ones, and that small changes in sunlight over the long term can have large impacts on ice sheets, while big changes over the short term have little impact.
Clearly, it's not as simple as "farther away during elliptical part of the orbit equals ice age" because where earth is during this orbit must also be understood in terms of whether an effect on climate is enhanced or muted by other aspects of orbital dynamics. For instance, another cycle involves the tilt of the planet as it travels through its orbit. During this journey, the angle of its spin axis gradually shifts between 22 degrees and 24 degrees and back. The inclination of this axis explains why the earth has seasons (one hemisphere will have nearly maximum tilt toward the sun at the start of the planet's annual orbit and then maximum tilt away 180 days later). Also, the degree of inclination toward the sun either exaggerates or diminishes the contrast of the seasons.
This is easy to envisage because the less tilt to the earth's axis, the less difference there will be in the amount of sunlight received by either pole at a given time. The greater the tilt, the more contrast between summer and winter temperatures. This constant seesaw between 22 and 24 degrees of inclination has a period of 41,000 years.
Then there is the wobble of earth's spin axis. Imagine the spin axis as a very long stick that entered the earth at the North Pole, went through the center of the planet, and exited the South Pole. The angle of that stick would vary between 22 and 24 degrees as just described, but the end of each stick would also trace a rough circle as the spin axis wobbled. This circle is called precession, and the wobble completes a circuit roughly every 19,000 to 23,000 years (the true period of precession is somewhat longer, about 26,000 years, but it is effectively shortened by the rotation of earth's orbit). Precession's effect on climate is to change which hemisphere has winter or summer when the earth is closest or farthest from the sun.
Precession becomes more or less significant depending on where we are in earth's 100,000-year oscillation between a flat and round orbit around the sun. The effect of precession is to change the angle of the rotation axis of the earth relative to the sun as it traces its 20,000-year circle, changing which hemisphere is tilted toward the sun at different points in earth's orbit. When earth is in the rounder part of its 100,000-year pulse between round and elliptical, precession is not so important, because distance from the sun would not vary much, only about 1.6 percent at present, versus 5 percent at the most elliptical. At those latter times, however, when earth's orbit tends more toward the squashed-circle shape, precession can change which hemisphere has greater contrast between summer and winter.
Imagine, for instance, the situation when earth's orbit is more squashed so that its distance from the sun varies more significantly over the year. If during this phase the precession of earth's axis is such that the northern hemisphere is tilted away from the sun when the earth is farthest from the sun in its orbit, and tilted toward the sun when earth is closest, then the net effect would be to exaggerate the difference between summer and winter in the northern hemisphere. In the southern hemisphere, however, the situation would be the opposite since it would be summer when earth was receiving less radiation and winter when it was receiving more, reducing the contrast between the seasons.
These big cycles have an impact on the occurrence of ice ages because they change how much solar radiation hits the earth, where it hits and when. The recipe for growing ice sheets, for instance, depends more on short, cool summers than it does on how cold it is in the winter. Key variables are how long temperatures are below freezing and how little they rise above it, not how far below freezing temperatures plunge.
The long climate cycles are like an ultralow-frequency ringing, and like any note when struck, they have harmonics. Ice core specialist Paul Mayewski of the University of Maine speculates that another 11,000-year cycle revealed by the ice core data represents just such a "harmonic" or ringing set in motion by the reverberations of the longest cycles.
Over the years, specialists in paleoclimate have noticed several other cycles out of ice cores taken from ice sheets and glaciers, from sea- and lake-bed sediments, from tree rings and other encrypted records of the past. For instance, during the recent ice ages, there was a regular cycle that saw temperatures fall and then rise over a roughly 6,100-year period. Described first by Hartmut Heinrich, a pioneering German marine geologist, these events occur during the coldest part of an ice age. According to Mayewski, Heinrich, Gerard Bond of Lamont-Doherty, and other climate scientists, this cycle is likely a lagging response of ice sheets to changes in solar radiation.
As the ice sheets grow, their internal dynamics eventually cause them to spread out into the seas and eventually peel off armadas of icebergs. Richard Alley argues that this is because, as ice sheets grow, they trap more and more heat underneath them, eventually melting their frozen bond with the underlying earth. This melted layer allows the sheet to surge outward. In turn, the flood of icebergs would cool the ocean and air. That would lead to an increase in sea ice, which would further cool the air by reflecting sunlight and trapping ocean heat beneath the ice.
Many scientists see the sea ice as a necessary amplifier of abrupt climate shifts. Jeff Severinghaus of the Scripps Institution of Oceanography describes one way this might happen. A shutdown of the conveyor sets the stage for a rapid increase in sea ice (in Antarctica, during winter, sea ice expands by 30,000 square miles a day), and once sea ice covers the northern oceans, "the atmosphere thinks it's a continent," he explains. Continents have no memory for temperatures. What this means is that they can't retain and dole out heat the way an ocean can, except on very short timescales. Moreover, the white surface of ice is about eight times more reflective of heat than open water, so the spread of sea ice provides a natural amplifier of cooling by both trapping the relative warmth of the ocean beneath it and by increasing the amount of heat reflected back into space.
The transfer of so much ice to the oceans sets in motion other cycles. Alley suggests that diminished ice in Hudson Bay, for instance, would reduce the cooling influence of winds flowing off the ice sheets. Moreover, all this fresh water introduced by icebergs would at first shut down thermohaline circulation and then, perhaps as more and more fresh water was taken up and sequestered in sea ice, eventually restart the conveyor on a roughly 1,500-year cycle. Noteworthy about these Dansgaard-Oeschger cycles (as they are named) is that the cooling and warming at either end occurs quite suddenly, with major warming and cooling shifts taking place in as little as a few years. The late Gerard Bond of Lamont-Doherty identified a pattern, now called a Bond cycle, in which these events become progressively cooler following each big Heinrich event, until the cycle ends with a very big warming.
Complicating all of this are both the big orbital cycles as well as more frequent cycles of solar variability caused by events such as sunspots. The effects of volcanic eruptions and the occasional comet striking the planet also skew the climate record. No wonder Richard Alley was moved to use the following metaphor for climate cycles, which sounds like something out of Gilbert and Sullivan: "You might think of a roller coaster riding the orbital rails, with Heinrich-Bond jumping off the roller coaster while playing with a Dansgaard-Oeschger yo-yo."
In the course of the past 100,000 years, there have been seven Heinrich events (they were marked by bigger intervals at the beginning because earth's orbital position was less conducive to growing ice sheets). The last one — dubbed Zero — was the Younger Dryas; it was, as noted, a doozy, plunging temperatures by as much as 27 degrees Fahrenheit in parts of the world in less than a decade. Beginning roughly 12,700 years ago, it ended about 1,300 years later with an equally dramatic warming that led into the current warm period.
Eclipsed by seismic climatic events such as the Younger Dryas are smaller perturbations of climate that take place over much shorter timescales. The North Atlantic Oscillation and the Pacific Decadal Oscillation describe regime shifts in atmospheric pressure over the North Atlantic and North Pacific that seem to change over twenty-year periods. The now familiar El Niño reworks storm tracks and rainfall patterns around the world every few years. When compared with the major climate shifts of Dansgaard-Oeschger cycles or Heinrich events, even the most severe El Niño would barely budge the needle. Even so, the 1997-98 El Niño did more than $100 billion damage to the global economy and killed tens of thousands of people.
There are other, more subtle influences on climate than changes in Earth's orbit, its attitude toward the sun, the oceans and ice, and the various harmonics of these systems. One is the composition of the atmosphere. If earth gets a fixed allowance of heat from the sun (at least on human timescales), the amount of the allowance that is retained has a great deal to do with the composition of the atmosphere. Incoming solar radiation is either reflected, absorbed by the earth, or sent back out toward space as infrared radiation. How much of that infrared escapes depends on the properties of the atmosphere. There are fourteen gases in the atmosphere, and most of them have little ability to trap outgoing infrared radiation. Those that do, such as CO2, methane, or nitrous oxide, are called greenhouse gases because, like a greenhouse, they keep heat from escaping back to space. Methane traps a lot of heat, but it is much rarer than carbon dioxide. It doesn't take a lot of a greenhouse gas to affect temperatures.
Atmospheric levels of carbon dioxide, by now the most familiar greenhouse gas, have risen or lowered in step with global temperatures for millions of years, although scientists only have reliable ice core data for the past 400,000 years. For instance, if there were 250 parts per million of CO2, earth's average temperature would level at about 57 degrees Fahrenheit. Now with 380 parts per million (the highest levels in 500,000 years), temperature has risen about a degree. If we reach 880 to 1,000 parts per million as expected within the next 100 years without reductions in emissions of carbon dioxide, the atmosphere will have heat-trapping abilities not seen for at least 30-to-40 million years, according to Daniel Schrag of Harvard, and at that time the planet was astoundingly hotter than it is now. Just a hundred or so ppm more than those levels and the atmosphere would resemble that of the Cretaceous period, when dinosaurs roamed on an earth described by Richard Alley as a "saurian steambath." In 2004, Schrag and Richard Alley published an article in Science in which they noted the disturbing fact that no present-day global climate model can produce the temperatures evident in the paleoclimate record from that time, raising the possibility of presently unknown feedbacks that might amplify a runaway warming.
Whether changes in CO2 lead or lag changes in temperature, and why levels rise and fall remains a matter of debate, but the correlations are strong. Writing in Science, Richard Kerr quotes Thomas Crowley of Duke University: "You can't say CO2 explains everything, but it does explain a heck of a lot." It is worth stressing that there is no dispute about the physics through which CO2 and other greenhouse gases retain heat in the lower atmosphere.
The most important greenhouse gas is water vapor, and it's also the most elusive. It's exceedingly difficult to reconstruct past cloud conditions. Clouds both reflect and trap heat, and whether they warm rather than cool the lower troposphere depends on how thick and how high they are. In the week following the terrorist attacks of September 11, 2001, the Federal Aviation Administration shut down air traffic, which meant that for at least that week there were no cloud contrails in the upper atmosphere. Not coincidentally, the contrast between day and night temperatures increased that week.
Any account of the orbital and other factors affecting climate naturally prompts the question "Where we are now?" Given all these moving and interacting parts, is it possible to pin down where we are now, or should be, according to the alignment of the earth, the dynamics of ice sheets, and other factors? The answer is yes. Sort of.
In terms of orientation in space, the earth is presently in the rounder part of the 100,000-year orbital pulse, which places us at the beginning of the 100,000-year ice age cycle. The tilt of earth's spin axis is inclined about 23.5 degrees, which by itself would accentuate the difference in seasons, but this effect is offset because of earth's current position in the precession of its axis. Presently, the northern hemisphere is tilted away from the sun when the earth is closest to the sun in its orbit, and toward the sun when it is farthest. Thus, during the northern winter when light hits the north less directly, that hemisphere is getting about 3 percent more radiation by virtue of earth being slightly closer to the sun, reducing summer-winter differences. At the same time, these seasonal differences are slightly exaggerated in the southern hemisphere, but here the effects of precession are tempered by the predominance of ocean water, which moderates seasonal shifts.
Given all the different factors that can screw up climate, we can thank our lucky stars for the rare syzygy of offsets that has been in place for 10,000 years or so. This is about as good as it gets in terms of orbital alignment. In The Two-Mile Time Machine, Richard Alley places us in the sweet spot of recent climate cycles.
Alley notes that the most stable periods in these 100,000-year cycles are the 10,000-year spans of coldest and warmest weather. We're now in that nice warm phase, and we would have to go back about 115,000 years to the Eemian interglacial era to find an equally warm and stable period. To extend Alley's metaphor of the climate roller coaster, these geologically brief warm periods are the equivalent of the pause at the top of the first big climb before the car begins its dizzying series of twists and turns and dips. If future climate carried forward what has happened in the past (and if humans had not evolved to rewrite the script), sometime in the future we would begin the next 90,000-year period of cooling, which would be an epoch marked by growing ice sheets and punctuated every few thousand years by violent, rapid, and extreme shifts as the ice grew. Based on the study of a European ice core project from Antarctica that provides a 740,000-year record, however, some paleoclimatologists argue that present conditions are more like the situation of about 400,000 years ago when a warm period lasted for over 25,000 years. If this turns out to be the case, we've got some time before we begin the plunge, unless we screw things up through our alterations of the atmosphere.
The Holocene may be a protective bubble of warmth among the ice ages, but ripples of climate change have occasionally intruded to upset the calm. Like mementi mori, abrupt climate events have periodically interrupted this halcyon period, reminding humanity that it thrives in a bubble. The instant chill of 8,200 years ago arrived after a warm stretch of a few thousand years. Big as this event was, however, Richard Alley estimates that it was only half as strong as the Younger Dryas.
Then, with the planet aligned so that conditions were favorable for hot summers and cold winters in the northern hemisphere, the climate entered a warm period that lasted until about 5200 B.P. (or 3200 B.C.). Warming early in this interregnum raised sea levels sufficiently to allow Mediterranean water to breach the Bosphorus and flow into the Black Sea, then about 500 feet below sea level. Although some paleoclimatologists challenge this idea, Lamont-Doherty marine geologists Walter Pitman and William Ryan argue in Noah's Flood that the breaching of the Bosphorus created an enormous cataract, two hundred times as powerful as Niagara Falls.* The monster waterfall raised the level of the Black Sea by 6 inches a day, drowning fields and trapping the unwary. According to this theory, refugees from the rising waters scattered both east and west, bringing agriculture to Europe. Moreover, Pitman and Ryan argue, ancestral memories of this unforgettable flood later surfaced in the book of Genesis and The Epic of Gilgamesh.
Apart from prompting biblical floods, the mild weather provided a benign context for the first civilizations, notably in China, Persia, and India about 6,000 years ago, and then 1,000 years later in Egypt. Human population, which UNESCO estimates grew from 5 million to 7 million between 10,000 B.P. and 6000 B.P., more than tripled in the next 2,000 years, as innovations such as irrigation increased agricultural production and provided some protection against the vagaries of the weather. By the dawn of the Christian era 2,000 years later, the population had grown by a factor of ten. During the next 2,000 years, human numbers grew by a factor of twenty-four. Now, every two weeks, we add numbers equivalent to the human population of the globe at the beginning of the present warm period. Clearly the Holocene has been a good time, at least for us.
There has been no event since the 8200 B.P. episode anywhere near the magnitude of that sudden freeze. Still, even small cyclical changes in climate (compared to the epic swings of climate during ice ages) can have outsized effects, particularly when a society has prospered around growing a particular crop in a particular place. There have been many such midsized events. Paul Mayewski's interpretation of ice cores taken from GISP2 in Greenland is that there were rapid climate-change events between 6,100 and 5,000 years ago, between 3,100 and 2,400 years ago, and one that began 600 years ago known to all as the Little Ice Age.
Copyright © 2006 by Eugene Linden
1. A Matter of Emphasis
2. The Deep Past: Climate as Creator
4. The First Victim
5. Weapons of Mass Destruction: Disease, Migration, Conflict, and Famine
6. Empty Promises of Water: The Collapse of the Mayans
7. The Little Ice Age: Five Hundred Years of Climate Chaos
8. Climate Comes into Focus
9. The Gears of Global Climate
10. Proxy Wars I: Ice
11. Proxy Wars II: Mud
Cross-Examination and Redirect
12. The Mystery of Tell Leilan
13. Scorched Earth
14. Is It Little Ice Age, or Ages?
El Niño: The Killer Next Door
15. El Niño: How It Works
16. El Niño Meets Empire
17. A Taste of Things to Come?
The Elephant in the Room
18. The Tides of Public Opinion
19. Water Moving Through Water
Closing Arguments: Are We Next?
20. Going Forward
Sea Change: Afterword to the Paperback Edition
Chronology: The Accelerating Pace of Climate Change and Scientific Discovery