Are ice age cycles of the Northern Hemisphere driven by processes in the Southern Hemisphere

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Are ice age cycles of the Northern Hemisphere driven by processes in the Southern Hemisphere?

Viewpoint: Yes, ice age cycles of the Northern Hemisphere are driven by complex forces in the Southern Hemisphere, and possibly even the tropics.

Viewpoint: No, the ice age cycles of the Northern Hemisphere are not driven by processes in the Southern Hemisphere; the Milankovitch cycle and disruption of the ocean's thermohaline circulation are the primary initiators.

Ice-age conditions—with a global temperature 9°F (5°C) lower than today's climate and glaciers that cover most of Europe, Asia, and North and South America in a deep blanket of ice—have predominated for 80% of the past 2.5 million years.

Ice ages come in 100,000-year cycles controlled by the shape of Earth's orbit around the Sun. The shape varies, becoming more circular or elliptical every 100,000 years. In the 1870s, the Scottish physicist James Croll suggested that ice ages were caused by insolation, or changes in the amount of solar radiation at the poles as a result of this 100,000-year change in orbit shape, and changes in orbital tilt (every 40,000 years) and wobble (every 20,000 years). By themselves, changes in the amount of solar energy reaching Earth are too small to affect the global climate. Somehow—no one knows exactly how—the changes interact with the atmosphere and oceans and grow into large global differences in average temperature. And no one knows yet why ice ages occur in both hemispheres simultaneously when changes in solar radiation from orbital variations have opposite effects in the north and south.

Despite gaps in knowledge, many hypotheses exist about what causes an ice age to begin or end. Some focus on the Northern Hemisphere as the connection between orbital variations and climate. In the 1930s, for example, the Serbian geophysicist Milutin Milankovitch suggested that orbital variations in solar radiation at 60°N drove the waxing and waning of ice sheets in North America and Europe. In 1912, Milankovitch had described the small but regular changes in the shape of Earth's orbit and the direction of its axis, a process now called the Milankovitch cycle. A confluence of these factors—maximum eccentricity (when Earth's orbit is most elliptical), extreme axial tilt (with the North Pole pointed most acutely away from the Sun), and precession, which delays and reduces solar radiation at high northern latitudes—could lead to a major ice age in the Northern Hemisphere.

A recent study of Antarctic seafloor sediment cores by an international team of scientists shows that changes in polar regions—particularly the advance and retreat of glaciers—follow variations in Earth's orbit, tilt, and precession as described in the Milankovitch cycle. The samples showed that Antarctic glaciers advanced and retreated at regular intervals during a 400,000-year period, and the glaciation and retreat cycle matched those predicted by Milankovitch, with increased glaciation at 100,000-and 40,000-year intervals.

Other researchers propose that the North Atlantic Deep Water circulation belt (NADW) may be an important amplifier of climatic variation that magnifies subtle temperature and precipitation changes near Greenland. The NADW is an enormous ocean current that moves huge quantities of equator-warmed water along the equator and up the coast of North America toward northwestern Europe. The engine that drives this current is near Iceland in the North Atlantic. Here, subsurface ocean water is very cold, and because it holds lots of salt, very dense. This cold, salty, dense North Atlantic current sinks to the depths and flows southward toward the equator, pushing immense volumes of water ahead of it. As it heads south the current warms, loses salt, becomes less dense, and rises toward the surface. At the equator the warm, low-saline current swings through the Caribbean and up again toward Europe. Near Greenland it is cooled by Arctic air masses and gets colder, saltier, and denser, then sinks and flows southward, completing the cycle.

Evidence shows that global warming affects the NADW and has led to periods of glaciation, and that this may be happening now. As air and sea surface temperatures increase, evaporation over the oceans increases. Greater amounts of water vapor rise, accumulate in clouds, and eventually fall as rain (fresh water). Much of the increased rainfall occurs over the North Atlantic, where it dilutes sea-water salinity and density, disrupting the NADW. As air temperatures increase, melting Arctic sea ice and Greenland glaciers further reduce North Atlantic salinity and density, pushing the North Atlantic current toward collapse. Collapse of the current would lower temperatures in northwestern Europe by 20°F (11°C) or more, giving Ireland a climate identical to that of Norway. Once snow and ice accumulate on the ground year-round, the onset of an ice age is rapid.

Other hypotheses about the origin of ice ages focus on processes in the Southern Hemisphere. In 2000, for example, Gideon Henderson of Lamont-Doherty Earth Observatory at Columbia University and Niall Slowey of Texas A&M University challenged the long-standing belief that processes in the Northern Hemisphere control ice age cycles. To understand what causes changes in Earth's climate, scientists must create a history of those changes. One way to do this involves removing long cores of soil or ice from Earth's crust. These cylindrical cores show layers of climate history, just as the rings of a tree trunk show its stages of growth. Long cores have been extracted from Greenland and the Antarctic, from marine sediment on ocean floors, and from the floor of a cave called Devil's Hole in Nevada, where terrestrial climate is preserved.

Henderson and Slowey said their study of marine sediment cores produced evidence that atmospheric CO2 (carbon dioxide) levels influenced the ice ages. The change in global atmospheric CO2 concentration was centered at 138,000 to 139,000 years ago, at the same time there was a peak in Southern Hemisphere insolation. This relationship suggested the change in CO2 was driven by a process in the Southern Hemisphere, and may initiate processes that eventually lead to the collapse of the Northern Hemisphere ice sheets.

Henderson and Slowey reported that ice-flow age models from samples taken from the Vostok ice core in Antarctica typically put the penultimate deglaciation earlier than the 127,000-year mark. Calculations by astronomers also indicated that peak insolation in the Southern Hemisphere occurred 138,000 years ago, in the same timeframe as Henderson and Slowey's findings about the end of the second-to-last ice age. Henderson and Slowey used an improved method of uranium thorium (U-th) dating to show that the midpoint of the end of this ice age was much older, at 135,000 years ago. They say this new, accurate date is consistent with deglaciation driven by orbital variations in solar radiation, either in the Southern Hemisphere or in the tropics, but not in the Northern Hemisphere. However, scientists still don't know how climatic change in the Southern Hemisphere could cause ice sheets in North America to melt.

—CHERYL PELLERIN

Viewpoint: Yes, ice age cycles of the Northern Hemisphere are driven by complex forces in the Southern Hemisphere, and possibly even the tropics.

For more than 80% of the last 2.5 million years, much of Earth's surface has been buried under a heavy blanket of ice. Periodically, Earth's atmosphere cools and great blankets of ice gouge their way south from the northern polar ice cap, covering Europe, Northern Asia, and much of North America. These ice ages are punctuated with interglacial periods, times when Earth's climate warms, and glaciers and ice caps recede. Understanding what drives these ice age cycles has proved to be an elusive task. Even as pieces to the puzzle are added, the picture remains incomplete.

However, when two researchers—Gideon M. Henderson from the Lamont-Doherty Earth Observation at Columbia University, and Niall C. Slowey from Texas A&M University—published the results of their research into the ice age phenomenon in Nature in March 2000), they added some important pieces to the puzzle. Henderson and Slowey's research on a marine sediment core led them to speculate that the alternating cycles of ice age and interglaciation (warm periods between ice ages) are driven by forces far removed from the huge ice sheets that cover the Northern Hemisphere every 100,000 years or so. By attributing those processes to events in the Southern Hemisphere, and possibly even the tropics, they challenged the longstanding hypothesis that processes in the Northern Hemisphere control ice age cycles.

Ice Ages and Insolation

The concept of the ice age is relatively new. Until the early 1830s, geological formations that we now know were caused by glaciation—the action of glaciers as polar ice caps grow—was thought to be caused by the Great Flood described in the Bible. The Swiss geologist Jean de Charpentier made the first scientific case for glaciation in the early 1830s. Several great geologists of that time became converts to this concept and, in 1842 the French mathematician Joseph Adhémar published Revolutions de la Mer, Deluges Periodics, a detailed hypothesis of ice ages. While his hypothesis was incorrect, his book excited other scientists who eagerly embarked on the process of discovering why ice ages occur. In particular, James Croll, a self-taught physicist from Scotland, made a significant contribution to unraveling the mystery. He developed several hypotheses in the 1870s, including the concept of astronomically based insolation, the intensity with which sunlight (solar radiation) hits Earth and variations in that intensity caused by latitudinal and cyclic changes as Earth journeys around the Sun. Based on his hypothesis of insolation, Croll predicted that ice ages would alternate between hemispheres because insolation would be greater in the Northern Hemisphere at one stage in the cycle, and in the next stage greater in the Southern Hemisphere. However, that prediction proved completely wrong; ice ages were shown to be synchronous, or occurring at the same time, and Croll's insolation hypothesis was tossed aside.

However, scientists still agree with several of Croll's ideas: that huge glaciers reflect rather than absorb the Sun's energy, further lowering Earth's temperature; that ocean currents influence global warming and cooling; and that precession and Earth's orbit influence climate. As it travels around the Sun, Earth's axis is tilted, but not always in the same direction. The tilt moves slowly, from 22.1° to 24.5° and back over a period of 41,000 years. Earth's axis also wobbles, much like the wobble of a top as it slows down, which means the North Pole is not always tilted in the direction that it is today. It wobbles back and forth in a process called precession; this happens over a period of 25,800 years. Also, Earth's orbit changes gradually over a period of 100,000 years, moving from slightly elliptical, or oval, to almost circular. These patterns combined mean that every 22,000 years, the hemisphere that is pointed toward the Sun at Earth's closest approach to the Sun cycles between North and South, creating variations in insolation.

The Milankovitch Hypothesis of Insolation

Croll's insolation hypothesis was revived again in 1938, when a Serbian engineer and professor teaching physics, mathematics, and astronomy at the University of Belgrade took up the ice age challenge. Milutin Milankovitch speculated that, because the Northern Hemisphere contained more than two-thirds of Earth's landmass, the effect of insolation on those landmasses at the mid-latitudes (on line with Greenland's southern tip) controlled the ice age phenomenon in both hemispheres at the same time

Using important new calculations of Earth's orbit made by the German scientist Ludwig Pilgrim in 1904, Milankovitch accurately described a dominating insolation cycle of 23,000 years, predicted that ice ages would be most severe when insolation fell below a certain threshold, and estimated dates of the ice ages. However, the invention of radiocarbon dating allowed precise age estimates that conflicted with the timing of the ice ages in Milankovitch's detailed calculations. Although the idea of Northern Hemisphere control remained, the astronomical hypothesis of insolation as the force that drove those controls was again abandoned.

In 2000, Richard A. Muller, professor in the physics department at the University of California, Berkeley, pointed out the daunting nature of Milankovitch's calculations. "Today these calculations are an interesting task for an undergraduate to do over the course of a summer using a desktop computer. But Milankovitch had to do all the calculations by hand, and it took him many years." He remarks that it was unfair to toss out Milankovitch's concepts: "Do we throw out the astronomical theory of the seasons simply because the first day of spring is not always spring-like? The warm weather of spring can be delayed by a month, or it can come early by a month; the important fact is that it always comes. We demand too much of a hypothesis or theory if we require it to predict all the details in addition to the major behavior."

The insolation hypothesis was again revived when scientists began to confirm a regularity in ice-age cycles. In a groundbreaking paper published in 1970, researchers Wally Broecker and Jan van Donk of Columbia University's Lamont Geological Observatory noted that an analysis of core samples taken from sea-floor sediment showed for the first time that a repeating 100,000-year cycle dominated ice age cycles. This frequency also appeared in the insolation hypothesis. In 1976, studies of seafloor sediment samples made by James D. Hays of Columbia University, John Imbrie of Brown University, and Nicholas Shackleton of the University of Cambridge, also showed evidence of 41,000-and 23,000-year cycles, and these cycles that were also evident in spectral analysis of insolation

New Dating Technique Builds a Case for the Southern Hemisphere

The insolation hypothesis still contained serious problems in relation to Northern Hemispheric processes driving ice age cycles. Of major concern was its implication that Northern Hemispheric deglaciation (the receding of the polar caps) should correspond with Northern Hemisphere June insolation. It did not. Astronomical studies showed that the northern June insolation during the penultimate, or next-to-last, deglaciation peaked 127,000 years ago, while scientific studies were suggesting deglaciation peaked much earlier—perhaps as much as 15,000 years earlier.

One way to confirm the growing evidence for early deglaciation was by examining oxygen isotopes contained in marine sediment. Using a new dating method based on the radioactive decay of uranium to thorium called the U-Th isochron technique, Henderson and Slowey studied marine sediments deposited during the penultimate deglaciation on the sea floor in the Bahamas. For the first time ever, the age of marine carbonate sediments more than 30,000 years old could be accurately determined. "It's like leaves falling on a forest floor," said Slowey. "If you were to figure out which season is which, you can look at the leaves to get a clue of what the seasons were like."

Kurt Sternlof, writing for Earth Institute News at the Columbia Earth Institute in New York City in 1999, explained that these marine sediments contain a record of "the total volume of global ice through time in the changing ratio of oxygen isotopes captured as they accumulated. Peaks in global ice volume correspond to ice ages; valleys correspond to interglacial periods." Thus, the U-Th technique allowed Henderson and Slowey to establish that the midpoint of the penultimate deglaciation and the end of the preceding ice age occurred 135,200 years ago, give or take 2,500 years. This was 8,000 years too early to have been affected by Northern Hemisphere peak insolation that occurred at 127,000 years. This timing assessment was supported by data from other researchers studying ice cores and sediment from cave floors.

Calculations by astronomers indicated that peak insolation in the Southern Hemisphere occurred 138,000 years ago, within the same timeframe as Henderson and Slowey's ocean sediment studies indicated as the end of the second-to-last ice age. Sternlof quotes Henderson as saying: "In our paper we demonstrate that, based on a simple argument of timing, the traditional model of ice ages as forced by climate amplifying mechanisms in the Northern Hemisphere cannot be correct."

Although Henderson and Slowey assert that the general association between orbital insolation and glacial timeframes remains obvious, they also assert that orbital insolation does not govern the ice-age phenomenon. In his article "Ice Cycle," in Nature News Service (2001), John Whitfield explains that variations in insolation are not great enough by themselves to drastically alter Earth's climate. By somehow interacting with our oceans and atmosphere, however, insolation causes huge changes in average temperature and therefore Earth's climate.

Digging Back into Climate History

To understand what causes changes in Earth's climate, scientists must discover the history of those changes. As Whitfield puts it, "The best way to go back is to dig a hole." Simplistically, digging holes involves removing long cores of material from Earth's crust. These cylindrical cores reveal layers of climate "history," just as the rings in a tree trunk reveal its stages of growth. Lengthy cores have been extracted from Greenland and the Antarctic where ice caps are several miles thick, from marine sediment on ocean floors, and from the floor of a cave called Devil's Hole in Nevada, where the terrestrial climate record is wonderfully preserved.

In their Nature article, Henderson and Slowey explain how their study of marine sediment cores produced evidence that atmospheric CO2 (carbon dioxide) levels influence the ice ages. The scientists write: "The change in global atmospheric CO2 concentration closely follows a hydrogen isotope, [.partialdiff] D, and is centered at 138,000 to 139,000 years ago, coincident with the peak in Southern Hemisphere insolation. This relationship suggests that the change in CO2 is driven by a process in the Southern Hemisphere. This change in CO2 may initiate the processes that eventually lead to the collapse of the Northern Hemisphere ice sheets. Southern Hemisphere mechanisms for the ice-age cycle are also suggested by the pattern of phasing between southern ocean sea-surface temperature changes and oxygen isotope [.partialdiff]18O, and are consistent with possible effects of sea-ice variability." Significantly, they point out that ice-flow age models from samples taken from the Vostok ice core also typically place the penultimate deglaciation as being earlier than the 127,000-year mark.

The Vostok ice core is the longest ice core ever obtained. Extracted from the Antarctic by a U.S.-Russian-French science team at Russia's Vostok research station, the coldest place on Earth, the core measures 2 mi (3.2 km) long and is composed of cylinder-shaped sections of ice deposits containing a record of snowfall, atmospheric chemicals, dust, and air bubbles. Previous cores taken from Antarctica and Greenland dated back only 150,000 years, revealing two ice ages. The Vostok core, which took from 1992 to 1998 to extract and contains ice samples dating back 420,000 years, shows Earth has undergone four ice ages. Significant to the search for driving forces behind the ice ages, however, was that, like marine sediments studied by Henderson and Slowey, the Vostok core allowed researchers to determine fluctuations in atmospheric CO2 levels throughout the ages.

Significance of Atmospheric CO2 Changes

CO2 is one of the most important greenhouse gases contributing to global warming. Microscopic oceanic plants and algae constantly remove CO2 that is absorbed readily from the air by cold ocean waters. When these life-forms die, they sink to the deep ocean floors taking their store of CO2 with them. The amount of CO2 returned to the atmosphere depends upon deep waters circulating to the ocean's surface and releasing their stores of CO2. Only at the end of the twentieth century did scientists discover that most deep-ocean waters do not return to the surface at low latitudes as was previously believed, but to the surface around Antarctica. Therefore, as the waters of Antarctica freeze and expand the southern polar ice cap, CO2 within those waters becomes imprisoned in the massive depths of ice. The CO2 lies trapped in the ice for several thousand years while Earth continues the journey through its orbital cycles. Henderson speculates that the "ultimate cause" of the penultimate deglaciation was the intensified amount of solar energy in the Southern Hemisphere that began to melt the Antarctic ice sheets. As carbon dioxide was released and entered the atmosphere, global temperatures began to rise, initiating deglaciation and ultimately the collapse of the Northern Hemisphere ice sheets.

The connection between CO2 and the Southern Hemisphere was further supported by another Nature article (March 9, 2000) by Britton Stephens, a University of Colorado researcher at the National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory in Boulder, Colorado, and Ralph Keeling of the Scripps Institution of Oceanography, University of California, San Diego. Stephens and Keeling found that atmospheric CO2 levels during an ice age fall by about 30%, from approximately 0.03 to 0.02%, keeping Earth cool by reducing the greenhouse effect. Using a computer model, Stephens and Keeling show how large ice sheets in the Antarctic, such as those that exist during an ice age, could prevent the usual release of carbon dioxide from the sea, thereby lowering atmospheric CO2 concentrations and causing global cooling. This process, they think, is "suggestive" of Southern Hemisphere forces lying behind climate changes.

To Be Continued

Although much is known about what drives the ice age cycle, much remains to be discovered. Even as Henderson and Slowey explore the CO2 hypothesis, they explore another involving the tropical ocean-atmosphere system, a system that is also consistent with the timescale of the penultimate deglaciation. "Recent modeling," they comment, "suggests that increasing insolation leads to a larger than average number of El Niño/Southern Oscillation (ENSO) warm events, starting at 137,000 years ago."

In his article for Nature News Service, John Whitfield writes that we expect too much if we expect one big idea to encompass the forces behind ice age cycles and changing climates. He closes his discussion with a quote from Henderson: "These cycles aren't controlled by one neat switch."

—MARIE L. THOMPSON

Viewpoint: No, the ice age cycles of the Northern Hemisphere are not driven by processes in the Southern Hemisphere; the Milankovitch cycle and disruption of the ocean's thermohaline circulation are the primary initiators.

In the past, scientists suggested that changes in the Southern Hemisphere—particularly those relating to ENSO (El Niño/Southern Oscillation) and glaciation trends in the Antarctic—were ultimately responsible for the advent of ice ages in the Northern Hemisphere. The latest research, arising largely from studies of current conditions, has pretty much exonerated the Southern Hemisphere of accusations of instigating glaciation in the north. The culprits responsible for northern ice ages are now believed to be cosmic or localized, but definitely not southern. Recent research points to two process as most likely to generate ice ages in the Northern Hemisphere: 1) Changes in Earth's tilt and/or orbit. 2) Climate change affecting the North Atlantic. This essay explores these hypotheses—which have widespread support in the scientific community—to show that they, not conditions south of the equator, are the primary triggers of northern glaciation.

Changes in Earth's Tilt and/or Orbit

The tilt of its rotational axis and the path of its orbit around the Sun have profound effects on Earth. Changes in either the axial tilt or the shape of the orbit cause enormous alterations in climate, which are sufficient to trigger ice ages. Several interrelated factors are involved.

Eccentricity.

The planet Earth revolves around the Sun in an orbit that is not perfectly circular, but elliptical. The exact shape of this elliptical orbit varies by 1 to 5% over time. This variation in Earth's orbit is known as eccentricity. The eccentricity of Earth's orbit affects the amount of sunlight hitting different parts of the planet's surface, especially during the orbit's aphelion and perihelion, the points in Earth's orbit at which it is farthest from the Sun and closest to the Sun, respectively. At times of maximum eccentricity—when Earth's orbit is most elliptical—summer and winter temperatures in both the Northern and Southern Hemispheres are extreme. Scientists studying Earth's eccentricity have found that, though the degree of change in solar radiation striking Earth during different orbital eccentricities seems rather small, only about 0.2%, this variation is sufficient to cause significant expansion or melting of polar ice. Earth's orbit varies periodically, with its eccentricity swinging from maximum to minimum about every 100,000 years.

Axial Tilt.

The rotational axis running through a globe from the North to the South Pole is tilted off center. Of course, a modern globe shows today's planetary tilt, which is 23.5°, but this degree of tilt has varied substantially over geologic time. Every 41,000 years or so, Earth's axis shifts, and its axial tilt varies, usually between 21.6 and 24.5°.

Changes in the tilt of Earth's rotational axis affect the planet's climate in the same way as changes in its eccentricity. As Earth's tilt changes, parts of the planet receive different amounts of solar radiation. The changes in the amount of sunlight striking the planet's surface are most extreme at high latitudes. When axial shifts turn the poles more acutely away from the Sun, polar regions may get up to 15% less solar radiation than they do today. The lightless polar winter is also much longer. The extremely long, dark, and severely cold winters at the poles result in increased glaciation.

Precession of the Equinoxes.

Changes in Earth's tilt are closely related to altered timing of the equinoxes, called precession. Earth's equinoxes occur twice a year, when the Sun is directly over the equator, and night and day have equal length. Today, the equinoxes occur on or about March 21 (the vernal equinox, or the first day of spring) and on or about September 21 (the autumnal equinox, or the first day of fall). The timing of the equinox depends on Earth's rotational axis. If the axial tilt changes, so does the time of the equinoxes. As Earth spins on its axis, the gravitational pull of the Sun and Moon may cause it to "wobble." Even slight wobbling produces changes in precession. Earth's axial wobbles follow a pattern that varies over a period of about 26,000 years.

Changes in precession caused by wobbling affect the timing and distribution of solar radiation on Earth's surface, again with polar regions most affected. For example, when aphelion occurs in January, winter in the Northern Hemisphere and summer in the Southern Hemisphere are colder. When a winter aphelion is coupled with changes in precession that delay, and thus reduce, solar radiation striking northern regions, glaciation increases. Longer, colder winters in the Northern Hemisphere lead almost inevitably to accumulation of ice and the onset of an ice age. Wobbling on its axis also causes greater or lesser changes in Earth's elliptical orbit. Orbital changes alter the timing of the aphelion and perihelion. The combination of altered precession and changes in the timing of aphelion and perihelion result in significant climate changes on Earth.

The tilt of Earth's rotational axis is closely related to precession, and precession is closely related to eccentricity. They are all closely related to climate changes on Earth—particularly northern ice ages. These factors were described by Serbian scientist Milutin Milankovitch in 1912, and together they are known as the Milankovitch cycle. Each factor may be capable of initiating northern glaciation on its own and in its own time frame (100,000 years; 41,000 years). A confluence of these factors—maximum eccentricity (when Earth's orbit is most elliptical), extreme axial tilt (with the North Pole pointed most acutely away from the Sun), and precession, which delays and reduces solar radiation at high northern latitudes—is thought to lead to a major ice age in the Northern Hemisphere.

The "Cosmic" Causation of Northern Ice Ages

On October 17, 2001, an international team of scientists led by New Zealand researcher Tim Naish, part of the Cape Roberts Project, published the results of their study of Antarctic seafloor sediment cores in the British journal Nature. The data show that changes in polar regions, particularly the advance and retreat of glaciers, follow variations in Earth's orbit, tilt, and precession as described in the Milankovitch cycle. The sea floor sediment samples indicate that Antarctic glaciers advanced and retreated at regular intervals during a 400,000-year period, and the cycle of glaciation and retreat matched those predicted by Milankovitch, with increased glaciation at intervals of 100,000 and 40,000 years. Ongoing research is fully expected to confirm these findings for Northern Hemisphere glaciation as well. The Cape Roberts Project study revealed surprising, and alarming, facts about the rapidity with which ice ages can occur. The seafloor cores showed that global climate changes—global warming or global cooling—can initiate a transition from a warm climate regime to intense glaciation in as few as 100 years.

The North Atlantic Deep Water Circulation

A Northern Hemisphere system affected by and thus leading to this transition is the North Atlantic Deep Water (NADW) circulation belt. The NADW is an enormous oceanic current that moves gargantuan quantities of water (estimated at 16 times more than the water in all the world's rivers) through the North Atlantic. The NADW flows along the equator and curls up the coast of North America carrying equator-warmed water toward northwestern Europe.

The engine that drives the NADW is located near Iceland in the North Atlantic. In this region, subsurface ocean water is extremely cold. This very cold water holds a lot of salt, so the water here is highly saline. Very salty water is also very dense—heavier than less salty, warmer water. This cold, salty, dense North Atlantic water sinks and flows southward toward the equator. As it heads south, the current warms. As it warms, it loses salt. As it loses salt, it becomes less dense. The warmer, less-dense current begins to rise toward the surface. At the equator, it is very warm and at its lowest density. This warm, low-saline current then swings through the Caribbean and up again toward Europe as the heat-transporting Gulf Stream. As it nears Greenland, the current is cooled by Arctic air masses, becomes colder, saltier, and denser, and then sinks and flows southward, completing the cycle of this perpetual oceanic conveyor belt.

The entire NADW system depends on the creation and sinking of the dense, cold, saline, current in the North Atlantic—thermohaline circulation. Thermohaline refers to the variations in temperature ("thermo") and salinity ("haline") that keep the oceanic conveyor belt going.

The NADW and Global Warming

The warmer surface water is and the warmer the air is above it, the greater the amount of water that will evaporate. There is no question that the global climate is currently warming. In 2000, scientists with the IPCC (Intergovernmental Panel on Climate Change) revised the agency's estimate of the rate and degree of global warming we are currently experiencing. They now believe that the global temperature is rising up to 38 to 40°F (3.5 to 4.5°C) per century. There is evidence to support the hypothesis that in the past, global warming affected the NADW and led to periods of glaciation. Could this be happening now?

As air temperature, and to a lesser extent sea surface temperature, increases, evaporation over the oceans also increases. Greater amounts of water vapor rise into the air and accumulate in clouds. Eventually the water vapor in clouds condenses and falls as rain. Much of this increased rainfall occurs over the North Atlantic. Rain is fresh water, and increased rainfall over the pivotal site of the sinking deep-water current in the North Atlantic dilutes the ocean water, reducing its salinity and its density. The less-dense, less-saline water is less likely to sink to the ocean depths, where it generates and drives the NADW. As air temperatures increase, Arctic sea ice and the immense Greenland glaciers melt more rapidly. Ice melt is also fresh water. Research has documented considerably increased input of sea ice and glacial meltwater into the North Atlantic. This additional injection of fresh water into the NADW further reduces its salinity and density, hinders its sinking, and weakens the flow of the entire current.

Although the surface waters of the North Atlantic that sink to drive the engine of the NADW have only 7% more salt than waters at similar latitudes in the Pacific, this is just sufficient to reach the threshold that causes these waters to sink and drive the Atlantic conveyor belt. However, even small reductions in salinity, or a few degrees increase in sea surface temperature, would prevent the North Atlantic current from reaching the sinking threshold. Scientists believe that the degree of freshwater input from increased precipitation and glacier melt, coupled with small, but documented increases in sea surface temperature, are beginning to push the NADW toward collapse.

Collapse of the NADW

Dr. Wallace S. Broecker from the Lamont-Doherty Earth Observation at Columbia University, one of the world's most eminent climate change experts, calls the NADW conveyor belt "the Achilles' heel of the climate system." In a paper published in the journal Science on November 28, 2001, Broecker described paleoclimate data that show that the NADW has collapsed several times in the past, initiating ice ages in the Northern Hemisphere. The evidence points to abrupt changes in the climate regime, perhaps a few decades or even a few years, after weakening or collapse of the NADW.

The part of the NADW that carries warm water northward from the Caribbean is the Gulf Stream. The Gulf Stream transports heat to northwestern Europe, which keeps the region considerably warmer than its latitude would otherwise permit (Britain has the same latitude as central Quebec, Canada). As the NADW weakens, less heat is carried to northwestern Europe. If the NADW collapses, northwestern Europe will begin to freeze over.

If the NADW weakens and collapses and the climate of northwestern Europe cools considerably, snow and ice that accumulate there during the winter will, in a relatively short time, remain on the ground during the region's colder summers. The collapse of the NADW would, in fact, lower overall temperatures in northwestern Europe by 20°F (11°C) or more, giving relatively balmy Dublin, Ireland, a frigid climate identical to that of Spitsbergen, Norway, a city 600 mi (965 km) north of the Arctic Circle. Once winter snows fail to melt, and snow and ice accumulate on the ground year-round, the onset of an ice age is quite rapid. Because albedo, or reflectivity, of ice is far greater than that of the bare ground, whose dark coloration absorbs more heat through solar radiation, the snow and ice create a brilliant white ground cover that reflects more heat away from Earth's surface. With less heat absorbed, the surface temperature falls further, so more snow and ice accumulate, and so on.

Paleoclimate data, particularly those obtained from Greenland ice cores, indicate that this process has occurred before and that it occurs quickly. There is abundant evidence documenting this process as a primary creator of Northern Hemisphere ice ages. There is no question that these two processes—the Milankovitch cycle and disruption of the ocean's thermohaline circulation—are the primary initiators of Northern Hemisphere ice ages.

—NATALIE GOLDSTEIN

Further Reading

Barber, D. C., et al. "Forcing of the Cold Event of 8,200 Years Ago by Catastrophic Drainage of the Laurentide Lakes." Nature 400 (1999): 344-48.

Berger, A. L. Milankovitch and Climate: Understanding the Response to Astronomical Forcing. Boston: D. Reidel Pub. Co., 1984.

Broecker, W. S., and G. M. Henderson. "The Sequence of Events Surrounding Termination II and Their Implications for the Cause of Glacial-Interglacial CO2 Change." Paleoceanography 13 (1999): 352-64.

Cronin, Thomas M. Principles of Paleoclimatology. New York: Columbia University Press, 1999.

Erickson, Jon. Ice Ages: Past and Future. New York: McGraw-Hill, 1990.

Hays, J. D., J. Imbrie, and N. J. Shackleton."Variations in the Earth's Orbit: Pacemaker of the Ice Ages." Science 194 (1976): 1,121-32.

Henderson, Gideon M., and Niall C. Slowey."Evidence against Northern-Hemisphere Forcing of the Penultimate Deglaciation from U-Th Dating." Nature 404 (2000): 61-6.

"Ice Ages in Sync." Scientific American (February 15, 2001).

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KEY TERMS

ALBEDO:

Degree to which a surface reflects light; its reflectivity; a lighter-colored surface has a higher albedo (reflects more light and heat) than a dark surface (low albedo: absorbs more light and heat).

APHELION:

Point in Earth's orbit where it is farthest from the Sun.

DEGLACIATION:

Melting of the polar ice caps following an ice age, or glaciation.

ECCENTRICITY:

Variation in the shape of Earth's orbit around the Sun.

ELLIPTICAL ORBIT:

An orbit that is elongated: oval-shaped rather than in a circle.

EQUINOX:

Day of the year when the sun is directly over the equator and day and night are of equal length.

GLACIATION:

Expansion of the polar ice caps; formation of massive glaciers and sheets of ice over much of the Northern Hemisphere landmass.

GULF STREAM:

Part of the NADW that flows from the Caribbean to northwestern Europe and carries huge amounts of heat to this region.

INSOLATION:

Rate at which solar radiation is delivered per unit of horizontal surface, in this instance, Earth's surface.

INTERGLACIATION:

Warmer periods between ice ages when Earth's climate is basically as we know it.

NADW:

North Atlantic Deep Water circulation; an enormous current of very cold, salty, dense ocean water that sinks and drives the conveyor belt of ocean currents.

PALEOCLIMATE:

Prehistoric climates.

PENULTIMATE DEGLACIATION:

Melting of the polar ice caps following the second-last, or next-to-last, ice age.

PERIHELION:

Point in Earth's orbit where it is closest to the Sun.

PRECESSION:

Alterations in the timing of the equinoxes on Earth. The major axis of Earth slowly rotates relative to the "fixed" stars; precession occurs when Earth's axis wobbles, much like the wobble of a top as it slows down.

SOLAR RADIATION:

Energy radiated from the Sun.

SYNCHRONOUS:

Occurring at the same time.

THERMOHALINE:

Temperature and salt content of ocean water; the thermohaline circulation is the ocean conveyor belt that is driven by differences in seawater's temperature and salinity.

IS ANOTHER ICE AGE IMMINENT?

Although there is no astronomical information indicating that we are in for a Milankovitch cycle-induced ice age, abundant data exists to show that warming of Earth's climate may plunge us into a deep freeze. And this is likely to happen far more quickly than anyone wants to believe.

No one questions the fact that the world climate is warming. A vast majority of scientists around the world believe this is due to the huge input of greenhouse gases from human combustion of fossil fuels.

The greenhouse effect—the accumulation of heat-trapping gases in the atmosphere—has enabled life to exist on Earth. Without naturally occurring greenhouse gases in the air, Earth would be a frozen, likely lifeless, planet. However, "too much of a good thing" can rapidly devastate life.

Important ice-core research shows that, throughout the past 420,000 years, the highest concentrations of significant greenhouse gases to occur in the atmosphere were carbon dioxide at 300 parts per million (ppm), and methane at 770 parts per billion (ppb). As of September 2001, the concentrations of these gases in the atmosphere were carbon dioxide at 369.4 ppm, and methane at 1,839 ppm. The greenhouse effect is not controversial. The above figures indicate that humans are releasing large amounts of greenhouse gases into the air. The global climate is predicted to warm as much as 8°F (4.5°C) in the next century. This warming will affect the thermohaline circulation in the North Atlantic; in fact, it is already doing so. There is clear evidence indicating that, as global warming worsens, weakening or collapse of the NADW will lead to the onset of another ice age.

Dr. Wallace Broecker, a leading climate expert, warns that: "Were [another ice age] to happen a century from now, at a time when we struggle to produce enough food to nourish the projected population of 12 to 18 billion, the consequences could be devastating."

—Natalie Goldstein

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Are ice age cycles of the Northern Hemisphere driven by processes in the Southern Hemisphere

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Are ice age cycles of the Northern Hemisphere driven by processes in the Southern Hemisphere