Introduction to Probing the History of Climate Change

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One of the major science stories in the late 1990's was the growing concern that Earth's climate is getting warmer. From the mid-1800's to 2000, the average temperature of the Earth's surface increased from 15 °C (59 °F) to 15.6 °C (60 °F)—its highest level in recorded history. If this warming trend continues, researchers warned, it could drastically alter the face of the planet. For example, the rising temperatures could begin to melt the icecaps of Greenland and Antarctica, flooding coastal regions around the world.

Although researchers in 2000 were still unable to fully explain what was causing the temperature increase, most climatologists (scientists who study climate) placed much of the blame on human activities, such as the burning of fossil fuels (coal, oil, and natural gas) and the clearing of forests for farmland or housing. On the other hand, a smaller number of scientists argued that though human-related factors have indeed increased the amounts of heat-trapping “greenhouse gases” such as carbon dioxide in the atmosphere, the increase had made no measurable difference in the Earth's climate. Those scientists said that the warming trend may be part of a normal cycle of change in the climate system. They cited studies of past climates indicating that the type of change now occurring is actually nothing new to the planet. In fact, a growing body of evidence from the study of climates long ago suggested that a stable climate may be the exception rather than the rule. In June 1999, a group of scientists led by French researcher Jean-Robert Petit reported that what we consider Earth's “normal” climate has been the norm for only about the past 11,500 years.

What Is Paleoclimatology?

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The study of ancient climates is known as paleoclimatology. In much the same way that some researchers study the prehistoric past by examining fossils and other physical clues, paleoclimatologists study several types of evidence in an attempt to understand what Earth's climate was like in the past and how—and why—–it has changed. The more scientists learn about how the climate has varied over the past several million years, the better their predictions about future changes will be.

The most common sources of data for paleoclimatologists include ice cores, long, narrow cylinders of ice drilled from an icecap; sediment cores, cylinders of mud and other matter drilled from the floors of oceans and lakes; the fossilized remains of plants and animals; samples from coral reefs; plant remains obtained from peat bogs; and the growth rings of trees. By studying these kinds of evidence, researchers have been able to reconstruct a timeline of the prehistoric climate of Earth.

For example, researchers now know that the Earth has gone through many glacial periods, or ice ages, during which average global temperatures dropped and the polar icecaps expanded, spreading thick sheets of ice across vast regions of land and sea. The earliest known ice ages occurred more than 2 billion years ago. The last few began about 600,000 years ago and lasted close to 100,000 years each. After each glacial period, global temperatures rose and the icecaps receded. The spans between ice ages, called interglacial periods, lasted from 10,000 to 20,000 years. At the height of the last glacial episode—usually called the Ice Age—glaciers spread across much of northern Europe and North America. The Ice Age ended about 11,500 years ago, giving way to the present interglacial, known as the Holocene Epoch.

Evidence From Ice Cores

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Of the many types of evidence that paleoclimatologists study, ice cores had by 2000 proved to be the most valuable. Drilling deep into the ice is like reaching back in time—the farther down you go, the farther back in time you can see. Ice cores offer a detailed record of climate change over long periods, and so they serve as excellent “yardsticks” for comparison with other evidence. The oldest ice on Earth is found near the North and South poles in the icecaps of Greenland and Antarctica, where drilling has reached ice that formed almost half a million years ago.

It was Antarctic ice that led Petit's team to their conclusion about the variability of Earth's ancient climate. The researchers studied a 3,600-meter (11,800-foot) cylinder of ice drilled near Vostok Station, a Russian scientific research base in Antarctica, 1,500 kilometers (930 miles) from the South Pole. The Vostok ice core spans 420,000 years and contains a continuous record of climate changes through four ice ages and the interglacial periods between them.

Antarctic ice is glacial ice, which begins as snow. As one snowfall after another piles up on top of the last, the increasing weight of the overlying snow eventually changes the buried snow into solid ice. This continues from one year to the next, forming a series of annual layers. The ice layers, which are sometimes so distinct that scientists can identify individual years, become a record of climatic changes that are literally "frozen in time." By measuring differences between one layer and the next, scientists can identify climatic shifts and gather clues about what could have caused them. The most common sources of evidence in an ice core, besides the composition of the ice itself, are air bubbles and dust.

Oxygen and Hydrogen Isotopes

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The most widely used method of determining ancient temperatures is based on the relative proportion of isotopes (variant forms) of oxygen and hydrogen in the ice. A molecule of water ice consists of two hydrogen atoms and one oxygen atom. The nucleus of all atoms consists of subatomic particles called protons, neutrons, and electrons. Isotopes of an element have the same number of protons, but a different number of neutrons, in the nucleus. All isotopes of an element exhibit the same chemical properties, but because they have different masses (a greater number of neutrons makes the isotope heavier), isotopes behave differently from one another under certain conditions.

The two most common isotopes of oxygen are oxygen 16 and oxygen 18, with either 8 or 10 neutrons, respectively. Both forms are found in water molecules, but oxygen 16 is far more common. Because oxygen 16 has fewer neutrons in its nucleus than oxygen 18, an oxygen-16 water molecule is lighter than an oxygen-18 water molecule. This difference causes oxygen-16 water molecules to evaporate at a faster rate than oxygen-18 water molecules. This oxygen-16-rich water vapor condenses to form raindrops or snowflakes. The snowflakes may then accumulate to make ice sheets rich in oxygen 16. Thus, when the climate is warmer (and evaporation rates become higher) ice becomes richer in oxygen 16. If a layer in an ice core contains a relatively high proportion of oxygen 16, scientists can generally infer that the layer is composed of ice that formed during a warm period.

After determining the relative proportions of oxygen isotopes in a layer of ice, paleoclimatologists use mathematical formulas to calculate the air temperature in that area when the ice formed. Applying this formula to the isotope record in ice cores indicates that during the Last Glacial Maximum (the farthest advance of the icecaps), about 22,000 years ago, northern Greenland was about 17 °C (30 °F) colder than it is today. Ice-core data from a glacier in the mountains of Peru indicate that temperatures in that area were about 11 °C (20 °F) lower at that time than they are now.

New Knowledge From the Ice

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The study of ice cores has led to great advances in understanding the history of Earth's climate. For the first time, it has become possible to study climatic shifts on a very small scale—periods of 10 years to 3,000 years. Although paleoclimatologists had found scattered evidence of considerable climatic variability over relatively short periods, they could not confirm that such shifts had occurred until ice-core studies made it possible to construct a sequence of past climatic events. Perhaps the most intriguing discovery stemming from ice-core analysis is the fact that global warming has occurred many times over the ages.

This realization stems, in part, from the work of two European paleoclimatologists, Willi Dansgaard of the University of Copenhagen in Denmark and Hans A. Oeschger of the University of Bern in Switzerland. In 1989, Dansgaard and Oeschger reviewed a number of earlier studies of oxygen-isotope ratios in ice cores from Greenland. By sifting through all the data, they discovered numerous episodes of global (or in some cases, regional) warming that began suddenly and lasted only 1,000 to 3,000 years. Later studies identified at least 24 of these rapid shifts, now known as Dansgaard-Oeschger events, between 100,000 and 11,500 years ago. The temperature in Greenland rose by as much as 15 °C (27 °F) during these episodes. In each case, the warming took only a few decades, but the return to cold conditions took place more gradually, over a few hundred years. In 2000, it was still unclear what caused these large, rapid, but short-lived climatic shifts.

Studying Air Bubbles, Dust, and Ocean Debris

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Ice cores also preserve another valuable type of evidence: bubbles containing the free gases present in the ancient atmosphere. Two of the most important gases scientists look for are carbon dioxide and methane. Both are so-called greenhouse gases, which keep the Earth's surface warm by trapping heat, which the Earth absorbs from the sun, and preventing it from reradiating out into space. The more of these gases there are in the atmosphere, the stronger the greenhouse effect. Comparing the levels of atmospheric greenhouse gases in ice-core samples with their present levels helps scientists estimate how strong the greenhouse effect was at the time the sample was locked into the ice. They incorporate this value into their temperature calculations.

Bubbles of air found in ice cores from both Greenland and Antarctica indicate that atmospheric concentrations of carbon dioxide and methane were low during cold times and high during warmer times. At first, scientists took this as confirmation that the burning of fossil fuels, which releases additional greenhouse gases and other chemicals into the atmosphere, had indeed caused the recent observed increase in global temperatures. However, more precise dating made possible by ice-core analysis has indicated that temperature changes in the distant past actually occurred prior to changes in greenhouse gas concentrations. Some scientists think that this surprising finding may mean that rising levels of greenhouse gases reinforced past global warming episodes, but were not necessarily responsible for triggering them.

Periodic Rises In Atmospheric Carbon Dioxide and Methane

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Studies of atmospheric gases from ice cores revealed that sharp increases in atmospheric carbon dioxide and methane occurred during Dansgaard-Oeschger events. The increases were so sudden and massive that the release of the gases must have been enormous. Paleoclimatologists think the increase in carbon dioxide was caused by changes in the deep circulation patterns of the ocean. Like the atmosphere,the ocean circulates in a regular pattern. Moreover, scientists have learned, the ocean's circulation is influenced by changes in the atmosphere. Researchers theorize that, as global temperatures rose during Dansgaard-Oeschger events, the vertical circulation of the ocean increased, carrying more carbon dioxide-rich water from the deep ocean up to the surface. At the surface, the carbon dioxide escaped into the air, raising global levels of atmospheric carbon dioxide.

The sudden increase in atmospheric methane during Dansgaard-Oeschger events may have been triggered by geologic activity, such as underwater earthquakes or landslides. Such incidents could have disturbed huge pockets of methane gas, produced by the decay of organic (from plants and animals) matter that had become buried beneath layers of sea-floor sediment. Another theory is that the rising temperatures melted huge deposits of methane hydrate, an icy substance formed when methane gas molecules are trapped in a “cage” of water ice. Huge deposits of methane hydrate exist today in the deep ocean and below the permafrost (a layer of permanently frozen soil) of Siberia.

Clues From Dust Particles

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Dust particles and other bits of debris are also common in both the Greenland and Antarctic ice cores, and they represent the third major source of information on past climates found in ice cores. The amount of dust in a layer of ice is an indication of how dry the region was and how strong the winds were at the time the ice formed. Dust is far more prevalent in ice deposited during glacial periods than in ice formed during interglacials. For example, layers of the Vostok ice core that were laid down in glacial periods contain 20 to 40 times as much dust as the layers from interglacials.

At about the same time that Dansgaard-Oeschger events were first being described, evidence from both ice and sediment cores led to the discovery of another kind of rapid climatic shift. While studying sediment cores from the floor of the North Atlantic Ocean, paleoclimatologist Hartmut Heinrich of the University of Gottingen in Germany noticed unusual layers of debris in some of the cores. These layers, which became known as Heinrich layers, consisted of pebbles, soil, and other material sandwiched between more typical sediment layers. However, this type of debris could only have come from land. How did it get to the bottom of the sea?

By comparing his sediment cores with ice-core data, Heinrich determined that the layers were associated with periods of rapid regional cooling, which are now known as “Heinrich events.” At least six Heinrich events, each lasting 1,000 to 2,000 years, have occurred at irregular intervals during the last 70,000 years. Heinrich theorized that the debris was carried into the oceans by icebergs, mostly from the ice sheet that covered what is now northeastern North America. As the ice sheet expanded during the periods of cooling, it picked up large amounts of soil and rock. Icebergs broke away from the edge of the ice sheet and drifted into the North Atlantic Ocean. The icebergs cooled the surface water of the entire North Atlantic, like ice cubes in a bowl of water. That led to further regional cooling of the atmosphere as the water absorbed heat from the air. As the icebergs melted, the soil and rocks trapped inside them came free and sank, forming layers of debris on the sea floor. The reason for the Heinrich events, each of which ended as abruptly as it began, remained a mystery in 2000.

Other Evidence From Beneath the Sea

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Heinrich was far from being the first paleoclimatologist to study ocean sediments. In fact, before the drilling of ice cores began in the mid-1960's, evidence recovered from cores of sea-floor sediments was the primary resource for paleoclimatic research. Sediment cores, however, are considered less accurate than ice cores because sediment layers accumulate far more slowly than ice layers and because sediment is often mixed around by burrowing marine animals. Nevertheless, sediment cores continue to be valuable to paleoclimatologists because they can be taken from sites around the world.

Moreover, despite their drawbacks, sediment cores can reveal much about what the ocean—and thus the overall climate—was like when each layer of sediment was laid down. A large proportion of sea-floor sediment consists of the fossilized shells of microscopic sea organisms called foraminifers. When these organisms die, their shells sink to the bottom and accumulate on the sea floor, where they are eventually incorporated into sediment. Scientists estimate the past temperature of the ocean by measuring the ratio of oxygen isotopes in foraminifer shells, which absorb oxygen from the water as they develop. In warmer times, the shells are more likely to take up the lighter isotope, oxygen 16. A layer containing foraminifer fossils rich in oxygen 16 therefore indicates that the layer was laid down during a warm period. The evaluation of this evidence, however, is complicated by the fact that the normal ratio of the two oxygen isotopes in the ocean changes between glacials and interglacials. During interglacials, when much of the glacial ice has melted, the ocean gains oxygen 16 from the inflow of meltwater from land, so the relative amount of oxygen 18 in the ocean drops.

Paleoclimatologists have gotten around this problem by distinguishing between two kinds of foraminifers: planktonic foraminifers, species that live near the surface of the ocean, and benthonic foraminifers, species that live on the deep-sea floor. Scientists can calculate what the temperature of the surface water was at a particular time in history using a formula based on the ratios of oxygen isotopes in the fossils of the different kinds of foraminifers in a layer of sediment. The shells of planktonic foraminifers that lived in warmer waters contain less oxygen 18 than the shells of those that lived in colder waters. A high ratio of oxygen 18 to oxygen 16 in the shells, therefore, is another indication that the creatures lived in cold water. On the other hand, because the temperature in the deep sea changed very little from the last ice age to the present, the changes in the isotope ratios in the shells of benthonic foraminifers reflect only the build-up and melting of the ice sheets.

By comparing the changes in isotope ratios in the benthonic foraminifers with the much larger ratio changes in the planktonic foraminifers, researchers can estimate the temperature changes in the ocean's surface waters at the time when the foraminifers lived. Using data from numerous studies of foraminifer shells from the 1960's and 1970's, researchers concluded that the surface waters of the North Atlantic were 4 to 8 °C (7 to 14 °F) colder during the Last Glacial Maximum.

Sediment Data Reveals Seven Glacial Cycles In Past 600,000 Years

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Sediment-core data studies identified seven glacial-interglacial cycles over the past 600,000 years. In each cycle, the buildup of ice took place gradually, over periods averaging about 80,000 years, while the melting that followed happened quickly, always taking less than 10,000 years. Dating of the cores using various methods revealed even earlier glacial periods. From about 3.4 million years ago, when the most recent series of glaciations began in the Northern Hemisphere, to 600,000 years ago, there were about 65 glacial-interglacial cycles. Each of those earlier cycles, however, was only about 40,000 years long. Why the later cycles became longer is an issue of major interest to paleoclimatologists.

Evidence from isotopes and foraminifer populations is of limited help in answering that question because it can provide only average temperature values over hundreds or a few thousands of years. The need for more precise data to study climate change over shorter periods led researchers to examine other kinds of paleoclimatic records.

They found much important evidence in coral reefs. Corals are tiny marine animals that form their skeletal structure out of calcium carbonate (limestone), which they extract from seawater. The coral animal, called a polyp, has a ring of tentacles surrounding a saclike body, which is partially enclosed by the carbonate skeleton. When a polyp dies, its skeleton remains in place and becomes part of a growing reef. Corals grow by adding new layers of carbonate on their outer surface. Core samples drilled from individual large corals therefore show growth bands that preserve a record of environmental changes. These records can cover more than 100 years, and fossil corals sometimes provide much older records. The oxygen-isotope ratio of each band indicates the temperature of the water in which the coral skeleton formed.

Scientists also can use corals to determine changes in sea level. Some species of coral live only in water about 1 meter (3 feet) deep. By tracking the presence of fossilized corals of these species at different depths, scientists can determine what the sea level was at various times in the past. In one major study, geologists took samples from fossil reefs off the coast of Barbados and used them to track changes in the global sea level since the Last Glacial Maximum. The evidence demonstrated that the melting of the ice sheets in the Northern Hemisphere began about 18,000 years ago and ended about 7,000 years ago. However, the melting did not always proceed at the same rate. On two occasions, it became very rapid, causing the global sea level to rise as much as 20 meters (66 feet) in 1,000 years. In addition, investigators found that the melting was interrupted by a return to near-glacial conditions between 12,800 to 11,500 years ago, a time known as the Younger Dryas period.

Climate Clues From the Land

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The paleoclimatic record on land tends to be much more variable than that in ocean sediments and glacial ice. Nonetheless, it provides valuable information about conditions in areas far from the oceans and icecaps. Peat bogs were among the first places on land where scientists sought information about the Ice Age and the beginning of the present interglacial period. Peat is composed of the organic matter left behind when mosses, herbs, and other plants die and rot in shallow, acidic water. Peat bogs form when these remains accumulate in swamps and marshes over long periods. Like ice and sediment, peat bogs accumulate plant debris in layers that can be studied by core sampling. Because some plants thrive in warm, wet conditions and others in colder, drier conditions, the plant species found in different peat layers indicate both temperature and rainfall at various times in the past.

In the early 1900's, scientists discovered that evidence from peat bogs was even more valuable than they had previously thought. They found that by analyzing pollen grains preserved in bogs, they could study changes in plant life through time in a region much larger than the area of the bog itself. That is because winds can carry pollen grains great distances. In addition, researchers learned to track climate shifts by identifying the types of fossilized pollen grains in different layers of a core sample taken from a peat bog. For example, a high ratio of pollen grains from beech trees indicates a cold climate. If the next layer contains more spruce or fir pollen, it suggests a warming trend.

Because they are less likely to be disturbed, sediment layers at the bottom of lakes preserve much more extensive and detailed climatic evidence than peat bogs do. But only since the 1990's have researchers made concerted efforts to obtain sediment cores from lakes. As with oxygen-isotope data from ocean-sediment cores, information from lake-sediment cores can be used to infer the temperature of the water—and thus of the atmosphere—at different times. In addition, like peat bogs, lake bed sediments preserve a wealth of pollen grains.

The trunks of trees are another excellent source of information on past climates. Each year, a tree adds a new layer of wood, called a growth ring, on the outer surface of the trunk beneath the bark. Wide rings indicate years of relatively favorable weather and adequate rainfall, while narrower ones indicate years of heat and drought.

Researchers have also found that the leaves of trees and other plants preserved in bogs and lake sediments provide a detailed record of changes in atmospheric carbon dioxide. The number of stomata (porelike openings) on the surface of a leaf is reduced during times when there is an elevated concentration of carbon dioxide in the air. Studies of leaf stomata have shown that modern atmospheric carbon dioxide concentrations are higher than they have been at any time during the last 16 million years.

Applying What Has Been Learned

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The ultimate goal of all this research is to learn how the Earth's climate system functions over long periods. That knowledge would help researchers predict how the climate will change in the future. Climatologists test their theories about the climate by creating computer models (simulations) of climatic conditions. The first models were created in attempts to predict the weather. In 2000, climatologists hoped that steadily improving models would soon make it possible to accurately predict future climate changes.

By 2000, models had helped paleoclimatologists develop a fairly good understanding of how a typical ice age progressed (though not why it began). They theorize that, as the ice sheets expanded and spread, they eventually became so large that they began to cool huge pools of air above them. These masses of cold air forced changes in the circulation patterns of the Earth's atmosphere, which in turn influenced changes in ocean circulation. A slowdown of vertical circulation in the ocean led to increased storage of carbon dioxide in the deep sea, causing carbon dioxide levels in the surface waters to drop. With less carbon dioxide entering the air from the ocean, atmospheric carbon dioxide levels dropped, enabling more heat to escape into space. This further contributed to the global cooling and expansion of the ice sheets. Later, another train of events, which researchers in 2000 were still trying to explain, led to gradual warming and a retreat of the ice. The mechanisms that caused the shorter-term naturally occurring changes in global climate were also still unexplained in 2000.

Nonetheless, most scientists agreed on one thing: Human activities do have an effect on Earth's climate system. Although research has now shown that ice ages and global warming have occurred many times as a part of the Earth's natural climate cycles, that knowledge must be tempered by the fact that the atmosphere now contains more carbon dioxide than it has for 16 million years. The massive burning of fossil fuels is a new factor in a very complex system. The human race could be setting the stage for future climate changes that are not part of a natural cycle.