Explain why unraveling past climate changes is important and discuss several ways in which such changes are detected.
Climate not only varies from place to place but is also naturally variableover time. During the great expanse of Earth history, and long before humans were roaming the planet, there were many shifts—from warm to cold and from wet to dry and back again.
Using fossils and many other geologic clues, scientists have reconstructed Earth’s climate going back hundreds of millions of years.
Over long time scales (tens to hundreds of millions of years), Earth’s climate can be broadly characterized as being a warm “greenhouse” or a cold “icehouse.”
During greenhouse times, there is little, if any, permanent ice at either pole, and relatively warm temperate climates are found even at high latitudes. During icehouse conditions, global climate is cool enough to support ice sheets at one or both poles. Earth’s climate has gradually transitioned between these two categories only a few times in the past 541 million years, the span known as the Phanerozoic (“visible life”) eon. The rocks and deposits of the Phanerozoic eon contain abundant fossils that document major environmental and evolutionary trends. The most recent transition occurred during the Cenozoic era.
The early Cenozoic was a time of greenhouse climates like those the dinosaurs experienced during the preceding Mesozoic era. By about 34 million years ago, permanent ice sheets were present at the South Pole, ushering in icehouse conditions (Figure 1).
Climate warmed during the Miocene epoch (about 20 million years ago) as mammal populations reached their greatest diversity. Climate then cooled. In North America, the lush “greenhouse” forests (there were palm trees in Wyoming and banana trees in Oregon) were replaced by open grasslands. Grassland ecosystems are better suited for a cooler, drier “icehouse” climate. By 2 million years ago (the start of the Quaternary epoch), Earth’s climate was cold enough to support vast ice sheets at both poles. In the Northern Hemisphere ice advanced nearly as far south as the present-day Ohio River, then subsequently retreated to Greenland. For the past 800,000 years, this cycle of ice advance and retreat has occurred about every 100,000 years.
The last major ice sheet advance reached a maximum about 18,000 years ago.
How do we know about these changes? What are the causes? The next sections take a look at how scientists decipher Earth’s climate history. Later we will explore some significant natural causes of climate change.
High-technology and precision instrumentation are now available to study the composition and dynamics of the atmosphere. But such tools are recent inventions and therefore have been providing data for only a short time span. To understand fully the behavior of the atmosphere and to anticipate future climate change, we must somehow discover how climate has changed over broad expanses of time.
Instrumental records go back only a couple of centuries, at best, and the further back we go, the less complete and more unreliable the data become. To overcome this lack of direct measurements, scientists must decipher and reconstruct past climates by using indirect evidence. Proxy data come from natural recorders of climate variability, such as seafloor sediments, glacial ice, fossil pollen, and tree-growth rings, as well as from historical documents (Figure 2).
Scientists who analyze proxy data and reconstruct past climates are engaged in the study of paleoclimatology.
The main goal of such work is to understand the climate of the past in order to assess the current and potential future climate in the context of natural climate variability.
Seafloor Sediment: A Storehouse of Climate Data
We know that the parts of the Earth system are linked so that a change in one part can produce changes in any or all of the other parts. In this section, you will see how changes in atmospheric and oceanic temperatures are reflected in the nature of life in the sea.
Most seafloor sediments contain the remains of organisms that once lived near the sea surface (the ocean– atmosphere interface). When such near surface organisms die, their shells slowly settle to the floor of the ocean, where they become part of the sedimentary record (Figure 3).
These seafloor sediments are useful recorders of worldwide climate change because the numbers and types of organisms living near the sea surface change with the climate.
For this reason, scientists are tapping the huge reservoir of data in seafloor sediments. The sediment cores gathered by drilling ships and other research vessels have provided invaluable data that have significantly expanded our knowledge and understanding of past climates.
One notable example of how seafloor sediments add to our understanding of climate change relates to unraveling the fluctuating atmospheric conditions of the Ice Age. The records of temperature changes contained in cores of sediment from the ocean floor have proven critical to our present understanding of this recent span of Earth history.
Oxygen Isotope Analysis
The isotopes of oxygen in water molecules or in the shells of marine organisms are an important source of proxy data on past climate conditions. Oxygen isotope analysis is based on precise measurement of the ratio between two isotopes of oxygen: 16O, which is the most common, and the heavier 18O. A molecule of H2O can form from either 16O or 18O, but the lighter isotope, 16O, evaporates more readily from the oceans. Because of this, precipitation (and hence the glacial ice that it may form) is enriched in 16O. This leaves a greater concentration of the heavier isotope, 18O, in the ocean water. Thus, during periods when glaciers are extensive, more of the lighter 16O is tied up in ice, so the concentration of 18O in seawater increases. Conversely, during warmer interglacial periods, when the amount of glacial ice decreases dramatically, more 16O is returned to the sea, so the proportion of 18O relative to 16O in ocean water also drops. Now, if we had some ancient recording of the changes of the 18O/16O ratio, we could determine when there were glacial periods and, therefore, when the climate grew cooler.
Fortunately, we do have such a recording. Certain marine microorganisms secrete their shells of calcium carbonate (CaCO3), and the prevailing oceanic 18O/16O ratio is reflected in the composition of these hard parts.
When these organisms die, their hard parts settle to the ocean floor, becoming part of the sediment layers there.
Consequently, periods of glacial activity can be determined from variations in the oxygen isotope ratio found in shells of certain microorganisms buried in deep-sea sediments. A higher ratio of 18O to 16O in shells indicates a time when ice sheets were growing larger.
The 18O/16O ratio also varies with temperature.
More 18O is evaporated from the oceans when temperatures are high, and less is evaporated when temperatures are low. Therefore, the heavy isotope is more abundant in the precipitation of warm eras and less abundant during colder periods. Using this principle, scientists studying the layers of ice and snow in glaciers have been able to determine past temperature changes.
Climate Change Recorded in Glacial Ice
Ice cores are an indispensable source of data for reconstructing past climates. Research based on vertical cores taken from the Greenland and Antarctic ice sheets has changed our basic understanding of how the climate system works. Scientists collect samples by using a drilling rig, like a small version of an oil drill. A hollow shaft follows the drill head into the ice, and an ice core is extracted.
In this way, cores that sometimes exceed 2000 meters (6500 feet) in length and may represent more than 200,000 years of climate history are acquired for study (Figure 4A).
Scientists are able to produce a record of changing air temperatures and snowfall by means of the oxygen isotope analysis described above. A portion of such a record is shown in Figure 4B.
Air bubbles trapped in the ice also record variations in atmospheric composition. Changes in carbon dioxide and methane are linked to fluctuating temperatures. The cores also include atmospheric fallout such as wind-blown dust, volcanic ash, pollen, and modern-day pollution.
Tree Rings: Archives of Environmental History
If you look at the end of a log, you will see that it is composed of a series of concentric rings (Figure 5A). Tree rings can be a very useful source of proxy data on past climates. Every year, in temperate regions, trees add a layer of new wood under the bark. Characteristics of each tree ring, such as thickness and density, reflect the environmental conditions (especially climate) that prevailed during the year when the ring formed. Favorable growth conditions produce a wide ring; unfavorable ones produce a narrow ring. Trees growing at the same time in the same region show similar tree-ring patterns.
Because a single growth ring is usually added each year, the age of the tree when it was cut can be determined by counting the rings. If the year of cutting is known, the age of the tree and the year in which each ring formed can be determined by counting back from the outside ring. Scientists are not limited to working with trees that have been cut down. Small, nondestructive core samples can be taken from living trees (Figure 5B).
To make the most effective use of tree rings, extended patterns known as ring chronologies are established. They are produced by comparing the patterns of rings among trees in an area. If the same pattern can be identified in two samples, one of which has been dated, the second sample can be dated from the first by matching the ring patterns common to both. Tree-ring chronologies extending back thousands of years have been established for some regions. To date a timber sample of unknown age, its ring pattern is matched against the reference chronology.
Tree-ring chronologies are unique archives of environmental history and have important applications in such disciplines as climate, geology, ecology, and archaeology. For example, tree rings are used to reconstruct climate variations within a region for spans of thousands of years prior to human historical records.
Knowing such long-term variations is of great value when interpreting the recent record of climate change.
Other Types of Proxy Data
In addition to the sources already discussed, other sources of proxy data that are used to gain insight into past climates include fossil pollen, corals, and historical documents.
Climate is a major factor influencing the distribution of vegetation, so the nature of the plant community occupying an area is a reflection of the climate. Pollen and spores are parts of the life cycles of many plants, and because they have very resistant walls, they are often the most abundant, easily identifiable, and best-preserved plant remains in sediments (Figure 6). By analyzing pollen from accurately dated sediments, scientists can obtain high-resolution records of vegetation changes in an area. Past climates can be reconstructed from such information.
Coral reefs consist of colonies of corals, invertebrates that live in warm, shallow waters and form atop the hard material left behind by past corals.
Corals build their hard skeletons from calcium carbonate (CaCO3) extracted from seawater. The carbonate contains isotopes of oxygen that can be used to determine the temperature of the water in which the coral grew. The portion of the skeleton that forms in winter has a different density than the portion that forms in summer because of variations in growth rates related to temperature and other environmental factors. Thus, corals exhibit seasonal growth bands very much like those observed in trees. The accuracy and reliability of the climate data extracted from corals has been established by comparing recent instrumental records to coral records for the same period. Oxygen isotope analysis of coral-growth rings can also serve as a proxy measurement for precipitation, particularly in areas where large variations in annual rainfall occur.
Think of coral as a paleothermometer that enables us to answer important questions about climate variability in the world’s oceans. The graph in Figure 7 is a 350-year sea-surface temperature record based on oxygen isotope analysis of a core extracted from a reef in the Galapagos Islands.
Historical documents sometimes contain helpful information. Although it may seem that such records should readily lend themselves to climate analysis, that is not the case. Most manuscripts were written for purposes other than climate description. Furthermore, writers understandably neglected periods of relatively stable atmospheric conditions and mention only droughts, severe storms, memorable blizzards, and other extremes. Nevertheless, records of crops, floods, and human migration have furnished useful evidence of the possible influences of changing climate (Figure 8).