Scientists learn about climate and how it has changed by studying climates of the past. By analyzing changes that have occurred in the earth's temperature over time, scientists can gain a better understanding of global warming, and make determinations about its possible causes.
Scientists have discovered ways to study the earth's climate, going back as far as thousands, or even millions, of years. Those who specialize in studying ancient climates are known as paleoclimatologists, a name derived from the Greek root word paleo , which means ancient. Paleoclimatologists use natural elements in the environment to find "proxy climate data" related to the past. When they study these types of data, these scientists typically use several different methods, so they are assured of forming the most accurate analysis possible.
One way that paleoclimatologists unlock the secrets of ancient climates is by studying the rings in certain types of trees, such as the redwoods and giant sequoias found in California and different varieties of pines. As a tree grows, it adds a new layer of wood to its trunk every year. This forms a ring, and the age of the tree can be determined by counting the number of these annual growth rings.
Many trees live to be hundreds of years old, and some live for thousands of years. The oldest trees on Earth are the bristlecone pines, many of which are found in the Ancient Bristlecone Pine Forest in California's White Mountains. The average age of these trees is 1,000 years, and a few are more than 4,000 years old. In 1964, before there were environmental laws to protect ancient trees, a particular bristlecone pine named Prometheus was cut down. After analyzing the tree's rings, scientists determined that the tree had been 4,862 years old—the oldest living thing on Earth.
Paleoclimatologists can learn more than just the age of a tree by studying its rings. They can determine what sort of climate conditions existed during its life by analyzing the thickness of each tree ring. Thick rings are a sign of favorable climate, abundant rainfall, and good growing conditions. Thin rings indicate poor growing conditions and lack of rain, as well as natural disasters such as droughts, floods, and volcanoes.
Samples from trees can be obtained in several different ways. Scientists do not want to needlessly destroy living trees, so they cut cross sections only from dead trees, logs, or stumps. These can be found intact on the ground, buried deep in the ground, or submerged in water. Tree remnants that have been buried for hundreds or even thousands of years have been found and analyzed. For samples from living trees, scientists use a tool known as an increment borer to drill a thin hole into the trunk. Then, a core sample of wood about the size of a drinking straw is extracted for analysis. This boring does not cause damage to the tree because when the sample has been removed, the tree naturally closes the small opening just as it would close a wound caused by insects or weather.
Once the wood samples are obtained, scientists return to the laboratory to measure and date them. Cross sections of dead trees are often old and brittle; and scientists may need to glue pieces together—or mount them on a hard wooden surface—for added protection. Cores that are taken from living trees are soft, so they must be dried before being mounted for examination. The next step is to sand the samples or trim them with razor blades to produce a smooth surface that makes the fine details of the rings more visible. Then scientists can examine the samples under a microscope and record their findings about the tree's history.
Another way paleoclimatologists analyze historical climates is by studying samples of varves—layers of silt and clay that are deposited year after year on the bottoms of glacial lakes and ponds. Varves provide natural climate records going back several thousand years. They consist of two layers: a thick, light-colored layer of silt and fine sand that forms in the spring and summer, and a thinner, dark-colored layer of clay that forms in the fall and winter and sinks to the bottom.
Varve thickness varies from year to year, usually according to the climate and the amount of rain that falls during a particular season. For example, when temperatures are especially hot and dry and there is little rain, less soil is washed into the water, and the varve layers are thinner. On the other hand, when spring and summer rains are heavy, a greater amount of soil is washed into lakes and ponds, and this causes thicker varves. Paleoclimatologists collect varve samples by using long, hollow tubes to drill into the soft bottoms of lakes and ponds. Once they extract this material, they analyze the different layers that have been deposited over time.
Clues about ancient climates are not found only in bodies of freshwater such as lakes and ponds, but are also buried in sediment that has settled in the earth's deep oceans. Robert B. Gagosian says that by studying these sediments, called deep-sea cores, scientists can reconstruct the history of ocean climates spanning thousands of years. He describes this research, and explains why it is so important:
Preserved in the sediments are the fossil remains of microscopic organisms that settle to the seafloor. They accumulate over time in layers . . . that delineate many important aspects of past climate. For instance, certain organisms are found only in colder, polar waters and never live in warmer waters. They can reveal where and when cold surface waters existed—and didn't exist—in the past. From records like these, we know that about 12,800 years ago, North Atlantic waters cooled dramatically—and so did the North Atlantic region. This large cooling in Earth's climate . . . lasted for about 1,300 years. This period is called the Younger Dryas, and it is just one of several periods when Earth's climate changed very rapidly from warm to cold conditions, and then back to warm again. 8
To gather data from oceans, scientists spend two to three months on research cruises. Using highly specialized equipment, they remove samples of deep-sea cores from beneath the surface of the ocean floor. These long cylinders of sediment provide valuable evidence about changes in ocean temperatures that were caused by fluctuations in climate.
Scientists also gather and study sediment from different bodies of water to gather pollen. This powdery substance, produced by flowering plants each growing season, is carried in the wind, and billions of grains of it end up buried at the bottoms of lakes, ponds, rivers, and oceans. The oldest pollen becomes fossilized, and is often found in sedimentary rocks that have formed over thousands of years. Since all plant species produce their own unique type of pollen, scientists can
Coral reefs can also provide important clues to climates of the past. There are many different types of corals, but "stony corals" build huge reefs in warm, tropical seas. Coral reefs are made up of millions of tiny animals called coral polyps, which are cousins of the jellyfish. Although polyps differ in size, they are usually quite small—about the size of a pinhead. The polyps form protective skeletons by extracting calcium carbonate—the same material that is found in teeth, bones, and shells—from the salty, tropical ocean waters in which they live. As the skeletons grow, coral reefs are formed, and become as hard as rocks. These huge structures are often called underwater
Every time a piece of coral skeleton is created, it leaves a record of the conditions under which it was created. For instance, when water temperatures change, the chemistry (or makeup) of the skeletons also changes. The result is that coral formed in the summer looks different than coral formed in the winter, so it is easy for paleoclimatologists to know in which season the coral was formed. As coral reefs grow, growth bands form that are very much like the growth rings found in trees. Sometimes these bands are visible to the naked eye, and sometimes scientists can only see the bands by x-raying them.
To gather samples of coral, scientists go on diving expeditions in tropical areas, where they search for massive coral reefs built by stony coral. Using drills that are connected to a compressor mechanism on a ship, the divers extract cores of the coral, much the same way cores are extracted from trees. Their goal is to drill in areas where the most growth has occurred, as the NOAA explains: "Think of the coral's structure as being very similar to an onion sliced in half, with a new ring added each year. If you wanted to drill into an onion to sample as many rings as possible, you would core from the surface directly towards the center. This is exactly how scientists go about getting as long a sample as possible from each coral." 9
Once scientists have carefully extracted the cores, they label and box them for shipment to their laboratories. There they x-ray the coral to examine the growth bands, which helps them determine the seasons in which the corals grew. With this proxy climate data, paleoclimatologists can analyze how climates fluctuated in the reef over hundreds of years.
Just as scientists gain clues about climate from warm, tropical seas, they can also gather knowledge from the coldest places on the earth. In fact, some of the most revealing indicators of historical climates come from studies of glaciers and ice sheets in the world's polar regions. To gather samples of ancient ice, scientists travel to remote areas of Antarctica, where temperatures can dip as low as -129 degrees Fahrenheit.
Massive ice domes, ice sheets, and glaciers are found in the Arctic and in Antarctica. These ice formations developed over hundreds of thousands of years as layers of snow pressed together. More precipitation continued to pile on top of the snow, squeezing the layers and slowly forming ice. As the layers accumulated, air bubbles were trapped inside, forming distinct lines that can be counted as easily as tree rings. Scientists examine the layers to determine the age of the ice and the approximate climate during a given period. They can also tell how much snow fell during a year, as well as what kind of air, dust, volcanic material, and other microscopic particles—including pollution—existed at the time the ice sheets were formed.
About 98 percent of the world's ancient ice is located in the polar regions, and most scientists choose to focus on those areas when they study ice. Others, however, believe that ice from tropical areas is even more crucial in order to understand how climates have changed over time. Lonnie Thompson is a glaciologist who studies ancient ice in areas such as South America and Africa. These regions have hot, tropical climates, but they also have very high mountain ranges where ice sheets and glaciers can be found. Thompson sometimes climbs mountains three or four miles high. On one expedition, he and his team worked for three weeks at an altitude above twenty-three thousand feet.
Thompson's work is challenging as well as dangerous. With the help of local porters and animals called yaks, he and his team haul about six tons of
During a typical expedition, Thompson and his team accumulate about four tons of ice samples, which means they must drag ten tons of equipment back down the mountain. He says it is well worth the effort, though, and he explains why he thinks ice is the best possible archive of the history of the earth's climate: "Understanding how the climate system works and has worked in the natural system is absolutely essential for any prediction of what's going to happen to the climate in the future." 10 Thompson adds that by examining ancient ice, scientists can determine climate conditions and changes over thousands of years in the past.
Whether they explore ice domes in Antarctica, glaciers in Tibet, or ice sheets at the top of Africa's Kilimanjaro, scientists gather samples by using powerful drills to bore into the ice. The deeper the drill goes—and that can be several miles—the further it travels back in time. (Thompson's oldest ice sample is more than seven hundred thousand years old.) After drilling, scientists extract cores of ice and carefully package them in insulated containers, so the samples can be sent to their laboratories for analysis. Thompson says that by collecting ice samples, scientists can compile a frozen history of the earth.
The reason scientists use proxy climate data obtained from ice, trees, coral reefs, and other products of nature is because they want to understand what the earth's climate was like long ago. Scientists use these types of data along with modern devices so they can learn more about how climate has changed over time, as well as how historical and current climates compare with each other.
Thermometers, which measure temperatures of the earth's surface, have been used to determine climate for only about 130 years. Some scientists, like Dr. S. Fred Singer, who is an atmospheric physicist, question the accuracy of thermometers because they are often used near cities, which are warmer than open country. Singer explains his views: "You have to be very careful with surface record. . . . As cities expand, they get warmer. And therefore they affect the readings. And it's very difficult to eliminate this—what's called the urban heat island effect." 11
Dr. John Firor, a senior scientist at the National Center for Atmospheric Research, says it is true that cities are generally warmer than open country. He adds, however, that thermometers can provide accurate measurements even in cities, and he explains how:
One can find empty holes in the ground—abandoned oil wells, for instance—and put down a long line of thermometers. This allows measurement of the temperature of soil or rocks many levels down. The reason this works is because over time, the warmth at the surface is conducted to deeper levels. So, the temperature deep down in the hole relates to the surface temperature of long ago. This is also true when the surface is cold—the coolness is conducted down over time. Many holes have been measured in recent years, and what we've found is that the record of past temperatures confirms what is measured from carefully placed surface thermometers. 12
A highly sophisticated way of monitoring the earth's climate is through the use of satellites. Since the 1950s, NASA satellites have been observing Earth's atmosphere, oceans, land, snow, and ice from high in space. The data they provide can help scientists develop a better understanding of how these different elements interact with each other to influence climate and weather.
One example is Terra , a satellite that was launched by NASA in 1999. Terra , named after the Latin word for land, is about the size of a small school bus, and its mission is to circle Earth for about six years. The satellite is fitted with a variety of sensitive instruments that are designed for specific purposes, such as measuring the chemical composition of clouds and gauging the temperature of the land. Terra 's MICR instrument has nine separate digital cameras that take pictures of Earth from different angles, while its MOPITT instrument uses light sensors to measure concentrations of methane gas and carbon monoxide, two heat-trapping gases. The satellite's instrument MODIS measures cloud cover and also monitors changes in Earth due to fires, earthquakes, droughts, or flooding. An instrument called CERES measures both incoming energy from the sun and reflected energy from Earth and studies the role that clouds play in this energy balance.
In the spring of 2002, NASA launched another satellite called Aqua , whose mission is to gather information about the earth's bodies of water. Aqua will circle the planet every sixteen days for six years, and its sophisticated instruments will measure such things as global precipitation, evaporation, humidity, and ocean circulation. This data will help scientists better understand the balance between the earth's oceans, land, and atmosphere, as well as how
In the future, NASA will launch more satellites to study global climate change. The organization describes the goal for these studies as follows:
As we learn more about our home planet, new questions arise, drawing us deeper into the complexities of Earth's climate system. We don't know the answers to many other important questions, like: Is the current warming trend temporary, or just the beginning of an accelerating increase in global temperatures? As temperatures rise, how will this affect weather patterns, food production systems, and sea level? Are the number and size of clouds increasing and, if so, how will this affect the amount of incoming and reflected sunlight, as well as the heat emitted from Earth's surface? . . . How will climate change affect human health, natural resources, and human economies in the future? NASA's Earth Observing System, and Terra in particular, will help scientists answer these questions, as well as some we don't even know to ask yet. 13
Scientists are the first to say that there are many unknown factors involved in the study of global climate change. Products of nature such as ice cores, coral reefs, ocean and lake sediments, and trees can offer valuable clues about changing climates in the ancient past. Modern instruments like satellites can provide knowledge about current activities affecting the earth's land, oceans, and atmosphere. Assembling the pieces of this global environmental puzzle is the focus of scientists and researchers all over the world. They know for sure that the earth is warming—and using the many tools available to them, it is their mission to find out why.