The Science of Paleoclimatology

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When climatologists study the current climate, they have a wealth of information at their fingertips. For example, they can obtain data from instruments, such as barometers, anemometers, thermometers, and rain gauges at weather observatories around the world to collect data for rainfall amounts, temperature, evapotranspiration rates, humidity, wind speed and direction, and major flow of air currents, such as the jet stream. Climate data can be collected from the mountains and valleys of all continents, including Antarctica; from the oceans; and from sophisticated satellite equipment in space. Because of this, and especially with the advances in computer technology, science has made great strides in recent years in being able to study, understand, and predict climate. Multiple types of data can be collected. Climatic data for areas can be gathered and put into computer models, such as temperature, precipitation, wind speed and direction, and humidity.

Other types of supporting data—called ancillary data—can also be collected to give scientists a better understanding of the weather. Ancillary data, such as pollution levels, population density, types and amounts of industry, modes of transportation, levels of development, types of dominant farming practices, land use changes, deforestation, and urbanization, are all pieces of information that can be used to provide a clearer understanding of climate change and global warming.

Fairly good records have been kept for the past 150 years by various international and government agencies and academic institutions. Beyond that time frame, however, data become much more scarce, and it becomes much more difficult to study and understand climatic interactions as they have related to change, including global warming.

THE PURPOSE OF PALEOCLIMATOLOGY

Before written records were kept, scientists did not have the convenience or luxury of accessing easily available, ready-to-use data. Instead, they used older, existing data that could have been interpreted in a meaningful way. This is where paleoclimatology comes into play. Paleoclimatol-ogy is the study of climate prior to the availability of recorded data, such as temperature data, precipitation data, wind data, storm data, and other measurements of the weather. The word comes from the Greek root paleo, which means "ancient," and the term climate. Paleoclimate research helps scientists better understand the evolution of the Earth's atmosphere, oceans, biosphere, and cryosphere. It also helps climatolo-gists quantify the various properties of the Earth's climate that force climate change and better understand the sensitivity of the environment to those forcings.

The National Aeronautics and Space Administration (NASA) uses paleoclimate data to test their computer models that attempt to portray climates different from what exists today. By being able to develop computer models that accurately simulate and portray past climatic activity accurately, scientists gain greater confidence in models they are building today to predict future climate scenarios for various places on Earth. If models are accurate on past incidents, then using these same models on current data raises confidence about future predictions. These models are extremely helpful because they are able to model variables that cannot be found in the geologic or fossil records, such as wind patterns, energy transportation, and cloud distribution.

For example, Gavin Schmidt, a climate modeler at NASA's Goddard Institute for Space Studies (GISS), created a climate model based on the atmosphere's response to the 1991 eruption of Mount Pinatubo in the Philippines. The massive amount of sulfur dioxide the volcano spewed into the stratosphere became sulfate aerosols (tiny reflective particles) that encircled the Earth for more than a year after the eruption, shielding it from the Sun's energy and causing the atmospheric temperature to cool by 0.8°F (0.5°C). The initial model worked well with one exception. According to Schmidt, "It turns out that most of the effects were well-modeled—it got cooler by about the right amount, and the water vapor feedback seemed to be well captured. The model, however, had one major flaw. In the winter following the eruption, actual temperatures in Eurasia were higher, not lower, than normal (the rest of the world was cooler). The model failed to reproduce this winter warming. Global climate models, however, do not generally do a good job with the stratosphere—the section of the atmosphere affected by Pinatubo's sulfate aerosols. Because the stratosphere does not influence weather, there are only a few models that have been built to describe it."

Because of this, Schmidt went back and looked closer at a phenomenon called the North Atlantic Oscillation (NAO), which is a permanent pressure system that exists over the Atlantic between the Azores Islands and Greenland. The NAO alternates between positive and negative conditions, and when the NAO is positive, it warms Eurasia, just as Mount Pinatubo's eruption did. Based on this information, Schmidt rebuilt the model, taking the stratosphere's reaction into account, and ran the model again. In order to check his results, he entered data from an era when known stratospheric changes had also taken place: the Maunder Minimum, a period of notable cooling in Europe between 1650 and 1710 when the Sun was relatively quiet. According to Schmidt, "This is an example where paleoclimate and satellite data came together to help scientists build a better model of how the stratosphere influences the NAO. This revised model was able to reproduce the unusually cold temperatures over Europe during the Maunder Minimum and was also able to reproduce the unusually warm temperatures over Europe after the Pinatubo eruption." Schmidt's goal for paleoclimatology is to provide the information needed to validate and refine other models, especially those designed to predict abrupt climate change—a critical issue in light of global warming. According to Schmidt, "We [currently] can't get the models to do some things like rapid climate change. We [humanity] owe our entire history to the fact that that happened, and we don't know why it did" (referring to the abrupt climate change after the Earth's last ice age).

NASA's GISS has been able to simulate various climate sequences throughout the Earth's history, such as the major glacial episodes, especially the Last Glacial Maximum and Holocene, which cover the past 18,000 years. These models can also be used to test for the climate system's sensitivity to change in carbon dioxide levels, a key component of global warming today.

Scientists want to be able to reconstruct past climate to gain a better understanding of what natural variations in climate have occurred over the past several thousand or more years, why they have occurred, and how these variations have affected the environment. It is also helpful in order to gain a better understanding of climate variation independent of human interference (because all historically recorded data have occurred during the time of human disturbance). Paleoclimatology includes both collecting evidence of past climate conditions and striving to understand the processes that caused the conditions—a "cause and effect" relationship.

Another important concept scientists have learned to appreciate from discoveries in paleoclimatology is that the Earth's climate is prone to frequent change. Throughout geologic time, there is evidence of floods, droughts, warm periods, and ice ages. By studying past climatic intervals, scientists are better able to reliably make predictions about how climatic changes will affect the environment and how long and how widespread their effects will be.

One key piece of knowledge of which scientists have gained a better understanding in recent years is that of abrupt climate change. They have been able to detect periods when the Earth was nearly frozen over and other times when it was a literal hothouse. Sometimes climate changes have happened gradually over very long periods of time, and other times significant changes have happened in a matter of decades or even years. It is important to know what kinds of changes are possible in a complex climate system in order to avoid unexpected surprises in light of recent global warming issues. This is one reason why being able to construct accurate models that depict abrupt climate change is so important.

Studying the past "natural" climatic cycles of the Earth also gives scientists a good base level of data against which to compare present-day situations. As humans interact with the atmosphere, they directly affect the climate system. The amount of pollution they add to the atmosphere, for instance, has a direct effect on global warming. By having a firm understanding of the Earth's climate throughout time, scientists can assess the specific effects of natural phenomena on the weather (such as volcanic eruptions) compared to human-induced phenomena (such as pollution, deforestation, and farming practices). One major finding is that the sharp rise in temperature seen in the 1900s is uncharacteristic compared to earlier time periods. Other time periods have not had such a sharp, distinct increase in temperature. A study led by Dr. James Hansen of NASA's GISS along with scientists from other organizations concluded that the Earth is now reaching and passing through the warmest levels it has seen in the past 12,000 years. They concluded that data show the Earth has been warming at the rapid rate of approximately 0.36°F (0.2°C) per decade for the past 30 years.

According to Dr. Hansen, "This evidence implies that we are getting close to dangerous levels of human-made pollution. In recent decades, human-made greenhouse gases have become the largest climate change factor." Even when current temperatures are compared to those of the last documented significant warm period, known as the Medieval Warm Period, which occurred from 800-1300 c.E. in Europe, temperatures today are 0.7°F (0.4°C) higher. This finding tells scientists that the human contribution to present-day global warming is significant and must be addressed if global warming is to be effectively dealt with.

Paleoclimatology assists computer modelers in refining their climate modeling programs. These computer models are extremely complex because the climate system has so many variables involved in it. When programmers design programs using paleoclimatic knowledge, it helps calibrate the models, making them more accurate overall and increasing their predictive power. With the interest today on rising temperatures and greenhouse gas concentrations, understanding the past is a way to compare it to the present and then be able to predict what is to come.

Scientists use several methods to study past climate. The type of method they use depends on how far back in time they want to go. If scientists are only looking backward less than 20 years, they can use available recorded data, including a vast database of satellite data and instrumental weather measurements. (The U.S. National Oceanic and Atmospheric Administration [NOAA] currently maintains this type of data.) As mentioned previously, other recorded data extends back into the 1800s and some other written records go back even further. For example, written records exist from the Middle Ages in Europe that record data on events such as grape harvests for wine making. If it is known what crops were farmed during a certain period, it is possible to make reasonable conclusions as to what the climate was probably like at the time.

This type of data, however, does not give much information on the long-term aspects of climate change. Some changes in climate take place in cycles of thousands or hundreds of thousands of years or even longer. In order to gain a good understanding of the processes that contributed to climate change and the results of it, climatologists must be able to study the records of broad sections of geologic time. Older climate data are also important to obtain because they give climatologists valuable information about natural climatic conditions before the beginning of the Industrial Revolution and subsequent large-scale human interference on climate. The beginning of the Industrial Revolution was about the same time climate records began to be kept. In order to look further into the past, scientists must use what is called proxy data.

Proxy data are simply natural data that can be used as markers, or indicators, about past climate, such as coral, tree rings, and layers of sediment. These items can contain preserved information about the Earth's atmospheric conditions and climate of the past. Proxy data will be dealt with in more detail in chapter 3.

Scientists have been able to learn much about the Earth's past climate through the tools available in climatology. They have been able to successfully determine that the Earth's climate is always fluctuating and has gone through several ice age cycles. Some ice age cycles have lasted thousands of years with glaciers advancing, then retreating. The last major ice age ended about 10,000 years ago. Since then, the Earth has fluctuated but generally warmed, although a Little Ice Age episode extended from approximately 1450 to 1890 c.E. in the Northern Hemisphere. This occurred after a warming period referred to as the Medieval Climate Optimum.

WHAT PREHISTORIC CHANGE REVEALS ABOUT THE FUTURE

An important part of being able to make forecasts of the Earth's future climate is to know how the Earth's climate has varied in the past and what mechanisms have made it vary. If paleoclimatology can help create past climate and give scientists a better idea of how climate has changed throughout time, it gives them important insight as to how today's actions will influence the trend of future climate, especially concerning global warming.

Since scientists have maintained a detailed record of the Earth's climate for the past 150 years, they have been able to determine that the temperature has warmed by 0.9°F (0.5°C). Because of its short duration, however, it is difficult to say how much of this warming can be directly attributed to humans, the burning of fossil fuels, and the enhanced greenhouse effect and how much of it is due to natural variations, such as solar variability and other factors.

Because the issue of global warming remains such a heated debate today among opposing groups, having paleoclimatic data is helpful because it gives scientists a handle on how much of the Earth's climate has naturally varied throughout time (under the influence of volcanic eruptions, orbital changes, and solar output) without any interference from humans.

According to NOAA, paleoclimatology helps scientists find answers to questions such as the following:

• Is the past 100 years of increased temperature and global warming normal?

• If the last century was abnormally warmer than it should have been, what does that mean?

• Is the recent rate of climate change normal?

• Is the recent warming something new or just part of an older, larger cycle?

• Is there any evidence of past climate forcings (outside factors forcing the climate to behave in a certain way) that could be actively contributing to the warming trend today of which we are currently unaware?

By being able to differentiate between natural and human input, approaching the issue of global warming can be much more scientific and yield better solutions. In other words, having a paleoclimatic perspective allows scientists to look back thousands of years to develop a more accurate picture of how the Earth's climate may change and affect everyone in the future.

RATE OF CHANGE

One important concept of paleoclimatology is to be able to track the changes in greenhouse gas concentrations by the heating or cooling of the Earth's surface. Scientists know that when the atmosphere warms up, carbon dioxide is released from the oceans. In addition, if the Earth's orbit changes and triggers a warming period, it can also increase greenhouse gases. When this happens, it triggers the greenhouse effect. This creates a positive feedback that encourages more warming. Conversely, when temperatures cool, CO2 is absorbed by the oceans, which contributes to additional cooling. According to the Intergovernmental Panel on Climate Change (IPCC) in their 2007 report, during the past 650,000 years, CO2 levels have been high, and during the cool glacial periods, CO2 levels have been low.

Because scientists can observe coinciding rates of change with temperatures and CO2 levels throughout the past several hundred thousand years and determine that there is a strong correlation, this also gives them a scientific basis and better understanding about the future. Through the correlation of temperatures and CO2 records, scientists can determine, through modeling, the types of effects that can be expected from various levels of CO2 in the atmosphere.

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Fluctuations in temperature (red line) and in the amount of carbon dioxide concentrations in the atmosphere (blue line) over the past 350,000 years. The temperature and carbon dioxide concentrations at the South Pole run roughly parallel to each other, showing the strong correlation between the two.

Ocean currents are also affected by changes in the Earth's surface temperature. It has been proven that melting glaciers and ice caps can add enough freshwater to the northern oceans to slow or stop the flow of key currents such as the Gulf Stream. Because ocean currents have a significant effect on climates around the world, these changes can cause significant changes to world climate. For instance, if the Gulf Stream were to slow down or stop due to excess melting of glacial and ice cap ice in the Arctic, it could bring cold, ice age-like weather to normally moderate temperature regions in Europe. If ocean currents are altered, the global distribution of heat will be altered, which will cause major changes in climate from one area of the Earth to another.

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This graph shows how the recent warm temperatures and rising carbon dioxide levels relate to those of the past 1,000 years. Experts believe this sharp rise is due largely to how humans have enhanced the greenhouse effect.

Scientists have gathered information from records preserved in layers of ocean sediments of the fossils of marine life that show that this has happened several times throughout the Earth's history and that these changes have caused significant climate changes over large areas.

NOAA has completed several studies that show that the Earth's earlier climate had periods of stability interrupted by periods of rapid change. They believe this is because interglacial climates (such as the current climate) are more stable than cooler, glacial climates. Disruption of this stability can be caused by variations in the Earth's orbit through time and the associated variability of solar radiation received at

This rock landscape of the American Southwest was shaped over eons of geologic time by the multiple forces of wind, water, and ice.

(Nature's Images)

the Earth's surface. Glacial periods occur when summer solar radiation is reduced in the Northern Hemisphere. Throughout the Pleistocene epoch (the past 1.8 million years), these cycles have occurred with a frequency of about 100,000 years. Precession of the equinoxes also affects solar radiation in cycles of 23,000 years, which encourages glacial periods. Warming at the end of glacial periods also happens abruptly due to the ice-albedo feedback mechanism. With less ice to reflect incoming solar radiation, more heat is absorbed by the Earth's surface, causing temperatures to rise. Once ice begins to melt and expose the land and water, additional solar radiation can be absorbed by the Earth's surface, raising temperatures and causing even more ice to melt in a positive

Clues to past climate can be seen in present-day landscapes. The basin this alpine lake is contained within was originally carved out by a glacier during the last ice age. The lake is in the Uinta Mountains in northern Utah, part of the Rocky Mountains. (Nature's Images)

feedback. Abrupt, rapid climate changes often accompany the transitions between glacial and interglacial periods. Most current civilizations came into existence during relatively stable periods of climate.

CLIMATE PATTERNS

Climatologists have determined that climate change is not only constant but that it also occurs at multiple time scales. It occurs at geologic time scales of millions of years, and nested within that at scales of hundreds of thousands of years, and within that at tens of thousands of years to thousands, hundreds, and even decades and annual scales. Long-term changes are seen in changes in the Earth's tilt, axis, and precession. Cycles are seen in ice ages. Shorter-term changes are seen in patterns of phenomena, such as El Niño. Historical climate change will be looked at in more detail in chapter 2.

Some scientists have even suggested that climate has a "memory" and that by understanding past conditions and behaviors it is possible to

Bryce National Park, Utah. This area was once covered by seas, mountains, deserts, and coastal plains. The geologic formations within this area testify to the various climates that existed when each layer was deposited. (Nature's Images)

predict and plan for future conditions. According to scientists at NOAA, "Understanding 'climate surprises' of the past is critical if we are to avoid being surprised by abrupt climatic change now." For example, abrupt climate change has had drastic consequences for ancient civilizations that can serve as warnings for society today. According to Dr. Harvey Weiss, a professor of archaeology at Yale University, as reported by CNN in April 2005, climate change was a fact of life for earlier civilizations. Egyptian pharaohs and medieval Vikings both had to deal with violent changes in weather patterns, which sometimes prompted mass migrations.

According to Dr. Weiss, "Those episodes proved to be the single most important stimulus for the major transformations in human history." As information is discovered about prehistoric climates, archaeologists are looking for connections between climate change and human development. They have linked the collapse of early Bronze Age civilizations in Greece and India to abrupt climate changes about 4,200 years ago. Drought has been cited as a factor in the collapse of the Anasazi civilization of the American Southwest during the 13 th century.

Dr. Weiss says results were not always negative, however—it was drought that drove farmers in ancient Mesopotamia to build irrigation channels, which enabled farmers to grow enough food so that for the first time everyone did not need to farm. Instead, it allowed some inhabitants to pursue other paths, such as architects, politicians, and artists. According to Weiss, "The historical lesson is that those societies had no knowledge of what was happening to them and certainly no historic knowledge of what could happen to them, where we have both." With today's technology, scientists are able to improve predictions for the future based on knowledge learned from incidents in the past.

Because different aspects of the environment respond in different ways, physical characteristics of areas play a significant role in determining climate. As an example, if a highly vegetated area were to have repeated years of lower-than-normal rainfall, the soil would dry out, lakes would shrink, and water sources would diminish. In response, the vegetation would die off. Over time, with less vegetation, less biomass would enter the soil, causing the soil to lose fertility. There would be less evapotranspiration, which would also change the characteristics of the area to become more arid. Evapotranspiration is the transfer of moisture from the Earth to the atmosphere through evaporation of water and transpiration from plants. Dry conditions would contribute to even further evaporation and shrinkage of lakes, cause grasslands to dry up, and cause desert areas to begin expanding. This is a process called "desertification" that is becoming common in the American Southwest as global warming continues, and it has caused enormous problems for the peoples of Africa. It has destroyed water supplies, drinking water, and farming and has subsequently contributed to the spread of disease and starvation. Desertlike conditions can then lead to less rainfall in subsequent years. When this works as a cycle, scientists view this as a self-perpetuating process. This happens because different parts of the environment respond over different time intervals and in different ways, sometimes feeding off of each other and either enhancing or minimizing the responses.

Some scientists in England have noticed that there is a correlation of summer rainfall amounts to the wintertime air pressure pattern of the North Atlantic Ocean; the summer climatic pattern seems to "remember" what the previous winter pattern was. Because of this correlation, farmers have been able to predict which years will be better for growing wheat, because they have been able to predict which years will have wetter or drier summers.

Being able to look at characteristics of climate in certain locations and to predict how climate will act in the future is a valuable tool. One thing that climatologists agree on is that the Earth's climate is always changing. Its variability changes on multiple timescales—short-, medium-, and long-term intervals. No matter what the scale of these changes, they have a significant effect on humans. Because of this, it is important that scientists study past climatic variability in order to better understand future climate change and the potential impacts it may have on society.

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