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.
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