Human Response to Holocene Climate Fluctuations: Have We Been Here Before?

HUMAN RESPONSE TO HOLOCENE CLIMATE FLUCTUATIONS: HAVE WE BEEN HERE BEFORE?

Patrick N. Smith

Dr. Elizabeth Shapiro, Advisor

2015

Masters project submitted in partial fulfillment of the requirements for the Master of Environmental Management degree in

the Nicholas School of the Environment of

Duke University

EXECUTIVE SUMMARY

The central proposition of this project is that archaeology has relevance and utility for general contemporary society, rather than just for the select group of academics who have chosen it as a field of study. The National Historic Preservation Act (NHPA) ensures that federal actions taken on behalf of the public will take into account the potential impact those actions might have on archaeological resources. As such, the discipline has real consequences for the public in the form of public expenditures. Archaeological research is often labor intensive, time-consuming, and expensive, so it is reasonable to ask what public benefit will be realized as a result of those expenditures. This is certainly something that professional archaeologists working in the field of cultural resources management have wrestled with. Unlike the public benefit of other regulatory laws, such as those enforcing clean air and clean water standards, the public benefit of conducting archaeological research and protecting archaeological sites is not self-evident. However, recent contributions by archaeologists to paleoclimate studies have given the discipline a new currency, with the public benefit being insights into one of the most controversial and far-reaching issues of our time: climate change.

Paleoclimate research has found that the Earth’s climate has undergone numerous oscillations during the current interglacial period (the Holocene). Until the modern era, these oscillations were, in all likelihood, caused by orbital forcing (eccentricity, axial tilt, and precession) working in conjunction with various feedback phenomena, such as volcanic emissions, surface reflectivity, atmospheric reflectivity, atmospheric chemistry, etc., with the net result being a somewhat predictable cycle of alternating cool and warm periods over the past 12,000 years. Archaeological research, augmented by historical documentation from Europe and Asia, has shown that these oscillations align with significant periods of culture change in both the Old and New Worlds. By comparing climate predictions for the future with the paleoclimate record and episodes of past culture change, this project offers the following qualified insights into what society might expect for the terminal 21st Century:

 The historic and archaeological records indicate the scale of climate events necessary to have a meaningful influence on human society is not very great.

 For the modern world, at-risk countries could face social collapse as a result of food stress brought on by increased aridification, while First World nations could have to deal with substantial population displacements from regions experiencing water shortages and an increased cost of living.

 By the end of the 21st Century, the predicted warming trajectory will probably push average global temperature past anything that has been experienced during the Holocene to date; there are no past conditions that are analogous to projected future conditions. The implication is that mankind should expect the need for significant cultural adaptation to future climate change.

TABLE OF CONTENTS

Executive Summary ...... i Table of Contents ...... iii List of Figures ...... iii Chapter 1 Introduction ...... 1 Chapter 2 Understanding The Mechanisms of Climate Change ...... 9 Historical Perspective ...... 9 The Mechanics of Orbital Forcing...... 12 Other Considerations ...... 16 Discussion ...... 21 Chapter 3 Climate Past and Future ...... 23 Climate Past ...... 23 Chapter 4 Evidence for Climate Change in Archaeological Record ...... 37 Evidence from the Southeastern United States...... 39 Interpretations ...... 42 Chapter 5 Conclusions ...... 45 Final Remarks ...... 47

LIST OF FIGURES

Figure 2.1: Stonehenge (Delso, 2014) ...... 9 Figure 2.2: Variation in Orbital Eccentricity (Simmon, n.d.) ...... 13 Figure 2.3: Variation in Axial Obliquity (Simmon, n.d.) ...... 14 Figure 2.4: Precession (Simmon, n.d.) ...... 15 Figure 3.1: Annual Mean Surface Air Temperature Change (IPCC, 2013c, p. 1063) ...... 33 Figure 3.2: Seasonal Mean Percentage Precipitation Change (RCP8.5) (IPCC, 2013e, p. 1078) ...... 34 Figure 3.3: Annual Mean Near Surface Soil Moisture Change (2081-2100) (IPCC, 2013b, p. 1080) ...... 35 Figure 3.4: Mean Regional Relative Sea-level Change (2081-2100) (IPCC, 2013d, p. 1196) .. 36 Figure 4.1: Mississippian Period Duck Bowl from Moundville (Bomar, n.d.) ...... 41 Figure 4.2: Rattlesnake Disk from Moundville (Moundville Decorative Disc, 2014) ...... 41

iii

CHAPTER 1 INTRODUCTION

But archaeology is not simply valuable as a purveyor of facts and evidences for the use of the historian. It elevates the mind of man; it enlarges his soul; it divests us of a part of our selfishness; it lifts us out of the rut of our every-day life; it makes our hearts beat in sympathy with those who cannot repay us even the “tribute of a sigh”; it educes affections which bless us and tend to make us blessings to all around, but which are apt to be dried up by too long and too intimate an acquaintance with the market-place and the exchange.

Rev. John Collingwood Bruce LL.D, F.S.A. (1857, p. 7)

This project is in some sense an attempt to validate archaeology as a useful discipline. As a person who has spent his entire career in the field of environmental compliance, first as a professional archaeologist and then as a National Environmental

Policy Act (NEPA) practitioner, I can say from experience that not all environmental disciplines or resources are viewed equally—not from the professional community, not from the government, and not from the public—nor is it reasonable to think they would be. Under the overarching framework of NEPA, the federal government must comply with a host of environmental laws and consider the impact of its actions on a broad range of environmental resources. Presumably, in each instance, there is some benefit to society; however, the value of these protections can be self-evident in some cases and seemingly very subjective in others.

Take for example the Clean Air Act (CAA) (1970), whose first purpose is “to protect and enhance the quality of the Nation’s air resources so as to promote the public health and welfare and the productive capacity of its population. . .” (p. 5675). Few people would object to such a law or be confused as to why it should be considered

important. Similarly, the need for the Clean Water Act (CWA) (1972) can also be

viewed as self-evident. It is probably fair to say that every mentally competent person

alive today knows that air and water are essential for life. And while they may not all

understand the science behind it, they know innately that breathing dirty air or drinking

dirty water is something they should strive to avoid if possible.

Now consider a law like the Endangered Species Act (ESA) (1973), which provides “a means whereby the ecosystems upon which endangered species and

threatened species depend may be conserved. . .” (p. 1). The value of such a law for

individuals and society is less self-evident than either the CAA or the CWA.

Nevertheless, the rationale for maintaining robust, diverse ecosystems—that is,

recognizing the connection between ecosystem services and human wellbeing—is not

so esoteric that it cannot be understood by most people through fairly straightforward

cause-and-effect reasoning.

There is also the issue of immediacy. Certainly, in some cases, people could

experience the health consequences of taking in polluted air or water almost at the point

of exposure. In the case of ecosystem degradation, the recognition of the

consequences may not be as immediate as for air and water pollution, but the damage

is something that people can recognize within their lifetime, if not within just a few

decades.

Finally, let us consider the National Historic Preservation Act (NHPA) (1966),

whose function, when distilled from the Act’s more broadly written statement of purpose,

is simply to preserve historical and archaeological sites in the United State. According

2 to the Advisory Council on Historic Preservation (ACHP) (n.d.), which was established by the NHPA, the Act, among other things, “expresses a general policy of supporting and encouraging the preservation of prehistoric and historic resources for present and future generations” and “encourages State and local preservation programs. . . “ (p. 1).

Based on this sort of language, one can hardly conclude that the value of the NHPA is self-evident or that the protections it affords are urgently needed. Some deeper explanation is required. After all, what are prehistoric and historic resources worth?

Moreover, what are the consequences for society if they are lost? The cost of preservation for these resources can often times be very high. The NHPA (1966) states:

The head of any Federal agency having direct or indirect jurisdiction over a proposed Federal or federally assisted undertaking in any State and the head of any Federal department or independent agency having authority to license any undertaking, prior to the approval of the expenditure of any Federal funds on the undertaking or prior to the issuance of any license, shall take into account the effect of the undertaking on any historic property. (p. 134)

What this means in the specific case of archaeology is that a jurisdictional authority overseeing a federally funded or permitted action for which ground-disturbing activities will take place (e.g., a new highway construction project), must identify any

National Register eligible archaeological resources within the project’s area of potential effect (APE) and either avoid them if practicable, or take measures to mitigate project impacts to these resources if avoidance is not possible or warranted. Because archaeological surveys and excavations are very labor-intensive undertakings, for large

projects, compliance with the NHPA can be a very costly and time-consuming

endeavor.

So what are we getting for all the time and money spent on archaeology? Life-

essential clean air and water? No. Preservation of ecosystem services essential for

maintaining a life-sustaining environment? No. If you ask people who work in this

discipline, often times you will get the rather broad and noncommittal statement about

“knowledge being its own reward.” Other times you will hear a more admonitory but still

hollow trope about “learning the mistakes of past civilizations, so as to not repeat them.”

While these are worthy objectives from a philosophical perspective, they do not really

address why the public’s best interests are served by spending hundreds of thousands

of tax dollars to excavate, for example, a 6,000-year-old archaeological site in central

Georgia. The peculiarities of regional lithic industries during the Middle Archaic Period

in the Southeast may seem like exciting stuff to a select group of academics, but what

can truly be learned from a stone-age hunter-gatherer culture that will improve the life of

the average 21st century citizen?

This dilemma concerning the relevance of archaeology is nearly as old as the

discipline itself. It is so old in fact, that Rev. John Collingwood Bruce felt the need to

write a monograph defending the discipline in 1857 entitled The Practical Advantages

Accruing from the Study of Archaeology. More recently, Honerkamp (1988) noted archaeologists need to “redefine what is important to know and why it is important” (p.

6) so we may offer our sponsors in the public and private sectors something more than

“the empty ‘greater appreciation of the past’ statements that usually characterize such efforts” (p. 6). More recently still, Noble (1996) worried that “it would not be surprising if

the entire federal historic preservation program is called into question during this period

of increasing fiscal conservatism and anti regulatory sentiment” (p. 80). The reason for

this, as Lees and King (2007) observed, may be that “the public is little aware of what

we do and why (at least not aware as they should be, given the level of public money

that is being spent on their behalf. . .)” (p. 59) and “that public projects do not equal a

public product” (p. 59).

Based on this, it would certainly appear that archaeologists have not done a very

good job of conveying to the public why archaeology is important. Fortunately for the

discipline, one of the most controversial and divisive subjects of the 21st century has

given archaeology a genuine currency, where lessons can be learned from the past that

have demonstrable relevance to the lives of contemporary citizens today. That subject,

of course, is climate change.

It is probably a reasonable statement to say the majority of Americans have

some understanding that the Earth’s climate has changed throughout its history. I

suspect, however, that this understanding is largely limited to profound events that have

long been studied and that have some currency in popular media—most notably ice

ages—and that very few people have much knowledge or awareness of Holocene1 climate fluctuations.2 This is based, admittedly, on anecdotal evidence (my own many

conversations on the topic with friends, family and co-workers over the years), as there

do not appear to be any studies dealing specifically with public awareness of

paleoclimates. Nevertheless, my conclusions don’t appear to be too great a stretch

1 The current interglacial period covering the last 12,000 years. 2 Malka, Krosnick and Langer (2009) suggested that the American “public’s low level of concern about GW results from a lack of public understanding of the problem” (p. 634).

insofar as general public awareness on the climate change issue appears to be lacking

(e.g., Malka et al., 2009; Akerlof et al., 2010). It may be that the ambivalent attitude and

apparent lack of urgency exhibited by many Americans3 regarding the current climate

dilemma is simply due to misconceptions about the scale of events necessary to impact

human society in a significant way. Compared to glacial-interglacial transitions,

Holocene climate fluctuations (e.g., stadial4 events) are miniscule; however, there is a growing body of research to show that these comparatively small events have had large impacts on human society throughout history (and prehistory) (Little, 2000; Smith, 2002;

Rosen, 2007; Foster 2012). Paleoclimate studies, to which archaeological research contributes significantly, may offer the modern world valuable insights into what we could be facing in the coming decades or centuries.

In the following chapters, I will discuss what we understand about the causes of climate change through history; how those changes have impacted human society over time, as revealed through the archaeological and historic records; what climate changes are predicted for the terminal 21st century; and if the past can be used to predict the

consequences human society may be facing as a result.

Specifically, the questions I wish to answer are these:

 Can we make useful predictions about human responses to future climate change by examining past human responses to climate change in the southeastern United States?

3 Akerlof et al. (2010) found that “Americans were far more likely to see climate change as a problem for people geographically and temporarily distant, rather than for themselves” (p. 2560). 4 Periods of lower temperatures during an interglacial.

o Are the predictions about the severity of future impacts to human society borne out by the historic and archaeological records?

o Do the historic and archaeological records also provide evidence of beneficial effects of climate change to human society and are any of these a reasonable predictor of future beneficial effects?

Ideally, the project will culminate in a paper suitable for popular consumption that will not only be informative, but will also trade on the appeal archaeological subject matter seems to have in popular culture.

CHAPTER 2 UNDERSTANDING THE MECHANISMS OF CLIMATE CHANGE

HISTORICAL PERSPECTIVE

It is a curious irony that humankind’s need for a cheap, efficient energy source during the last three centuries helped open the door to the study of climate change. To be sure, our interest in climate—at least annual climate—long predates the Industrial

Age. One need only to consider that there are over a hundred known ancient observatories scattered across the six inhabited continents to realize that Man has long believed his fate is closely bound to the movement of the sun (Traditions of the Sun,

2005; Wikipedia, 2015a). Among these, Stonehenge is perhaps most recognized in popular culture (Figure 2.1), but there are others reputed to have similar functions. There is Wurdi Youang in southeastern Australia, an Aboriginal ovoid arrangement of stones with features thought to mark the position of the setting sun at the longest and shortest days of the year (Hamacher & Norris, 2011). There is the astronomic clock at Machu Picchu in Peru, known as the Hitching Post of the Sun

(Machu Picchu, n.d.). There is the controversial 5,000-year-old open-air sun temple in

Alberta known as Canada's Stonehenge (Weber, 2009). And then, of course, there is

Stonehenge itself, an early Bronze Age astronomical calculator rising stoically out of the

Figure 2.1: Stonehenge (Delso, 2014) 9

Salisbury Plain in southern England (Stanford SOLAR Center, 2008).

Though the time of construction for these many observatories is often separated

by millennia, what they collectively reveal is that ancient man, no matter where or when he lived, was keenly interested in tracking the sun’s annual progress across the sky.

And while the full range of purpose served by these observatories can be debated, few would argue with the assertion that early astronomers sought to accurately predict the seasons—the annual climate cycles that marked, for example, the arrival of migratory animals, or the cessation of rains, or the most advantageous times for planting and harvest (Traditions of the Sun, 2005).

However, it was not until the Industrial Revolution, when the mining industry’s search for valuable mineral resources, which included fossil fuels, led early geologists like James Hutton (known as the Father of Modern Geology) and Charles Lyell to speculate on the great antiquity of the Earth (Understanding Evolution, 2008). Armed with fossil evidence and the principle of superposition5, these men rejected Biblical

timescales, which were based on human life spans, and embraced a vastly more

ancient genesis (Understanding Evolution, 2008; Braterman, 2013). It suddenly

became possible to imagine the Earth not as a static body 6,000 years old, but rather as

a monumental stratified palimpsest eons in the making (Understanding Evolution, 2008;

Braterman, 2013).

5The principle of superposition states that “in any undisturbed sequence of rocks deposited in layers, the youngest layer is on top and the oldest on bottom, each layer being younger than the one beneath it and older than the one above it” (Law of Superposition, n.d.). It is attributed to Danish scientist Nicolas Steno.

As our understanding of the Earth’s formation grew, so did its age from thousands of years to millions, and then finally, by the 20th century, from millions to

billions (Braterman, 2013). Careful study of the geologic evidence, strengthened by

contributions from paleontology and later chemistry, also revealed that the Earth had

undergone many substantial and lengthy climate fluctuations, alternating between warm ice-free periods and major ice ages (Water Resources Foundation, 2009; National

Oceanic and Atmospheric Administration (NOAA), 2014). The Earth is currently within one of these major ice ages—the Quaternary Period—which began some 2.59 million years ago (Cohen, Finney, & Gibbard, 2013). The Quaternary Period itself has

undergone a series of climate fluctuations, characterized by cool, dry glaciations

interrupted by comparatively warm and wet interglacial periods (National Geographic,

2015c).

While the cause(s) of these fluctuations is not entirely understood, significant

contributors are thought to be astronomical cycles that change the Earth’s relationship

to the sun over many thousands of years. Among climate scientists, these cycles are

commonly referred to as Milankovitch Cycles, after Milutin Milankovitch, the Serbian

polymath and university professor who was the first person to fully elaborate the theory

(Graham, 2000).

Milankovitch noted that three cyclical phenomena—eccentricity, axial tilt, and

precession—influence the amount of solar radiation received by any spot at the surface

of the planet at any given time (Graham, 2000). He further determined that this is more

significant for northern latitudes because a large majority of the Earth’s landmass is in

the northern hemisphere (NOAA, 2009). Insofar as land heats and cools more rapidly

than water, Milankovitch concluded that orbital variations favoring more solar radiation

for higher northern latitudes would result in warmer climatic conditions (interglacial

periods), whereas cooler conditions would persist if the opposite were true, resulting in

glaciations (Berger, 1988; Graham, 2000).

THE MECHANICS OF ORBITAL FORCING

Eccentricity. Simply put, eccentricity describes how much the shape of Earth’s

orbit around the sun deviates from a perfect circle (Figure 2.2) (Eccentricity, n.d.). Were

the Earth the only planet in the solar system, its orbital path around the sun would

remain constant (Wikipedia, 2015b). However, because there are other gravitationally

large bodies out there pulling on the Earth, most notably Jupiter and Saturn, the shape of Earth’s orbit varies from nearly circular to slightly eliptical (Climatica, 2015).

Furthermore, because Jupiter and Saturn are not stationary, and their orbital periods

differ, their positions relative to each other and the Earth change over time. This means that sometimes their gravitational pulls on the Earth are additive, having a greater influence on eccentricity (i.e., stretching the Earth’s orbital ellipse), while at other times, their gravitational pulls are in opposition, resulting in less influence on eccentricity

(Riebeek, 2006; Villanueva, 2009; Bloomberg, 2013).

With regard to climate, eccentricity is meaningful because it influences the length

of the seasons. At times of low eccentricity (circular orbit), the length of each season is

roughly equal, while during periods of high eccentricity, seasons can vary by several

days (Bloomberg, 2013). An eccentric orbit also means that the Earth’s distance

from the sun changes throughout the year, thereby affecting the amount of solar

radiation it receives at any given time. And insofar as the solar system is one great

12 clockwork, the Earth’s orbital perturbations are, of course, cyclical, requiring approximately 100,000 years to complete (i.e., point of greatest eccentricity to point of greatest eccentricity) (Graham, 2000, Riebeek, 2006; NOAA, 2009).

Figure 2.2: Variation in Orbital Eccentricity (Simmon, n.d.)

Axial Tilt. Axial tilt, also known as obliquity, describes the difference by which the Earth’s rotational axis deviates from the perpendicular in relation to the Earth’s orbital plane (Figure 2.3) (Wikipedia, 2014). The Earth’s axis is always tilting, but the degree of that tilt changes over time. Again, this is cyclical, with the tilt alternating between a minimum of 22.1 and a maximum of 24.5 degrees (Graham, 2000). A complete cycle (e.g., maximum to maximum) takes approximately 41,000 years

(Graham, 2000, Riebeek, 2006; NOAA, 2009).

With regard to climate, axial tilt is significant because it impacts the amount of

solar radiation received by higher latitudes (northern and southern). When the tilt is more pronounced, more direct solar radiation will reach the northern hemisphere landmass and Antarctica (NOAA, 2009).

Figure 2.3: Variation in Axial Obliquity (Simmon, n.d.)

Precession. Precession describes the wobble in the Earth’s rotational axis, such that the line of the axis traces out a cone as it completes a cycle (Figure 2.4) (Precession, n.d.). It can be visualized by imagining the motion of a top as its spinning begins to slow and its rotational axis begins to wobble, deviating from the vertical. This phenomenon is caused by the tidal forces exerted by the sun and the moon on the

Earth, with a cycle completing approximately once every 19,000 to 23,000 years

(Riebeek, 2006; NOAA, 2009). With regard to climate, axial precession is important because it determines seasons relative to the Earth’s distance from the sun. For example, at our current point in the cycle, the northpole is tilted toward the sun at

Figure 2.4: Precession (Simmon, n.d.)

roughly its maximum distance from the sun (McClure, 2011). That is to say, the summer solstice for the northern hemisphere occurs when the Earth is near its maximum distance (alphelion) from the sun along its elliptical orbital path, and the winter solstice occurs when the Earth is near its minimum distance from the sun

(perihelion) (McClure, 2011). This results in relatively mild winters and cool summers.

Conversely, for the southern hemisphere, the summer solstice occurs near perihelion, while the winter solstice occurs near alphelion, resulting in comparatively hot summers and cold winters (Lomb, 2013). In 10,000 years or so, the situation will be reversed

(McClure, 2011). As with the other two phenomena discussed, precession bears upon the amount of solar radiation each hemisphere receives (Graham, 2000, Riebeek, 2006;

NOAA, 2009).

Milankovitch’s Contribution. Milankovitch’s true genius was not in recognizing

that astronomical cycles could influence climate change—others had advanced similar

theories before him—it was rather in how he formulated a comprehensive model that

predicted when incoming solar radiation should favor glacial advances and when it

should favor interglacial periods, such as the one we are in now (Graham, 2000,

Riebeek, 2006). Unfortunately for Milankovitch, it took scientists another 50 years to

validate his theory using core samples from deep-sea sediments. In their paper

published in Science, Hays, Imbrie, and Shackleton (1976) concluded that “changes in

the Earth's orbital geometry are the fundamental cause of the succession of Quaternary ice ages”6 (p. 1131).

OTHER CONSIDERATIONS

It is worth noting here that, while Hays et al. (1976) consider orbital geometry as

the fundamental cause of glaciations, there is by no means consensus among scientists

that it is the only cause, or that orbital forcing is sufficient in itself to explain such radical

climate swings as those defining the transition between glacial and interglacial periods

(Muller & MacDonald, 1997; Berger, 2012). Generally speaking, orbital variations are

thought to work in concert with or in opposition to various other climate-influencing

phenomena. Among these are solar output, cosmic dust, volcanic emissions, orogeny

(uplift or mountain building), continental drift, surface reflectivity, atmospheric reflectivity,

and atmospheric chemistry (Pidwirny, 2006; The Azimuth Project, 2011). Both orogeny and continental drift have had major influences on global climate, affecting such things

6 Insofar as the Quaternary Period is itself a major ice age, Hays et al.’s use of the term refers to the many periods of glaciation that have occurred within the Quaternary Period.

as insolation7, ocean currents, and global weather patterns (Global Climate Change,

n.d.). However, since both operate on such exceedingly long timescales, they do not

warrant further consideration here. It is sufficient to note that the positions of the Earth’s

landmasses and mountain ranges have remained essentially unchanged for the entire time Homo sapiens have existed (Conner & Harrison, 2003; Smithsonian Institution,

2015). Therefore, their role in the development of human civilization vis-à-vis climate remains constant.

Solar Output. The amount of solar radiation given off by the sun is not constant.

Scientists have linked declines in solar radiation with increased sunspot activity, which has been shown to be cyclical. Models predict that just a one percent change in solar output over a century would result in an average global temperature change of 0.5 to 1

degree Celsius (Pidwirny, 2006). However, directly observed changes in the sun’s

recent output and reconstructed variations based on proxy data indicate that for the past

2000 years, solar radiation has only varied by 0.1 to 0.2 percent (National Research

Council, 2006). This alone is probably not enough to account for significant climate

change but could serve to amplify or mitigate other climate forcing phenomena.

Cosmic Dust. The cosmic dust theory holds that there is a region of space

through which the Earth periodically passes, which contains dust that blocks some of

the incoming solar radiation (Muller & MacDonald, 1997). Whether or not the Earth is

within the dust zone depends on the inclination of the Earth's orbital plane, which varies

over time due to the gravitational pull of other planets (Muller & MacDonald, 1995,

7 The amount of solar radiation reaching the Earth by latitude and by season measured in Watts per square meter (W/m2) (Stine and Geyer, 2013).

1997). As with the period for eccentricity, the variation in the Earth’s orbital inclination completes a full cycle approximately once every 100,000 years (Muller & MacDonald,

1997). That is, Earth will occupy the dusty region of space for a period of time every

100,000 years (Cronin, 2010; Muller & MacDonald, 1997, 1999).

Volcanic Emissions. Volcanic eruptions can influence climate but typically for only short periods of time, a matter of a few years, after a major eruption (Pidwirny,

2006). Furthermore, it is not the dust ejected into the atmosphere that is the principal cause, as is commonly believed, but rather the sulfur dioxide emitted, which reacts with water vapor in the stratosphere to create a haze that reflects some of the incoming solar radiation back into space (Pidwirny, 2006).

A good example of this is the Mount Pinatubo eruption in 1991, which ejected about 20 million tons of sulfur dioxide in the atmosphere (Pidwirny, 2006). This caused the average global air temperature to drop by an estimated 0.8 degrees Celsius in 1992 and 1993 (Pidwirny, 2006). Isolated eruptions are probably not significant enough to have lasting effects on climate but they can work as an amplifying or mitigating factor to other phenomena (Hilderman, 2010; United States Geological Survey (USGS), 2015).

Surface Reflectivity. This phenomenon, also known as albedo, refers to the amount of solar radiation reflected back into space from the Earth’s surface. Different surfaces have different albedo values (Earth System Science Education Alliance

(ESSEA), 2015). Where climate change is concerned, the albedo measure that is most commonly cited is that for snow and ice at the Earth’s poles. Here, approximately 60 to

90 percent of the incoming solar radiation is reflected back into space (ESSEA, 2015).

Conversely, forest cover has a very low albedo value, reflecting only about 8 to 15

percent of the incoming radiation, while deserts reflect around 30 percent (ESSEA,

2015). Currently, the Earth’s average albedo value is about 31 percent (ESSEA, 2015).

Depending on which other phenomena predominate, albedo can function as a

positive or negative feedback (Samson, n.d.). For example, if the Milankovitch Cycles

were at a point favoring glaciation, the increasing ice cover at the Earth’s poles would

act as a positive feedback for cooling by reflecting more of the incoming solar radiation

back into space (NOAA, 2008). On the other hand, were the Earth’s orbital variations in

a phase favoring extreme warming that promoted large-scale desertification of what was

formerly forestland, the increased reflectivity of the desert land could somewhat mitigate

the warming effect (Ellsaesser, MacCracken, Potter & Luther, 1976).

Atmospheric Reflectivity. The main agents of atmospheric reflectivity are

clouds. A number of factors can affect a cloud’s albedo value including height and size

(ESSEA, 2015). Furthermore, the size and number of water droplets forming the cloud

will affect its reflectivity (ESSEA, 2015). For example, towering low-level cumulonimbus

clouds have a high albedo value compared to wispy high-altitude cirrus clouds (ESSEA,

2015). However, while dense low-level clouds tend to reflect more incoming solar radiation, they also prevent more infrared radiation from leaving the Earth’s surface, which causes a greenhouse effect (Watkins, n.d.). Consequently, during warmer wetter conditions favoring more cloud cover, clouds could act as both a positive and negative feedback for warming.

Atmospheric Chemistry. Perhaps the most controversial climate change phenomenon of the last few decades is atmospheric chemistry. The composition of the

Earth’s atmosphere directly affects the strength of the greenhouse effect, which is the ability of the atmosphere to absorb infrared radiation and re-radiate it, in part back to the

Earth’s surface (NOAA, n.d.). The most commonly identified greenhouse gas is carbon dioxide (CO2), but there are other major greenhouse contributors including water vapor, methane, and ozone (NOAA, n.d.). Among these, water vapor contributes most to the greenhouse effect, followed by CO2, methane, and ozone (NOAA, n.d.). There are other greenhouse gases as well, such as nitrous oxide and chlorofluorocarbons (CFCs), but they contribute very little to the greenhouse effect at their current atmospheric concentrations (NOAA, n.d.).

Due to human’s exploitation of fossil fuels, atmospheric concentrations of CO2 have become of great concern in recent decades. During the transition from the last glacial period to the current interglacial, atmospheric CO2 grew from about 180 parts per million (ppm) to about 280 ppm (Jasper and Hayes, 1990). This 100 ppm variation is thought to have had a major amplifying effect on orbital forcing and is also thought to be responsible for the speed with which the climate shifted from glacial conditions to interglacial conditions, which cannot be accounted for by the Milankovitch Cycles alone

(Jansen et al., 2007). Atmospheric CO2 levels of around 280 ppm have characterized the entirety of the Holocene until the early 18th century, when levels began to rise, quite likely due to anthropogenic forces including deforestation, urbanization, industrialization, and the use of fossil fuels (NOAA, n.d.).

With regard to climate change, CO2 concentration can itself act as a forcing

agent or as a positive or negative feedback for another climate forcing phenomenon

(Hausfather, 2007). An example of the former would be the climbing levels of CO2 in

the atmosphere today, introduced largely by man’s use of fossil fuels. In this case, CO2

does not appear to be a feedback response to another phenomenon acting as a prime mover, but is itself the prime mover triggering its own set of feedbacks such as the reduction in albedo from the loss of polar ice (Hausfather, 2007).

An example of the latter would be the CO2 response to orbital forcing. When orbital conditions favor glaciation, polar ice grows and ocean temperature drops

(Oceans at MIT, n.d.). As ice caps grow, they encroach on the lower latitudes of North

America and Europe replacing forestland with ice (Royal Geographical Society, n.d.).

Insofar as forestland acts as a carbon sink, a reduction in forestland reduces the amount of CO2 taken out of the atmosphere, creating a negative feedback to cooling

(Bonan, 2008). However, cooler ocean waters are able to absorb more CO2, making them a more effective carbon sink than warmer waters, thereby creating a positive feedback for cooling (Jansen et al., 2007).

DISCUSSION

As is evident from the foregoing discussion, identifying the cause of climate

change is an extremely complex problem. While there is a fair amount of consensus in

the scientific community that the Milankovitch Cycles have played a prominent role in

the glacial/interglacial transitions of the Quaternary Period (Lowe & Walker, 2015), the

roles of other climate forcing phenomena and feedbacks are not as well understood.

This is especially true in regard to the many relatively minor climate fluctuations that

have occurred during the Holocene since the end of the last glacial period. Because the

Milankovitch Cycles act over such long periods, they cannot adequately account for

most of the Holocene fluctuations (Jansen et al., 2007). However, identifying the precise causes of these fluctuations is not necessary for this project; it is sufficient that we recognize that the Holocene has been characterized by periodic and seemingly

cyclical climate oscillations, in all likelihood caused by some combination of the

phenomena discussed above, and that these oscillations have had a significant effect

on the development of human civilization over the past 10,000 years or so (Anderson,

Maasch, Sandweiss, & Mayewski, 2007).

CHAPTER 3 CLIMATE PAST AND FUTURE

If paleoclimate studies have demonstrated any one fact, it is the complexity of unraveling the dynamic nature and extent of past climate changes. There are so many interrelated variables that influence global meteorological conditions that a comprehensive consideration of them all at once is next to impossible.

Keith Little, Ph.D (2000, p. 5)

CLIMATE PAST

Few would deny that the warming of the globe from the last glacial period to its

current interglacial condition had a profound effect on human civilization. During the

last glacial period, sea levels were as much as 130 meters lower than they are today

(Lambeck, Esat, & Potter, 2002). This made it possible for modern Homo sapiens to

leave Africa and the Near East and move across southern Asia and into Australia

(National Geographic, 2015a). Then, as the Earth warmed, glaciers retreated and sea

levels began to rise. In northeast Asia and northwest North America coastal areas that

were formerly under ice opened up, allowing coastal migrations of people from Asia into

the New World (National Geographic, 2015b). The retreating ice also allowed the

passage of isolated populations in Beringia into the continental interior of North America

before the rising water inundated the land connecting the two continents (Bonnichsen &

Turnmire, 1999). By some estimates, it took only about 1000 years for the human

population to radiate throughout North and South America once these passages were

open (Kelly & Todd, 1988).

Retreating ice sheets in northern North America, Europe, and Asia opened up

huge expanses of inhabitable land in the northern latitudes, while rising sea levels

inundated low-lying coastal areas, no doubt displacing Mesolithic populations and

creating islands where there were once continuous landmasses (Upton & Climate

Central, 2015). Perhaps the best example of this is Doggerland, an estimated 18,000 square mile area that once connected the island of Great Britain with Europe. This area, which is known to have been inhabited by Mesolithic people, existed in one form or another from about 18,000 years ago (the end of the last glacial maximum in Europe) to about 6,000 years ago, when the last remnants of it were finally swallowed by the

North Sea (Spinney, 2012).

By contrast, the Holocene is characterized by much more modest climate oscillations that are not only of much shorter duration but that also appear to have often been regionally variable rather than global in scale (Masson-Delmotte et al., 2013).

Despite the fact that, unlike ice ages, these fluctuations are most likely poorly understood by or entirely unknown to the general public8, they nevertheless appear to

have had a significant influence on the development of human civilization (Anderson et

al., 2007; Foster, 2012).

Generally speaking, the Holocene is characterized by a series of alternating

warm and cool periods, but with an overall cooling trend that began following the

warmest pre-modern period, known as the Holocene Climatic Optimum (HCO)

8 Brulle, Carmichael and Jenkins (2012) found that “information-based science advocacy has had only a minor effect on public concern” (p. 1) about climate change, due in part to a lack of engagement on the part of the public.

(Masson-Delmotte et al., 2013). This overall trend is quite likely a result of precessional forcing; over the last 10,000 years high-latitude solar insolation during the Northern

Hemisphere summer has declined by about 10 percent (Nesje et al., 2005). According to Bond et al. (1997), during the Holocene, periods of relatively stable climate are interrupted by periods of cooling and/or aridification on a 1470+/-500-year cycle. These are sometimes referred to as Bond Events after the author. The causes of Bond Events are not well understood but variation in solar output has been offered as one, albeit controversial, explanation (Bond et al., 2001). These events are also not of equal magnitude, with some appearing only weakly in climate change proxy datasets (Cox,

2005). Consequently, the following discussion of climate oscillations focuses on those events/periods that are at least reasonably well established.

The HCO is thought to have peaked at different times in different regions, but is broadly considered to have spanned the period between 9,000 and 5,000 years ago

(Renssen, Seppa, Crosta, Goosse, & Roche, 2012). Orbital forcing is thought to have been the primary cause of the HCO; within this period the Earth was near its maximum axial tilt and the northern hemisphere summer occurred during perihelion (Lowe &

Walker, 2015). The average annual temperature for North America at the time is estimated to have been 2 degrees Celsius warmer than the pre-modern warming period

(100 BP) (Wanner, Mercolli, Grosjean & Ritz, 2014). Based on relict dunes dating to around 6,000 years ago, the Midwest and Great Plains regions are known to have been more arid than today, possibly even desert-like (Dean, Ahlbrandt, Anderson, &

Bradbury, 1996).

The HCO was interrupted by two abrupt periods of cooling and drying referred to

as the 8.2 kiloyear event and the 5.9 kiloyear event (Bond et al. 1997; Kobashi et al.,

2007). The second of these events, thought to have been brought on in some measure

by orbital forcing, was also the most intense period of aridification during the Holocene

and resulted in the formation of the Sahara Desert from what was previously fertile

grasslands (Claussen, et al., 1999; deMenocal et al., 2000). While conditions warmed

again after the 5.9 kiloyear event, arid conditions in the Sahara persisted and the region

never returned to a grassland ecosystem (deMenocal et al., 2000).

The HCO was followed by another cool period of intense aridification referred to

as the 4.2 kiloyear event (deMenocal, 2001; Wanner et al., 2014). This is thought to

have had a significant impact on the climates of North Africa, the Middle East, the Red

Sea, the Arabian Peninsula, the Indian Subcontinent, and midcontinental North America

(deMenocal, 2001; Wanner et al., 2014).

The next significant warming is known as the Roman Warm Period, which spans

the period of about 300 BC to AD 500 (Wang, Surge, & Walker, 2011). During this

period, the climate was mild and generally warmer and wetter in Europe and North

America. This was followed by what is referred to as the Vandal Minimum or Dark Ages

Cold Period (Biancci & McCave, 1999; Wang, Surge, & Walker, 2011). The overall

cooling of this period, which lines up well with the Bond Event cycle (Berglund, 2003),

was punctuated by the extreme weather events of AD 535 to AD 536 (Little, 2000;

Graslund & Price, 2012). These weather events were caused by atmospheric dust or

aeresols thought to have been the result of a large volcanic eruption or comet/meteor

impact (Little, 2000; Graslund & Price, 2012). During these years, crop failures

occurred throughout Europe, the Middle East, and China as a result of unseasonably

low temperatures and a persistent dry fog or dust veil (Little, 2000; Graslund & Price,

2012). The Vandal Minimum coincides with the Migration Period in European history

Berglund, 2003; Graslund & Price, 2012).

The last two periods prior to the current warming are known as the Medieval

Warm Period (MWP) and the Little Ice Age (LIA), which date to about AD 950 to AD

1400 and AD 1400 to AD 1850, respectively (Desprat, Goni, & Loutre, 2003). With

regard to Holocene climate studies, these two periods have probably been the most

researched; however, there is by no means consensus on the terminal dates of either

(see Table 3 in Desprat et al., 2003). There is strong evidence that these two events

had a broad reach, affecting climate over the Northern Hemisphere, but there is also

growing evidence to suggest that they were in fact global in scale, particularly the LIA

(Mann et al., 2009; Chambers, Brain, Mauquoy, McCarroll, & Daley, 2014). Mann et al.

(2009) found that the MWP was characterized by “warmth over a large part of the North

Atlantic, Southern Greenland, the Eurasian Arctic, and parts of North America” (p.

1258), but that “certain regions, such as central Eurasia, northwestern North America,

and (with less confidence) parts of the South Atlantic, exhibit anomalous coolness” (p.

1259). Furthermore, there is evidence that prolonged droughts affected the western

United States during the MWP (Bradley, Hughes, & Diaz, 2003). In recognition of this variability, some researches refer to the period as the Medieval Climate Anomaly (MCA) rather than identifying it as strictly a warming event (Bradley, Hughes, & Diaz, 2003).

By contrast, the LIA was one of the coldest periods of the entire Holocene, characterized by glacial advances in Europe (Nesje et al., 2005) and pronounced

27 cooling over the Northern Hemisphere continents. Nevertheless, Mann et al. (2009) note that during this period parts of the Middle East, central North Atlantic, Africa, isolated parts of the United States, tropical Eurasia, and the extratropical Pacific Ocean displayed warmth comparable to that of the present day.

CLIMATE FUTURE

For this project, I have chosen to rely on climate projections developed by the

Intergovernmental Panel on Climate Change (IPCC), which is the leading international body for the assessment of climate change. The IPCC was established in 1988 by the

United Nations Environment Programme (UNEP) and the World Meteorological

Organization (WMO) “to provide the world with a clear scientific view on the current state of knowledge in climate change and its potential environmental and socio- economic impacts” (Intergovernmental Panel on Climate Change (IPCC), n.d., p. 1).

IPCC contributors include scientists from all over the world who serve as researchers, authors, and reviewers (IPCC, n.d.). Contributions are voluntary and uncompensated

(IPCC, n.d.). The guiding principles of the organization are comprehensiveness, objectivity, openness, and transparency (IPCC, n.d.).

Specifically, I will be referring to the contributions of the IPCC Working Group I

(WG I), whose research topics include, among other things, historical and paleoclimatic perspectives on climate change, and climate projections. Insofar as the IPCC has been in existence since 1988, the latest findings represent over a quarter century of climate research and refinement. The most recent IPCC publication containing information relevant to this chapter is the IPCC’s Fifth Assessment Report (AR5) (IPCC, 2013f).

The reporting methodology in AR5 uses probability values for climate predictions

“based on statistical analysis of observations or model results, or both, and expert

judgement” (Cubasch et al., 2013, p. 121). These values include the following: virtually

certain 99–100% probability, very likely 90–100%, likely 66–100%, about as likely as not

33–66%, unlikely 0–33%, very unlikely 0–10%, and exceptionally unlikely 0–1%

(Cubasch et al., 2013, p. 121). Furthermore, climate is modeled across four

Representative Concentration Pathway (RCP) scenarios9, which offer projections for

progressively more extreme inputs into the climate system, most notably greenhouse gases from human activity (IPCC, 2013a). For the purpose of this project, I have tried when possible to use only those predictions whose chances are considered likely or better.

In overview, AR5 offers the following general climate statements that are relevant to this study (Collins et al., 2013; Church et al., 2013):

 “Global mean temperatures will continue to rise over the 21st century if greenhouse gas (GHG) emissions continue unabated” (Collins et al., 2013, p. 1031).

 “Temperature change will not be regionally uniform” (Collins et al., 2013, p. 1031).

9 “RCP2.6 One pathway where radiative forcing peaks at approximately 3 W m–2 before 2100 and then declines” (ICPP 2013a, p. 1461).

“RCP4.5 and RCP6.0 Two intermediate stabilization pathways in which radiative forcing is stabilized at approximately 4.5 W m–2 and 6.0 W m–2 after 2100” (ICPP 2013a, p. 1461) .

“RCP8.5 One high pathway for which radiative forcing reaches greater than 8.5 W m–2 by 2100 and continues to rise for some amount of time” (ICPP 2013a, p. 1461) .

 “It is virtually certain that, in most places, there will be more hot and fewer cold temperature extremes as global mean temperatures increase” (Collins et al., 2013, p. 1031).

 “It is virtually certain that, in the long term, global precipitation will increase with increased global mean surface temperature” (Collins et al., 2013, p. 1032).

 “Changes in average precipitation in a warmer world will exhibit substantial spatial variation. Some regions will experience increases, other regions will experience decreases, and yet others will not experience significant changes at all. There is high confidence that the contrast of annual mean precipitation between dry and wet regions and that the contrast between wet and dry seasons will increase over most of the globe as temperatures increase” (Collins et al., 2013, p. 1032).

 “It is very likely that NH [Northern Hemisphere] snow cover will reduce as global temperatures rise over the coming century. A retreat of permafrost extent with rising global temperatures is virtually certain” (Collins et al., 2013, p. 1032).

 “Ocean thermal expansion and glacier melting have been the dominant contributors to 20th century global mean sea level rise” (Church et al., 2013, p. 1139).

 “It is very likely that in the 21st century and beyond, sea level change will have a strong regional pattern, with some places experiencing significant deviations of local and regional sea level change from the global mean change” (Church et al., 2013, p. 1140).

 “It is very likely that the rate of global mean sea level rise during the 21st century will exceed the rate observed during 1971–2010 for all

Representative Concentration Pathway (RCP) scenarios due to increases in ocean warming and loss of mass from glaciers and ice sheets. . . For the period 2081–2100, compared to 1986–2005, global mean sea level rise is likely (medium confidence) to be in the 5 to 95% range of projections from process based models, which give 0.26 to 0.55 meter for RCP2.6, 0.32 to 0.63 meter for RCP4.5, 0.33 to 0.63 meter for RCP6.0, and 0.45 to 0.82 meter for RCP8.5. For RCP8.5, the rise by 2100 is 0.52 to 0.98 meter with a rate during 2081–2100 of 8 to 16 millimeter per year” (Church et al., 2013, p. 1140).

Of most interest here are regions around the world for which the models show presumably beneficial conditions for humanity in the future, namely, marginally warmer and wetter climates in more northern latitudes, and regions for which presumably unfavorable conditions are forecast, namely, those indicative of repeated and/or prolonged drought cycles, or those where rising sea level will put coastal populations at greater risk for displacement. Ultimately, in the following chapter, selected regional predictions will be compared to the historical and archaeological records to see if past conditions can offer insight into future impacts to human populations for those same regions.

The following three figures depict annual mean surface temperature change, seasonal mean precipitation change, and soil moisture change, respectively. Based on this graphically depicted IPCC climate modeling, average surface air temperature is

expected to rise over the entire planet for all RCP scenarios (Figure 3.1) (IPCC, 2013c).

The relative increases by region are also consistent across all RCP scenarios, with

average temperature increasing much more dramatically in more northern latitudes than

in tropical and subtropical zones (IPCC, 2013c). Comparing end-of-the-century (2081-

2100) temperature conditions for RCP8.5 with seasonal precipitation change (Figure

3.2), we can see that most of the United States, northern Europe, and Asia will be

generally warmer with more precipitation, while the Mediterranean region, southwestern

United States, Mexico and Central America, northeastern and southern South America,

and southern Africa will be warmer and drier. In light of this, the findings depicted in

Figure 3.3 are generally to be expected. Areas most at risk for agricultural drought are the Mediterranean region, northeast and southwest South America, southern Africa, and the southwestern United States.

The increased global temperatures of the 21st century are expected to cause

substantial loss of polar ice, thermal expansion of ocean water, and a reduction of water

storage on land (Church et al., 2013). Collectively, this is anticipated to result in the sea

level rises depicted in Figure 3.4. For the most extreme scenario (RCP8.5 [d]), the end-

of-the-century (2081-2100) prediction indicates a sea-level rise of at least 0.5 meter for

essentially all inhabited coastal zones (IPCC, 2013d).

Figure 3.1: Annual Mean Surface Air Temperature Change (IPCC, 2013c, p. 1063)

Figure 3.2: Seasonal Mean Percentage Precipitation Change (RCP8.5) (IPCC, 2013e, p. 1078)

Figure 3.3: Annual Mean Near Surface Soil Moisture Change (2081-2100) (IPCC, 2013b, p. 1080)

Figure 3.4: Mean Regional Relative Sea-level Change (2081-2100) (IPCC, 2013d, p. 1196)

CHAPTER 4 EVIDENCE FOR CLIMATE CHANGE IN THE ARCHAEOLOGICAL RECORD

This chapter will focus on some examples from the archaeological record

demonstrating the putative influence of climate change on the development of human

culture in the Southeastern United States. I’ve chosen the Southeast because it is the

region of the country with which I have the most experience, having spent my entire

professional career here. I’m familiar with prehistoric cultural developments here and

know that the region is extremely rich in cultural resources. Furthermore, the Southeast

is home to what is perhaps the most complex prehistoric society in North America north

of Mexico: the Mississippian Mound Builders (Teltser, 1996). I’ve also conducted some

of my own paleoclimate climate and culture change research in this area of the country as part of an earlier master’s degree in anthropology. The examples I have chosen are from the Late Holocene (the past 2000 years or so) because this period is fairly well researched and because the written record from the Old World for this period is well established and can be useful for comparative purposes. However, there is a sizeable body of literature on the subject of climate and culture change, particularly from the last decade or so, that examines not only the Late Holocene, but also the more distant past.

Some good examples of this research have been assembled by Anderson,

Maasch, and Sandweiss (2007) into a book focusing on the period from roughly 9000 to

5000 years ago (i.e., the HCO). They reasoned that the HCO was a more suitable

focus because, while examining later periods (e.g., the Medieval Warm Period) could be

potentially instructive, they were of relatively short duration and thus did not offer a

basis for understanding “the consequences of sustained higher than average

37 temperatures. . .” (Anderson et al., 2007, p. 3). The logic here is that the future climate may not be characterized by the same sorts of warm and cold cycles that characterized the last 2000 years, but rather may be more consistently warm for a much longer period of time (Anderson et al., 2007).

The research compiled in the book offers a truly global perspective as well, with chapters devoted to Mid-Holocene climate and culture change in North, Central, and

South America; Africa; Mesopotamia; Central China; the basin of the Sea of Japan; the

Western Pacific; and Northern Europe. This well illustrates the growing importance of the subject among archaeologists across all regions of study. Of some relevance to this project is the chapter devoted to the southeastern North America (Anderson, Russo &

Sassaman, 2007) and, to a lesser degree, the chapter devoted to Northern Europe

(Karlen & Larsson, 2007).

Anderson, Russo and Sassaman (2007) have shown that the Mid-Holocene

Southeast exhibited some of the same developments noted for the later warm periods discussed below, such as monumental architecture (mounds); long-distance trade; greater political complexity; and more elaborate mortuary practices. They also noted that “areas rich in resources. . . tended to have the highest levels of sociopolitical complexity. . .” (Anderson, Russo and Sassaman, 2007, p. 478).

Interestingly, Karlen and Larsson (2007) offer a contradictory point of view to

Anderson et al.’s (2007) rationale for choosing the HCO as a more suitable period of study (i.e., that it was a period of sustained higher than average temperatures) than later periods. They note that in northern Europe at least, the HCO was interrupted by

38 several relatively cool periods, and it may in fact have been an abrupt and significant warming after one of these cool periods that triggered the adoption of agriculture in

Northern Europe (Karlen & Larsson).

Anderson et al.’s (2007) opinions notwithstanding, understanding climate and culture change during the Late Holocene is considered a useful pursuit by researchers interested in gaining insights into future climate and culture change (deMenocal, 2001;

Rosen, 2007; Foster, 2012). In his concluding remarks in a study of the drought-related collapses of the Akkadian (Mesopotamia), Maya (Yucatan), Mochica (coastal Peru), and

Tiwanaku (Bolivian-Peruvian altiplano) civilizations, deMenocal (2001) notes that

“[e]fforts to understand past cultural responses to large and persistent climate changes may prove instructive for assessing modern societal preparedness for a changing and uncertain future” (p. 672).

EVIDENCE FROM THE SOUTHEASTERN UNITED STATES

The timeframe I will examine here spans what are known in the archaeology of the Southeast as the Middle Woodland (350 BC-AD 500), Late Woodland (AD 450-AD

950), Mississippian (AD 950-AD 1700), and Protohistoric (AD 1500-AD 1700) Periods

(National Park Service, n.d.a). These generally coincide with the Roman Warm Period,

Vandal Minimum, Medieval Warm Period and Little Ice Age, respectively (Foster, 2012).

Divisions among these periods are based on a number of things including significant changes in material culture, settlement patterns, subsistence strategies, mortuary practices, sphere of influence, and putative political structure (Walthall, 1980; Knight;

1996; Teltser, 1996; Anderson & Mainfort, 2002; Nassaney, 2000; National Park

Service, n.d.a). Insofar as the first three periods predate European contact, no written

history exists; what is known about them is largely due to the work of archaeologists.

Middle Woodland. The Middle Woodland is characterized by elaborate

ceremonial earthworks; burials containing exotic mortuary goods indicative of elite

status individuals (i.e., possible evidence of hierarchical society); and extensive

interregional trade networks extending from the Rockies to the eastern United States

and the Great Lakes to the Gulf Coast. Subsistence was based on seasonal hunting

and gathering; however, there is some evidence to suggest maize horticulture was

practiced in some areas (Walthall, 1980; Anderson & Mainfort, 2002; National Park

Service, n.d.b).

Late Woodland. By contrast, the Late Woodland exhibits significantly curtailed

trade; nucleation of villages; evidence of increased conflict (e.g., injuries noted on

human remains, adoption of the bow); greater exploitation of so-called hinterland

environments (e.g., uplands) and expedient shelters (e.g., rock shelters); greater

subsistence breadth (both faunal and botanical species); and a move toward greater

use of undecorated utilitarian wares (ceramics vessels) (Walthall, 1980; Little, 2000;

Nassaney, 2000; Anderson & Mainfort, 2002; Smith, 2002; National Park Service, n.d.a).

Mississippian. The Mississippian Period is generally regarded as the zenith of

Native American cultural expression in the pre-contact Southeast. It is characterized by hierarchical chiefdom-level societies that practiced maize agriculture and whose political spheres of influence centered on towns known for their monumental residential and

40 ceremonial architecture consisting of earthen platform mounds (Walthall, 1980; Knight,

1996; Teltser, 1996). The material remains (e.g., pottery, personal adornments, iconography, etc.) are far more elaborate and diverse than anything recovered from

Late Woodland contexts (Figure 4.1 and 4.2) (Walthall; 1980; Knight, 1996; Teltser,

1996; National Park Service, n.d.a). The period shares some similarities with the

Middle Woodland, namely, elaborate mortuary practices and extensive trade networks.

Figure 4.1: Mississippian Period Figure 4.2: Rattlesnake Disk from Duck Bowl from Moundville (Bomar, Moundville (Moundville Decorative Disc, n.d.) 2014)

Protohistoric. By 1500 AD, Mississippian society had largely collapsed, but it is difficult to know precisely what the cause was (Walthall, 1980). Remnant of the culture persisted in some areas until the late seventeenth century (United States Environmental

Protection Agency (EPA), 2014). The onset of the Little Ice Age could well have created conditions such that the carrying capacity of the land could no longer support the standing population (Foster, 2012). But the terminal Mississippian coincides with early

European contact, which introduced diseases, which by some estimates may have

reduced the indigenous population to only a small fraction of pre-contact numbers

(Bianchine & Russo, 1992; Teltser, 1996; Betts, 2006; Foster, 2012).

INTERPRETATIONS

Regarding the Middle-to-Late Woodland transition discussed above, Smith

(2002) interpreted the move toward the exploitation of hinterland environments, greater subsistence breadth (specifically, a transition from first-line to second-line resources), and the focus on simple utilitarian ceramics and evidence of a cultural response to a climatic downturn. He reasoned that the less favorable cooler and drier climate of Late

Woodland reduced the land’s carrying capacity, making it necessary for people to

expand their resource base both geographically and taxonomically. More labor-

intensive and mobile subsistence strategies led to a more pragmatic attitude toward fashioning and decorating ceramic vessels, resulting in simpler utilitarian plainware pottery sets (Smith, 2002).

Little (2000) made similar observations in relation to the exploitation of shellfish during the Middle and Late Woodland in the Tennessee Valley. The depositional sequencing noted in shell middens appears to closely follow the climate oscillations characterizing the progression from the Roman Warm Period (Middle Woodland) to the

Vandal Minimum (Late Woodland), where absence of shell is associated with the former period while a preponderance of shell is associated with the latter (Little, 2000). This was interpreted to be evidence for a warm wet period followed by a cool dry period

(Little, 2000). Presumably, more shoals form during drier times when river levels are lower and siltation is reduced, favoring the propagation of shellfish, which was a readily exploited resource during the Late Woodland in that area.

While the examples from the Southeast are compelling in their own right, it bears

mentioning here that similar patterns of culture change have been gleaned from the archaeological records of the American Southwest, Southern Plains, Trans-Mississippi

South, and Central Mississippi Valley as well (Foster, 2012). And of course, European historians and climate scientists have long recognized a similar pattern of cultural expansion and contraction across these same temporal boundaries in Europe. Rosen

(2007) writes “[t]he Late Holocene, although more stable than previous periods, does have enough variability to greatly impact human societies as seen in the influence of the

Medieval Warm Period and the Little Ice Age on European civilization in the last 1,500

years” (p. 177). And Foster (2012) was compelled enough by the evidence to write the

following:

These dramatic changes in climate and the natural environment prompted distinct cultural responses in the southern temperate zone of North America. Warm periods are often characterized in agrarian societies by population and economic expansion, increased subsistence and settlement security, the extension of long-distance networks of trade and interaction, and the construction of monumental architecture. Cool and mesic [dry] periods are often characterized by reduced social complexity and by general social stress and contraction. (p. 167)

It is probably worthwhile to point out here that each cultural period discussed

above did not exist in a vacuum. That is, the fortunes of one period surely must have

borne upon the fortunes of the following period. In times of plenty (i.e. warm and wet

periods), we can reasonably conclude that populations would have grown as a result of

the increased carrying capacity of the environment. It is not difficult to imagine how this

43 standing population would have magnified the resource stress experienced during a subsequent climatic downturn, especially if that downturn was a particularly abrupt one

(Smith, 2002). Interestingly, this may point to population control as one potential strategy for mitigating the adverse effects of future climate change.

CHAPTER 5 CONCLUSIONS

Here I will return to the research questions raised in Chapter 1:

Can we make useful predictions about human responses to future climate change by examining past human responses to climate change in the southeastern United States?

The answer to this is a qualified yes. In general, the historic and archaeological records show a pattern of social decline/collapse and/or increased conflict as a result of climate-related stresses brought on by cooler and/or drier conditions over the past 2000 years (Little, 2000; Smith, 2002; Foster, 2012). For the modern world, this could certainly be the case for politically unstable regions or developing countries where increased aridification results in greater food stress (United Nations High Commission for Refugees, 2009; Oliver-Smith, 2012). Even for First World countries, prolonged drought in certain regions (e.g., the American Southwest) could result in population migrations due to water shortages and an ever-increasing cost of living (National

Climate Assessment, 2014).

Are the predictions about the severity of future impacts to human society borne out by the historic and archaeological records?

As it turns out, this is a difficult question to answer because credible predictions about the severity of future impacts are difficult to parse from the available literature.

The climate modeling and reporting methodology of the IPCC offers only heavily qualified and broadly stated predictions across multiple input scenarios. However, one useful takeaway here is that the historic and archaeological records indicate the scale of climate events necessary to have a meaningful influence on human society is not very

great. McMichael (2012) writes that “[b]eyond impacts of weather disasters, great

undulations in the fates and fortunes of societies throughout the Holocene epoch have

been associated with seemingly small but sustained climatic changes, affecting crops, livestock, epidemic outbreaks, social unrest, and conflict” (p. 4730). Given this, it seems likely that climate changes predicted for even the IPCC’s most favorable

(modest) input scenario (RCP 2.6) will have significant and far-reaching impacts on society.

Do the historic and archaeological records also provide evidence of beneficial effects of climate change to human society and are any of these a reasonable predictor of future beneficial effects?

The use of term “beneficial effects” here requires a value judgment; therefore,

I’ve chosen to accept that things like agriculture, expansion, and increased political organization are positive developments. I should also point out that the causal role of

climate in these developments is by no means a foregone conclusion, particularly for

the latter two. Nevertheless, there is evidence that climatic conditions favoring

agriculture coincided with the adoption of it in parts of the New World and the expansion

of it in parts of the Old World, and that these developments could be responsible for

expansion (spheres of influence and trade) and increased political sophistication

(Foster, 2012).

With regard to second part of the question, the answer is a qualified no. The

periods of favorable climatic conditions discussed in Chapter 4, namely, the Roman

Warm Period (Middle Woodland) and the Medieval Warm Period (Mississippian), were

warming and wetting trends emerging from preceding periods that were considerably

cooler than today (Foster, 2012). Furthermore, the maximum warming for the Roman

Warm Period and the Medieval Warm Period was in all likelihood no greater than

modern era conditions and quite possibly cooler than current conditions (Masson-

Delmotte et al., 2013). Considering that by 2100, the predicted warming trajectory will

probably push average global temperature past anything that has been experienced

during the Holocene to date, there are no past conditions that are analogous to

projected future conditions (extrapolated from data provided in Collins et al., 2013 and

Wanner et al. 2014).

FINAL REMARKS

Archaeology is the study of human culture through its material remains, and if done correctly is by necessity a discipline of synthesis. Ask a bridge engineer how the study of human anatomy, or linguistics, or astronomy might help him solve a problem in

his field, and he’ll probably throw you a funny look. Ask the same question of an

archaeologist, and you’re likely to get an answer many times longer than you bargained

for. This is quite naturally to be expected of course. After all, it was human endeavor

that shaped the material before it went into the ground; it stands to reason that virtually

any field of human endeavor could have some bearing on understanding that material

once it comes back out.

Climate science is one field of interest that may bring new relevance to

archaeology. In a 2006 publication entitled Surface Temperature Reconstructions for

the Last 2,000 Years, The National Research Council (NRC) noted,

In areas where writing was not widespread or preserved, archeological evidence such as excavated ruins can also sometimes offer clues as to how climate may have been changing at certain times in history and how human societies may have responded to those changes. (p. 6)

In my view, this understates the potential contribution of the archaeologist. The strength

of archaeology as a valuable discipline lies in its comprehensiveness, its willingness to engage any field of study in pursuit of “clues” that will help solve a problem or answer a question. Not only is it a conduit between the past and the present, but it is also a conduit between seemingly disparate fields of interest. Ultimately, it is this power to connect diverse interests that accounts for its popular appeal.

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