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1
Introduction
The atmosphere’s concentration of carbon dioxide—a potent green-
house gas—has been increasing in recent years faster than had been
forecast by even the most extreme projections of a decade ago. At cur-
rent carbon emission rates, Earth will experience atmospheric CO2 levels
within this century that have not occurred since the warm “greenhouse”
climates of more than 34 million years ago. Atmospheric pCO2 could reach
as high as 2000 ppmv1 if fossil fuel emissions remain unabated, all fossil
fuel resources are used, and carbon sequestration efforts remain at present-
day levels (Kump, 2002; Caldeira and Wicket, 2003). As oceanographer
Roger Revelle noted more than 50 years ago, humans are launching an
uncontrolled “Great Geophysical Experiment” with the planet to observe
how burning fossil fuels will affect all aspects of the climate, chemistry,
and ecology of Earth (Revelle and Suess, 1957). Despite the high stakes
for humans and the natural environment that will result from forcing
an “icehouse” planet into “greenhouse” conditions, we still have only a
poor idea of what this rapidly approaching greenhouse world will be like.
However, studies of past climate states do provide a vision of this climate
future and the substantial and abrupt (years to decades) climate shifts that
are likely to usher in these changed climate conditions.
This projected rise in atmospheric CO2 levels—perhaps at unprec-
edented rates—raises a series of major questions with direct implications
for human civilization:
1 The atmospheric concentration of carbon dioxide—the partial pressure of the gas
(pCO2)—is expressed in units of ppmv (parts per million by volume).
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17
INTRODUCTION
• What is the sensitivity of air and ocean (both shallow and deep)
temperatures to dramatically increased CO2 levels?
• How high will atmospheric CO2 levels rise, and for how long will
these high levels persist?
• How quickly do ice sheets decay and vanish, and consequently
how rapidly does sea level change? Also, if the Arctic is to become perma -
nently ice-free, how will this affect thermohaline circulation and regional
and global climate patterns?
• Are there processes in the climate system that are not currently
apparent or understood that will become important in a warmer world?
• How will global warming affect rainfall and snow levels, and
what will be the regional consequences for flooding and drought?
• What effect will these changes have on the diversity of marine biota?
What will be the impact on—and response of—terrestrial ecosystems?
• Has climate change become inevitable? How long will it take to
reverse the projected changes through natural processes?
How Earth’s climate system has responded to past episodes of increas-
ing and elevated atmospheric CO2 is a critical element of the answers to
these questions.
Temperature Response to Increasing CO2
Recent syntheses suggest that climate sensitivity—the response of
global mean surface temperature to a doubling of atmospheric CO2
levels—lies between 1.5 and 6.2°C (Hegerl et al., 2006; IPCC, 2007; Hansen
et al., 2008). The lower end of this range (≤3°C) is based on modern data
and paleoclimate records extending back no further than the Last Glacial
Maximum of 20,000 years ago, and therefore these estimates factor in
only the short-term climate feedbacks—such as water vapor, sea ice, and
aerosols—that operate on subcentennial timescales. Climate sensitivity,
however, is likely to be enhanced under higher atmospheric CO2 and
significantly warmer conditions due to long-term positive feedbacks that
typically are active on much longer timescales (thousands to tens of thou-
sands of years) (Hansen et al., 2008; Zachos et al., 2008; Pagani et al., 2010).
These physical and biochemical feedbacks—such as changes in ice sheets
and terrestrial biomes as well as greenhouse gas release from soils and
from methane hydrates in tundra and ocean sediments—however, may
become increasingly more relevant on human timescales (decades) with
continued global warming (Hansen and Sato, 2001; Hansen et al., 2008).
Determining the deep-time record of equilibrium climate sensitivity—in
particular during periods of elevated CO2 and at timescales at which long-
term climate feedbacks operate—is thus a critical element in evaluating
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18 UNDERSTANDING EARTH’S DEEP PAST
climate theories more thoroughly and for constraining the magnitude and
effects of future temperature increases (Box 1.1).
Alternating Icehouse and Greenhouse—Earth’s Climate History
Earth is currently in an “icehouse” state—a climate state characterized
by continental-based ice sheets at high latitudes. Human evolution took
place in this bipolar (i.e., with ice sheets at each pole) icehouse (NRC,
2010), and civilizations arose within its most recent interglacial phase.
Such icehouse states, however, account for far less of Earth’s history than
“hothouse” states (Figure 1.2).
Most paleoclimate studies have focused on the interglacial-glacial
cycles that have prevailed during the past 2 million years of the current
icehouse, to link instrumental records with geological records of the
recent past and to exploit direct records of atmospheric gases preserved
in continental glaciers. These relatively recent (Pleistocene) records docu -
ment systematic fluctuations in atmospheric greenhouse gases in near
concert with changes in continental ice volume, sea level, and ocean
temperatures. Their decadal- to millennial-scale resolution has improved
scientific understanding of the complex climate dynamics of the current
bipolar glacial state, including the ability of climate to change extremely
rapidly—in some cases over a decade or less (Taylor et al., 1993; Alley et
al., 2003). Perhaps most importantly, recent ice core archives reveal that
during the past 800,000 years—prior to the industrial rise in pCO2—the
current icehouse has been characterized by atmospheric CO2 levels of less
than 300 parts per million (Siegenthaler et al., 2005).
In contrast to this reasonably well documented record of recent cli -
mate dynamics and at least partial understanding of the short-term (sub -
centennial) feedbacks that have operated in icehouse states of the near
past, scientific understanding of the climate dynamics for past periods
of global warming—when Earth was in a “greenhouse” climate state—is
much less advanced. The paleoclimate records of deep-time worlds,2 how-
ever, are the closest analogue to Earth’s anticipated future climate—one
that will be warmer and greenhouse gas forced beyond that experienced
in the past 2 million years, as atmospheric CO2 contents have already
surpassed by about 35 percent those that applied during the Pleistocene
glacial-interglacial cycles. This deep-time geological archive records the
full spectrum of Earth’s climate states and uniquely captures the ecosys-
2 The deep-time geological record that is the subject of this report refers to that part of
Earth’s history that must be reconstructed from rocks, older than historical or ice core records.
Although the past 2 million years of the Pleistocene are included in deep time, most of the
focus of the research described or advocated here is the long record of Earth’s history prior
to the Pleistocene.
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19
INTRODUCTION
BOX 1.1
Societal Effects—
What Do the Projected Temperature Changes Really Mean?
Global temperatures are projected to rise by at least 1°C, and per-
haps up to 6°C (Figure 1.1), by the end of this century (IPCC, 2007). The
human consequences of this steep rise in greenhouse gases are likely
to be substantial, with decreased precipitation in already drought-prone
regions and widespread social, economic, and health effects (IPCC, 2007).
One yardstick to better appreciate these effects is to consider the roughly
0.2-0.5°C rise in global temperatures that accompanied the Medieval
Warm Period at ~1000 A.D. This modest rise in temperatures resulted in
meadows and stunted beech forests in fjords in southwest Greenland, as
well as ice-free shipping lanes that allowed Vikings to colonize Greenland
between 982 A.D and 1400 A.D. Drought throughout the Americas and
Southeast Asia, coincident with this warming event, has been invoked as
a contributing factor in the collapse of the Anasazi, the Classic Mayan, the
South American Moche civilization, and the Khmer empire of Angkor Wat
(e.g., Haug et al., 2003; Hodell et al., 2005; Ekdahl et al., 2008; Zhang
et al., 2008).
Fluctuations in average global temperatures during the glacial-interglacial
cycles of the past several hundred thousand years caused major shifts in
the areal extent of continental ice sheets and greater than 100-m sea level
changes, with some interglacial periods up to 2-3°C warmer than the pres-
ent day (Otto-Bliesner et al., 2006). Large-scale changes in carbon cycling
and overall greenhouse gas contents, including 50 percent variations in
atmospheric CO2, occurred in response to interglacial warmings (Sigman
and Boyle, 2000; Lea, 2004; Siegenthaler et al., 2005), highlighting the
potential for amplification of future CO2-driven global warming through
climate-CO2 feedbacks. In fact, estimates of temperature response to all
modern forcings, including human and naturally induced factors, indicate
the potential for ~0.6-1.4°C of additional warming—with no additional
greenhouse gas forcing—as the long-term feedbacks that typically operate
on thousands to tens of thousands of years (e.g., changes in surface albedo
feedback with variation in ice sheet and vegetation coverage) become
operative on human timescales (Hansen et al., 2008). The substantial
societal impacts from past temperature increases that were of lesser mag-
nitude than those anticipated during this century raise obvious questions
about the societal impacts that are likely to result from future temperature
rise.
continued
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20 UNDERSTANDING EARTH’S DEEP PAST
BOX 1.1 Continued
FIGURE 1.1 Projections (colored lines), with uncertainty bounds of ±1
standard deviation (shading), for future surface temperature rise from
models that use different economic scenarios. Scenario A2 represents
“business as usual” where temperature is projected to rise by the end of
the century between 2° an
d 5.5°C if no effort is made to constrain the rise of CO2 levels. The solid
bars at right indicate the best estimate (solid line) and possible ranges
(shading) for each scenario.
SOURCE: IPCC (2007, Figure SPM.5, p. 14).
tem response to, and interaction with, this full range of climate changes.
The deep-time record thus offers the potential for a much improved under-
standing of the long-term equilibrium sensitivity of climate to increasing
pCO2, and of the impact of major climate change on atmospheric and
ocean circulation; ice sheet stability and sea level response; ocean acidifi -
cation and hypoxia; regional hydroclimates; and the diversity, radiation,
and decline of marine and terrestrial organisms (see Box 1.2). Furthermore,
the deep-time paleoclimate records uniquely offer the temporal continuity
required to understand how both short- (subcentennial) and long-term
(millennial-scale) climate system feedbacks have played out over the
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?
?
?
Pliocene
Ediacaran Cryogenian
Eocene
Miocene
Oligocene
Paleocene
Silurian
Triassic
Jurassic
Permian
Devonian
Cambrian
Carboniferous
Ordovician
Cretaceous
Cenozoic Neoproterozoic
0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850
Millions of Years
FIGURE 1.2 Although warmer greenhouse conditions (red-brown intervals) have dominated most of the past ~1 billion years of
Earth’s history, there have been extended periods of cool “icehouse” conditions (light-blue intervals) including intervals for which
there is evidence of continental ice sheets at one or both poles (shown as darker blue bars). The question marks in the Cryogenian
reflect uncertainties associated with the geographic extent and duration of inferred glacial events during this time (Allen and
Etienne, 2008; Kendall et al., 2009). The Paleocene-Eocene Thermal Maximum and Mid-Eocene Thermal Maximum are shown as
red bars. The current icehouse began ~34 million years ago with increased glaciation in Antarctica and accelerated with northern
hemisphere glaciation over the past 3 million years.
SOURCES: Compiled based on Miller et al. (2003); Montañez et al. (2007), Bornemann et al. (2008); Brezinski et al. (2008); Fielding
et al. (2008); Zachos et al. (2008); and Macdonald et al. (2010).
21
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22 UNDERSTANDING EARTH’S DEEP PAST
BOX 1.2
The Nonlinear Development of Life on Earth
The Earth has been populated with life, as we know it today, through
a protracted and nonlinear history of evolution characterized by repeated
extinctions and radiations (Figure 1.3). The biosphere in the Precambrian
was dominated by single-cellular organisms such as microbes and cyano-
bacteria, capable of building massive reefal structures and living in the
overall oxygen-poor conditions of the oceans of the time. In contrast,
Phanerozoic faunal life—the past 542 million years—has been character-
ized by a metazoan fauna, rich in diversity, that arose following the geo-
logically rapid radiation of life in the latest Precambrian (see Figure 1.2
for timescale). Floral ecosystems of equal diversity soon populated much
of the Earth following their evolution ~450 million years ago. Throughout
Earth history, physiological evolution and ecosystem dynamics have been
intricately linked to various surface processes and systems (e.g., landscapes,
ocean and atmospheric composition and circulation, soil and hydrological
processes) through interactions and feedbacks, examples of which are pre-
sented in this report and are a fundamental component of interdisciplinary
deep-time studies. Earth’s deep-time history offers numerous examples of
how ecosystems, geosystems, and climate systems have operated in the
absence of various major groups of life and under conditions far more ex-
treme than those of the present day—time intervals when oceans lacked the
major elements of their current buffering capacity or were hypoxic, when
the poles lacked ice sheets, and/or when atmospheric CO2 levels were
higher by hundreds to thousands of parts per million of volume.
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23
INTRODUCTION
Cretaceous-
Paleogene Triassic Permo-Triassic Ordovician Cambrian
PETM Extinction Extinction Extinction “Explosion”
Extinction
Fossil record Chicxulub
Siberian traps
impact
Mammals
Non-avian Dinosaurs
Reptiles
Amphibians
Fish teeth
Land arthropods
Fish
Diatoms
Deep sea calcifers Stromatolites
Calcareous algae
Metazoan fossils
Molecular clock
Grasses
estimates
Forests
Vascular plants
Terrestrial vegetation
Pollen
Ocean hypoxia
Oil
Coal “Snowball” Events
10
δ13C%vPDB
5
0
-5
300
100 400 600
0 200 700 800
500
Geological time (millions of years)
FIGURE 1.3 Timing of major events in late Precambrian and Phanerozoic
evolution. From bottom to top: record of the carbon cycle from carbon
isotopes, showing the transition from the high-amplitude cycles of the
late Neoproterozoic—including several “snowball Earth” episodes—to
the much more muted trends of the Phanerozoic (Hayes et al., 1999).
Periods of abundant coal and oil formation, which include the extensive
coal units of the Carboniferous and Permian, Cretaceous coal, and exten -
sive coal deposits of the Paleogene Arctic, as well as oil deposits formed
during Jurassic and Cretaceous oceanic anoxic events and Mio-Pliocene
oil deposits of the Pacific Rim (Windley, 1995), are shown in black. Ocean
hypoxia (red line) illustrates the reduction in the extent of anoxic or
hypoxic conditions in the deep sea with time, with low oxygen common
in the Paleozoic and intermittent episodes of basinwide to global oceanic
anoxic events in the Mesozoic. The extent of vegetation cover is shown
with green lines, and major groups of oceanic organisms that contribute
to global geochemical cycles either through burrowing (metazoans;
Sheehan, 2001), marine calcifiers that buffer ocean pH (Ridgwell et al.,
2003), or diatoms with their role in the silica cycle (Ridgwell et al., 2002;
Cortese et al., 2004; Lazarus et al., 2009), are shown in blue. Fish (brown
line) appear in the early Cambrian (Shu, 1999) and give rise to terrestrial
amphibians in the Devonian (Selden, 2001). The invasion of land is
accomplished by terrestrial arthropods well before the appearance of
terrestrial vertebrates. Non-avian dinosaurs and mammals evolved from
reptiles in the early Mesozoic (Sereno, 1999; Brusatte et al., 2008). Reptiles
show multiple invasions of the oceans in the Mesozoic, and mammalian
groups invade the ocean several times in the Cenozoic.
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24 UNDERSTANDING EARTH’S DEEP PAST
longer periods of time (millennia to hundreds of thousands of years) that
are necessary to fully understand how Earth’s climate responds to, and
recovers from, the levels of greenhouse gas forcing that will result from
fossil fuel burning over the next century.
COMMITTEE CHARGE AND SCOPE OF THIS STUDY
The National Science Foundation, U.S. Geological Survey, and Chevron
Corporation, with input from the Geosystems initiative3 and the broader
research community, commissioned the National Research Council to
describe the present state of understanding of Earth’s geological record of
past climates, as well as to identify focused research initiatives that would
enhance the understanding of this record and thereby improve predictive
capabilities for the likely parameters and impacts of future climate change.
The study committee was also charged to present advice on research
implementation and public outreach strategies (Box 1.3).
To address this charge, the National Research Council assembled a
committee of 12 members with broad disciplinary expertise; committee
biographical information is presented in Appendix A. The committee held
four meetings between February 2008 and February 2009, convening in
Washington, D.C.; Boulder, Colorado; and twice in Irvine, California. The
major focal point for community input to the committee was a 2-day open
workshop held in May 2008 (see Appendix B), where concurrent breakout
sessions interspersed with plenary addresses enabled the committee to
gain a thorough understanding of community perspectives regarding the
status of existing research as well as future research priorities. Additional
briefings by sponsors and keynote addresses from other speakers were
presented at the initial meeting of the committee (see Appendix C).
The paleoclimate archive contained in the geological record both
offers an opportunity and assigns a responsibility for Earth and climate
science to effectively predict what is likely to happen as Earth warms and
to offer projections with enough precision to assist society to mitigate
and/or adapt to future changes. The examination of climate states in the
deep-time geological record has the potential to provide unique informa -
tion about how Earth’s climate dynamics operate over long time frames
and during changes of large magnitude. Earth’s pending transition into
warmer climates provides the motivation for the description of the under-
standing of past warm periods presented in Chapter 2, and the transitions
into and out of different climate states over differing timescales is the
3 The Geosystems initiative is an interdisciplinary, community-based initiative focused on
understanding the wealth of “alternative-Earth” climatic extremes archived in older parts
of the geological record, as the basis for understanding Earth’s climate future. See http://
www.geosystems.org/.
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25
INTRODUCTION
BOX 1.3
Statement of Task
The geologic record contains physical, chemical, and biological indi-
cators of a range of past climate states. As recent changes in atmospheric
composition cause Earth’s climate to change, and amid suggestions that
future change may cause the Earth to transition to a climatic state that is
dramatically different from that of the recent past, there is an increasing
focus on the geologic record as a repository of critical information for
understanding the likely parameters and impacts of future change. To
further our understanding of past climates, their signatures, and key envi-
ronmental forcing parameters and their impact on ecosystems, an NRC
study will:
• Assess the present state of knowledge of Earth’s deep-time paleo-
climate record, with particular emphasis on the transition periods of major
paleoclimate change.
• Describe opportunities for high-priority research, with particular
emphasis on collaborative multidisciplinary activities.
• Outline the research and data infrastructure that will be required to
accomplish the priority research objectives.
The report should also include concepts and suggestions for an effective
education and outreach program.
focus of Chapter 3. The capabilities and limitations of existing models and
proxies used to describe and understand past climates are addressed in
Chapter 4, providing the backdrop for the recommendations for a high-
priority deep-time climate research agenda and strategies to implement
this agenda which are contained in Chapter 5. Some elements of this
report—particularly the descriptions of existing scientific understanding
of the paleoclimate record and the processes that have controlled that
record—are necessarily technical; nevertheless, every effort has been made
to present the material in terms that are accessible to the broadest possible
audience.
