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Summary
By the end of this century, without a reduction in emissions,
atmospheric CO2 is projected to increase to levels that Earth has
not experienced for more than 30 million years. Critical insights
to understanding how Earth’s systems would function in this
high-CO2 environment are contained in the records of warm
periods and major climate transitions from Earth’s geological
past.
Earth is currently in a cool “icehouse” state, a climate state character-
ized by continental-based ice sheets at high latitudes. When considering
the immense expanse of geological time, an icehouse Earth has not been
the norm; for most of its geological history Earth has been in a warmer
“greenhouse” state. As increasing levels of atmospheric CO2 drive Earth
toward a warmer climate state, an improved understanding of how
climate dynamics could change is needed to inform public policy deci -
sions. Research on the climates of Earth’s deep past can address several
questions that have direct implications for human civilization: How high
will atmospheric CO2 levels rise, and how long will these high levels
persist? Have scientists underestimated the sensitivity of Earth surface
temperatures to dramatically increased CO2 levels? How quickly do ice
sheets decay and vanish, and how will sea level respond? How will global
warming affect rainfall and snow patterns, and what will be the regional
consequences for flooding and drought? What effect will these changes,
possibly involving increasingly acidic oceans and rapidly modified con -
tinental climates, have on regional and global ecosystems? Because of
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6 UNDERSTANDING EARTH’S DEEP PAST
the long-lasting effects of this anthropogenic perturbation on the climate
system, has permanent change—from a human point of view—become
inevitable? How many thousands of years will it take for natural processes
to reverse the projected changes?
The importance of these questions to science and to society prompted
the National Science Foundation, the U.S. Geological Survey, and Chevron
Corporation to commission the National Research Council (NRC) to report
on the present state of scientific understanding of Earth’s geological record
of past climates, and to identify focused research initiatives to better under-
stand the insights offered by Earth’s deep-time record into the response of
Earth systems to potential future climate change. Throughout this report,
the “deep time” geological record refers to that part of Earth’s history that
is older than historical or ice core records, and therefore must be recon-
structed from rocks. Although the past 2 million years of the Pleistocene are
included in “deep time,” most of the research described or called for in this
report focuses on the long record of Earth’s history prior to the Pleistocene.
The study of climate history and processes during the current glacial
(icehouse) state shows the sensitivity of Earth’s climate system to various
external and internal factors and the response of key components of the
Earth system to such change. The resolution and high precision of these
datasets capture environmental change on human timescales, thereby
providing a critical baseline against which future climate change can be
assessed. However, this more recent paleoclimate record captures only
part of the known range of climate phenomena; the waxing and waning
of ice sheets in a bipolar glaciated world under atmospheric CO2 levels at
least 25 percent lower than current levels.
In contrast to this reasonably well documented record of recent cli -
mate dynamics and at least partial understanding of the short-term feed -
backs that have operated in icehouse states of the recent past, the climate
dynamics of past periods of global warming and major climate transitions
are considerably less well understood. Paleoclimate records offer poten-
tial for a much improved understanding of the long-term equilibrium
sensitivity of climate to increasing atmospheric CO2 and of the impact of
global warming on atmospheric and ocean circulation, ice sheet stability
and sea level response, ocean acidification, regional hydroclimates, and
the health of marine and terrestrial ecosystems. Deep-time paleoclimate
records uniquely offer the temporal continuity required to understand
how both short-term (decades to centuries) and long-term (millennia
to tens of millennia) climate system feedbacks have played out over the
longer periods of time in Earth’s history. This understanding will provide
valuable insights on how Earth’s climate would respond to, and recover
from, the levels of greenhouse gas forcing that are projected to occur if no
efforts are made to reduce emissions.
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SUMMARY
Although deep-time greenhouse climates are not exact analogues for
the climate of the future, past warm climates—and in particular abrupt
global warming events—could provide important insights into how physi-
cal, biogeochemical, and biological processes operate under warm condi-
tions. These insights particularly include the role of greenhouse gases in
causing—or “forcing”—global warming; the impact of warming on ice
sheet stability, on sea level, and on oceanic and hydrological processes;
and the consequences of global warming for ecosystems and the global
biosphere.
As Earth continues to warm, it may be approaching a critical climate
threshold beyond which rapid and potentially permanent—at least on a
human timescale—changes not anticipated by climate models tuned to
modern conditions may occur. Components of the climate system that are
particularly vulnerable to being forced across such thresholds by increas -
ing atmospheric CO2 include the following: the loss of Arctic summer sea
ice, the stability of the Greenland and West Antarctic Ice Sheets, the vigor
of Atlantic thermohaline circulation, the extent of Amazon and boreal
forests, and the variability of El Niño-Southern Oscillation. The deep-time
geological archive of climate change concerning such thresholds could
provide insight to several major societal questions—How soon could
abrupt and dramatic climate change occur, and how long could such
change persist?
The deep-time geological record provides several examples of such
climate transitions, perhaps the most dramatic of which—with potential
parallels to the near future—was the Paleocene-Eocene Thermal Maximum.
This abrupt greenhouse gas-induced global warming began ~55 million
years ago with the large and rapid releases of “fossil” carbon and major
disruption of the carbon cycle. Global warming was accompanied by
extreme changes in hydroclimate and accelerated weathering, deep-ocean
acidification, and possible widespread oceanic anoxia. Whereas regional
climates in the mid- to high latitudes became wetter and were character-
ized by increased extreme precipitation events, other regions, such as the
western interior of North America, became more arid. With this intense
climate change came ecological disruption, with the appearance of modern
mammals (including primates), large-scale floral and faunal ecosystem
migration, and widespread extinctions in the deep ocean.
A key requirement for an improved understanding of deep-time cli-
mate dynamics is the integration of high-resolution observational records
across critical intervals of paleoclimate transition with more sophisticated
modeling. Projections of climate for the next century are based on general
climate models (GCMs) that have been developed and tuned using records
of the “recent” past. In part, this reflects the high levels of radiometric cali-
bration and temporal resolution (subannual to submillennial) offered by
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8 UNDERSTANDING EARTH’S DEEP PAST
near-time paleoclimate archives. A critical prerequisite for accurate projec-
tions of future regional and global climate changes based on GCMs, how -
ever, is that these models use parameters that are relevant to the potential
future climate states we seek to better understand. In this context, the
recent climate archive captures only a small part of the known range of
climate phenomena because it has been derived from a time dominated by
low and relatively stable atmospheric CO2 and bipolar glaciations. Model-
ing efforts will have to be expanded to capture the full range of variability
and climate-forcing feedbacks of the global climate system, in particular
for the past “extreme climate events” and warmer Earth intervals that may
serve as analogues for future climate. An additional requirement is the
need to improve the more comprehensive Earth system models to better
capture regional climate variability, particularly with different boundary
conditions (e.g., paleogeography, paleotopography, atmospheric pCO2
[partial pressure of carbon dioxide], solar luminosity).
HIGH-PRIORITY DEEP-TIME CLIMATE RESEARCH AGENDA
The committee, with substantial contributions from a broad cross
section of the scientific community, has identified the following six ele -
ments of a deep-time scientific research agenda as having the highest
priority to address enduring scientific issues and produce exciting and
critically important results over the next decade:
Improved Understanding of Climate Sensitivity and
CO2-Climate Coupling
Determining the sensitivity of the Earth’s mean surface temperature to
increased greenhouse gas levels in the atmosphere is a key requirement for
estimating the likely magnitude and effects of future climate change. The
paleoclimate record, which captures the climate response to a full range
of radiative forcing, can uniquely contribute to a better understanding
of how climate feedbacks and the amplification of climate change have
varied with changes in atmospheric CO2 and other greenhouse gases.
A critical scientific focus is the determination of long-term equilibrium
climate sensitivity on multiple timescales, in particular during periods of
greenhouse gas forcing comparable to that anticipated within and beyond
this century. Using the deep-time geological archive to address these ques-
tions will require focused efforts to improve the accuracy and precision
of existing proxies, together with efforts to develop new proxies for past
atmospheric pCO2, surface air and ocean temperatures, non-greenhouse
gases, and atmospheric aerosol contents. With these improved data, a
hierarchy of models can be used to test how well processes other than
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SUMMARY
CO2 forcing (e.g., non-CO2 greenhouse gases; solar and aerosol forcing)
can explain anomalously warm and cold periods and to critically evaluate
the degree to which feedbacks and climate responses are characteristic of
greenhouse gas forcing.
Climate Dynamics of Hot Tropics and Warm Poles
Although paleoclimate reconstructions indicate that the tropics have
been much warmer in the past and that anomalous polar warmth char-
acterized some warm paleoworlds, the current understanding of how
tropical and higher-latitude temperatures respond to increased CO2 forcing
remains limited. This reflects the mismatch between modern observational
data, which have been used to define the hypothesis of thermostatic regula-
tion of tropical surface temperatures, and climate model simulations run
with large radiative forcings that argue against tropical climate stability.
The current consensus is that tropical ocean temperatures seem not to
have been regulated by such a tropical thermostat. Notably, the deep-time
record indicates that the mechanisms and feedbacks in the modern ice-
house climate system, which have controlled tropical temperatures and a
high pole-to-equator thermal gradient, may not straightforwardly apply in
warmer worlds, suggesting that additional feedbacks probably operated
under warmer mean temperatures. An improved understanding of these
processes, which may drive further changes in surface temperatures in a
future warmer world, is important in light of the substantial effects that
higher temperatures would have on tropical ecosystems and ultimately on
regional extratropical climates through teleconnections.
Sea Level and Ice Sheet Stability in a Warm World
Large uncertainties in the theoretical understanding of ice sheet
dynamics currently hamper the ability to predict future ice sheet and sea
level response to continued climate forcing. For example, paleoclimate
studies of intervals within the current icehouse climate state document
variations in the extent of ice sheet coverage that cannot be reproduced
by state-of-the-art coupled climate-ice sheet models. These studies further
indicate that equilibrium sea level in response to current warming may be
substantially higher than model projections. Efforts to address these issues
will have to focus on past periods of ice sheet collapse that accompanied
transitions from icehouse to greenhouse conditions, in order to provide
context and understanding of the “worst-case” forecasts for the future.
Such studies will also refine scientific understanding of long-term equilib -
rium sea level change in a warmer world, the nature of ice age termination,
and the timescales at which such feedbacks and responses could operate
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10 UNDERSTANDING EARTH’S DEEP PAST
in the future. An integral component of such studies should be a focus on
improving the ability to separate the temperature and seawater signals
recorded in biogenic marine proxies, including refinement of existing
paleotemperature proxies and the development of new geochemical and
biomarker proxies.
Understanding the Hydrology of a Hot World
Studies of past climates and climate models strongly suggest that
the greatest impact of continued CO2 forcing would be regional climate
changes, with consequent modifications of the quantity and quality of
water resources—particularly in drought-prone regions—and impacts
on ecosystem dynamics. A fundamental component of research to under-
stand hydrology under warmer conditions is the requirement—because
of its potential for large feedbacks to the climate system—for an improved
understanding of the global hydrological cycle over a full spectrum of CO2
levels and climate conditions. The deep-time record uniquely archives the
processes and feedbacks that influence the hydrological cycle in a warmer
world, including the effect of high-latitude unipolar glaciation or ice-free
conditions on regional precipitation patterns in lower latitudes. Under-
standing these mechanisms and feedback processes requires the collection
of linked marine-terrestrial records that are spatially resolved and of high
temporal resolution, precision, and accuracy.
Understanding Tipping Points and
Abrupt Transitions to a Warmer World
Studies of past climates show that Earth’s climate system does not
respond linearly to gradual CO2 forcing, but rather responds by abrupt
change as it is driven across climatic thresholds. Modern climate is
changing rapidly, and there is a possibility that Earth will soon pass
thresholds that will lead to even larger and/or more rapid changes in
its environments. Climate system behavior whereby a small change in
forcing leads to a large change in the system represents a “tipping phe -
nomenon” and the threshold at which an abrupt change occurs is the
“tipping point.” The question of how close Earth is to a tipping point,
when it could transition into a new climate state, is of critical impor-
tance. Because of their proven potential for capturing the dynamics of
past abrupt changes, intervals of abrupt climate transitions in the geo -
logical record—including past hyperthermals—should be the focus of
future collaborative paleoclimate, paleoecologic, and modeling studies.
Such studies should lead to an improved understanding of how various
components of the climate system responded to abrupt transitions in
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SUMMARY
the past, in particular during times when the rates of change were suf -
ficiently fast to imperil diversity. This research also will help determine
whether there exist thresholds and feedbacks in the climate system of
which we are unaware—especially in warm worlds and past icehouse-
to-greenhouse transitions. Moreover, targeting such intervals for more
detailed investigation is a critical requirement for constraining how long
any abrupt climate change might persist.
Understanding Ecosystem Thresholds and
Resilience in a Warming World
Both ecosystems and human society are highly sensitive to abrupt
shifts in climate because such shifts may exceed the tolerance of organ -
isms and, consequently, have major effects on biotic diversity, human
investments, and societal stability. Modeling future biodiversity losses
and biosphere-climate feedbacks, however, is inherently difficult because
of the complex, nonlinear interactions with competing effects that result
in an uncertain net response to climatic forcing. How rapidly biological
and physical systems can adjust to abrupt climate change is a fundamental
question accompanying present-day global warming. An important tool to
address this question is to describe and understand the outcome of equiva-
lent “natural experiments” in the deep-time geological record, particularly
where the magnitude and/or rates of change in the global climate system
were sufficiently large to threaten the viability and diversity of many spe-
cies, which at times led to mass extinctions. The paleontological record of
the past few million years does not provide such an archive because it does
not record catastrophic-scale climate and ecological events. As with the
other elements of a deep-time research agenda, improved dynamic mod-
els, more spatially and temporally resolved datasets with high precision
and chronological constraint, and data-model comparisons are all critical
components of future research efforts to better understand ecosystem
processes and dynamic interactions.
STRATEGIES AND TOOLS TO IMPLEMENT
THE RESEARCH AGENDA
Implementing the deep-time paleoclimate research agenda described
above will require four key infrastructure and analytical elements:
• Development of additional and improved quantitative estimates
of paleoprecipitation, paleoseasonality, paleoaridity, and paleosoil condi -
tions (including paleoproductivity). This will require targeted efforts to
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12 UNDERSTANDING EARTH’S DEEP PAST
refine existing proxies and develop new proxies1 in particular where the
level of precision and accuracy—and thus the degree of uncertainty in
inferred climate parameter estimates—can be significantly reduced. Proxy
improvement efforts should include strategies for better constraining the
paleogeographic setting of proxy records, including latitude and altitude
or bathymetry, as well as the development of proxies for greenhouse
gases other than CO2 (e.g., methane). Ultimately, such new and/or refined
mineral and organic proxies will permit the collection of multiproxy paleo-
climate time series that are spatially resolved, temporally well constrained,
and of high precision and accuracy.
• A transect-based deep-time drilling program designed to iden -
tify, prioritize, drill, and sample key paleoclimate targets—involving
a substantially expanded continental drilling program and additional
support for the existing scientific ocean drilling program—to deliver
high-resolution, multiproxy archives for the key paleoclimate targets
across terrestrial-paralic-marine transects and latitudinal or longitudinal
transects. Such a drilling strategy will permit direct comparison of the
marine and terrestrial proxy records that record fundamentally different
climate responses—local and regional effects on continents compared with
homogenized oceanic signals.
• Enhanced paleoclimate modeling with a focus on past warm
worlds and extreme and/or abrupt climate events, including improved
parameterization of conditions that are relevant to future climate, devel -
opment of higher-resolution modules to capture regional paleoclimate
variability, and an emphasis on paleoclimate model intercomparison
studies and “next-generation” paleoclimate data-model comparisons. An
increase in model spatial resolution will be required to capture smaller-
scale features that are more comparable to the highly spatially resolved
geological data that can be obtained through ocean and continental drill -
ing. Downscaling techniques using either statistical approaches or nested
fine-scale regional models, will be required to better compare simulated
climate variables to site-specific observational data. Dedicated computa -
tional resources for model development and the application of models to
specific time periods are important requirements to address this element
of the research agenda.
• An interdisciplinary collaboration infrastructure to foster broad-
based collaborations of observation-based scientists and modelers. This
collaboration will allow team-based studies of important paleoclimate
time slices, incorporating climate and geochemical models; will expand
1 When used in the scientific sense (rather than the more common context of stock owner -
ship or voting delegation), proxies refer to parameters that “stand in” for other parameters
that cannot be directly measured. For example, tree ring width is commonly considered to
be primarily a proxy for past temperature.
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SUMMARY
capabilities for the development, calibration, and testing of highly precise
and accurate paleoclimate proxies; and will allow the continued develop -
ment of digital databases to store proxy data and facilitate multiproxy and
record comparisons across all spatial and temporal scales. Such broad-
based and interdisciplinary cultural and technological infrastructure will
require acceptance and endorsement by both the scientific community
and the funding agencies that support deep-time paleoclimatology and
paleobiology-paleoecology studies. Making the transition from single-
researcher or small-group research efforts to a much broader-based inter-
disciplinary collaboration will be possible only through a modification
of scientific research culture, and will require substantially increased
programmatic and financial support.
EDUCATION AND OUTREACH STRATEGIES—
STEPS TOWARD A BROADER COMMUNITY UNDERSTANDING
OF CLIMATES IN DEEP TIME
Despite the potential of the deep-time geological record to provide
unique insights into the global climate system’s sensitivity, response,
and ability to recover from perturbation, the public—and indeed many
scientists—have minimal appreciation of the value of understanding
deep-time climate history and appear largely unaware of the relevance of
far distant times for Earth’s future. The deep-time climate research com -
munity has not made the point strongly enough that the record of the past
can be inspected both for insights into the Earth’s climate system and for
making more informed projections for the future. A strategy for education
and outreach to convey the lessons contained within deep-time records
should be tailored to several specific audiences:
• K-12 elementary and secondary education, with teachers and
children requiring different strategies. Museums are a key resource for
educating children, providing access to the inherently interesting dinosaur
story as a window into deep time. The involvement of teachers in scien -
tific endeavors (e.g., the teacher-at-sea program of the Integrated Ocean
Drilling Program) provides opportunities to demystify science and convey
the excitement of scientific discovery, as well as disseminating scientific
information.
• Colleges and universities, where distinguished lecture tours and
summer schools can add to the more traditional learning elements in
geoscience courses. The integration of deep-time paleoclimatology into
environmental science curricula offers an additional opportunity to con -
vey the relevance of the deep-time record.
• To involve and educate the general public, the deep-time obser-
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14 UNDERSTANDING EARTH’S DEEP PAST
vation and modeling communities have opportunities to break into the
popular science realm by emphasizing their more compelling and under-
standable elements. Immediate opportunities exist for the popularization
of ice cap and ocean drilling research, both of which occur in dramatic
settings that are unfamiliar and interesting to the general public. These
illustrate “science in action,” showing scientists undertaking interesting
activities in the pursuit of knowledge.
• Potential scientific collaborators from the broader climate science
community can obtain an increased understanding of the potential offered
by paleoclimate data and modeling through the creation or use of forums
in which scientists from different disciplines exchange information and
perspectives. This is effectively done within disciplines by talks and sym-
posia at national disciplinary meetings and between disciplines at meetings
of broader groups, such as those hosted by the American Association for
the Advancement of Science.
• Policy makers require scientifically credible and actionable data
on which to base their policies. Faced with a diversity of opinions, they
need credible sources of information. This report and other NRC reports
attempt to play this role, but in a much broader sense the scientific com-
munity must strive to make the presentation of deep-time paleoclimate
information as understandable as possible.
The paleoclimate record contains facts that are surprising to most
people: for example, there have been times when the poles were forested
rather than being icebound; there were times when the polar seas were
warm, and there were times when tropical forests grew at midlatitudes.
For the majority of Earth’s history, the planet has been in a greenhouse
state rather than in the current icehouse state. Such concepts provide
an opportunity to help disparate audiences understand that the Earth
has archived its climate history and that this archive, while not fully
understood, provides crucial lessons for improving our understanding
of Earth’s climate future. Such relatively simple but relevant messages
provide a straightforward mechanism for an improved understanding in
the broader community of the importance of paleoclimate studies.
The possibility that our world is moving toward a “greenhouse”
future continues to increase as anthropogenic carbon builds up in
the atmosphere, providing a powerful motivation for understand-
ing the dynamics of Earth’s past “greenhouse” climates that are
recorded in the deep-time geological record. It is the deep-time
climate record that has revealed feedbacks in the climate system
that are unique to warmer worlds—and thus are not archived
in more recent paleoclimate records—and might be expected to
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SUMMARY
become increasingly relevant with continued warming. It is the
deep-time record that has revealed the thresholds and tipping
points in the climate system that have led to past abrupt climate
change, including amplified warming, substantial changes in
continental hydroclimate, catastrophic ice sheet collapse, and
greatly accelerated sea level rise. Also, it is uniquely the deep-
time record that has archived the full temporal range of climate
change impacts on marine and terrestrial ecosystems, including
ecological tipping points. An integrated research program—a
deep-time climate research agenda—to provide a considerably
improved understanding of the processes and characteristics over
the full range of Earth’s potential climate states offers great prom-
ise for informing individuals, communities, and public policy.
