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6
Conclusions and Recommendations
The current rate of atmospheric increase of CO2—~3 ppmv per year
(IPCC, 2007)—is an order of magnitude or more greater than the increase
in atmospheric CO2 during the last deglaciation, when rapid retreat of
northern hemisphere ice sheets led to rates of sea level rise of up to 5 m
per century (Stanford et al., 2006). The last time the atmosphere contained
CO2 levels comparable to today’s values, during the Pliocene, surface
temperatures were on average ~3°C warmer, the Greenland ice sheet col -
lapsed, and sea level rose by up to 30 m (Pagani et al., 2010; Seki et al.,
2010). With the combination of continued burning of fossil fuels and the
additional contribution of greenhouse gases to the atmosphere through
positive feedbacks in the climate system, future atmospheric CO 2 levels
could exceed 1,000 ppmv (Kump et al., 2009)—levels well above the sta -
bility threshold values for continental ice on Earth (Hansen et al., 2008).
In fact, it is necessary to look back at least 34 million years—prior to the
current icehouse—to examine climate change under such CO2 levels. In
this context, the magnitude and rate of the present greenhouse gas increase
place the climate system in what could be one of the most severe increases
in radiative forcing of the global climate system in Earth history.
To fully evaluate climate forcing feedbacks and tipping points that
may characterize Earth’s future, and to better understand climate change
impacts and recovery, it is necessary to examine the records from past
warm periods when there were similar magnitudes and rates of green-
house gas forcing. The deep-time paleoclimatology record contains a
rich archive of such warm worlds, and the associated transitions into
and out of “greenhouse” conditions. For example, climate reconstruc -
138
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CONCLUSIONS AND RECOMMENDATIONS
tions of the end-Paleocene (~55 Ma), mid to late Cretaceous (~120 to 90
Ma), end-Triassic (~200 Ma), and Late Paleozoic (~300 to 251.2 Ma)—all
periods associated with the massive release of greenhouse gases to the
atmosphere—reveal dramatic changes in oceanic conditions and terrestrial
climates. These changes brought about extensive restructuring of marine
and terrestrial ecosystems that in many cases involved mass extinctions.
These deep-time records also reveal that some of the feedbacks in the cli -
mate system may be unique to warmer worlds—and thus are not archived
in more recent paleoclimate records—and accordingly might be expected
to become increasingly relevant with continued warming. In particular,
long-term feedbacks that are typically active on millennial scales are
likely to become important at the human timescale, leading to substantial
and abrupt (years to centuries) climate modifications. Reconstructions of
past climates show that civilization has evolved in an anomalously stable
period unrepresentative of the climate system’s natural variability. There -
fore, refining current understanding of climate dynamics (e.g., the range,
rates, and magnitudes of feedbacks and change) during past periods of
global warming, particularly times associated with epic deglaciations,
is critical for assessing future risks. Improved understanding of climate
dynamics will also aid efforts to mitigate the impact of continued warm -
ing on regional hydroclimates and water resources, ice sheet and sea level
stability, and the health of marine and terrestrial ecosystems. Exciting
research opportunities to help accomplish this task exist in the untapped
potential of the deep-time geological record.
This report identifies a six-element research agenda designed to
describe past climate variability and to better constrain how Earth’s
climate system has responded to episodes of changing greenhouse gas
levels. The knowledge gained by this scientific agenda will be important
for addressing questions regarding the projected rise in atmospheric CO 2
and the societal implications of this rise. The report also describes the
research infrastructure necessary for successful implementation of the
deep-time paleoclimatology agenda, as well as an education and outreach
strategy designed to broaden our collective understanding of the unique
perspective that the full range of the geological record provides for future
climate change.
Improved Understanding of Climate Sensitivity and
CO2-Climate Coupling
Determining the sensitivity of 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 current understanding of climate sensitivity, defined on the basis of
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modern data and relatively recent paleoclimate records (≤20,000 years), is
associated with large uncertainty (1.5 to ≥6°C). Positive feedbacks typically
considered to have been active on longer timescales, but that may become
increasingly relevant with continued warming, are not considered in
these estimates. An improved definition of long-term equilibrium climate
sensitivity—including more refined constraints on its lower boundary—
over the full range and timescales of past radiative forcing is a major
research priority. An associated focus is on gaining an improved under-
standing of how climate feedbacks and their role in amplifying climate
change have varied with changes in greenhouse gas forcing. Accomplish-
ing this objective will require the development of more accurate and pre-
cise paleo-CO2 and paleotemperature proxies, as well as the development
of new proxies for the full range of greenhouse gases. A complementary
requirement is for high-resolution and high-precision time-series records,
based on integrating multiproxy techniques. Data-model comparisons are
needed to rigorously test non-CO2 forcing mechanisms of global warming,
as well as to refine the understanding of how the Earth’s climate system
would respond to increasing levels of atmospheric CO2.
Climate Dynamics of Hot Tropics and Warm Poles
Recent climate modeling and deep-time paleoclimatology studies have
demonstrated that the long-standing paradigm that the temperatures of
tropical climates do not rise significantly during warm periods because of
some type of temperature buffering mechanism is probably incorrect. Con-
sequently, the mechanisms and feedbacks in the modern climate system
that have controlled tropical and polar surface temperatures—ultimately
leading to the existing relatively high pole-to-equator thermal gradient—
may not operate in warmer worlds. A decreased latitudinal gradient in the
future, which would almost certainly be associated with polar sea ice and
continental ice sheet losses, would change atmospheric wind patterns and,
in turn, ocean circulation—all having potential detrimental effects through
teleconnections (Hay, 2010). To refine knowledge of the processes and
climate feedbacks that may influence surface temperatures under higher
atmospheric pCO2, it is important that high-temporal-resolution, higher-
precision proxy time series be developed across latitudinal transects, with
a focus on reconstructing terrestrial-marine linkages. This will require a
greatly increased effort in high-precision geochronological dating, cou -
pled with substantially more spatially resolved proxy records. A more
comprehensive understanding of the limits of tropical climate stability,
the origin of anomalous polar warmth, and an understanding of how a
weaker thermal gradient is established and maintained in warmer climate
regimes will require further climate model development and deep-time
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CONCLUSIONS AND RECOMMENDATIONS
data-model comparisons. These comparisons would also provide a much
needed test of the efficacy of model projections of future climate.
Sea Level and Ice Sheet Stability in a Warm World
Study of the current icehouse climate state has provided better con-
straints on CO2 and surface temperature threshold levels for ice sheet
stability (Pagani et al., 2005, 2010; Pearson et al., 2009; Seki et al., 2010).
Large gaps, however, remain in the understanding of ice sheet dynamics,
with resulting limitations on the applicability of current coupled climate-
ice sheet models. These issues highlight the uncertainties that still exist in
projections regarding the timescales at which ice sheets would respond to
continued warming and in understanding the influence of feedbacks not
revealed by recent paleoclimate records or considered by future climate
model projections (e.g., the projections used in IPPC, 2007). Consequently,
the magnitude of sea level rise, once climate equilibrium is reached,
remains elusive despite deep-time paleoclimate evidence that it could be
substantially higher than model projections (Rohling et al., 2009). To mark-
edly improve the understanding of climate–ice sheet–sea level dynamics
relevant to a warming Earth, it will be necessary to probe deeper into
Earth’s history to the periods of truly catastrophic ice sheet collapse that
accompanied past icehouse-to-greenhouse transitions. To fully exploit
such deep-time archives will require radiometrically constrained and
spatially resolved marine, paralic, and terrestrial records for both high and
low latitudes. In addition, improved methods for deconvolving tempera -
ture and seawater δ18O from proxy records are needed, as well as targeted
efforts to couple land-ice component models with complex global climate
models that are capable of integrating the atmospheric hydrological cycle.
Understanding the Hydrology of a Hot World
There is broad scientific consensus that one of the largest impacts of
continued CO2 forcing would be major regional climate modifications, with
the likelihood of substantial societal impacts (e.g., water shortages, flood -
ing). The insights gained from reconstructing the processes and climate
feedbacks that influence surface temperatures under higher atmospheric
pCO2 levels are an important element of this research agenda, particularly
because of the sensitivity of climate to small changes in high-latitude and
tropical surface temperatures as a consequence of teleconnections. The
deep-time geological record provides a critical and unique component of
research focused on this issue, because it is the only source of information
regarding how marine-terrestrial carbon and water cycle dynamics have
influenced the global climate system during periods of radiative forcing
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comparable to those projected for the future, including periods of unipo -
lar glacial or fully deglaciated greenhouse conditions. This will require a
greatly expanded effort to develop linked marine-terrestrial records that
are spatially resolved and of high temporal resolution, precision, and
accuracy. New and improved quantitative estimates of paleoprecipitation,
paleoseasonality, paleoaridity and humidity, and paleosoil conditions
(including paleoproductivity) are critical components of this effort.
Understanding Tipping Points and Abrupt Transitions
to a Warmer World
Studies of past climates and climate models show that Earth’s cli-
mate 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 very rapidly, and there is a possibility that
Earth will soon pass thresholds that will lead to even more rapid changes
in Earth’s environments. Consequently, the question of how close Earth is
to a tipping point, and when it could transition into a new climate state, is
of critical importance. Because of their proven potential for capturing the
dynamics of past abrupt changes, intervals of tipping-point climate tran -
sitions in the geological record—including past hyperthermals—should
be the focus of future collaborative paleoclimate, paleoecological, and
modeling studies. Such studies should lead to an improved understand-
ing of how various components of the climate system responded to abrupt
transitions, in particular during times when the rates of change were suf -
ficiently large to imperil diversity. This research will also help determine
whether there exist thresholds and feedbacks in the climate system of
which we are currently 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 as well as
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
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CONCLUSIONS AND RECOMMENDATIONS
fundamental question accompanying present-day global warming. An
important tool to address this question is to describe and understand the
outcome of equivalent “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 species, which at times led to mass extinctions. The paleon-
tological record of the past few million years does not provide such an
archive because it does not record catastrophic-scale climate and ecologi-
cal events. As with the other elements of a deep-time research agenda,
improved dynamic models, more spatially and temporally resolved data-
sets 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 A DEEP-TIME
CLIMATE RESEARCH AGENDA
Four key infrastructure and analytical elements will be required to
implement this high-priority research agenda.
Improved Proxies and Multiproxy Records
Refinement of existing and development of new mineral and
organic proxies for environmental and ecological parameters,
coupled with an enhanced effort to chronologically calibrate tar-
geted intervals with high-precision radiometric ages, are critical
requirements for developing the spatially resolved, multiproxy
paleoclimate and paleoecological time series described in the
research agenda.
Despite exponential advances in the development of paleoclimate
proxies over the past two decades, the precision and accuracy of existing
organic and mineral paleotemperature and paleo-CO2 proxies are compro-
mised by their calibrations to extant analogues, by incompletely under-
stood biological and environmental controls on geochemical signatures,
and/or by their sensitivity to postdepositional alteration. Moreover, paleo-
barometer proxies are limited to CO2, and there is a need for the existing
very limited complement of proxies for estimating past terrestrial climatic
conditions to be expanded and refined. A focused effort to improve exist-
ing proxies and develop new proxies is at the core of the proposed research
agenda, in particular where the level of precision and accuracy—and thus
the degree of uncertainty in inferred climate parameter estimates—can
be quantified and significantly reduced. Such an effort will need to be
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highly collaborative, requiring calibration studies in modern marine and
terrestrial environments as well as laboratory systems. The Critical Zone
Observatories initiative funded by the National Science Foundation may
offer opportunities to integrate such calibration studies into existing
observatories. Ultimately, comparison studies of plant-mineral proxy
estimates that are characterized by differing sensitivities and uncertain -
ties are necessary to test the veracity and sensitivity of each of the proxies.
Proxy development efforts must be complemented by studies that apply
emerging imaging and analytical technology to critically evaluate the
effects of postdepositional alteration on the compositions of isotopic and
geochemical proxies.
Deep-Time Drilling Transects
The recovery of high-quality cores to provide the sample resolu-
tion and preservational quality needed to develop multiproxy
archives for key paleoclimate targets across terrestrial-paralic-
marine transects and latitudinal or longitudinal transects will
require substantially increased investment in scientific conti-
nental drilling and continued support for scientific ocean drill -
ing. Continental drilling will permit direct comparison of the
marine and terrestrial proxy records that record fundamentally
different climate responses (local and regional effects on con-
tinents compared with homogenized oceanic signals) and will
provide the continuous records necessary for high-resolution
dating of critical climate transition intervals.
T he requirement for well-preserved and chronologically well-
constrained proxy records with high spatial and temporal resolution and
precision to analyze environmental and ecological systems in climate
transition is a recurrent theme throughout the research agenda. A transect-
based deep-time drilling program designed to identify, prioritize, drill,
and sample key paleoclimate targets—involving a substantially expanded
continental drilling program and additional support for the existing sci -
entific ocean drilling program—is a high priority for implementing the
recommended research agenda. Although scientific ocean drilling has
provided much of the basis for what is presently known about Neogene
climate dynamics and ocean-climate linkages, there is still a pressing
need for high-resolution sections that carry clear signals of orbital forcing
in older parts of the record, particularly the Paleogene and Cretaceous.
Sections representing the greenhouse intervals for climatically sensitive
regions are still required, specifically in the Arctic and proximal to Ant -
arctica. Continental drilling of cyclic successions, of extended duration
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CONCLUSIONS AND RECOMMENDATIONS
and with high potential for preservation of volcanic ashes, will greatly
expand the opportunity for radiometric and nonradiometric dating and
correlation, thereby facilitating comparison of paleoclimate records across
marine-paralic-terrestrial gradients as a function of time.
Improved Paleoclimate Modules and Models
An enhanced paleoclimate modeling effort, with a focus on
past warm worlds and extreme and/or abrupt climate events,
is critical for refining scientific understanding of the complex
dynamics of past climates and for producing models that can
be adjusted to include forcings or feedbacks not revealed by
shallower-time paleoclimate reconstructions.
As critical boundary conditions of the climate system—greenhouse
gas concentrations, polar ice mass, distribution of biomes—change in the
coming century, calibrations of climate models based on modern systems
and the recent past will become increasingly less relevant. The deep-time
geological record of past climates and major transitions provides the only
test of climate models and their predictions against the range of back-
ground conditions most likely to be relevant to Earth’s anticipated future
climate state if emissions are not reduced. Modeling of ancient climates
characterized by boundary conditions substantially different from those
of the present day, however, presents a substantial challenge to the model-
ing community. In turn, how well such models simulate past climates and
feedbacks inferred from deep time influences the community’s confidence
in the ability of global climate models to forecast future regional and global
climate changes.
To that end, a markedly enhanced effort in deep-time paleoclimate
modeling involving development of higher-resolution modules, improved
parameterization of conditions relevant to future climate, and an emphasis
on paleoclimate model intercomparisons and “next-generation” data-
model comparisons is a fundamental component of the proposed research
agenda. An increase in model spatial resolution will be required to capture
smaller-scale features and regional climate changes comparable in scale to
the spatially resolved geological data that can be obtained through con -
tinental drilling and proxy development. Deep-time data also uniquely
offer the opportunity to carry out model-model-data comparisons for
past warm climates characterized by elevated CO2. Such comparisons will
permit an assessment of the credibility of the performance and param-
eterizations of various community models in a way that future climate
experiments are presently incapable of doing. Achieving this component
of the deep-time initiative will require new tools to facilitate model-data
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146 UNDERSTANDING EARTH’S DEEP PAST
comparisons (e.g., prognostic modules for proxies, geographic informa -
tion system–based tools, refined dynamic vegetation models, metadata
techniques), dedicated computational resources for deep-time climate
simulations, and the development and application of Earth system models
of intermediate complexity that can be integrated as subsystem models
within more complex three-dimensional Earth system models.
Strategies for Fostering Focused Deep-Time Scientific Interaction
Implementing the research agenda described in this report will
require a synergistic research culture among the broad range of
disciplines that can contribute to solving the numerous puzzles
of deep-time paleoclimatology, focusing on specific paleoclimate
time slices as natural laboratories for team-based analyses of
deep-time climates and their impact on Earth systems. Establish-
ment of a cultural and technological infrastructure to support
team-based projects offers the potential for discoveries unattain-
able by single-discipline research or even by more conventional
integrated efforts.
Establishing the scientific collaboration, cross-disciplinary syntheses,
widespread and open data exchange, cross-training of scientists and stu-
dents, and dedicated and focused outreach activities required to address
the research agenda described in this report will require the development
of natural observatories for team-based studies of important paleoclimate
time slices, incorporating climate and geochemical models; capabilities for
the development, calibration, and testing of highly precise and accurate
paleoclimate proxies; and the continued development 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. With-
out the addition of targeted new resources—in addition to existing pro-
grammatic resources—the scientific breakthroughs that can be made by this
broad-based research community will be unlikely to ever come to fruition.
EDUCATION AND OUTREACH—STEPS TOWARD A BROADER
COMMUNITY UNDERSTANDING OF CLIMATES IN DEEP TIME
Despite the potential and importance of the deep-time geological
record, as articulated throughout this report, the public has minimal
appreciation of the relevance of deep-time climates for Earth’s future. This
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CONCLUSIONS AND RECOMMENDATIONS
largely reflects the limited efforts by the scientific community to ensure
that the importance and relevance of scientific efforts and results are con-
veyed to students, teachers, scientific and media partners, policy makers,
and the general public. Barriers such as disciplinary jargon (geological
time, paleoclimate proxies, and numeric climate models), imperfect inter-
pretations and solutions created by uncertainties in temporal resolution,
patchwork spatial resolution, and incompletely calibrated climate proxies,
all present significant challenges for conveying complex messages to the
general public with sufficient simplification but without losing accuracy.
To resolve this issue, a strategy for education and outreach, to convey
the lessons contained within deep-time records, should be tailored to the
range of specific target audiences:
• K-12 elementary and secondary students. Museums are a key
resource for educating students. Involving teachers in scientific endeavors
can help demystify science and convey the excitement of scientific dis-
covery, as well as being a method of disseminating scientific information.
• For colleges and universities, distinguished lecture tours, topi-
cal summer schools, and the integration of deep-time paleoclimatology
into traditional and nontraditional earth science courses offer additional
opportunities to convey the relevance of the deep-time record.
• To involve and educate the general public, the deep-time obser-
vation and modeling communities have opportunities to break into the
popular science realm by emphasizing their more compelling and under-
standable elements. Immediate opportunities to illustrate “deep-time
paleoclimatology in action” to the general public abound, whether the
irreversible impact of past major climate changes on life, extreme glacia -
tions and catastrophic deglaciations, or the mysteries of the ocean. The
scientific community needs to proactively pursue pathways to the public
provided by various multimedia opportunities.
• Potential scientific collaborators from the broader climate science
community can obtain increased understanding of the potential offered
by paleoclimate data and modeling through the creation or use of forums
where scientists from different disciplines exchange information and
perspectives. This can be effectively done between disciplines at meetings
of broader groups (e.g., American Association for the Advancement of
Science) and industry, environmental, ecology, and physical anthropology
conferences.
• 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 National
Research Council reports attempt to play this role, but in a much broader
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148 UNDERSTANDING EARTH’S DEEP PAST
sense the scientific community 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 under-
stood, provides crucial lessons to improving our understanding of Earth’s
climate future.
The possibility that our world is moving toward a “green-
house” future continues to increase as anthropogenic carbon
builds up in the atmosphere, providing a powerful motivation
for understanding 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 that might be expected to become increasingly relevant
with continued warming. It is the deep-time record that has
revealed the thresholds and tipping points in the climate sys-
tem that have led to past abrupt climate change, including
amplified warming, substantial changes in continental hydro -
climate, catastrophic ice sheet collapse, and greatly accelerated
sea level rise. Further, 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 tip -
ping points. An integrated research program—a deep-time cli -
mate 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 promise for
informing individuals, communities, and public policy.
