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 characterized 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 decisions. 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 continental climates, have on regional and global ecosystems? Because of
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 understand 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 reconstructed 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 climate dynamics and at least partial understanding of the short-term feedbacks 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 potential 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.
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 physical, biogeochemical, and biological processes operate under warm conditions. 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 increasing 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 characterized 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 climate 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 calibration and temporal resolution (subannual to submillennial) offered by
near-time paleoclimate archives. A critical prerequisite for accurate projections of future regional and global climate changes based on GCMs, however, 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. Modeling 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 elements 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 questions 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
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 characterized 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 regulation 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 icehouse 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 equilibrium sea level change in a warmer world, the nature of ice age termination, and the timescales at which such feedbacks and responses could operate
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 understand 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. Understanding 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 phenomenon” 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 importance. Because of their proven potential for capturing the dynamics of past abrupt changes, intervals of abrupt climate transitions in the geological 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
the past, in particular during times when the rates of change were sufficiently 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 organisms 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 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 many species, 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 models, 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 conditions (including paleoproductivity). This will require targeted efforts to
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 paleoclimate time series that are spatially resolved, temporally well constrained, and of high precision and accuracy.
• 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 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, development 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 drilling. 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 computational 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 ownership 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.
capabilities for the development, calibration, and testing of highly precise and accurate paleoclimate proxies; and will allow 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. Making the transition from single-researcher or small-group research efforts to a much broader-based interdisciplinary 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 community 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 scientific 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 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 understandable 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 symposia 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 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 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 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 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 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 promise for informing individuals, communities, and public policy.