The atmosphere’s concentration of carbon dioxide—a potent greenhouse gas—has been increasing in recent years faster than had been forecast by even the most extreme projections of a decade ago. At current 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 unprecedented 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).
• 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 permanently 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 increasing 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 thousands 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
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 document 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 climate dynamics and at least partial understanding of the short-term (subcentennial) 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 however, 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.
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 perhaps 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 present 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 magnitude than those anticipated during this century raise obvious questions about the societal impacts that are likely to result from future temperature rise.
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 understanding 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 acidification 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
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).
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 cyanobacteria, 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 characterized by a metazoan fauna, rich in diversity, that arose following the geologically 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 presented 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 extreme 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.
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 extensive 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.
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.
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 information 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 understanding 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/.
BOX 1.3 Statement of Task
The geologic record contains physical, chemical, and biological indicators 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 environmental forcing parameters and their impact on ecosystems, an NRC study will:
• Assess the present state of knowledge of Earth’s deep-time paleoclimate 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.