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Executive Summary By the end of this century, without a reduction in emissions, atmo - spheric CO2 is projected to increase to levels that Earth has not experi- enced 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. Throughout its long geological history, Earth has had two fundamen- tally different climate states—a cool “icehouse” state characterized by the waxing and waning of continental-based ice sheets at high latitudes, and a “greenhouse” state characterized by much warmer temperatures globally and only small—or no—ice sheets. Although Earth has been in an icehouse state throughout the time that humans evolved and for the previous 30 million years, Earth has been in the warmer greenhouse state for most of the past 600 million years of geological time. As greenhouse gas emissions propel Earth toward a warmer climate state, an improved understanding of climate dynamics in warm envi- ronments 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 sci - entists underestimated the sensitivity of Earth’s 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 involv- 1

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2 UNDERSTANDING EARTH’S DEEP PAST ing 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 per- manent 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 to describe the existing understanding of Earth’s past climates, and to identify focused research initiatives to better understand the insights that the deep-time record offers into the response of Earth systems to projected future climate change. Throughout this report, “deep time” refers to that part of Earth’s history that must be reconstructed from rock, and is 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 called for in this report is on the long record of Earth’s history prior to the Pleistocene. Although deep-time greenhouse climates are not exact analogues for the climate of the future, past warm climates—and particularly abrupt global warming events—provide important insights into how physical, 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, 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 may occur, prompting major societal questions: How soon could abrupt and dramatic climate change occur, and how long could such change persist? HIGH-PRIORITY DEEP-TIME CLIMATE RESEARCH AGENDA The following six elements of a deep-time scientific research agenda have the potential to address enduring scientific issues and produce excit- ing and critically important results over the next decade: • To understand how sensitive climates are to increased atmo - spheric CO2. • To understand how heat is transported around the globe and the controls on pole-to-equator thermal gradients.

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3 EXECUTIVE SUMMARY • To understand sea level and ice sheet stability in a warm world. • To understand how water cycles will operate in a warm world. • To understand abrupt transitions across tipping points into a warmer world. • To understand ecosystem thresholds and resilience in a warming world. 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 estimates of precipita- tion, seasonality, aridity, and soil productivity in the geological past. • Continental and ocean drilling transects to collect high-resolution records of past climate events and transitions, to determine climate parameters before and after these events, and to model the dynamic pro - cesses causing these transitions. • Paleoclimate modeling focusing on past warm worlds and extreme and/or abrupt climate events, at high resolution to capture regional paleo- climate variability. Model outputs will be compared with climate records from drilling transects and fine-tuned. • A transition from single-researcher or small-group research efforts to a much broader-based interdisciplinary collaboration of observation- based scientists with climate modelers for team-based studies of important paleoclimate events. ENCOURAGING A BROADER COMMUNITY UNDERSTANDING OF CLIMATES IN DEEP TIME The public—and indeed many scientists—have minimal apprecia- tion of the value of understanding deep-time climate history and appear largely unaware of the relevance of far distant past times for Earth’s future. The paleoclimate record contains surprising facts—there have been times when the poles were forested rather than being icebound; there were times when the polar seas were warm; there were times when tropical forests grew at midlatitudes; more of Earth history has been greenhouse than icehouse. Such straightforward 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, is perhaps science’s best tool to understand Earth’s climate future.

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4 UNDERSTANDING EARTH’S DEEP PAST 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. 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 indi- viduals, communities, and public policy.