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. Throughout its long geological history, Earth has had two fundamentally 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 environments 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’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 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 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 conditions. 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?