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 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?

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 exciting and critically important results over the next decade:

• To understand how sensitive climates are to increased atmospheric CO2.

• To understand how heat is transported around the globe and the controls on pole-to-equator thermal gradients.



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