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.