impacts on ecological systems, and typically involve hysteresis (Lenton et al., 2008).

ICEHOUSE-GREENHOUSE TRANSITIONS

The following sections describe four periods of past climate change—icehouse-to-greenhouse or greenhouse-to-icehouse transitions—that were driven by slow (long-term) climate forcing across a critical threshold that led to abrupt and highly variable climate responses, as examples of what can be gleaned from the deep-time geological record of climate change and the scientific challenges that persist.

Initiation of the Cenozoic Icehouse

The early Cenozoic greenhouse Earth was plunged from a protracted state of warmth into its current glacial state 33.7 million years ago (Ma), at the Eocene-Oligocene boundary. The transition from a relatively deglaciated climate state to one in which the Antarctic ice sheet grew to between 40 and 160 percent of its modern size occurred within ~200,000 to 300,000 years (Coxall et al., 2005; Liu et al., 2009b). A long-term decrease in CO2, commencing after the Early Eocene Climate Optimum at 52 Ma, has been proposed as the main cause of this cooling trend (Box 3.2) (Edmond and Huh, 2003; Kent and Muttoni, 2008). A CO2 decrease through yet another apparent threshold (from as high as 415 ppmv [parts per million by volume] in the early Pliocene to ~280 ppmv; Pagani et al., 2010; Seki et al., 2010) most probably accounted for the initiation and growth of northern hemisphere ice sheets at around 3 Ma (DeConto et al., 2008; Lunt et al., 2008).

All of the elements of a tipping point climate transition are recorded by this greenhouse-to-icehouse turnover (Kump, 2009). As the climate system reorganized itself, it experienced an overshoot (the Oi-1 climate event) into a deep glacial, which was colder and with larger ice sheets than would be sustained during the less extreme conditions of the glaciated Oligocene (Zachos et al., 1996). The calcium carbonate compensation depth in the oceans deepened substantially in two 40-thousand-year (ky) long steps (separated by 200 ky) that occurred synchronously with the stepwise onset of major permanent ice sheets in Antarctica (Coxall et al., 2005). This instability in the climate system persisted for ~200 to 300 ky (Zachos et al., 2001b) and caused major changes in ocean and atmosphericic circulation with widespread effects on most marine and terrestrial ecosystems (Pearson et al., 2008). Such a characteristic response of a homeostatic feedback system implies an underlying dynamic that still remains to be fully understood but could result from changes in, and the interplay between,



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