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provide insight into the duration over which elevated greenhouse gas levels have persisted in
the atmosphere—both issues of direct societal relevance. For example, studies of long-term
equilibrium sensitivity of surface temperatures to rising atmospheric CO 2 levels indicate
temperature has been enhanced during times of higher atmospheric CO 2 due to the switching
on of long-term positive feedbacks (Royer et al., 2007; Pagani et al., 2010). Feedbacks such
as changes in ice sheet volume, distribution and composition of terrestrial biomes, and
greenhouse gas release from soils, tundra, and ocean sediments typically operate on
timescales much longer than that of humans and are projected to become increasingly more
relevant on human timescales (decades) with continued global warming (Hansen and Sato,
2001; Hansen et al., 2008).
Figure B2.6 Top: Atmospheric p CO 2 and continental glaciation over the past 800 million
years. Vertical white and gray bars indicate the timing and extent of continental ice sheets
(after Crowley, 1998; Evans, 2000). CO 2 trends are inferred from mineral and biological
proxies (see Royer, 2006, for details of compilation). Plausible ranges of CO 2 estimated using
the GEOCARB III model are also plotted (Berner and Kothavala, 2001). All data have been
adjusted to the Gradstein et al. (2004) timescale. Bottom: Global compilation of deep-sea
benthic foraminifera 18 O isotope records from 40 Deep Sea Drilling Program (DSDP) and
Ocean Drilling Program (ODP) sites (Zachos et al., 2001) updated with high-resolution
records for the Eocene through Miocene intervals (Billups et al., 2002; Bohaty and Zachos,
2003; Lear et al., 2004). Much of the post-Oligocene
18 O variability (~70 percent) reflects
changes in Antarctic and Northern Hemisphere ice volume, which is represented by white
and gray horizontal bars (e.g., Hambrey et al., 1991; Wise et al., 1991; Ehrmann and
Mackensen, 1992). The dashed bars represent periods of ephemeral ice or ice sheets
smaller than present, whereas the solid bars represent ice sheets of modern or greater size.
The evolution and stability of the West Antarctic Ice Sheet (e.g., Lemasurier and Rocchi,
2005) remain uncertain and could affect estimates of future sea-level rise. SOURCE: Caption
adapted from Jansen et al. (2007). Diagram courtesy of Linda Sohl and Mark Chandler.
δ
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