IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group I: The Physical Science Basis
Figure 6.1

Figure 6.1. (Top) Atmospheric CO2 and continental glaciation 400 Ma to present. Vertical blue bars mark the timing and palaeolatitudinal extent of ice sheets (after Crowley, 1998). Plotted CO2 records represent five-point running averages from each of the four major proxies (see Royer, 2006 for details of compilation). Also plotted are the plausible ranges of CO2 from the geochemical carbon cycle model GEOCARB III (Berner and Kothavala, 2001). All data have been adjusted to the Gradstein et al. (2004) time scale. (Middle) Global compilation of deep-sea benthic foraminifera 18O isotope records from 40 Deep Sea Drilling Program and Ocean Drilling Program sites (Zachos et al., 2001) updated with high-resolution records for the Eocene through Miocene interval (Billups et al., 2002; Bohaty and Zachos, 2003; Lear et al., 2004). Most data were derived from analyses of two common and long-lived benthic taxa, Cibicidoides and Nuttallides. To correct for genus-specific isotope vital effects, the 18O values were adjusted by +0.64 and +0.4 (Shackleton et al., 1984), respectively. The ages are relative to the geomagnetic polarity time scale of Berggren et al. (1995). The raw data were smoothed using a five-point running mean, and curve-fitted with a locally weighted mean. The 18O temperature values assume an ice-free ocean (–1.0‰ Standard Mean Ocean Water), and thus only apply to the time preceding large-scale antarctic glaciation (~35 Ma). After the early Oligocene much of the variability (~70%) in the 18O record reflects changes in antarctic and Northern Hemisphere ice volume, which is represented by light blue horizontal bars (e.g., Hambrey et al., 1991; Wise et al., 1991; Ehrmann and Mackensen, 1992). Where the bars are dashed, they represent periods of ephemeral ice or ice sheets smaller than present, while 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) remains an important area of uncertainty that could affect estimates of future sea level rise. (Bottom) Detailed record of CO2 for the last 65 Myr. Individual records of CO2 and associated errors are colour-coded by proxy method; when possible, records are based on replicate samples (see Royer, 2006 for details and data references). Dating errors are typically less than ±1 Myr. The range of error for each CO2 proxy varies considerably, with estimates based on soil nodules yielding the greatest uncertainty. Also plotted are the plausible ranges of CO2 from three geochemical carbon cycle models.