6.4.1.4 How Realistic Are Simulations of Terrestrial Carbon Storage at the Last Glacial Maximum?

There is evidence that terrestrial carbon storage was reduced during the LGM compared to today. Mass balance calculations based on ¹³C measurements on shells of benthic foraminifera yield a reduction in the terrestrial biosphere carbon inventory (soil and living vegetation) of about 300 to 700 GtC (Shackleton, 1977; Bird et al., 1994) compared to the pre-industrial inventory of about 3,000 GtC. Estimates of terrestrial carbon storage based on ecosystem reconstructions suggest an even larger difference (e.g., Crowley, 1995). Simulations with carbon cycle models yield a reduction in global terrestrial carbon stocks of 600 to 1,000 GtC at the LGM compared to pre-industrial time (Francois et al., 1998; Beerling, 1999; Francois et al., 1999; Kaplan et al., 2002; Liu et al., 2002; Kaplan et al., 2003; Joos et al., 2004). The majority of this simulated difference is due to reduced simulated growth resulting from lower atmospheric CO₂. A major regulating role for CO₂ is consistent with the model-data analysis of Bond et al. (2003), who suggested that low atmospheric CO₂ could have been a significant factor in the reduction of trees during glacial times, because of their slower regrowth after disturbances such as fire. In summary, results of terrestrial models, also used to project future CO₂ concentrations, are broadly compatible with the range of reconstructed differences in glacial-interglacial carbon storage on land.

6.4.1.5 How Long Did the Previous Interglacials Last?

The four interglacials of the last 450 kyr preceding the Holocene (Marine Isotope Stages 5, 7, 9 and 11) were all different in multiple aspects, including duration (Figure 6.3). The shortest (Stage 7) lasted a few thousand years, and the longest (Stage 11; ~420 to 395 ka) lasted almost 30 kyr. Evidence for an unusually long Stage 11 has been recently reinforced by new ice core and marine sediment data. The European Programme for Ice Coring in Antarctica (EPICA) Dome C antarctic ice core record suggests that antarctic temperature remained approximately as warm as the Holocene for 28 kyr (EPICA community members, 2004). A new stack of 57 globally distributed benthic δ¹⁸O records presents age estimates at Stage 11 nearly identical to those provided by the EPICA results (Lisiecki and Raymo, 2005).

It has been suggested that Stage 11 was an extraordinarily long interglacial period because of its low orbital eccentricity, which reduces the effect of climatic precession on insolation (Box 6.1) (Berger and Loutre, 2003). In addition, the EPICA Dome C and the recently revisited Vostok records show CO₂ concentrations similar to pre-industrial Holocene values throughout Stage 11 (Raynaud et al., 2005). Thus, both the orbital forcing and the CO₂ feedback were providing favourable conditions for an unusually long interglacial. Moreover, the length of Stage 11 has been simulated by conceptual models of the Quaternary climate, based on threshold mechanisms (Paillard, 1998). For Stage 11, these conceptual models show that the deglaciation was triggered by the insolation maximum at about 427 ka, but that the next insolation minimum was not sufficiently low to start another glaciation. The interglacial thus lasts an additional precessional cycle, yielding a total duration of 28 kyr.