FIGURE 3-4 Changes in the greenhouse gases CO₂ and methane over the last 12,000 years. Methane (CH₄) data from GRIP ice core, Summit Greenland (Blunier et al., 1995), and CO₂ from Taylor Dome ice core, Antarctica (Indermühle et al., 1999).

thought to have occurred as the terrestrial biomass began to increase after the end of the glaciation. This is consistent with the view of Smith et al. (2004) that growth of the large methane source in the early Holocene drew down atmospheric CO₂. The subsequent overall increase in CO₂ to the present is thought to reflect a change to colder and drier conditions in tropical and subtropical regions associated with the Neoglacial trend described above. In this scenario, from about 7,000 years ago to the present, the growing peatlands of the northern latitudes are a source of methane, while the decrease in tropical biomass causes an increase in atmospheric CO₂.

Estimates of solar irradiance variations during the pre-modern Holocene assume that cosmogenic isotope information recorded in tree rings (¹⁴C) and in ice cores (¹⁰Be) provide useful irradiance proxies (Figure 3-5). Yet, whereas the source of irradiance variations are magnetically active