9.3.3.3 Other Forcings and Sources of Uncertainties

In addition to forcing uncertainties discussed above, a number of other uncertainties affect the understanding of pre-industrial climate change. For example, land cover change may have influenced the pre-industrial climate (Bertrand et al., 2002; Bauer et al., 2003), leading to a regional cooling of 1°C to 2°C in winter and spring over the major agricultural regions of North America and Eurasia in some model simulations, when pre-agriculture vegetation was replaced by present-day vegetation (Betts, 2001). The largest anthropogenic land cover changes involve deforestation (Chapter 2). The greatest proportion of deforestation has occurred in the temperate regions of the NH (Ramankutty and Foley, 1999; Goldewijk, 2001). Europe had cleared about 80% of its agricultural area by 1860, but over half of the forest removal in North America took place after 1860 (Betts, 2001), mainly in the late 19th century (Stendel et al., 2006). During the past two decades, the CO₂ flux caused by land use changes has been dominated by tropical deforestation (Section 7.3.2.1.2). Climate model simulations suggest that the effect of land use change was likely small at hemispheric and global scales, estimated variously as –0.02°C relative to natural pre-agricultural vegetation (Betts, 2001), less than –0.1°C since 1700 (Stendel et al., 2006) and about –0.05°C over the 20th century and too small to be detected statistically in observed trends (Matthews et al., 2004). However, the latter authors did find a larger cooling effect since 1700 of between –0.06°C and –0.22°C when they explored the sensitivity to different representations of land cover change.

Oceanic processes and ocean-atmosphere interaction may also have played a role in the climate evolution during the last millennium (Delworth and Knutson, 2000; Weber et al., 2004; van der Schrier and Barkmeijer, 2005). Climate models generally simulate a weak to moderate increase in the intensity of the oceanic meridional overturning circulation in response to a decrease in solar irradiance (Cubasch et al., 1997; Goosse and Renssen, 2004; Weber et al., 2004). A delayed response to natural forcing due to the storage and transport of heat anomalies by the deep ocean has been proposed to explain the warm Southern Ocean around the 14th to 15th centuries (Goosse et al., 2004).