Lgm

+BIQME>1

(asynchronous)

• exp. dryland veget. —* add. cooling Africa and Australia

• conifer —► tundra —► add, cooling in W. Europe

• vegetation-snow albedo —► improved high lat. climate

• 2 stable soL: bright and dark desert

• Greatly reduced forests at all latitudes

• Extensive tundra and steppes in mid-latitudes

• Expanded deserts

Tundra and polar deserts decrease by 3 x 10 km" as compared w ith the present day, and the vegetation snow-albedo feedback amplifies the high northern latitude w arming, In the sub tropics, precipitation increases with a reduction of the desert areas (between 10 and 30 N, the desert fraction becomes 14% instead of 71% as at the present). The results of their coupled atmosphere-ocean-1vegetation model show strong interactions between the sea surface temperature, sea ice, and vegetation cover at boreal latitudes, but in the subtropics the atmosphere-vegetation feedback is the most important.

As already cautioned, it must be stressed that such results cannot be considered definitive, and more research must be done before robust conclusions can be drawn. Indeed, Kutzbach and Liu (1997), for example, showed that, if we allow a dynamical ocean to interact with the atmosphere, the increased amplitude ot the seasonal cycle of insolation in the Northern Hemisphere 6 kyr ago could have increased tropical Atlantic-sea surface temperature in late summer and, in turn, could have enhanced the summer monsoon precipitation of northern Africa, The processes mainly responsible for this precipitation increase in north Africa are therefore different from those of Ganopolski et aL (1998), involving aspects of changes in sea surface temperature and in atmospheric circulation that cannot be simulated in models other than the GCMs. The explanation of a given climate change is therefore stil! model-dependent, and the effort must be sustained to build climate models in which the most important processes acting in the real world would be treated equally well.

8.4 Impact of Sea Level Change in the LLN Model

To investigate the sea level-ice volume feedback, the NH climate model of LLN has heen extended to the whole Earth, and sea level has been allowed to vary. As a consequence, during glacials the ice sheets can grow up to the shoreline, leading to a larger ice volume (Figure 8.J). At the Last Glacial Maximum, for example (Table 8.3), the North American ice sheet increases by about 7%, the Eurasian ice sheer by about 25%, and the Greenland ice sheet by 20% when compared w ith the experiment using a fixed sea level. Consequently, when the sea level is allowed to change, the simulated total ice volume amounts to 71 x 10f] km \ with 17 over North America, 14 over Eurasia,

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