Vn Voi Vto Vn

The reverse is true during interglaeials, but with a mueh smaller magnitude of the difference — Vqq. That is because the sea level is only slightly above the present-day level during these warm climates, whereas it is much lower during cold conditions (in and Foo, sea level is fixed to its present-day value).

On the other hand, the two experiments that include the vegetation-snow albedo feedback (V\\ and seem to be more realistic and lead to very similar ice volumes, except at the Last Glacial Maximum. In a classical sense, one might conclude that the sea level change plays no role most of the time during the last glacial-interglacial cycle, something that is contradicted by the importance of Fqi described above, In these V\\ and J'I« experiments, taking into account the sea level change (Fjj) as compared with keeping it fixed (Fig) increases the ice volume during the glacials (V\ i > Fiq) at isotopic stages 5d, 4, and 2 (^110, 60, and 20 kyr BP, respectively) and decreases it (I \ [ < V\o) during interglaeials, the difference being smaller during the interglaeials than during

A a the glacials. Actually, V\\ — V]{) — Fqi + V\\, and this difference therefore represents the sum of the pure contribution of sea level (strictly positive only during glacial times)

and the synergism, Fn, between sea level and VSAF

If we compare the experiments in which the vegetation-snow albedo feedback is considered (V\\ and Fio) to the experiments in which it is not, all the rest being kept the same (respectively, i'oi and Joo)* we can see that the vegetation effect helps to moderate the ice volume most of the time (with V\\ < Vq\ and Vm < ifa), reinforcing it only at stage 5d ( where V\\ > I 'm and > frfa). At this isotopic stage 5d, the VSAF can play its normal role, amplifying the initial cooling through an increase of the tundra fraction and of the albedo in winter. This is because before 5d - that is at isotopic stage 5e -the Northern Hemisphere ice volume was indeed minimum, and the pure VSAF could not be masked by the albedo-temperature feedback. This is not the case at the other glacial stages. After 5d, in the experiments with no VSAF (Pin and f'ou), ice is much less removed than in V\\ and Fjo and the ice-albedo feedback rapidly helps to produce more ice in Vq\ and ¿oo than in V\] and Km, hiding the importance of the VSAF. This screening effect of the ice-albedo feedback is actually responsible for the trend visible a a in Fio and Fqi .

Actually, the difference Fio — Vqo is exactly equal to the pure contribution of the VSAF feedback in a two-factor problem. Here, it is negative everywhere except at stage 5d (for the reason explained above) and is larger at the present day than at the LGM (Figure 8.5): in Fio* there are 16 x 10f> km* of ice less than in Ifa now, whereas there are only 9 x 10ft km3 less at the LGM.

This means that this feedback produces in Fjo, when compared with less ice at LGM but much less during the Holocene, reinforcing the amplitude of the variation between these two climatic states.

The difference between the ice volumes of V\\ and is more difficult to explain a a because it reflects both the pure contribution of the VSAF, Fm, and the synergism, V\ \. As seen in Figure 8.5, when the sea level varies, the total vegetation-snow albedo feed-

a a back, F10 ~f V\ \, decreases the ice volume significantly, whether the climate is glacial or mterglacial. Both at the LGM and at the peak of the Holocene, there arc 18 x 10* km * of ice less in V\\ than in ^oi-

two factors being significant but almost counterbalancing each other, and > 0 and a

V\\ < 0 being of the same order of magnitude. Bet ween 30 and about 10 kyr BP (i.e., over the w hole isotoptc stage 2 and slightly more), the sea level factor becomes increasingly important and controls the increase of the ice volume up to its maximum at around 19 kyr BP where f'o\ = I ho + V\\ I

Over the last 10 kyr the entire difference (V\\ — Vqq) can be explained mainly by the vegetation-snow albedo feedback, as was also the case during isotopic stage 5. However, there is a difference between the Holocene and stage 5: over the last 10 kyr, a the synergism between the vegetation and the sea level mechanisms, T 1is significantly

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