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summer, respectively) is caused by a decrease of the planetary albedo related to an extension of the boreal forests and a decrease of the subtropical deserts. In high northern Latitudes, a northward expansion of the boreal forest due to summer warming leads to an annual w arming via the vegetation-snow -albedo feedback, strongly amplified by the sea-ice feedback. In the subtropics, a strong positive feedback between vegetation and precipitation leads to a greening of the Sahara. (Contrary to the temperature, precipitation changes in summer over the Nil continents. North Africa in particular, are related mainly to the pure contribution of vegetation changes, as show n in Table 8,4b. Finally, a strong w arming of the North Atlantic, together with an increased freshw ater flux into the Atlantic basin, leads to a weakening of the thcrmo-halinc circulation, which in turn results in a warming of the Southern Hemisphere (up to 2 C near the Antarctic) and in a negative feedback for the Northern f lemisphere high latitudes.

8.6 Conclusions

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