Amplification Of Change By Vegetation Cover

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So far, relatively little modeling has been done on other grassland and desert regions of the world, but the suspicion must be that some of these also show instability in climate that is amplified by vegetation. Apart from the Sahara and Arabia, there are certainly some regions that have a history of large, repeated changes in climate over the past 10,000 years or so. One example is semi-arid northwestern China, which shows great instability in climate on the timescale of millennia. At various times, the climate on the fringes of the Mongolian desert became much moister, moist enough to grow an abundance of crops in areas that are now too dry to cultivate. Farming communities thrived in areas now barely inhabited and mostly devoid of vegetation. Fossil pollen and wood from wild plants such as trees, and changes in the iron oxide chemistry of the soils, confirms that the climate was much wetter at these times. Using a combination of different indicators, geologists have put together a general history of climate change in the region (Table 5.2).

So, it seems that there were several separate moist phases when trees and crop-growing spread out across northwest China, the main times being between about 9,400 and 7,900 years ago, and between 6,500 and 4,900 years ago. Although these broadly fall within the same phase as the moist Sahara, when summer sunlight was at its greatest, in China there are some striking fluctuations in the climate that do not occur in the Green Sahara. At these times, the climate in western China switched from much moister than present to drier than present, before later switching back again.

14.7

13.8

13.5

14.7

13.8

13.5

L JJM'Mip* 1 ^

(b)

-1-1-I-1-1-1-1-1-1

to T3

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(c)

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1 '

F

0.8-

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c

0,6

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-3000 -2000

-5000 -4000 voir

Figure 5.9. In the Sahara, during the last 9,000 years, the summer solar energy input changes slowly (a) but because of vegetation feedback the rainfall (c) and especially the vegetation cover (d) changes much faster, flipping from quite dense vegetation to virtually no vegetation. After Bonan.

-3000 -2000

-5000 -4000 voir

Figure 5.9. In the Sahara, during the last 9,000 years, the summer solar energy input changes slowly (a) but because of vegetation feedback the rainfall (c) and especially the vegetation cover (d) changes much faster, flipping from quite dense vegetation to virtually no vegetation. After Bonan.

Table 5.2. Climate history of northwestern China over the last 10,000 years. From: Petit-Maire and Guo.

Drier than present

9,900-9,400 yr

Moist

9,400-7,900 yr

Drier than present

7,900-6,500 yr

Moist

6,500-4,900 yr

Drier than present

4,900 yr

Moist

3,200 yr

Relatively dry phase (but still moister than present)

3,200-2,800 yr

About the same as at present (fairly arid)

Since 2,800 yr

From what we know from attempts at modeling the Green Sahara, it seems reasonable that the moist phases in northwestern China might have been accentuated by vegetation feedbacks. When the climate models are applied to this region they do indeed show that the greener landscapes would have helped to pull in more rainfall. However, while these same models produce stable moist conditions during the phase between 9,000 and 6,000 years ago in the Sahara (and instability only later, as the summer sunlight decreased), they do not predict the instability of rainfall in northwestern China. The climate instability seen in the environmental record of northwestern China is suggestive of a system where climate-vegetation feedbacks are playing an important role, but the current models at least do not show it. It may be that a future generation of more sophisticated models will show up hidden feedbacks that are occurring with vegetation.

So far then, modeling attempts in western China have not revealed any sign of multiple steady states, but the closely alternating history of arid and moist phases suggests that such instability must also exist there. Whatever the underlying cause, it is likely that part of this variability is caused by amplification of background climate changes by vegetation.

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