mechanism for very-long-period changes. How would you cycle in and out, and what would the trigger be?

NICHOLSON: Well, if you assume some large-scale trigger in the general atmospheric circulation, maybe a major change in SST, you could get a continental-scale pattern like the wet 1950s all over Africa. But in other areas the rainfall goes back to normal, whereas in the Sahel the anomalies persist for 10 or even 20 years. There has to be some other smaller-scale forcing that reinforces the existing conditions until it's overridden by some larger trigger in the atmosphere-ocean circulation.

COLE: Ropelewski and Halpert's analysis of global rainfall suggests that the ENSO teleconnection with African rainfall is strongest in the east. Do you see a strong ENSO signal in the variability in the area you've been looking at?

NICHOLSON: There's a small one in the coastal region, but nothing in the central Sahel.

RIND: Shukla, your hypothesis suggests two questions to me. The first is, can it be documented—through satellite observations, for instance—that the vegetation really changed so extremely in those 10° latitude belts? And the second is, did those British experiments in which SST anomalies related so well to the Sahel changes, also produce the drying conditions in China and India between the 1950s and 1980s?

SHUKLA: We have exaggerated the extent of land degradation in our experiments. But there is a very clear shift in the rainfall between the 1950s and the 1980s, and Tucker and his colleagues have shown from the vegetation data that there is a very clear relationship between the two. If you look at the latitude of an isoline, it is definitely going down; the 200-mm isoline has dropped by 2° or 2.5°. So there are changes at the boundary, albeit of less magnitude than what I have chosen for this experiment.

BERGMAN: Sharon, when I was at the Climate Analysis Center, we found amazing discontinuities in precipitation time series in Western Africa that suggested that the station sites for some locations must have been moved, although there was no record of a change. Did you find this, and were you able to correct for it?

NICHOLSON: There were only a couple of stations in my network that had to be thrown out because of that sort of discontinuity, though we did find differences at the same station because of varied sources. Our approach has been to take aerial averages and hope that the outliers get factored out. But let me make a pitch here for the need to put together an archive of some of the old, forgotten data sources, such as pilot balloon data, to help us understand decadal and longer-scale variability.

RASMUSSON: It is still not clear to me that we are seeing the effect of human intervention. The vegetation seems to move north again in wetter years. Can you see an underlying southward trend'?

NICHOLSON: Shukla and I differ somewhat in our views here. I think that so far human intervention has very little to do with it. Many of the papers on desertification are essentially inferring continent-wide desertification from two data points in West Africa. But I don't think that the vegetation's recovery changes any of the ideas about the relation between low-frequency forcing and land surface.

One other thing I'd like to mention with respect to Shukla's argument is that I think one variable has been left out: dust. If I had to put money on any of those land-surface variables, I'd pick dust. It responds to both rainfall and land-surface factors, it has the best memory in the system, and it has shown the most consistent relationship over time with rainfall fluctuations.

SHUKLA: That is certainly a point that ought to be discussed further, if we had time. I'd like to think a little more before I put my money on dust. But let me add just a couple of things. The population pressure has been higher than ever before in the past 20 or 30 years, and that period coincides with the most severe drought in the 100 or so years for which we have reliable instrumental data. Proxy-data evidence of persistent droughts by no means rules out a human role in the current situation. It is also possible that human intervention, such as large-scale agriculture and afforestation, has the potential for reversing the present tendency, whatever its origin.



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