20K yrs ago was during the Last Glacial Aridity Maximum/Maxima 22-13,000 14C y.a. I don't think the "greening" of the Sahara happened till after the end of the Ice Age:
SAHARA
(dates in Guo et al are given in 14C years ago on the left, approximate calibrated of 'real' dates are given on the right) Moist 9,500-8,200 14C ya (10,400-9,100 ya)
Slight drying 8,200-8,000 14C ya (9,100-8,900 ya)
Moist 8,000-7,000 14C ya (8,900-7,900 ya)
Moderately dry 7,000-5,700 14C ya (7,900-6,500 ya)
Moist 5,700-4,000 14C ya (6,500-4,500 ya)
Very dry - as dry as at present - 4,000-3,800 14C ya (4,500-4,100 ya)
Slightly moister than present 3,800-3,500 14C ya (4,100-3,700 ya)
After 3,500 14C ya (3,700 ya). Remaining about as dry as at present
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