Science Daily writes:

New research investigating the transition of the Sahara from a lush, green landscape 10,000 years ago to the arid conditions found today, suggests that humans may have played an active role in its desertification. […] As more vegetation was removed by the introduction of livestock, it increased the albedo (the amount of sunlight that reflects off the earth’s surface) of the land, which in turn influenced atmospheric conditions sufficiently to reduce monsoon rainfall. The weakening monsoons caused further desertification and vegetation loss, promoting a feedback loop which eventually spread over the entirety of the modern Sahara.

Similar effects are being discussed in The Science Show of January 21, about current day South Western Australia:

Europeans began altering the landscape following settlement of the region in 1829. Vast areas of eucalypt forest were cleared for crops and pasture. Cleared land produces less turbulence as storms move across the landscape. And fewer big trees means less transpiration. The atmosphere is drier, the storms slip by, with less rain produced. Now global effects mean cold fronts are passing further south, some missing the continent altogether. The water table is falling by half a metre a year. If the trend continues, there are predictions of a 17% drop in agricultural production in future decades.