The desertification assessment presented at the 1977 UN Conference on Desertification used the ratio of annual precipitation to potential evapotranspiration (P/PE) as a simple and appropriate index of humidity to define the drylands of the world. Using global data sets of temperature and precipitation an annual humidity index (HI) was calculated on a 5° resolution for two independent 30 yr periods, 1931-60 and 1961-90. PE was calculated from surface air temperature using the Thornthwaite method, adjusted using selected regional data sets of Penman PE estimates for Europe and Sudan. An assessment was made of the sensitivity of dryland boundaries in Africa to recent climatic change. Between 1931-60 and 1961-90, 7.3% of the continent has shifted into a drier moisture zone and only 1.7% into a wetter zone. The net areas of hyper-arid and arid lands in Africa have increased from 1931-60 to 1961-90 by 50 and 3 million hectares respectively.