Conclusions

1. A history of the climate indicates that the present interglacial period corresponds to a desertification period in arid lands. There is much evidence that the climate of arid lands has been very variable since the last glacial maximum. Climatic changes are different from region to region, as indicated by changes in precipitation in the Dead Sea and the Indus River basin.

2. The SST has increased markedly in the low latitude ocean during the past 60 years. The global energy and water cycle has intensified in response to the increase in SST, resulting in an increase in precipitation in some regions and a decrease in other regions. Generally speaking, the precipitation in arid lands seems to be on a decreasing trend.

3. The residence time of groundwater in arid lands is very long, which suggests a relatively small amount of natural recharge compared with the huge amount of groundwater reserve. High salinity in groundwater is a result of long residence time in the aquifer but, in arid lands, shallow groundwater with shorter residence time is also highly saline in some cases.

4. Basic scientific research is needed for the proper management of soil and water in arid lands. Recently, interesting results about the role of surface dry layer that reduces evaporation loss from the soil surface have been obtained.