FOREWORD: DESERTIFICATION
With deserti®cation now being acknowledged as a major environmental problem, it seemed an appropriate time for the Study Group on Erosion and Deserti®cation in Mediterranean Regions (MED) and the Commission on Geomorphological Response to Environmental Change (GERTEC) of the International Geographical Union (IGU) to hold a joint session on the theme `Understanding and combatting desert-i®cation under global change' at the General Assembly of IGU, which took place in August 1996 in The Hague (The Netherlands). Several scientists, who are playing a prominent role in deserti®cation research, agreed to present keynote papers dealing with various perspectives or aspects of deserti®cation. The following four papers, presented at the Conference, are complementary to each other. They are written by authors actively concerned with the problems and contain insights derived from their practical experience.
The main issues addressed are: What are the main causes of deserti®cation? What triggers lead to the emergence of mutual feedback processes and the loss of the ability of a system to recover from a stressor? Or is deserti®cation another Tragedy of the Commons? What are the changes necessary for rehabilitation and how can this rehabilitation be achieved?
Mainguet and Gomez da Silva consider all of the related issues in an introductory review of the concept of the deserti®cation processes and their underlying causes. Within this they outline the fundamental diculties and opportunities for remediation strategies. Their paper is chie¯y based on long practical experience in Africa and Northeast Brazil.
Even well-meaning governments can sow the seeds of deserti®cation as long as quick pro®ts have priority over sustainable land-use policies, and at the same time can be instrumental in its remediation. McClure describes the important role of various policy issues, with special reference to the United States. He illustrates the speci®c problems encountered in highly industrialized countries as diering from certain developing countries, and highlights the equally speci®c dierences for remediation and rehabilitation.
McClure has played a role in the negotiations by the United States in the Deserti®cation Convention, and provides a good overview of the past and current state of aairs in the United States.
Remediation policies must be underpinned by a sound knowledge of ecosystem processes. Puigdefa bregas directs what is perhaps the most complete arid lands research ®eld studies site in Europe, at the Rambla Honda (Almeria, Spain). From years of measuring both abiotic and biotic processes, he has developed an ecosystem perspective to the problem. This is clearly re¯ected in his paper in which he considers the fundamental problems of loss of resilience when surpassing threshold conditions. He gives a conceptual basis for research on ecosystem rehabilitation.
Finally, what can be learned from deserti®cation processes and the impact of global change on deserts? Lavee, et al. have been undertaking measurements of deserti®cation processes along a climatological transect across the Judean Desert from Jerusalem to Jericho. Although full of geomorphological riddles, it was clearly demonstrated that there are fundamental dierences between arid, semiarid and semihumid process domains.
It remains for us, as organizers of the IGU Joint Session, to thank the authors for contributing to this volume. We also thank the editors of this journal for facilitating the publication and major editing of these papers.