[1] Beijing Normal Univ, Coll Global Change & Earth Syst Sci, State Key Lab Earth Surface Proc & Resource Ecol, Beijing 100875, Peoples R China
[2] Univ Cambridge, Mongolian & Inner Asia Studies Unit, Cambridge CB2 3RF, England
[3] Leibniz Inst Agr Dev Transit Econ IAMO, D-06120 Halle, Saale, Germany
[4] Beijing Normal Univ, Sch Syst Sci, Beijing 100875, Peoples R China
[5] Univ Coll Dublin, Sch Math & Stat, Dublin 4, Ireland