1. Department of Geological and Atmospheric Sciences; Iowa State University; Ames IA 50011 USA
2. Pacific Northwest National Laboratory; Joint Global Change Research Institute; College Park MD 20740 USA
3. Climate Change Science Institute; Oak Ridge National Laboratory; Oak Ridge TN 37831 USA
4. Ministry of Education Key Laboratory for Earth System Modeling; Center for Earth System Science; Tsinghua University; Beijing 100084 China