Expansion or shrinking of Sahara Desert and frozen lands in the Arctic and Tibetan Plateau

Global climate change has extensively modified landforms and terrestrial ecosystems in many parts of the world during the past decades. Expansion of the Sahara Desert (SD) and greening of the Arctic tundra-glacier region (ArcTG) and of the Tibetan Plateau alpine tundra-grassland region (TPATG) have been hot subjects under extensive investigations. The terrestrial conditions in these regions have had substantial climate effects. However, quantitative and comprehensive assessments of the landform changes and reliable future projections in these regions are lacking. Here we use both observations and climate and ecosystem models to quantify/project changes in the extents and boundaries of the SD, ArcTG, and TPATG based on climate and vegetation indices. The biophysical-ecosystem model, SSiB version 4 coupled with TRIFFID is coupled with the NCEP Climate Forecast System. It is found that, based on observed climate indices, the SD expands 8% and the ArcTG shrinks 16% during 1950-2015, respectively; and TPATG reduces 10% during 1981-2015. SD southern boundaries advance 100 km southward during 1950-2015. ArcTG boundaries are displaced about 50 km poleward in 1950-2015. The simulated climate indices show similar results and also reveal the roles of anthropogenic forcing and two-way vegetation-climate feedbacks. The projected climate indices show these trends will continue in 2015-2050. Assessments based on vegetation indices in SD and ArcTG areas have shown similar advance/retreat and provide cross-validation and uncertainty estimation.