Late Holocene Landscape Collapse of a Trans‐Himalayan Dryland: Human Impact and Aridification

• Published in: Geophysical research letters 2019-12, Vol.46 (23), p.13814-13824
• Main Authors: Menges, Johanna, Hovius, Niels, Andermann, Christoff, Dietze, Michael, Swoboda, Charlie, Cook, Kristen L., Adhikari, Basanta R., Vieth‐Hillebrand, Andrea, Bonnet, Stephane, Reimann, Tony, Koutsodendris, Andreas, Sachse, Dirk
• Format: Article
• Language: English
• Subjects: Arid lands; Arid zones; Aridification; Climate and land use; Climate change; Collapse; Ecosystem services; Environmental degradation; Food security; Geomorphology; Holocene; human activity; Human impact; Human influences; Humidity; Land use; Landscape; late Holocene; paleoclimate; Plant cover; Precipitation; Pressure; Relative humidity; Sciences of the Universe; Slope stability; Soil degradation; Soil erosion; Soil formation; Soil loss; Soil stability; Soils; Thresholds; Tibetan plateau; Vegetation; Vegetation cover
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2019GL084192

Soil degradation is a severe and growing threat to ecosystem services globally. Soil loss is often nonlinear, involving a rapid deterioration from a stable eco‐geomorphic state once a tipping point is reached. Soil loss thresholds have been studied at plot scale, but for landscapes, quantitative constraints on the necessary and sufficient conditions for tipping points are rare. Here, we document a landscape‐wide eco‐geomorphic tipping point at the edge of the Tibetan Plateau and quantify its drivers and erosional consequences. We show that in the upper Kali Gandaki valley, Nepal, soil formation prevailed under wetter conditions during much of the Holocene. Our data suggest that after a period of human pressure and declining vegetation cover, a 20% reduction of relative humidity and precipitation below 200 mm/year halted soil formation after 1.6 ka and promoted widespread gullying and rapid soil loss, with irreversible consequences for ecosystem services. Plain Language Summary Two billion people live in drylands, where small changes in climate and land use can have large impacts on soil stability and food security. It is important to know the thresholds of eco‐geomorphic stability in such settings. Here, we determine the conditions that tipped a trans‐Himalayan dryland into irreversible degradation. We show that in the upper Kali Gandaki valley, Nepal, sustained soil formation terminated after 1.6 ka. Human pressure in the preceding period had reduced vegetation cover, but a 20% drop in relative humidity and a precipitation decrease to below 200 mm/year promoted widespread, rapid badland formation. These values may serve as indicators elsewhere, but local eco‐geomorphic threshold values are likely to differ between landscapes due to other essential variables. Key Points We document an eco‐geomorphic tipping point, plunging a landscape at the southern edge of the Tibetan plateau into an erosional state The tipping point occurred 1.6 ka ago, it was facilitated by a period of human land use and triggered by late Holocene aridification At sustained modern erosion rates of 1 mm/year, soils are removed on a millennial time scale, precluding fast recovery of the landscape