Because soils are an important sink for greenhouse gasses, we subjected sub-alpine grassland to a six-level climate change treatment. Two independent methods showed that at warming > 1.5 °C the grassland ecosystem lost ca. 14 % or ca. 1 kg C m⁻² in 5 years. This shrinking of the terrestrial C sink implies a substantial positive feedback to the atmospheric greenhouse effect. It is likely that this dramatic C loss is a transient effect before a new, climate-adjusted steady state is reached.